Patent Publication Number: US-2023141955-A1

Title: Terminal Protective Case and Audio Playback System

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage of International Application No. PCT/CN2021/074057, filed on Jan. 28, 2021, which claims priority to Chinese Patent Application No. 202010130959.2, filed on Feb. 28, 2020. Both of the aforementioned applications are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     This application relates to the field of audio technologies, and in particular, to a terminal protective case and an audio playback system. 
     BACKGROUND 
     As a user&#39;s requirement for volume of a loudspeaker sound of a terminal (such as a mobile phone or a tablet) increases, an amplitude of a speaker (which may also be referred to as a loudspeaker) of the terminal continuously increases. Currently, amplitudes of some speakers reach 0.4 millimeter to 0.5 millimeter or even greater, and an increase in the amplitude of the speaker causes a corresponding increase in a size of the speaker. However, because a current terminal pursues lightweight and thinness, a size of a speaker mounted in the terminal is strictly limited. In this case, it is difficult to greatly increase an amplitude of the speaker, so that volume of the speaker is limited. Consequently, it is relatively difficult to increase loudspeaker volume of the terminal. 
     SUMMARY 
     An objective of this application is to provide a terminal protective case and an audio playback system. The audio playback system includes a terminal protective case and a terminal mounted in the terminal protective case. The terminal protective case has a piezoelectric speaker, and the piezoelectric speaker can make a sound together with a speaker of the terminal, so that the audio playback system has high loudspeaker volume. 
     According to a first aspect, this application provides a terminal protective case, including a housing, a diaphragm, a piezoelectric sheet, and a drive circuit. The housing includes a main body part and a fastening frame part, where a side of the fastening frame part is fastened to a periphery of the main body part, the fastening frame part and the main body part jointly enclose an accommodating groove, and the accommodating groove is configured to accommodate a terminal. When the terminal is accommodated in the terminal protective case, the terminal and the protective case jointly form an audio playback system. The diaphragm is located within the main body part, the piezoelectric sheet is fastened to the diaphragm, a front cavity is formed on a side of the diaphragm, a sound outlet hole is disposed on the main body part, and the sound outlet hole connects the front cavity to the outside of the main body part. The drive circuit is located within the main body part and is electrically connected to the piezoelectric sheet, and the drive circuit is configured to drive, in response to a communication signal of the terminal, the piezoelectric sheet to drive the diaphragm to vibrate. 
     In this application, the terminal protective case forms a piezoelectric speaker including a diaphragm and a piezoelectric sheet. The drive circuit can form a corresponding drive signal based on the communication signal of the terminal. The drive signal drives the piezoelectric sheet to deform, to drive the diaphragm to vibrate. The diaphragm pushes air in the front cavity to vibrate, to make a sound. The sound is propagated to the outside of the main body part through the sound outlet hole, so that the terminal protective case may make a sound. The drive circuit can drive, in response to the communication signal of the terminal, the piezoelectric sheet to drive the diaphragm to vibrate, so that the piezoelectric speaker makes a sound. Therefore, the piezoelectric speaker of the terminal protective case may make a sound together with a speaker of the terminal, to greatly increase loudspeaker volume of the audio playback system. In other words, the audio playback system has higher loudspeaker volume, and still has a good loudspeaker effect in a noisy environment or an outdoor environment. 
     The terminal can control, by using the communication signal, the piezoelectric speaker of the terminal protective case to make a sound or not to make a sound, so that the audio playback system can implement a plurality of loudspeaker modes such as making a sound by the terminal protective case alone, making a sound by the terminal alone, and making a sound by the terminal protective case and the terminal together. When the piezoelectric speaker of the terminal protective case and the speaker of the terminal make a sound together, the piezoelectric speaker and the speaker of the terminal may cooperate to form a multi-channel loudspeaker system. This improves sound field rendering and a stereo effect. Because the piezoelectric speaker has better high-frequency performance, the piezoelectric speaker can supplement and enhance high-frequency performance of the speaker of the terminal, so that the audio playback system has better sound quality. 
     Because a piezoelectric speaker core has a small thickness, when the terminal protective case forms the piezoelectric speaker, a thickness of the main body part of the terminal protective case can be controlled within a small range. This improves a hand holding feeling and portability of the audio playback system. 
     In a possible implementation, there is one piezoelectric sheet, and the piezoelectric sheet is fastened to a side of the diaphragm. In another possible implementation, there are two piezoelectric sheets, and the two piezoelectric sheets are respectively fastened on two sides of the diaphragm. 
     In a possible implementation, the main body part includes a bottom plate portion, a top plate portion, and a connection portion. The bottom plate portion and the top plate portion are disposed opposite to each other. The connection portion is connected between a periphery of the bottom plate portion and a periphery of the top plate portion, and the fastening frame part is connected to the bottom plate portion and/or the connection portion. The sound outlet hole is disposed on the connection portion. 
     In this implementation, the terminal protective case makes a sound from a side surface. When the terminal protective case is held by a user or placed on an object, the sound outlet hole is not easily blocked, so that the terminal protective case can normally make a sound. This improves user experience. 
     In a possible implementation, there may be a plurality of sound outlet holes, and the plurality of sound outlet holes are disposed at intervals. Each sound outlet hole is roughly in a rectangular shape. In another possible implementation, there may also be one sound outlet hole. In another possible implementation, the sound outlet hole may be alternatively in another shape such as a circle or an ellipse. 
     In a possible implementation, the housing includes a main housing and a cover plate, and the cover plate is fixedly connected to the main housing. The fastening frame part, and the bottom plate portion and the connection portion of the main body part are formed on the main housing, and the top plate portion of the main body part is formed on the cover plate. In this case, because the main housing and the cover plate can be manufactured separately, and the main housing and the cover plate can meet different performance requirements, the top plate portion of the main body part, the fastening frame part, and the bottom plate portion and the connection portion of the main body part can meet different performance requirements. For example, a top plate of the main body part has better hardness to improve anti-wear and anti-scratch performance. The fastening frame part, and the bottom plate portion and the connection portion of the main body part have better toughness to improve anti-impact performance, so that overall performance of the housing is better, and more diversified function requirements can be met. In another possible implementation, the fastening frame part and the bottom plate portion of the main body part are formed on the main housing, and the connection portion and the top plate portion are formed on the cover plate. 
     In a possible implementation, the fastening frame part is connected to the bottom plate portion, and the fastening frame part and the bottom plate portion may form a large connection area, to increase connection firmness between the fastening frame part and the bottom plate portion, and improve reliability of the terminal protective case. In another possible implementation, the fastening frame part is connected to the connection portion. In still another possible implementation, the fastening frame part is connected to both the bottom plate portion and the connection portion, to increase a connection area with the main body part. 
     In a possible implementation, the bottom plate portion includes a bottom plate body and a first protrusion. The first protrusion is fastened to a side that is of the bottom plate body and that faces the top plate portion. The top plate portion includes a top plate body and a second protrusion. The second protrusion is fastened to a side that is of the top plate body and that faces the bottom plate portion. The second protrusion is disposed opposite to the first protrusion; and a periphery of the diaphragm is fastened between the first protrusion and the second protrusion. 
     In this implementation, the diaphragm is fastened to the first protrusion and the second protrusion, so that the diaphragm is directly fastened to the main body part of the housing. In this case, a front cavity and a rear cavity of the piezoelectric speaker are formed by using space in the main body part. This improves space utilization of the terminal protective case and facilitates thinness of the terminal protective case. 
     In a possible implementation, the main body part may be made of a hard material, for example, a polycarbonate plastic or a metal material, to reduce housing vibration generated due to vibration of the diaphragm. 
     In a possible implementation, the main body part further includes one or more holders. One end of each holder is fixedly connected to the bottom plate portion, and the other end is fixedly connected to the top plate portion. 
     In this implementation, the one or more holders help improve support strength and connection firmness between the top plate portion and the bottom plate portion, so that overall structural strength of the main body part is significantly improved, and structural reliability of the terminal protective case is higher. The structural strength of the main body part is improved, which is also conducive to normal sound-making of the piezoelectric speaker. 
     In a possible implementation, the front cavity is located between the diaphragm and the bottom plate body and is located in the first protrusion. The first protrusion is in a ring shape, the first protrusion is provided with a first notch, and the first notch connects the front cavity and the sound outlet hole. First space is formed between the diaphragm and the top plate body, and the first space is located in the second protrusion. Second space is formed between the bottom plate body and the top plate body, and the second space is located on the outside of the second protrusion. The second protrusion is in a ring shape, the second protrusion is provided with a second notch, and the second notch connects the second space and the first space to form a rear cavity. In this implementation, space of the rear cavity formed within the main body part is large, so that the piezoelectric speaker has a good low-frequency sound pressure level. 
     In a possible implementation, there may be a plurality of first notches, and the plurality of first notches are disposed at intervals. In this case, a portion that is of the first protrusion and that is located between the two adjacent first notches can still fasten the diaphragm, so that a connection area between the first protrusion and the diaphragm is increased, and connection reliability between the first protrusion and the diaphragm is higher. For example, when there are a plurality of sound outlet holes, the plurality of first notches are disposed in a one-to-one correspondence with the plurality of sound outlet holes; or when there is one sound outlet hole, the sound outlet hole is connected to the plurality of first notches. In another possible implementation, there may be alternatively one first notch, and one first notch is connected to all sound outlet holes. 
     In a possible implementation, the second protrusion is provided with a second notch, and the second notch connects the first space of the rear cavity and the second space of the rear cavity. There may be a plurality of second notches, and the plurality of second notches are disposed at intervals. In this case, a portion that is of the second protrusion and that is located between the two adjacent second notches can still fasten the diaphragm, so that a connection area between the second protrusion and the diaphragm is increased, and connection reliability between the second protrusion and the diaphragm is higher. In another possible implementation, there may be one second notch. 
     In a possible implementation, the bottom plate portion may further include a limiting rib, and the limiting rib is fastened to a side that is of the bottom plate body and that faces the top plate portion. The limiting rib is located on a side of the first protrusion, and the limiting rib is higher than the first protrusion. The limiting rib may be configured to limit or assist in limiting a position of the second protrusion, so that positioning and fastening between the top plate portion and the top plate portion are more reliable. A gap is formed between the limiting rib and the top plate body, where the limiting rib is away from all or a portion of the top surface of the bottom plate body. 
     In another possible implementation, the front cavity is located between the diaphragm and the top plate body and is located in the second protrusion. A portion of the rear cavity is located between the diaphragm and the bottom plate portion, and is located in the first protrusion, and another portion of the rear cavity is located between the top plate portion and the bottom plate portion, and is located on the outside of the first protrusion. The two portions of the rear cavity are connected to each other. 
     In a possible implementation, the terminal protective case further includes a first cover body and a second cover body that are located within the main body part. The first cover body and the second cover body are disposed opposite to each other, and a periphery of the diaphragm is fastened between the first cover body and the second cover body. The front cavity is formed between the diaphragm and the first cover body. A connection hole is disposed on the first cover body, and the connection hole connects the front cavity and the sound outlet hole. A rear cavity is formed between the diaphragm and the second cover body. A leakage hole is disposed on the second cover body, and the leakage hole connects the rear cavity to the outside of the second cover body. 
     In this implementation, the first cover body, the second cover body, the diaphragm, and the piezoelectric sheet jointly form the piezoelectric speaker. The piezoelectric speaker may be first formed in the terminal protective case, and then the piezoelectric speaker is mounted into the main body part, to reduce assembly difficulty of the terminal protective case. In this way, a product yield of the terminal protective case is high. In addition, when the piezoelectric speaker is independently assembled into a module, assembly precision of the piezoelectric speaker is also ensured, so that the piezoelectric speaker has a higher product yield. 
     In a possible implementation, the leakage hole is disposed near middle space of the main body part, so that the rear cavity is connected to space that is within the main body part, that is located between the top plate portion and the bottom plate portion, and that is located on the outside of the second cover body. In this way, the space is used to expand space of the rear cavity of the piezoelectric speaker and improve a low-frequency sound pressure level of the piezoelectric speaker. In this case, the rear cavity of the piezoelectric speaker is an open rear cavity. In another possible implementation, the leakage hole may be alternatively disposed close to an edge of the main body part, that is, a connection portion facing the main body part, so that the leakage hole is directly connected to the outside of the main body part through a breather hole of the connection portion. The leakage hole of the second cover body may be disposed opposite to the breather hole. 
     In a possible implementation, the diaphragm and the piezoelectric sheet fastened to the diaphragm jointly form a piezoelectric speaker core. The terminal protective case includes a plurality of piezoelectric speaker cores. In this implementation, the plurality of piezoelectric speaker cores are disposed in the terminal protective case, to form a plurality of piezoelectric speakers. Therefore, loudspeaker volume of the terminal protective case and the audio playback system is significantly increased. This helps improve user experience. 
     In a possible implementation, the front cavities of the plurality of piezoelectric speaker cores are respectively connected to a plurality of sound outlet holes on different positions. In this case, the plurality of piezoelectric speakers may cooperate with each other to implement stereo. For example, one of the two piezoelectric speakers is connected to a sound outlet hole at the top of the terminal protective case and is set as a left sound channel, and the other is connected to a sound outlet hole at the bottom of the terminal protective case and is set as a right sound channel. In another possible implementation, the front cavities of the plurality of piezoelectric speaker cores are connected to one or more same sound outlet holes. In this case, volume of sounds made by the plurality of piezoelectric speakers is superimposed. 
     In a possible implementation, the plurality of sound outlet holes includes a first sound outlet hole and a second sound outlet hole. The first sound outlet hole and the second sound outlet hole are connected to the front cavities of different piezoelectric speaker cores. The first sound outlet hole and the second sound outlet hole are respectively located at two ends of the connection portion. In this implementation, a distance between the first sound outlet hole and the second sound outlet hole is long. This helps improve a stereo playback sound effect of the terminal protective case. In both a landscape use scenario and a portrait use scenario, the terminal protective case can implement stereo playback. 
     In a possible implementation, the terminal protective case further includes an electrical connector. The electrical connector is configured to plug in a connector of the terminal. The drive circuit is electrically connected to the electrical connector, and the communication signal of the terminal is transmitted to the drive circuit through the connector of the terminal and the electrical connector. 
     In this implementation, the terminal protective case is directly connected to the connector of the terminal through the electrical connector, to obtain the communication signal of the terminal, so that quality of communication between the terminal protective case and the terminal is good. In addition, the terminal protective case can further obtain a power signal from the terminal through the electrical connector, so that no battery or a related charging circuit needs to be additionally disposed on the terminal protective case. This helps simplify a hardware structure and a circuit structure of the terminal protective case. 
     In a possible implementation, the electrical connector includes a male connector and a female socket, one end of the male connector is fixedly and electrically connected to the female socket, and the female socket projects from the terminal protective case because the male connector is plugged in the connector of the terminal. In this implementation, another device (for example, a charger or a headset) may be electrically connected to the connector of the terminal by plugging in the female socket, to implement a communication connection and/or a power connection to the terminal. 
     In a possible implementation, the terminal protective case further includes a wireless communication module and a battery that are located within the main body part. The wireless communication module is electrically connected to the drive circuit. The wireless communication module is configured to perform wireless communication with the terminal, to receive the communication signal of the terminal. For example, the wireless communication module is configured to communicate with the wireless communication module of the terminal by using a wireless communication technology. The wireless communication technology may include but is not limited to a Bluetooth technology, an ultra-wideband technology, a wireless local area network technology, and the like. The wireless communication module may include a radio frequency circuit and an antenna electrically connected to the radio frequency circuit. The antenna is configured to convert a received electromagnetic wave into a radio frequency signal, and the radio frequency circuit is configured to demodulate the radio frequency signal into a baseband signal. 
     The battery is electrically connected to the wireless communication module and the drive circuit. The battery is configured to supply power to the terminal protective case. A charging module is electrically connected to the battery, and an external charging device charges the battery through the charging module. For example, the charging module may include an electrical connector, for example, a universal serial bus connector. The main body part is provided with a charging socket, and the external charging device may be connected to the charging module through the charging socket. 
     In this implementation, because the terminal protective case communicates with the terminal through wireless communication, and the battery is disposed in the terminal protective case. Therefore, the terminal protective case can work normally in a scenario in which the terminal protective case is separated from the terminal. This improves user experience. For example, the terminal protective case may be charged or play music after being detached from the terminal. 
     In a possible implementation, the terminal protective case further includes a wireless communication module, a charging coil, and a wireless charging circuit that are located within the main body part. The wireless communication module is electrically connected to the drive circuit. The wireless communication module is configured to perform wireless communication with the terminal, to receive the communication signal of the terminal. An input terminal of the wireless charging circuit is connected to the charging coil, and an output terminal of the wireless charging circuit is connected to the drive circuit and the wireless communication module. The wireless charging circuit is configured to: control the charging coil to be coupled to a charging coil of the terminal, and supply power to the drive circuit and the wireless communication module. 
     In this implementation, no battery needs to be disposed in the terminal protective case, and a power supply is obtained by using a wireless charging technology. The charging coil and the wireless charging circuit are small in size and light in weight, thereby facilitating thinness and lightweight of the terminal protective case. 
     In a possible implementation, the drive circuit includes a positive power interface, a negative power interface, a first capacitor, a boost circuit, a second capacitor, a power amplifier, a signal interface, a demodulator, and filtering matched impedance. The positive power interface is connected to a first input terminal of the boost circuit, and the negative power interface is connected to a second input terminal of the boost circuit. One terminal of the first capacitor is connected between the positive power interface and the first input terminal of the boost circuit, and the other terminal of the first capacitor is connected between the negative power interface and the second input terminal of the boost circuit. A first output terminal of the boost circuit is connected to a positive input terminal of the power amplifier, and a second output terminal of the boost circuit is connected to a negative input terminal of the power amplifier. One terminal of the second capacitor is connected between the first output terminal of the boost circuit and the positive input terminal of the power amplifier, and the other terminal of the second capacitor is connected between the second output terminal of the boost circuit and the negative input terminal of the power amplifier. The demodulator is connected between the signal interface and a signal input terminal of the power amplifier. The demodulator is configured to demodulate a signal received by the signal interface into an audio signal, and the power amplifier is configured to amplify the audio signal and form a drive signal. The filtering matched impedance is connected between an output terminal of the power amplifier and the piezoelectric sheet, and the filtering matched impedance is configured to send the drive signal to the piezoelectric sheet after filtering and attenuating the drive signal. 
     In this implementation, the boost circuit is configured to boost an output voltage of the battery and then provide the boosted output voltage to the power amplifier. The first capacitor and the second capacitor can regulate an output voltage of the battery and an output voltage of the boost circuit, and are also configured to provide a large transient peak current, so as to meet a driving current requirement of the piezoelectric sheet of the piezoelectric speaker. 
     According to a second aspect, this application further provides an audio playback system, including a terminal and the terminal protective case according to any one of the foregoing. Because the piezoelectric speaker of the terminal protective case can make a sound together with a speaker of the terminal, the audio playback system has high loudspeaker volume, and still has a good loudspeaker effect in a noisy environment or an outdoor environment. 
     According to a third aspect, this application further provides an audio playback system, including a terminal and a terminal protective case. The terminal protective case includes a housing, a diaphragm, a piezoelectric sheet, and a first conductive contact. The housing includes a main body part and a fastening frame part. A side of the fastening frame part is fastened to a periphery of the main body part, and the fastening frame part and the main body part jointly enclose an accommodating groove. The diaphragm is located within the main body part, the piezoelectric sheet is fastened to the diaphragm, a front cavity is formed on a side of the diaphragm, a sound outlet hole is disposed on the main body part, and the sound outlet hole connects the front cavity to the outside of the main body part. The first conductive contact is fastened to the fastening frame part and/or the main body part and projects from the accommodating groove, and the first conductive contact is electrically connected to the piezoelectric sheet. 
     The terminal includes a drive circuit and a second conductive contact electrically connected to the drive circuit. The terminal is accommodated in the accommodating groove. The second conductive contact abuts against the first conductive contact. The drive circuit is configured to convert an audio signal into a drive signal. The drive signal is transmitted to the piezoelectric sheet through the second conductive contact and the first conductive contact, to drive the piezoelectric sheet to drive the diaphragm to vibrate. 
     In this application, the terminal may transmit the drive signal to the piezoelectric sheet of the terminal protective case, to drive the piezoelectric sheet to drive the diaphragm to vibrate. Therefore, the piezoelectric speaker of the terminal protective case may make a sound under the control of the terminal. The piezoelectric speaker of the terminal protective case may make a sound together with a speaker of the terminal to greatly increase loudspeaker volume of the audio playback system. In other words, the audio playback system has high loudspeaker volume, and the audio playback system still has a good loudspeaker effect in a noisy environment or an outdoor environment. 
     The terminal can control, by using the drive signal, the piezoelectric speaker of the terminal protective case to make a sound or not to make a sound, so that the audio playback system can implement a plurality of loudspeaker modes such as making a sound by the terminal protective case alone, making a sound by the terminal alone, and making a sound by the terminal protective case and the terminal together. In addition, when the piezoelectric speaker of the terminal protective case and the speaker of the terminal make a sound together, the piezoelectric speaker and the speaker of the terminal may cooperate to form a multi-channel loudspeaker system. This improves sound field rendering and a stereo effect. 
     Because the piezoelectric sheet of the piezoelectric speaker of the terminal protective case is directly driven by the drive signal sent by the terminal, a structure such as a battery, a charging circuit, or a piezoelectric sheet drive circuit does not need to be disposed on the terminal protective case. Therefore, the terminal protective case has a simple structure and low costs. 
     In some possible implementations, the first conductive contact may be of a protrusion structure, a dome structure, or a pad structure, and the first conductive contact may be made of a material such as metal. The second conductive contact may be of a protrusion structure, a dome structure, or a pad structure, and the first conductive contact may be made of a material such as metal. 
     In a possible implementation, the main body part includes a bottom plate portion, a top plate portion, and a connection portion. The bottom plate portion and the top plate portion are disposed opposite to each other. The connection portion is connected between a periphery of the bottom plate portion and a periphery of the top plate portion, and the fastening frame part is connected to the bottom plate portion and/or the connection portion. The sound outlet hole is disposed on the connection portion. 
     In this implementation, the terminal protective case makes a sound from a side surface. When the terminal protective case is held by a user or placed on an object, the sound outlet hole is not easily blocked, so that the terminal protective case can normally make a sound. This improves user experience. 
     In a possible implementation, there may be a plurality of sound outlet holes, and the plurality of sound outlet holes are disposed at intervals. Each sound outlet hole is roughly in a rectangular shape. In another possible implementation, there may also be one sound outlet hole. In another possible implementation, the sound outlet hole may be alternatively in another shape such as a circle or an ellipse. 
     In a possible implementation, the housing includes a main housing and a cover plate, and the cover plate is fixedly connected to the main housing. The fastening frame part, and the bottom plate portion and the connection portion of the main body part are formed on the main housing, and the top plate portion of the main body part is formed on the cover plate. In this case, because the main housing and the cover plate can be manufactured separately, and the main housing and the cover plate can meet different performance requirements, the top plate portion of the main body part, the fastening frame part, and the bottom plate portion and the connection portion of the main body part can meet different performance requirements. For example, a top plate of the main body part has better hardness to improve anti-wear and anti-scratch performance. The fastening frame part, and the bottom plate portion and the connection portion of the main body part have better toughness to improve anti-impact performance, so that overall performance of the housing is better, and more diversified function requirements can be met. In another possible implementation, the fastening frame part and the bottom plate portion of the main body part are formed on the main housing, and the connection portion and the top plate portion are formed on the cover plate. 
     In a possible implementation, the fastening frame part is connected to the bottom plate portion, and the fastening frame part and the bottom plate portion may form a large connection area, to increase connection firmness between the fastening frame part and the bottom plate portion, and improve reliability of the terminal protective case. In another possible implementation, the fastening frame part is connected to the connection portion. In still another possible implementation, the fastening frame part is connected to both the bottom plate portion and the connection portion, to increase a connection area with the main body part. 
     In a possible implementation, the bottom plate portion includes a bottom plate body and a first protrusion. The first protrusion is fastened to a side that is of the bottom plate body and that faces the top plate portion. The top plate portion includes a top plate body and a second protrusion. The second protrusion is fastened to a side that is of the top plate body and that faces the bottom plate portion. The second protrusion is disposed opposite to the first protrusion. A periphery of the diaphragm is fastened between the first protrusion and the second protrusion. 
     In this implementation, the diaphragm is fastened to the first protrusion and the second protrusion, so that the diaphragm is directly fastened to the main body part of the housing. In this case, a front cavity and a rear cavity of the piezoelectric speaker are formed by using space in the main body part. This improves space utilization of the terminal protective case and facilitates thinness of the terminal protective case. 
     In a possible implementation, the main body part may be made of a hard material, for example, a polycarbonate plastic or a metal material, to reduce housing vibration generated due to vibration of the diaphragm. 
     In a possible implementation, the main body part further includes one or more holders. One end of each holder is fixedly connected to the bottom plate portion, and the other end is fixedly connected to the top plate portion. 
     In this implementation, the one or more holders help improve support strength and connection firmness between the top plate portion and the bottom plate portion, so that overall structural strength of the main body part is significantly improved, and structural reliability of the terminal protective case is higher. The structural strength of the main body part is improved, which is also conducive to normal sound-making of the piezoelectric speaker. 
     In a possible implementation, the front cavity is located between the diaphragm and the bottom plate body and is located in the first protrusion. The first protrusion is in a ring shape, the first protrusion is provided with a first notch, and the first notch connects the front cavity and the sound outlet hole. First space is formed between the diaphragm and the top plate body, and the first space is located in the second protrusion. Second space is formed between the bottom plate body and the top plate body, and the second space is located on the outside of the second protrusion. The second protrusion is in a ring shape, the second protrusion is provided with a second notch, and the second notch connects the second space and the first space to form a rear cavity. In this implementation, space of the rear cavity formed within the main body part is large, so that the piezoelectric speaker has a good low-frequency sound pressure level. 
     In a possible implementation, there may be a plurality of first notches, and the plurality of first notches are disposed at intervals. In this case, a portion that is of the first protrusion and that is located between the two adjacent first notches can still fasten the diaphragm, so that a connection area between the first protrusion and the diaphragm is increased, and connection reliability between the first protrusion and the diaphragm is higher. For example, when there are a plurality of sound outlet holes, the plurality of first notches are disposed in a one-to-one correspondence with the plurality of sound outlet holes; or when there is one sound outlet hole, the sound outlet hole is connected to the plurality of first notches. In another possible implementation, there may be alternatively one first notch, and one first notch is connected to all sound outlet holes. 
     In a possible implementation, the second protrusion is provided with a second notch, and the second notch connects the first space of the rear cavity and the second space of the rear cavity. There may be a plurality of second notches, and the plurality of second notches are disposed at intervals. In this case, a portion that is of the second protrusion and that is located between the two adjacent second notches can still fasten the diaphragm, so that a connection area between the second protrusion and the diaphragm is increased, and connection reliability between the second protrusion and the diaphragm is higher. In another possible implementation, there may be one second notch. 
     In a possible implementation, the bottom plate portion may further include a limiting rib, and the limiting rib is fastened to a side that is of the bottom plate body and that faces the top plate portion. The limiting rib is located on a side of the first protrusion, and the limiting rib is higher than the first protrusion. The limiting rib may be configured to limit or assist in limiting a position of the second protrusion, so that positioning and fastening between the top plate portion and the top plate portion are more reliable. A gap is formed between the limiting rib and the top plate body, where the limiting rib is away from all or a portion of the top surface of the bottom plate body. 
     In another possible implementation, the front cavity is located between the diaphragm and the top plate body and is located in the second protrusion. A portion of the rear cavity is located between the diaphragm and the bottom plate portion, and is located in the first protrusion, and another portion of the rear cavity is located between the top plate portion and the bottom plate portion, and is located on the outside of the first protrusion. The two portions of the rear cavity are connected to each other. 
     In a possible implementation, the terminal protective case further includes a first cover body and a second cover body that are located within the main body part. The first cover body and the second cover body are disposed opposite to each other, and a periphery of the diaphragm is fastened between the first cover body and the second cover body. The front cavity is formed between the diaphragm and the first cover body. A connection hole is disposed on the first cover body, and the connection hole connects the front cavity and the sound outlet hole. A rear cavity is formed between the diaphragm and the second cover body. A leakage hole is disposed on the second cover body, and the leakage hole connects the rear cavity to the outside of the second cover body. 
     In this implementation, the first cover body, the second cover body, the diaphragm, and the piezoelectric sheet jointly form the piezoelectric speaker. The piezoelectric speaker may be first formed in the terminal protective case, and then the piezoelectric speaker is mounted into the main body part, to reduce assembly difficulty of the terminal protective case. In this way, a product yield of the terminal protective case is high. In addition, when the piezoelectric speaker is independently assembled into a module, assembly precision of the piezoelectric speaker is also ensured, so that the piezoelectric speaker has a higher product yield. 
     In a possible implementation, the leakage hole is disposed near middle space of the main body part, so that the rear cavity is connected to space that is within the main body part, that is located between the top plate portion and the bottom plate portion, and that is located on the outside of the second cover body. In this way, the space is used to expand space of the rear cavity of the piezoelectric speaker and improve a low-frequency sound pressure level of the piezoelectric speaker. In this case, the rear cavity of the piezoelectric speaker is an open rear cavity. In another possible implementation, the leakage hole may be alternatively disposed close to an edge of the main body part, that is, a connection portion facing the main body part, so that the leakage hole is directly connected to the outside of the main body part through a breather hole of the connection portion. The leakage hole of the second cover body may be disposed opposite to the breather hole. 
     In a possible implementation, the diaphragm and the piezoelectric sheet fastened to the diaphragm jointly form a piezoelectric speaker core. The terminal protective case includes a plurality of piezoelectric speaker cores. In this implementation, the plurality of piezoelectric speaker cores are disposed in the terminal protective case, to form a plurality of piezoelectric speakers. Therefore, loudspeaker volume of the terminal protective case and the audio playback system is significantly increased. This helps improve user experience. 
     In a possible implementation, the front cavities of the plurality of piezoelectric speaker cores are respectively connected to a plurality of sound outlet holes on different positions. In this case, the plurality of piezoelectric speakers may cooperate with each other to implement stereo. For example, one of the two piezoelectric speakers is connected to a sound outlet hole at the top of the terminal protective case and is set as a left sound channel, and the other is connected to a sound outlet hole at the bottom of the terminal protective case and is set as a right sound channel. In another possible implementation, the front cavities of the plurality of piezoelectric speaker cores are connected to one or more same sound outlet holes. In this case, volume of sounds made by the plurality of piezoelectric speakers is superimposed. 
     In a possible implementation, the plurality of sound outlet holes includes a first sound outlet hole and a second sound outlet hole. The first sound outlet hole and the second sound outlet hole are connected to the front cavities of different piezoelectric speaker cores. The first sound outlet hole and the second sound outlet hole are respectively located at two ends of the connection portion. In this implementation, a distance between the first sound outlet hole and the second sound outlet hole is long. This helps improve a stereo playback sound effect of the terminal protective case. In both a landscape use scenario and a portrait use scenario, the terminal protective case can implement stereo playback. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of a structure of an audio playback system according to an embodiment of this application; 
         FIG.  2    is a schematic diagram of a structure of a terminal of the audio playback system shown in  FIG.  1    in some embodiments; 
         FIG.  3    is a schematic diagram of a structure of a terminal protective case of the audio playback system shown in  FIG.  1    in some embodiments; 
         FIG.  4    is a schematic diagram of a structure of the terminal protective case shown in  FIG.  3    from another angle; 
         FIG.  5    is a schematic diagram of an exploded structure of the terminal protective case shown in  FIG.  4   ; 
         FIG.  6    is a schematic diagram of a state of the audio playback system shown in  FIG.  1    in some use scenarios; 
         FIG.  7    is a schematic diagram of a structure of a portion of a terminal protective case that is shown in  FIG.  4    and that is cut along A-A; 
         FIG.  8    is an enlarged view of a structure at a position B of the structure shown in  FIG.  7   ; 
         FIG.  9    is a schematic diagram of a structure of a main housing of the terminal protective case shown in  FIG.  5   ; 
         FIG.  10    is a schematic diagram of a structure of a cover plate of the terminal protective case shown in  FIG.  5    in some embodiments; 
         FIG.  11    is a schematic diagram of a structure of a cover plate of the terminal protective case shown in  FIG.  5    in some other embodiments; 
         FIG.  12    is a schematic diagram of a portion of a circuit of the terminal protective case shown in  FIG.  4   ; 
         FIG.  13    is a schematic diagram of a structure of a terminal protective case of the audio playback system shown in  FIG.  1    in some other embodiments; 
         FIG.  14    is a schematic diagram of a portion of a circuit of the terminal protective case shown in  FIG.  13   ; 
         FIG.  15    is a schematic diagram of a structure of a terminal protective case of the audio playback system shown in  FIG.  1    in still some other embodiments; 
         FIG.  16    is a schematic diagram of a portion of a circuit of the terminal protective case shown in  FIG.  15   ; 
         FIG.  17    is a schematic diagram of a structure of a terminal protective case of the audio playback system shown in  FIG.  1    in yet some other embodiments; 
         FIG.  18    is a schematic diagram of a structure of a terminal of the audio playback system shown in  FIG.  1    in some other embodiments; 
         FIG.  19    is a schematic diagram of a structure of the terminal protective case shown in  FIG.  17    from another angle; 
         FIG.  20    is a schematic diagram of a structure of a terminal protective case of the audio playback system shown in  FIG.  1    in still yet some other embodiments; 
         FIG.  21    is a schematic diagram of a structure of a terminal of the audio playback system shown in  FIG.  1    in still some other embodiments; 
         FIG.  22    is a schematic diagram of a structure of a terminal protective case of the audio playback system shown in  FIG.  1    in some further embodiments; 
         FIG.  23    is a schematic diagram of a structure of a terminal of the audio playback system shown in  FIG.  1    in yet some other embodiments; and 
         FIG.  24    is a schematic diagram of an internal structure of a terminal protective case of the audio playback system shown in  FIG.  1    in still some further embodiments. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The following describes embodiments of this application with reference to the accompanying drawings in embodiments of this application. 
     Embodiments of this application provide a terminal protective case and an audio playback system to which the terminal protective case is applied. The audio playback system further includes a terminal, and a terminal protective case that detachably sleeves over the outside of the terminal. The terminal protective case has a piezoelectric speaker, and the piezoelectric speaker may make a sound together with a speaker of the terminal, thereby greatly increasing loudspeaker volume of an audio playback system. The terminal may be an electronic product having a sound playback function, such as a mobile phone or a tablet computer. 
       FIG.  1    is a schematic diagram of a structure of an audio playback system  1000  according to an embodiment of this application. 
     The audio playback system  1000  has a sound playback function. The audio playback system  1000  includes a terminal  100  and a terminal protective case  200 . The terminal protective case  200  detachably sleeves on the outside of the terminal  100 . In other words, the terminal protective case  200  may sleeve on the outside of the terminal  100 , to protect the terminal  100 . The terminal protective case  200  may be alternatively detached from the terminal  100 , to be separated from the terminal  100 . In this embodiment, an example in which the terminal  100  is a mobile phone is used for description. 
       FIG.  2    is a schematic diagram of a structure of a terminal  100  of the audio playback system  1000  shown in  FIG.  1    in some embodiments. 
     The terminal  100  may include a housing  11 , and function modules such as a display  12 , a circuit board  13 , a battery  14 , a wireless charging coil  15 , a wireless communication module  16 , an infrared sensing module  17 , a front-facing camera module  18 , a rear-facing camera module  19 , a flash module no, a screen sound-making module  111 , a speaker  114 , a first microphone (which may also be referred to as a microphone)  115 , a second microphone  116 , a connector  117 , and a card holder  118  that are mounted on the housing  11 . 
     The display  12  is configured to display an image, and the display  12  may further integrate a touch function. The display  12  is fastened to a side of the housing  11 , and the display  12  and the housing  11  jointly enclose an internal cavity of the entire terminal  100 . Other function modules of the terminal  100  may be accommodated in the internal cavity of the entire system, and the housing  11  can fasten and protect these function modules. For example, the housing  11  may include a middle frame and a rear cover, and the rear cover and the display  12  are respectively fastened on two opposite sides of the middle frame. The middle frame may include a frame and a middle plate fastened within the frame, and the frame is connected between a periphery of the rear cover and a periphery of the display  12 . The frame and the middle plate may be integrally formed, or may be assembled to form an integrated structure. The frame and the rear cover may be integrally formed, or may be assembled to form an integrated structure. 
     There may be one or more circuit boards  13 . In embodiments of this application, “a plurality of” means at least two. A plurality of components may be fastened on the circuit board  13 . The plurality of components include but are not limited to a processor  119 , a memory, and the like. The memory is coupled to the processor  119 . The memory is configured to store computer program code. The computer program code includes computer instructions. The processor  119  is configured to invoke the computer instructions to enable the terminal  100  to perform corresponding operations. The processor  119  is coupled to another function module of the terminal  100 , and is configured to control work of the another function module. 
     The battery  14  is electrically connected to the another function module of the terminal  100 , and is configured to supply power to the terminal  100 . The wireless charging coil  15  is electrically connected to the battery  14 , and an external power supply may charge the battery  14  through the wireless charging coil  15 . In some embodiments, the battery  14  may alternatively charge another device through the wireless charging coil  15 . 
     The wireless communication module  16  is configured to communicate with a network or another device by using a wireless communication technology. The wireless communication technology includes a global system for mobile communication (global system for mobile communication, GSM), a general packet radio service (general packet radio service, GPRS), a code division multiple access (code division multiple access, CDMA), a wideband code division multiple access (wideband code division multiple access, WCDMA), a time-division code division multiple access (time-division code division multiple access, TD-SCDMA), long term evolution (long term evolution, LTE), Bluetooth (Bluetooth, BT), a global navigation satellite system (global navigation satellite system, GNSS), wireless local area networks (wireless local area networks, WLAN) (such as a wireless fidelity (wireless fidelity, Wi-Fi) network), ultra wide band (ultra wide band, UWB), near field communication (near field communication, NFC), and/or frequency modulation (frequency modulation, FM), and the like. In embodiments of this application, “A and/or B” includes three cases: “A”, “B”, and “A and B”. For example, the wireless communication module  16  may include a plurality of antennas configured to transmit and receive an electromagnetic wave signal, and each antenna may be configured to cover one or more communication frequency bands. Different antennas may be reused to improve utilization of the antennas. 
     The infrared sensing module  17  is configured to transmit an infrared (infrared, IR) signal, to communicate with another device by using an infrared technology. 
     The front-facing camera module  18  is configured to collect light in front of the terminal  100  to perform image shooting. The front of the terminal  100  is space on a side facing the display  12 . The rear-facing camera module  19  is configured to collect light at the rear of the terminal  100  for image shooting. The rear of the terminal  100  is space on a side opposite to the front of the terminal  100 . The flash module no is configured to supplement light. 
     The screen sound-making module  111  is configured to drive the display  12  to vibrate to make a sound. The speaker  114  is configured to convert an electrical signal into a sound signal. In some embodiments, the speaker  114  of the terminal  100  is configured to make a loudspeaker sound, and the screen sound-making module  111  is configured to drive the display  12  to vibrate to make a non-loudspeaker sound. For example, the loudspeaker sound may be a hands-free call sound, loudspeaker voice information, loudspeaker music, navigation voice, broadcast, or the like. The non-loudspeaker sound may be a non-hands-free call sound, non-loudspeaker voice information, or the like. 
     The first microphone  115  and the second microphone  116  may be located at the top and the bottom of the terminal  100  respectively, and the first microphone  115  and the second microphone  116  may work cooperatively or work independently of each other. It may be understood that, in this embodiment of this application, orientation words such as “top” and “bottom” in related descriptions of the terminal  100  are described based on a perspective of the terminal  100  shown in  FIG.  1    and  FIG.  2   , and do not limit a factor such as a placement location of the terminal  100  in an actual use scenario. 
     The connector  117  is configured to connect to an electrical connector of another device, to electrically connect to the another device. This implements power transmission and/or signal transmission. The connector  117  may be a universal serial bus (universal serial bus, USB) connector. 
     The card holder  118  is configured to mount a subscriber identity module (subscriber identification module, SIM) card, and the terminal  100  identifies a subscriber of a global system for mobile communication through the subscriber identity module card. 
     It may be understood that, in some other embodiments, the terminal  100  may include all or some of the foregoing function modules, or the terminal  100  may also include another function module such as a button, a sensor module, or a motor. The button is configured to receive a user operation, so that the terminal  100  can respond to the user operation. The sensor module may include one or more of a pressure sensor, a gyro sensor, a barometric pressure sensor, a magnetic sensor, an acceleration sensor, a distance sensor, an optical proximity sensor, a fingerprint sensor, a temperature sensor, a touch sensor or an ambient light sensor. The motor may generate a vibration prompt. The motor may be used for an incoming call vibration prompt, or may be used for a touch vibration feedback. A quantity, a type, a location, and the like of a specific function module of the terminal  100  are not strictly limited in this embodiment of this application. 
       FIG.  3    is a schematic diagram of a structure of a terminal protective case  200  of the audio playback system  1000  shown in  FIG.  1    in some embodiments. 
     The terminal protective case  200  includes a housing  2 . The housing  2  is configured to protect a part (described below) that is of the terminal protective case  200  and that is mounted within the housing  2 . The housing  2  includes a main body part  21  and a fastening frame part  22 . A side of the fastening frame part  22  is fastened to a periphery of the main body part  21 . The fastening frame part  22  and the main body part  21  jointly enclose an accommodating groove  23 , and the accommodating groove  23  is configured to accommodate the terminal  100 . 
     In some embodiments, as shown in  FIG.  3   , the fastening frame part  22  includes a top frame part  221  and a bottom frame part  222 , and the top frame part  221  and the bottom frame part  222  are disposed at intervals. The main body part  21  is roughly rectangular. The top frame part  221  is connected to a top short side of the main body part  21 , and two ends of the top frame part  221  are respectively connected to top ends of two long sides of the main body part  21 . In other words, the top frame part  221  is connected to two top corners of the main body part  21 , so that the top frame part  221  forms two wrapping top corners. The bottom frame part  222  is connected to a bottom short side of the main body part  21 , and two ends of the bottom frame part  222  are respectively connected to bottom ends of two long sides of the main body part  21 . In other words, the bottom frame part  222  is connected to two bottom corners of the main body part  21 , so that the bottom frame part  222  forms two wrapping bottom corners. The two wrapping bottom corners and the two wrapping top corners can jointly wrap four corners of the terminal  100 , to better protect the terminal  100 . This reduces a damage risk and a damage degree of the terminal  100  in scenarios such as falling and collision. It may be understood that, in this embodiment of this application, orientation words such as “top” and “bottom” in related descriptions of the terminal protective case  200  are described based on a perspective of the terminal protective case  200  shown in  FIG.  1    and  FIG.  3   , and do not limit a factor such as a placement location of the terminal protective case  200  in an actual use scenario. 
     In some embodiments, as shown in  FIG.  3   , the fastening frame part  22  further includes two connection frame parts  223 . The two connection frame parts  223  are disposed opposite to each other and are respectively connected between two ends of the top frame part  221  and two ends of the bottom frame part  222 . The connection frame part  223  is lower than the top frame part  221  and the bottom frame part  222 , that is, the connection frame part  223  is recessed relative to the top frame part  221  and the bottom frame part  222  to form an exposed region. When the terminal  100  is mounted in the accommodating groove  23  of the terminal protective case  200 , a button on a side surface of the terminal  100  may be exposed through the exposed region of the terminal protective case  200 . The two connection frame parts  223  may fasten and protect the side of the terminal  100  to some extent, so that the terminal protective case  200  protects the terminal  100  more comprehensively. 
     In some embodiments, as shown in  FIG.  3   , the main body part  21  is provided with a camera hole  211  and a flash hole  212  that are spaced apart from each other. The top frame part  221  of the fastening frame part  22  is provided with an infrared hole  2211  and a first microphone hole  2212  that are spaced apart from each other. The bottom frame part  222  of the fastening frame part  22  is provided with a speaker hole  2221 , a second microphone hole  2222 , a connector hole  2223 , and a card holder hole  2224  that are spaced apart from each other. With reference to  FIG.  2    and  FIG.  3   , when the terminal  100  is accommodated in the accommodating groove  23  of the terminal protective case  200 , the rear-facing camera module  19  corresponds to the camera hole  211 , so as to perform image shooting through the camera hole  211 . The flash module no corresponds to the flash hole  212 , so as to emit light through the flash hole  212 . The infrared sensing module  17  corresponds to the infrared hole  2211 , so as to transmit an infrared signal through the infrared hole  2211 . The first microphone  115  corresponds to the first microphone hole  2212 , so as to collect a sound signal through the first microphone hole  2212 . The speaker  114  corresponds to the speaker hole  2221 , so as to make a loudspeaker sound signal through the speaker hole  2221 . The second microphone  116  corresponds to the second microphone hole  2222 , so as to collect a sound signal through the second microphone hole  2222 . The connector  117  corresponds to the connector hole  2223 , so that an external electrical connector can be plugged in the connector  117  through the connector hole  2223 . The card holder  118  corresponds to the card holder hole  2224 , so that the subscriber identity card can be mounted in the terminal  100  through the card holder hole  2224 . In some other embodiments, the two connection frame parts  223  of the fastening frame part  22  may also be as high as the top frame pall  221  and the bottom frame pall  222 , and a button hole is disposed at a position that is on the connection frame part  223  and that corresponds to a button of the terminal  100 . A specific structure of the fastening frame pall  22  is not strictly limited in this application. 
     A shape of the main body part  21  of the terminal protective case  200  adapts to a shape of the terminal  100 , and a shape of the fastening frame part  22  also adapts to a shape of the terminal  100 . For example, when the terminal  100  is in a circular shape, the main body part  21  is also in a circular shape, the fastening frame part  22  is in a shape of a circular ring (which may be a continuous circular ring, or may be formed by a plurality of concentric arc-shaped segments), and the main body part  21  and the fastening frame pall  22  roughly enclose the circular accommodating groove  23  to accommodate the terminal  100 . Positions, a quantity, and shapes of holes on the main body pall  21  and the fastening frame part  22  are set according to a requirement of the terminal  100 . 
     Refer to  FIG.  4    and  FIG.  5    together.  FIG.  4    is a schematic diagram of a structure of the terminal protective case  200  shown in  FIG.  3    from another angle, and  FIG.  5    is a schematic diagram of an exploded structure of the terminal protective case  200  shown in  FIG.  4   . 
     In some embodiments, the terminal protective case  200  includes a main housing  2   a  and a cover plate  2   b . The cover plate  2   b  is fixedly connected to the main housing  2   a , and the cover plate  2   b  and the main housing  2   a  jointly form the housing  2  of the terminal protective case  200 . For example, the fastening frame part  22  of the housing  2  is formed on the main housing  2   a , a portion of the main body part  21  of the housing  2  is formed on the main housing  2   a , and another portion of the main body part  21  is formed on the cover plate  2   b . When the cover plate  2   b  is fastened to the main housing  2   a , the two portions of the main body part  21  are fastened to each other, and accommodation space is formed within the main body part  21 . The accommodation space is used to accommodate another component of the terminal protective case  200 . The main housing  2   a  and the cover plate  2   b  each include a plurality of structures (refer to the following description). The plurality of structures may be an integrated structure, or may be an integrated structure formed by assembly and fastening. 
     In some embodiments, the terminal protective case  200  may further include a piezoelectric speaker core  3 , a drive circuit  4 , a wireless communication module  5 , a battery  6 , and a charging module  7  that are located within the main body part  21 . 
     In some embodiments, as shown in  FIG.  5   , the piezoelectric speaker core  3  includes a diaphragm  31  and a piezoelectric sheet  32 , and the piezoelectric sheet  32  is fastened to the diaphragm  31 . The piezoelectric speaker core  3  may include a piezoelectric sheet  32 , where the piezoelectric sheet  32  is fastened to a side of the diaphragm  31 . Alternatively, the piezoelectric speaker core  3  may include two piezoelectric sheets  32 , where the two piezoelectric sheets  32  are respectively fastened to two sides of the diaphragm  31 . The piezoelectric sheet  32  may be a complete integrated piezoelectric sheet, or may be formed by splicing several small piezoelectric sheet units. 
     For example, the diaphragm  31  is fastened between the main housing  2   a  and the cover plate  2   b , so that both the diaphragm  31  and the piezoelectric sheet  32  are located within the main body part  21 . In this embodiment, the main body part  21  may be used as a housing to form a piezoelectric speaker together with the piezoelectric speaker core  3 . The diaphragm  31  divides space within the main body part  21  into two independent spaces. One side of the diaphragm  31  forms a front cavity of the piezoelectric speaker, and the other side of the diaphragm  31  forms a rear cavity of the piezoelectric speaker. When the piezoelectric speaker works, the piezoelectric sheet  32  deforms according to a drive signal, to drive the diaphragm  31  to vibrate, and the diaphragm  31  pushes air in the front cavity to vibrate, so as to make a sound. 
     A sound outlet hole  213  is disposed on the main body part  21 , and the sound outlet hole  213  connects space within the main body part  21  and space on the outside of the main body part  21 . For example, the sound outlet hole  213  connects the front cavity to the outside of the main body part  21 . In this case, a sound made by the piezoelectric speaker can be propagated to the outside of the main body part  21  through the sound outlet hole  213 , so that the terminal protective case  200  makes a sound. 
     In this embodiment, the terminal protective case  200  uses the piezoelectric speaker as a sound-making component. Because a thickness of the piezoelectric speaker core  3  is very small, when the terminal protective case  200  forms the piezoelectric speaker, the thickness of the main body part  21  of the terminal protective case  200  may be controlled within a small range. This improves a hand holding feeding and portability of the audio playback system  1000 . 
     For example, the main body part  21  may be made of a hard material, for example, a polycarbonate (polycarbonate, PC) plastic or a metal material, to reduce housing vibration generated due to vibration of the diaphragm  31 . 
     In some embodiments, the drive circuit  4  is electrically connected to the piezoelectric sheet  32 . The drive circuit  4  is configured to respond to a communication signal of the terminal  100 , and drive the piezoelectric sheet  32  to drive the diaphragm  31  to vibrate. The drive circuit  4  can form a corresponding drive signal based on the communication signal. The drive signal drives the piezoelectric sheet  32  to deform, and drives the diaphragm  31  to vibrate, so that the piezoelectric speaker core  3  makes a sound. 
     In this embodiment, the drive circuit  4  can drive, in response to the communication signal of the terminal  100 , the piezoelectric sheet  32  to drive the diaphragm  31  to vibrate, so that the piezoelectric speaker makes a sound. The piezoelectric speaker of the terminal protective case  200  may make a sound together with the speaker  114  of the terminal  100 , to greatly increase loudspeaker volume of the audio playback system  1000 . In other words, the audio playback system  1000  has high loudspeaker volume, and still has a good loudspeaker effect in a noisy environment or an outdoor environment. In addition, the terminal  100  can control, by u sing the communication signal, the piezoelectric speaker of the terminal protective case  200  to make a sound or not to make a sound, so that the audio playback system  1000  can implement a plurality of loudspeaker modes, such as making a sound by the terminal protective case  200  alone, making a sound by the terminal  100  alone, and making a sound by the terminal protective case  200  and the terminal  100  together. When the piezoelectric speaker of the terminal protective case  200  and the speaker  114  of the terminal  100  make a sound together, the piezoelectric speaker and the speaker of the terminal may cooperate to form a multi-channel loudspeaker system. This improves sound field rendering and a stereo effect. 
     In addition, because the piezoelectric speaker has better high-frequency performance, the piezoelectric speaker can supplement and enhance high-frequency performance of the speaker  114  of the terminal  100 , so that the audio playback system  1000  has better sound quality. 
       FIG.  6    is a schematic diagram of a state of the audio playback system  1000  shown in  FIG.  1    in some use scenarios. 
     In some embodiments, when the terminal  100  needs to play a loudspeaker sound (for example, playing music or making a hands-free call), the terminal  100  displays a loudspeaker mode selection interface  1001 . The interface includes an icon  1002  for separately playing a loudspeaker sound through the terminal, an icon  1003  for separately playing a loudspeaker sound through the terminal protective case, and an icon  1004  for playing a loudspeaker sound through both the terminal and the terminal protective case. A user may trigger a corresponding icon based on a requirement, and the terminal  100  controls, in response to a user operation, the terminal  100  and/or the terminal protective case  200  to play a loudspeaker sound, to meet a use requirement and improve loudspeaker experience. 
     In some other embodiments, the terminal  100  may alternatively select a loudspeaker mode in another manner, for example, by using a button or a sound. In some other embodiments, the user may select the loudspeaker mode, or may set a default loudspeaker mode. A selection manner and a setting manner of the loudspeaker mode are not strictly limited in this embodiment of this application. 
     Refer to  FIG.  4   ,  FIG.  5   , and  FIG.  7    together,  FIG.  7    is a schematic diagram of a structure of a portion of a terminal protective case  200  that is shown in  FIG.  4    and that is cut along A-A. 
     In some embodiments, the main body part  21  includes a bottom plate portion  214 , a top plate portion  215 , and a connection portion  216 . The bottom plate portion  214  and the top plate portion  215  are disposed opposite to each other, and the connection portion  216  is connected between a periphery of the bottom plate portion  214  and a periphery of the top plate portion  215 . The connection portion  216  is roughly in a shape of a frame, and a specific shape of the connection portion  216  varies with shapes of the top plate portion  215  and the bottom plate portion  214 . This is not strictly limited in this embodiment of this application. The bottom plate portion  214 , the connection portion  216 , and the top plate portion  215  jointly enclose space within the main body part  21 . The fastening frame part  22  is connected to the bottom plate portion  214 . When the terminal  100  is mounted in the accommodating groove  23  of the terminal protective case  200 , the bottom plate portion  214  faces the terminal  100  and may be in contact with the terminal  100 , the top plate portion  215  faces the terminal  100  in a backward manner, and the top plate portion  215  forms an appearance structure of the audio playback system  1000 . 
     In this embodiment, the fastening frame part  22 , and the bottom plate portion  214  and the connection portion  216  of the main body part  21  are formed on the main housing  2   a , and the top plate portion  215  of the main body part  21  is formed on the cover plate  2   b . In this case, because the main housing  2   a  and the cover plate  2   b  can be manufactured separately, and the main housing  2   a  and the cover plate  2   b  can meet different performance requirements, the top plate portion  215  of the main body part  21  can meet different performance requirements with the fastening frame part  22 , and the bottom plate portion  214  and the connection portion  216  of the main body part  21 . For example, the top plate of the main body part  21  has better hardness, so as to improve anti-wear and anti-scratch performance. The fastening frame part  22 , and the bottom plate portion  214  and the connection portion  216  of the main body part  21  have better toughness, so as to improve impact resistance performance. In this way, overall performance of the case  2  is better, and more diversified function requirements may be met. 
     In addition, the fastening frame part  22  is connected to the bottom plate portion  214 , and the fastening frame part  22  and the bottom plate portion  214  may form a large connection area, to increase connection firmness between the fastening frame part  22  and the bottom plate portion  214 , and improve reliability of the terminal protective case  200 . In some other embodiments, the fastening frame part  22  may be alternatively connected to the connection portion  216 ; or the fastening frame part  22  may be alternatively connected to both the bottom plate portion  214  and the connection portion  216 , to increase a connection area to the main body part  21 . This is not strictly limited in this embodiment of this application. 
     In some other embodiments, the fastening frame part  22  and the bottom plate portion  214  of the main body part  21  may be formed on the main housing  2   a , and the connection portion  216  and the top plate portion  215  may be formed on the cover plate  2   b . It may be understood that each portion of the housing  2  may be formed in a plurality of manners. A specific forming manner of the housing  2  is not strictly limited in this application. 
     In some embodiments, as shown in  FIG.  4    and  FIG.  5   , a mounting groove  2161  may be disposed at an end that is of the connection portion  216  and that is away from the bottom plate portion  214 . The mounting groove  2161  is disposed close to the inside of the connection portion  216 , and continuously surrounds inside space of the connection portion  216 . The top plate portion  215  is mounted on the mounting groove  2161 , so as to better position and fasten the top plate portion  215  with the connection portion  216 . In addition, the connection frame part  223  may also protect the top plate portion  215 . In some other embodiments, alternatively, no mounting groove  2161  may be disposed on the connection portion  216 , and the top plate portion  215  is directly fastened to a top side surface or an inner side surface of the connection portion  216 . 
     In some embodiments, as shown in  FIG.  4    and  FIG.  5   , the flash hole  212  is disposed on the main body part  21 , and the flash hole  212  penetrates through the connection portion  216  and the bottom plate portion  214 . The flash hole  212  is located on the outside of the mounting groove  2161 . The flash hole  212  and the top plate portion  215  do not overlap. The camera hole  211  is disposed on the main body part  21 , and the camera hole  211  penetrates through the top plate portion  215  and the bottom plate portion  214 . 
     In some embodiments, as shown in  FIG.  5    and  FIG.  7   , the top plate portion  215  is provided with a first through hole  2151 . The top plate portion  215  forms an annular protrusion  2152  around a position of the first through hole  2151 . The annular protrusion  2152  protrudes in a direction close to the bottom plate portion  214  and abuts against the bottom plate portion  214 . The bottom plate portion  214  is provided with a second through hole  2141 . The bottom plate portion  214  surrounds a position of the second through hole  2141  to form a limiting protrusion  2142 . The second through hole  2141  is connected to the first through hole  2151 , and the limiting protrusion  2142  is at least partially embedded within the annular protrusion  2152 . 
     In this embodiment, the first through hole  2151  and the second through hole  2141  jointly form the camera hole  211  of the terminal protective case  200 . The annular protrusion  2152  abuts against the bottom plate portion  214 . This helps increase support strength between the top plate portion  215  and the bottom plate portion  214 , and improve structural strength and pressure resistance performance of the main body part  21 . Embedding fit between the limiting protrusion  2142  and the annular protrusion  2152  facilitates better mutual positioning and fastening between the top plate portion  215  and the bottom plate portion  214 , and may also increase structural strength of the main body part  21 . In addition, because the diaphragm  31  is directly fastened to the main body part  21 , structural strength of the main body part  21  is increased. This helps reduce housing vibration caused by vibration of the diaphragm  31 , so that the piezoelectric speaker can make sound normally, and structural reliability of the terminal protective case  200  can be also improved, thereby improving holding experience and use experience of a user. 
     In some embodiments, as shown in  FIG.  5    and  FIG.  7   , the main body part  21  further includes one or more holders  217 . One end of each holder  217  is fixedly connected to the bottom plate portion  214 , and the other end is fixedly connected to the top plate portion  215 . In this case, the one or more holders  217  help improve support strength and connection firmness between the top plate portion  215  and the bottom plate portion  214 , so that overall structural strength of the main body part  21  is significantly improved, and structural reliability of the terminal protective case  200  is higher. In addition, the structural strength of the main body part  21  is improved, which is also conducive to normal sound-making of the piezoelectric speaker. 
     For example, the one or more holders  217  are integrally formed with the bottom plate portion  214 , and are fastened to the top plate portion  215  in a manner of bonding or the like. In some other embodiments, the one or more holders  217  may be alternatively integrated with the top plate portion  215 , and are fastened to the bottom plate portion  214  in a manner of bonding or the like. In some other embodiments, the one or more holders  217 , the top plate portion  215 , and the bottom plate portion  214  are fastened to each other in a manner of bonding or the like. In some other embodiments, there are a plurality of holders  217 . At least one holder  217  is integrally formed with the bottom plate portion  214 , and is fastened to the top plate portion  215  in a manner of bonding or the like. At least one holder  217  is integrally formed with the top plate portion  215 , and is fastened to the bottom plate portion  214  in a manner of bonding or the like. 
     The one or more holders  217  may be disposed in a gap within the main body part  21 , namely, space that is within the main body part  21  and that is not used to arrange another device. A specific arrangement location, an arrangement spacing, and an arrangement quantity of the holders  217 , and a specific shape of the holders  217  are not strictly limited in this application. For example, the terminal protective case  200  may include a plurality of holders  217 , and the plurality of holders  217  are arranged in an array at intervals, to support the top plate portion  215  and the bottom plate portion  214  evenly. 
     Refer to  FIG.  5    and  FIG.  8    together,  FIG.  8    is an enlarged view of a structure at a position B of the structure shown in  FIG.  7   . 
     In some embodiments, the sound outlet hole  213  is disposed on the connection portion  216  of the main body part  21 . The sound outlet hole  213  penetrates through the connection portion  216 , to connect inner space and outer space of the connection portion  216 , that is, to connect space within the main body part  21  and space on the outside of the main body part  21 . For example, there may be a plurality of sound outlet holes  213 , and the plurality of sound outlet holes  213  are disposed at intervals. Each sound outlet hole  213  is roughly in a rectangular shape. In some other embodiments, there may be alternatively one sound outlet hole  213 . In some other embodiments, the sound outlet hole  213  may be alternatively in another shape such as a circle or an ellipse. In this case, the terminal protective case  200  makes a sound from a side surface. When the terminal protective case  200  is held by a user or placed on an object, the sound outlet hole  213  is not easily blocked, so that the terminal protective case  200  can normally make a sound. This improves user experience. 
     For example, the sound outlet hole  213  may be roughly disposed at the bottom of the terminal protective case  200 , for example, may be located near the connector hole  2223 , so that when the user holds the audio playback system  1000 , a risk that the sound outlet hole  213  is blocked is reduced, and user experience is improved. 
     In some other embodiments, the sound outlet hole  213  may be alternatively disposed on the top plate portion  215  of the main body part  21 . Alternatively, the sound outlet hole  213  may be partially disposed on the connection portion  216  of the main body part  21 , and partially disposed on the top plate portion  215  of the main body part  21 . A position of the sound outlet hole  213  is not strictly limited in this application. 
     Refer to  FIG.  8    to  FIG.  10    together.  FIG.  9    is a schematic diagram of a structure of a main housing  2   a  of the terminal protective case  200  shown in  FIG.  5   , and  FIG.  10    is a schematic diagram of a structure of a cover plate  2   b  of the terminal protective case  200  shown in  FIG.  5    in some embodiments. 
     In some embodiments, as shown in  FIG.  8    and  FIG.  9   , the bottom plate portion  214  includes a bottom plate body  2143  and a first protrusion  2144 . The first protrusion  2144  is fastened on a side that is of the bottom plate body  2143  and that faces the top plate portion  215 . As shown in  FIG.  8    and  FIG.  10   , the top plate portion  215  includes a top plate body  2153  and a second protrusion  2154 . The second protrusion  2154  is fastened on a side that is of the top plate body  2153  and that faces the bottom plate portion  214 , and the second protrusion  2154  is disposed opposite to the first protrusion  2144 . As shown in  FIG.  8   , a periphery of the diaphragm  31  is fastened between the first protrusion  2144  and the second protrusion  2154 . The diaphragm  31  may be fastened to the first protrusion  2144  and the second protrusion  2154  in a manner of bonding or the like. The diaphragm  31  divides space within the main body part  21  into a front cavity  311  and a rear cavity  312  that are independent of each other. The front cavity  311  is formed on one side of the diaphragm  31 , and the rear cavity  312  is formed on the other side of the diaphragm  31 . 
     In this embodiment, the diaphragm  31  is fastened to the first protrusion  2144  and the second protrusion  2154 , so that the diaphragm  31  is directly fastened to the main body part  21  of the housing  2 . In this case, a front cavity  311  and a rear cavity  312  of the piezoelectric speaker are formed by using space in the main body part  21 . This improves space utilization of the terminal protective case  200  and facilitates thinness of the terminal protective case  200 . 
     In some embodiments, as shown in  FIG.  8   , the front cavity  311  is located between the diaphragm  31  and the bottom plate body  2143  and is located in the first protrusion  2144 . First space  312   a  is formed between the diaphragm  31  and the top plate body  2153 , and the first space  312   a  is located in the second protrusion  2154 . Second space  312   b  is formed between the bottom plate body  2143  and the top plate body  2153 , and the second space  312   b  is located on the outside of the second protrusion  2154 . The second space  312   b  and the first space  312   a  are connected to form the rear cavity  312  of the piezoelectric loudspeaker. In this embodiment, space of the rear cavity  312  formed within the main body part  21  is large, so that the piezoelectric speaker has a good low-frequency sound pressure level. For example, with reference to  FIG.  5    and  FIG.  8   , a breather hole  2162  (as shown in  FIG.  5   ) is further disposed on the connection portion  216  of the main body part  21 , and the second space  312   b  of the rear cavity  312  is connected to the outside of the main body part  21  through the breather hole  2162 . 
     As shown in  FIG.  8    and  FIG.  9   , the first protrusion  2144  is in a ring shape. A shape of the first protrusion  2144  adapts to a shape of the diaphragm  31 , to better fasten the first protrusion  2144  to a periphery of the diaphragm  31 . For example, the diaphragm  31  is rectangular, and the first protrusion  2144  is roughly in a rectangular ring shape. In some other embodiments, when the diaphragm  31  is in a circular shape, the first protrusion  2144  is roughly in a circular ring shape. 
     The first protrusion  2144  is provided with a first notch  2145 , and the first notch  2145  connects the front cavity  311  and the sound outlet hole  213 . There may be a plurality of first notches  2145 , and the plurality of first notches  2145  are disposed at intervals. In this case, a portion that is of the first protrusion  2144  and that is located between the two adjacent first notches  2145  can still fasten the diaphragm  31 , so that a connection area between the first protrusion  2144  and the diaphragm  31  is increased, and connection reliability between the first protrusion  2144  and the diaphragm  31  is higher. For example, when there are a plurality of sound outlet holes  213 , the plurality of first notches  2145  are disposed in a one-to-one correspondence with the plurality of sound outlet holes  213 ; or when there is one sound outlet hole  213 , the sound outlet hole  213  is connected to the plurality of first notches  2145 . In some other embodiments, there may be alternatively one first notch  2145 , and one first notch  2145  is connected to all sound outlet holes  213 . 
     For example, as shown in  FIG.  9   , the first protrusion  2144  is fixedly connected to the inner side surface of the connection portion  216 , to improve structural stability. For example, the first notch  2145  is located on a same side of the first protrusion  2144 , and the other three sides of the first protrusion  2144  are continuously fastened to a periphery of the diaphragm  31 , to improve connection reliability to the diaphragm  31 . In this case, the diaphragm  31  better isolates the front cavity  311  from the rear cavity  312 , so that the piezoelectric speaker has better sound quality. 
     As shown in  FIG.  8    and  FIG.  10   , the second protrusion  2154  is in a ring shape. A shape of the second protrusion  2154  adapts to a shape of the diaphragm  31 , to better fasten the second protrusion  2154  to a periphery of the diaphragm  31 . For example, the diaphragm  31  is rectangular, and the second protrusion  2154  is roughly in a rectangular ring shape. In some other embodiments, when the diaphragm  31  is in a circular shape, the second protrusion  2154  is roughly in a circular ring shape. 
     The second protrusion  2154  is provided with a second notch  2155 , and the second notch  2155  connects the first space  312   a  of the rear cavity  312  and the second space  312   b  of the rear cavity  312 . There may be a plurality of second notches  2155 , and the plurality of second notches  2155  are disposed at intervals. In this case, a portion that is of the second protrusion  2154  and that is located between the two adjacent second notches  2155  can still fasten the diaphragm  31 , so that a connection area between the second protrusion  2154  and the diaphragm  31  is increased, and connection reliability between the second protrusion  2154  and the diaphragm  31  is higher. In some other embodiments, there may be one second notch  2155 . 
     For example, as shown in  FIG.  10   , the second notch  2155  is located on a same side of the second protrusion  2154 , and the other three sides of the second protrusion  2154  are continuously fastened to a periphery of the diaphragm  31 , so that a connection between the diaphragm  31  and the second protrusion  2154  is more reliable. 
       FIG.  11    is a schematic diagram of a structure of a cover plate  2   b  of the terminal protective case  200  shown in  FIG.  5    in some other embodiments. When no conflict occurs, the cover plate  2   b  in this embodiment may include most features of the cover plate  2   b  in the foregoing embodiment. The following mainly describes differences between this embodiment and the foregoing embodiment, and most same content is not described again. 
     In some other embodiments, there are a plurality of second notches  2155 , and the plurality of second notches  2155  are arranged at intervals on three sides of the second protrusion  2154 . A side that is of the second protrusion  2154  and on which no notch is disposed is configured to directly face a side (refer to  FIG.  9   ) on which a notch is disposed of the first protrusion  2144 , to balance connection strength between each side of the diaphragm  31  and the main body part  21 . In this way, the connection between the diaphragm  31  and the main body part  21  is firmer and more reliable. 
     It may be understood that, in the foregoing embodiment, the first notch  2145  and the second notch  2155  are configured to meet a spatial connection requirement. In addition to the foregoing embodiment, there may also be a plurality of other design manners for quantities and arrangement positions of the first notch  2145  and the second notch  2155 . This is not strictly limited in this application. 
     In some embodiments, refer to  FIG.  8    and  FIG.  9    again, the bottom plate portion  214  may further include a limiting rib  2146 , and the limiting rib  2146  is fastened to a side that is of the bottom plate body  2143  and that faces the top plate portion  215 . The limiting rib  2146  is located on a side of the first protrusion  2144 , and the limiting rib  2146  is higher than the first protrusion  2144 . The limiting rib  2146  may be configured to limit or assist in limiting a position of the second protrusion  2154 , so that positioning and fastening between the top plate portion  215  and the top plate portion  215  are more reliable. A gap is formed between the limiting rib  2146  and the top plate body  2153 , where the limiting rib  2146  is away from all or a portion of the top surface of the bottom plate body  2143 . 
     In some other embodiments, the limiting rib  2146  may not be disposed on the bottom plate portion  214 , but a limiting structure is disposed on the top plate portion  215 . The limiting structure is configured to limit a position of the first protrusion  2144 . 
     In some other embodiments, positions of the front cavity and the rear cavity of the piezoelectric speaker may have different designs, for example: The front cavity is located between the diaphragm  31  and the top plate portion  215  and is located in the second protrusion  2154 . A portion of the rear cavity is located between the diaphragm  31  and the bottom plate portion  214  and is located in the first protrusion  2144 . The other portion of the rear cavity is located between the top plate portion  215  and the bottom plate portion  214  and is located on the outside of the first protrusion  2144 . The two parts of the rear cavity  312  are connected to each other. In this case, the second protrusion  2154  is roughly in a ring shape, and the second notch of the second protrusion  2154  is configured to connect the front cavity to the sound outlet hole  213 . The first protrusion  2144  is roughly in a ring shape, and the first notch of the first protrusion  2144  is configured to connect two parts of the rear cavity. 
     Refer to  FIG.  4    and  FIG.  12    together.  FIG.  12    is a schematic diagram of a portion of a circuit of the terminal protective case  200  shown in  FIG.  4   . 
     In some embodiments, the wireless communication module  5  is electrically connected to the drive circuit  4 , and the wireless communication module  5  is configured to perform wireless communication with the terminal  100 , to receive a communication signal of the terminal  100 . For example, the wireless communication module  5  is configured to communicate with the wireless communication module  16  of the terminal  100  by using a wireless communication technology. The wireless communication technology may include but is not limited to a Bluetooth technology, an ultra-wideband technology, a wireless local area network technology, and the like. The wireless communication module  5  may include a radio frequency circuit  51  and an antenna  52  electrically connected to the radio frequency circuit  51 . The antenna  52  is configured to convert a received electromagnetic wave into a radio frequency signal. The radio frequency circuit  51  is configured to demodulate the radio frequency signal into a baseband signal. In this case, the communication signal of the terminal  100  is an electromagnetic wave signal. 
     The battery  6  is electrically connected to the wireless communication module  5  and the drive circuit  4 . The battery  6  is configured to supply power to the terminal protective case  200 . A charging module  7  is electrically connected to the battery  6 , and an external charging device charges the battery  6  through the charging module  7 . For example, the charging module  7  may include an electrical connector, for example, a universal serial bus connector. As shown in  FIG.  4   , a charging socket  218  is disposed on the main body part  21 , and the external charging device may be connected to the charging module  7  through the charging socket  218 . 
     In this embodiment, the terminal protective case  200  communicates with the terminal  100  through wireless communication, and the battery  6  is disposed in the terminal protective case  200 . Therefore, the terminal protective case  200  can work normally in a scenario in which the terminal protective case  200  is separated from the terminal  100 . This improves user experience. For example, the terminal protective case  200  may be charged or play music after being detached from the terminal  100 . 
     As shown in  FIG.  12   , in some embodiments, the drive circuit  4  includes a positive power interface  41 , a negative power interface  42 , a first capacitor  43 , a boost circuit  44 , a second capacitor  45 , a power amplifier  46 , a signal interface  47 , a demodulator  48 , and filtering matched impedance  49 . For example, the positive power interface  41  is connected to a positive electrode of the battery  6 , the negative power interface  42  is connected to a negative electrode of the battery  6 , and the signal interface  47  is connected to the radio frequency circuit  51  of the wireless communication module  5 . 
     The positive power interface  41  is connected to a first input terminal  441  of the boost circuit  44 , and the negative power interface  42  is connected to a second input terminal  442  of the boost circuit  44 . One terminal of the first capacitor  43  is connected between the positive power interface  41  and the first input terminal  441  of the boost circuit  44 , and the other terminal of the first capacitor  43  is connected between the negative power interface  42  and the second input terminal  442  of the boost circuit  44 . A first output terminal  443  of the boost circuit  44  is connected to a positive input terminal  461  of the power amplifier  46 , and a second output terminal  444  of the boost circuit  44  is connected to a negative input terminal  462  of the power amplifier  46 . One terminal of the second capacitor  45  is connected between the first output terminal  443  of the boost circuit  44  and the positive input terminal  461  of the power amplifier  46 , and the other terminal of the second capacitor  45  is connected between the second output terminal  444  of the boost circuit  44  and the negative input terminal  462  of the power amplifier  46 . The demodulator  48  is connected between the signal interface  47  and a signal input terminal  463  of the power amplifier  46 , the demodulator  48  is configured to demodulate a signal (for example, a baseband signal) received by the signal interface  47  into an audio signal, and the power amplifier  46  is configured to amplify the audio signal and form a drive signal. The filtering matched impedance  49  is connected between an output terminal  464  of the power amplifier  46  and the piezoelectric sheet  32 , and the filtering matched impedance  49  is configured to send the drive signal to the piezoelectric sheet  32  after filtering and attenuating the drive signal. 
     In this embodiment, the boost circuit  44  is configured to boost an output voltage of the battery  6  and then provide the output voltage to the power amplifier  46 . The first capacitor  43  and the second capacitor  45  can regulate the output voltage of the battery  6  and an output voltage of the boost circuit  44 , and are also configured to provide a large transient peak current, so as to meet a driving current requirement of the piezoelectric sheet  32  of the piezoelectric speaker. 
     For example, the battery  6  may provide an output voltage of 5 V. The first capacitor  43  is a supercapacitor. For example, a capacitance value of the first capacitor  43  may be 1 F (F). The boost circuit  44  may boost a voltage of 5 V to 25 V. The second capacitor  45  is a supercapacitor. For example, a capacitance value of the second capacitor  45  may be 0.2 F. 
     For example, the drive circuit  4  may further include a control unit (not shown in the figure). The control unit configures and controls a power link, a power amplifier link, and a data link of the drive circuit  4 . 
     In some other embodiments, the power amplifier  46  in the drive circuit  4  may also be a smart power amplifier (smart power amplifier). In some other embodiments, the charging module  7  may alternatively include a wireless charging coil, and the external charging device charges the battery  6  through the charging module  7  by using a wireless charging technology. In this case, the charging socket may not be disposed on the main body part  21 . In some other embodiments, the charging module  7  may further include a discharge circuit, and the battery  6  may supply power to the terminal  100  through the discharge circuit, to improve an endurance capability of the terminal  100 . 
     In some embodiments, the terminal protective case  200  may further include a recording module (not shown in the figure) located within the main body part  21 . The recording module can communicate with the terminal  100  through the wireless communication module  5 . It may be understood that, in some other embodiments, a manner of communication between the recording module and the terminal  100  may change with a manner of communication between the drive circuit  4  and the terminal  100 , to simplify hardware and a circuit of the terminal protective case  200 . 
     Refer to both  FIG.  13    and  FIG.  14   .  FIG.  13    is a schematic diagram of a structure of the terminal protective case  200  of the audio playback system  1000  shown in  FIG.  1    in some other embodiments, and  FIG.  14    is a schematic diagram of a portion of a circuit of the terminal protective case  200  shown in  FIG.  13   . When no conflict occurs, the terminal protective case  200  in this embodiment may include most features of the terminal protective case  200  in the foregoing embodiment. The following mainly describes differences between this embodiment and the foregoing embodiment, and most same content is not described again. The terminal protective case  200  shown in  FIG.  13    may sleeve the terminal  100  shown in  FIG.  2   . 
     In some embodiments, no battery is disposed in the terminal protective case  200 , and instead, an external charging device directly supplies power to the terminal protective case  200  by using a wireless charging technology. For example, the terminal protective case  200  further includes a charging coil  8  and a wireless charging circuit  9  that are located within the main body part  21 . Both the charging coil  8  and the wireless charging circuit  9  are located within the main body part  21 . An input terminal of the wireless charging circuit  9  is connected to the charging coil  8 , and an output terminal of the wireless charging circuit  9  is connected to the drive circuit  4  and the wireless communication module  5 . The wireless charging circuit  9  is configured to: control the charging coil  8  to be coupled to a wireless charging coil  15  of the terminal  100 , and supply power to the drive circuit  4  and the wireless communication module  5 . The drive circuit  4  drives the piezoelectric speaker core  3  to work. 
     In this embodiment, no battery needs to be disposed in the terminal protective case  200 , and the power supply is obtained by using a wireless charging technology. Compared with the foregoing embodiment, the charging coil  8  and the wireless charging circuit  9  in this embodiment are small in size and light in weight, thereby facilitating thinness and lightweight of the terminal protective case  200 . 
     The wireless charging circuit  9  includes a positive input terminal  91  and a negative input terminal  92 . The positive input terminal  91  and the negative input terminal  92  are respectively connected to two ends of the charging coil  8 . The wireless charging circuit  9  further includes a positive output terminal  93  and a negative output terminal  94 . The positive output terminal  93  is connected to the positive power interface  41  of the drive circuit  4 , and the negative output terminal  94  is connected to the negative power interface  42  of the drive circuit  4 . 
     A third capacitor  95  is further disposed between the output terminals ( 93  and  94 ) of the wireless charging circuit  9  and the wireless communication module  5 , and two terminals of the third capacitor  95  are respectively connected to the positive output terminal  93  and the negative output terminal  94  of the wireless charging circuit  9 . The third capacitor  95  is configured to regulate a power supply voltage transmitted by the wireless charging circuit  9  to the wireless communication module  5 . 
     In some embodiments, the wireless charging circuit  9  can be further configured to control the charging coil  8  to be coupled to a wireless charging coil of another external charging device (for example, a power bank or a charging base), so that charging manners of the terminal protective case  200  are more diversified, and charging convenience is improved. 
     Refer to both  FIG.  15    and  FIG.  16   .  FIG.  15    is a schematic diagram of a structure of the terminal protective case  200  of the audio playback system  1000  shown in  FIG.  1    in still some other embodiments, and  FIG.  16    is a schematic diagram of a portion of a circuit of the terminal protective case  200  shown in  FIG.  15   . When no conflict occurs, the terminal protective case  200  in this embodiment may include most features of the terminal protective case  200  in the foregoing embodiment. The following mainly describes differences between this embodiment and the foregoing embodiment, and most same content is not described again. The terminal protective case  200  shown in  FIG.  15    may sleeve the terminal  100  shown in  FIG.  2   . 
     The terminal protective case  200  further includes an electrical connector  10 . The electrical connector  10  is plugged in the connector  117  of the terminal  100 . The drive circuit  4  is electrically connected to the electrical connector  10 . A communication signal of the terminal  100  is transmitted to the drive circuit  4  through the connector  117  and the electrical connector  10 . In this case, the communication signal of the terminal  100  is a universal serial bus signal, the signal interface  47  in the drive circuit  4  is connected to the electrical connector  10 , and the demodulator  48  is configured to demodulate the universal serial bus signal into an audio signal. 
     In this embodiment, the terminal protective case  200  is directly connected to the connector  117  of the terminal  100  through the electrical connector  10 , to obtain a communication signal of the terminal  100 , so that quality of communication between the terminal protective case  200  and the terminal  100  is good. 
     For example, the terminal protective case  200  may not include a battery, and the terminal protective case  200  simultaneously obtains a power signal from the terminal  100  through the electrical connector  10 . In this case, both the positive power interface  41  and the negative power interface  42  of the drive circuit  4  are connected to the electrical connector  10 . In this embodiment, a battery and a related charging circuit do not need to be additionally disposed in the terminal protective case  200 . This helps simplify a hardware structure and a circuit structure of the terminal protective case  200 . 
     As shown in  FIG.  15   , the electrical connector  10  includes a male connector  101  and a female socket  102 . An end of the male connector  101  is fixedly and electrically connected to the female socket  102 . The male connector  101  is configured to plug in a connector  117  of the terminal  100 . The female socket  102  projects from the terminal protective case  200 . In this embodiment, another device (for example, a charger or a headset) may be electrically connected to the connector  117  of the terminal  100  by plugging in the female socket  102 , to implement a communication connection and/or a power connection to the terminal  100 . 
     Refer to both  FIG.  17    and  FIG.  18   .  FIG.  17    is a schematic diagram of a structure of a terminal protective case  200  of the audio playback system  1000  shown in  FIG.  1    in yet some other embodiments, and  FIG.  18    is a schematic diagram of a structure of a terminal  100  of the audio playback system  1000  shown in  FIG.  1    in some other embodiments. When no conflict occurs, the terminal  100  in this embodiment may include most features of the terminal  100  in the foregoing embodiment, and the terminal protective case  200  in this embodiment may include most features of the terminal protective case  200  in the foregoing embodiment. The following mainly describes differences between this embodiment and the foregoing embodiment, and most same content is not described again. The terminal protective case  200  shown in  FIG.  17    may sleeve the terminal  100  shown in  FIG.  18   . 
     In some embodiments, the rear-facing camera module  19  of the terminal  100  is disposed at a position close to a side edge. In this case, the flash module no and the rear-facing camera module  19  may be arranged vertically or horizontally. The terminal protective case  200  includes a plurality of piezoelectric speaker cores  3 . The plurality of piezoelectric speaker cores  3  may be spaced apart from each other. For example, the terminal protective case  200  includes two piezoelectric speaker cores  3 , and the two piezoelectric speaker cores  3  are separately disposed close to the top and the bottom of the terminal protective case  200 . The camera hole  211  and the flash hole  212  of the terminal protective case  200  are arranged close to a side edge of the terminal protective case  200 , and positions of the camera hole  211  and the flash hole  212  correspond to positions of the rear-facing camera module  19  and the flash module no of the terminal  100 . 
     In this embodiment, the plurality of piezoelectric speaker cores  3  are disposed in the terminal protective case  200 , to form a plurality of piezoelectric speakers. Therefore, loudspeaker volume of the terminal protective case  200  and the audio playback system  1000  is significantly increased. This helps improve user experience. 
     In some other embodiments, the terminal protective case  200  may alternatively include at least three piezoelectric speaker cores  3 . A quantity of the piezoelectric speaker cores  3  is not strictly limited in this application. 
     In some embodiments, front cavities of the plurality of piezoelectric speaker cores  3  (that is, front cavities of piezoelectric speakers) are connected to one or more same sound outlet holes. In this case, volume of sounds made by the plurality of piezoelectric speakers is superimposed. In some other embodiments, the front cavities of the plurality of piezoelectric speaker cores  3  (that is, front cavities of the piezoelectric speaker) are respectively connected to a plurality of sound outlet holes at different positions. In this case, the plurality of piezoelectric speakers may cooperate with each other to implement stereo. For example, one of the two piezoelectric speakers is connected to a sound outlet hole at the top of the terminal protective case  200  and is set as a left sound channel, and the other is connected to a sound outlet hole at the bottom of the terminal protective case  200  and is set as a right sound channel. 
     Refer to  FIG.  17    and  FIG.  19    together.  FIG.  19    is a schematic diagram of a structure of the terminal protective case  200  shown in  FIG.  17    from another angle. 
     For example, the plurality of sound outlet holes  213  include a first sound outlet hole  2131  and a second sound outlet hole  2132 . The first sound outlet hole  2131  and the second sound outlet hole  2132  are connected to front cavities of different piezoelectric speaker cores  3 . The first sound outlet hole  2131  and the second sound outlet hole  2132  are respectively located at two ends of the connection portion  216 . For example, the first sound outlet hole  2131  and the second sound outlet hole  2132  may be respectively located at the top and the bottom of the terminal protective case  200 . 
     In this embodiment, a distance between the first sound outlet hole  2131  and the second sound outlet hole  2132  is long. This helps improve a stereo playback sound effect of the terminal protective case  200 . It may be understood that, in both a landscape use scenario and a portrait use scenario, the terminal protective case  200  can implement stereo playback. 
     In some other embodiments, a quantity of piezoelectric speaker cores  3  may be alternatively greater than three or equal to three. This is not strictly limited in this application. It may be understood that, in this embodiment of this application, when there are a plurality of piezoelectric speaker cores  3 , a related structure of the main body part  21  of the housing  2  is correspondingly adjusted, and the drive circuit  4  is also correspondingly adjusted. For example, the plurality of piezoelectric speaker cores  3  may have rear cavities independent of each other, or may share a rear cavity. 
     In some other embodiments, the rear-facing camera module  19  of the terminal  100  may also be arranged near a middle position or arranged in a middle position. In this case, a diaphragm with a small area may be disposed on some piezoelectric speaker cores  3  of the terminal protective case  200 , so as to arrange the piezoelectric speaker cores  3  by using space around the camera hole  211  (corresponding to the rear-facing camera module  19 ). In this way, internal space of the terminal protective case  200  is fully used, and there can be a large spacing between a plurality of piezoelectric speakers, to improve a stereo sound effect. In other words, the terminal protective case  200  may include a plurality of piezoelectric speaker cores  3  with different diaphragm areas. It may be understood that there may be a plurality of design solutions for a position of the camera hole  211  of the terminal protective case  200 , a size of the camera hole  211 , a diaphragm area relationship of the plurality of piezoelectric speaker cores  3 , and an arrangement position relationship of the plurality of piezoelectric speaker cores  3 , and the solutions are not limited to the foregoing solutions. 
     Refer to both  FIG.  20    and  FIG.  21   .  FIG.  20    is a schematic diagram of a structure of a terminal protective case  200  of the audio playback system  1000  shown in  FIG.  1    in still yet some other embodiments, and  FIG.  21    is a schematic diagram of a structure of a terminal  100  of the audio playback system  1000  shown in  FIG.  1    in still some other embodiments. When no conflict occurs, the terminal  100  in this embodiment may include most features of the terminal  100  in the foregoing embodiment, and the terminal protective case  200  in this embodiment may include most features of the terminal protective case  200  in the foregoing embodiment. The following mainly describes differences between this embodiment and the foregoing embodiment, and most same content is not described again. The terminal protective case  200  shown in  FIG.  20    may sleeve the terminal  100  shown in  FIG.  21   . 
     In some embodiments, the terminal protective case  200  includes a housing  2 , a diaphragm  31 , a piezoelectric sheet  32 , and a first conductive contact  20 . The housing  2  includes a main body part  21 , a fastening frame part  22 , and an accommodating groove  23  jointly encircled by the fastening frame part  22  and the main body part  21 . The diaphragm  31  is located within the main body part  21 , and the piezoelectric sheet  32  is fastened to the diaphragm  31 . The first conductive contact  20  is fastened on the fastening frame part  22  and projects from the accommodating groove  23 . The first conductive contact  20  is electrically connected to the piezoelectric sheet  32 . 
     The terminal  100  may be accommodated in the accommodating groove  23  of the terminal protective case  200 , and the terminal  100  and the terminal protective case  200  jointly form the audio playback system  1000 . The terminal  100  further includes a drive circuit  120  and a second conductive contact  121  that is electrically connected to the drive circuit  120 . The drive circuit  120  may be fastened to the circuit board  13 , and the drive circuit  120  is coupled to a processor  119 . The drive circuit  120  is configured to convert an audio signal into a drive signal. The second conductive contact  121  partially or totally projects from the terminal  100 . When the terminal  100  is accommodated in the accommodating groove  23 , the second conductive contact  121  abuts against the first conductive contact  20 . The drive signal formed by the drive circuit  120  is transmitted to the piezoelectric sheet  32  through the second conductive contact  121  and the first conductive contact  20 , to drive the piezoelectric sheet  32  to drive the diaphragm  31  to vibrate. 
     In this embodiment, the terminal  100  may transmit the drive signal to the piezoelectric sheet  32  of the terminal protective case  200 , to drive the piezoelectric sheet  32  to drive the diaphragm  31  to vibrate. Therefore, the piezoelectric speaker of the terminal protective case  200  may make a sound under control of the terminal  100 . The piezoelectric speaker of the terminal protective case  200  may make a sound together with the speaker  114  of the terminal  100  to greatly increase loudspeaker volume of the audio playback system  1000 . In other words, the audio playback system  1000  has high loudspeaker volume, and the audio playback system  1000  still has a good loudspeaker effect in a noisy environment or an outdoor environment. In addition, the terminal  100  can control, by using the drive signal, the piezoelectric speaker of the terminal protective case  200  to make a sound or not to make a sound, so that the audio playback system  1000  can implement a plurality of loudspeaker modes, such as making a sound by the terminal protective case  200  alone, making a sound by the terminal  100  alone, and making a sound by the terminal protective case  200  and the terminal  100  together. In addition, when the piezoelectric speaker of the terminal protective case  200  and the speaker  114  of the terminal  100  make a sound together, the piezoelectric speaker and the speaker of the terminal may cooperate to form a multi-channel loudspeaker system. This improves sound field rendering and a stereo effect. 
     In this embodiment, because the piezoelectric sheet  32  of the piezoelectric speaker of the terminal protective case  200  is directly driven by the drive signal sent by the terminal  100 , a structure such as a battery, a charging circuit, or a piezoelectric sheet drive circuit does not need to be disposed on the terminal protective case  200 . Therefore, the terminal protective case  200  has a simple structure and low costs. 
     In some embodiments, the first conductive contact  20  may be of a protrusion structure, a dome structure, or a pad structure, and the first conductive contact  20  may be made of a material such as metal. The second conductive contact  121  may be of a protrusion structure, a dome structure, or a pad structure, and the first conductive contact  20  may be made of a material such as metal. 
     In some embodiments, there are two first conductive contacts  20 , and the two first conductive contacts  20  may be relatively fastened on two sides of the bottom frame part  222  of the fastening frame part  22 . In this case, there are also two second conductive contacts  121 , and the two second conductive contacts  121  are respectively fixed on two sides of the frame of the terminal  100 . 
     In some other embodiments, the two first conductive contacts  20  may also be fastened on two sides of the top frame part  221  of the fastening frame part  22 , or the two first conductive contacts  20  may be alternatively fastened on a same side of the top frame part  221  or the bottom frame part  222  of the fastening frame part  22 . In this case, the position of the second conductive contact  121  of the terminal  100  is adaptively adjusted. In some other embodiments, the first conductive contact  20  may be alternatively fastened to the main body part  21 , and the second conductive contact  121  may be alternatively fastened to the rear cover of the terminal  100 . In some other embodiments, one of the two first conductive contacts  20  may be alternatively fastened to the fastening frame part  22  and the other may be fastened to the main body part  21 . Positions of the two second conductive contacts  121  of the terminal  100  are adaptively adjusted. 
     In some other embodiments, there may also be one first conductive contact  20 , and the first conductive contact  20  includes a positive input terminal and a negative input terminal. In this case, there is also one second conductive contact  121 , and the second conductive contact  121  includes a positive output terminal and a negative output terminal. In some other embodiments, there may also be three or another quantity of first conductive contacts  20 , which may be designed according to a drive signal requirement of the piezoelectric sheet  32 . It may be understood that a specific quantity and arrangement locations of the first conductive contacts  20  are not strictly limited in this embodiment of this application, and a quantity and locations of the second conductive contacts  121  are adaptively adjusted and designed according to the first conductive contact  20 . 
     In some other embodiments, the terminal protective case  200  and the terminal  100  may further include one or more other conductive contact groups that may abut against each other. For example, the terminal protective case  200  may further include a third conductive contact electrically connected to the recording module. The terminal  100  may further include a fourth conductive contact configured to abut against the third conductive contact, and the fourth conductive contact is coupled to the processor  119 . 
     Refer to both  FIG.  22    and  FIG.  23   .  FIG.  22    is a schematic diagram of a structure of a terminal protective case  200  of the audio playback system  1000  shown in  FIG.  1    in some further embodiments, and  FIG.  23    is a schematic diagram of a structure of a terminal  100  of the audio playback system  1000  shown in  FIG.  1    in yet some other embodiments. When no conflict occurs, the terminal  100  in this embodiment may include most features of the terminal  100  in the foregoing embodiment, and the terminal protective case  200  in this embodiment may include most features of the terminal protective case  200  in the foregoing embodiment. The following mainly describes differences between this embodiment and the foregoing embodiment, and most same content is not described again. The terminal protective case  200  shown in  FIG.  22    may sleeve the terminal  100  shown in  FIG.  23   . 
     In some embodiments, the terminal protective case  200  includes a plurality of piezoelectric speaker cores  3  located within the main body part  21 . Each piezoelectric speaker core  3  includes a diaphragm  31  and a piezoelectric sheet  32  fastened to the diaphragm  31 . The terminal protective case  200  further includes a plurality of groups of first conductive contacts  20 , and the plurality of groups of first conductive contacts  20  are respectively connected to piezoelectric sheets  32  of a plurality of piezoelectric speaker cores  3 . The terminal  100  includes a plurality of groups of second conductive contacts  121  that are connected to the plurality of groups of first conductive contacts  20  in a one-to-one correspondence manner. 
     For example, the terminal protective case  200  includes two piezoelectric speaker cores  3  and two groups of first conductive contacts  20 . The two piezoelectric speaker cores  3  are respectively disposed close to the top and the bottom of the terminal protective case  200 . The two groups of first conductive contacts  20  are respectively disposed close to the top and the bottom of the terminal protective case  200 . The terminal  100  includes two groups of second conductive contacts  121 , and the two groups of second conductive contacts  121  are respectively disposed near the top and the bottom of the terminal  100 . 
     In some embodiments, the front cavities of the plurality of piezoelectric speaker cores  3  may be connected to one or more same sound outlet holes. In this case, volume of sounds made by the plurality of piezoelectric speakers is superimposed. In some other embodiments, the front cavities of the plurality of piezoelectric speaker cores  3  (that is, front cavities of the piezoelectric speaker) are respectively connected to a plurality of sound outlet holes at different positions. In this case, the plurality of piezoelectric speakers may cooperate with each other to implement stereo. 
       FIG.  24    is a schematic diagram of an internal structure of a terminal protective case  200  of the audio playback system  1000  shown in  FIG.  1    in still some further embodiments. When no conflict occurs, the terminal protective case  200  in this embodiment may include most features of the terminal protective case  200  in the foregoing embodiment. The following mainly describes differences between this embodiment and the foregoing embodiment, and most same content is not described again. The protective case in this embodiment may be used together with any one of the foregoing terminals  100 . 
     In some embodiments, the terminal protective case  200  further includes a first cover body  20  and a second cover body  30  that are located within the main body part  21 . The first cover body  20  and the second cover body  30  are disposed opposite to each other, and a periphery of the diaphragm  31  is fastened between the first cover body  20  and the second cover body  30 . A front cavity  311  is formed between the diaphragm  31  and the first cover body  20 , a connection hole  201  is disposed on the first cover body  20 , and the connection hole  201  connects the front cavity  311  to the sound outlet hole  213 . A rear cavity  312  is formed between the diaphragm  31  and the second cover body  30 , a leakage hole  301  is disposed on the second cover body  30 , and the leakage hole  301  connects the rear cavity  312  to the outside of the second cover body  30 . 
     In this embodiment, the first cover body  20 , the second cover body  30 , the diaphragm  31 , and the piezoelectric sheet  32  jointly form the piezoelectric speaker  300 . The piezoelectric speaker  300  may be first formed in the terminal protective case  200 , and then the piezoelectric speaker  300  is mounted into the main body part  21 , to reduce assembly difficulty of the terminal protective case  200 . In this way, a product yield of the terminal protective case  200  is high. In addition, when the piezoelectric speaker  300  is independently assembled into a module, assembly precision of the piezoelectric speaker  300  is also ensured, so that the piezoelectric speaker  300  has a higher product yield. 
     It may be understood that, compared with the foregoing embodiment, in this embodiment, a first protrusion may not be disposed on the bottom plate portion  214  of the main body part  21 , and a second protrusion may not be disposed on the top plate portion  215 . To improve positioning and fastening reliability between the piezoelectric speaker  300  and the main body part  21 , the bottom plate portion  214  may further include a first limiting piece  2147  fastened to the bottom plate body  2143 . The top plate portion  215  may further include a second limiting piece  2156  fastened to the top plate body  2153 . Both the second limiting piece  2156  and the first limiting piece  2147  are configured to limit the piezoelectric speaker  300 . 
     In some embodiments, the leakage hole  301  is disposed near middle space of the main body part  21 , so that the rear cavity  312  is connected to space that is within the main body part  21 , that is located between the top plate portion  215  and the bottom plate portion  214 , and that is located on the outside of the second cover body  30 . In this way, the space is used to expand space of the rear cavity  312  of the piezoelectric speaker  300  and improve a low-frequency sound pressure level of the piezoelectric speaker  300 . In this case, the rear cavity of the piezoelectric speaker  300  is an open rear cavity. In some other embodiments, the leakage hole may be alternatively disposed close to an edge of the main body part  21 , that is, a connection portion facing the main body part  21 , so that the leakage hole is directly connected to the outside of the main body part  21  through a breather hole  2162  (refer to  FIG.  4   ) of the connection portion. For example, the leakage hole of the second cover body  30  is disposed opposite to the breather hole  2162 . 
     For example, the first cover body  20  may be located between the diaphragm  31  and the bottom plate portion  214  of the main body part  21 . The second cover body  30  may be located between the diaphragm  31  and the top plate portion  215  of the main body part  21 . The connection hole  201  of the first cover body  20  is close to and opposite to the sound outlet hole  213 . 
     In some embodiments, the first cover body  20  includes a body  202  and a fastening frame  203  fastened on a periphery of the body  202 . The second cover body  30  includes a body  302  and a fastening frame  303  fastened on a periphery of the body  302 . The fastening frame  203  of the first cover body  20  and the fastening frame  303  of the second cover body  30  are respectively fastened on two sides of a periphery of the diaphragm  31 . The connection hole  201  is disposed on the fastening frame  203  of the first cover body  20 , and the leakage hole  301  is disposed on the fastening frame  303  of the second cover body  30 . 
     In this embodiment, because the piezoelectric speaker  300  has a small overall thickness, when the terminal protective case  200  forms the piezoelectric speaker  300 , a thickness of the main body part  21  of the terminal protective case may be controlled within a small range. In this case, when the terminal protective case  200  sleeves the outside of the terminal  100 , a hand holding feeling and portability of a user are improved. 
     Generally, the overall thickness of the piezoelectric speaker  300  may be controlled to be less than 5 millimeters or less than 4 millimeters. For example, a thickness of the body  202  of the first cover body  20  may be 0.5 millimeter, a height of the front cavity  311  may be 1 millimeter, a thickness of the piezoelectric sheet  32  may be 0.27 millimeter, a thickness of the diaphragm  31  may be 0.15 millimeter, a height of the rear cavity  312  may be 1 millimeter, and a thickness of the body  302  of the second cover body  30  may be 0.5 millimeter. In this case, the overall thickness of the piezoelectric speaker  300  is 3.42 millimeters. 
     In some embodiments, the first cover body  20  and the second cover body  30  are made of a hard material, for example, a steel sheet with a thickness of 0.4 millimeter to 0.5 millimeter may be selected. The material and the thickness of the first cover body  20  and the second cover body  30  are not strictly limited in this application. 
     It may be understood that the first cover body  20  and the second cover body  30  jointly form a speaker housing of the piezoelectric speaker  300 . When the terminal protective case  200  includes a plurality of piezoelectric speaker cores  3 , the terminal protective case  200  also includes a plurality of speaker housings. The plurality of speaker housings are configured to accommodate and fasten the plurality of piezoelectric speaker cores  3  in a one-to-one correspondence manner, to form the plurality of piezoelectric speakers  300 . 
     The foregoing descriptions are merely specific implementations of this application, but are not intended to limit the protection scope of this application. Any variation or replacement readily figured out by a person skilled in the art within the technical scope disclosed in this application shall fall within the protection scope of this application. When no conflict occurs, embodiments of this application and the features in the embodiments may be mutually combined. Therefore, the protection scope of this application shall be subject to the protection scope of the claims.