# Patent

## Title
Method of controlling sound reproduction of enclosure mounted loudspeakers

## Abstract

A method of controlling sound reproduction may include applying an audio signal to a voice coil of the electrodynamic loudspeaker to produce sound, detecting one of an impedance and admittance of the loudspeaker across a predetermined audio frequency range based on a detected voice coil current and voice coil voltage and determining a fundamental resonance frequency of the loudspeaker based on the detected impedance or admittance. The fundamental resonance frequency of the loudspeaker may be compared with a nominal fundamental resonance frequency of the loudspeaker representing a nominal acoustic operating condition of the loudspeaker. A change of operating condition of the loudspeaker may be detected based on a frequency deviation between the determined fundamental resonance frequency and a nominal fundamental resonance frequency of the loudspeaker. The level of the audio signal may be attenuated in response to the frequency deviation meets a predetermined frequency error criterion.

## Background
RELATED APPLICATIONS
This application is a continuation-in-part of U.S. patent application Ser. No. 13/948,663, “Method of Detecting Enclosure Leakage of Enclosure Mounted Loudspeakers,” filed Jul. 23, 2013, the disclosure of which is incorporated herein in its entirety.


BACKGROUND OF THE INVENTION
The present invention relates to a method of controlling sound reproduction of an enclosure mounted electrodynamic loudspeaker of a portable communication device and a corresponding sound reproduction assembly. The method of controlling sound reproduction comprises steps of applying an audio signal to a voice coil of the electrodynamic loudspeaker through an output amplifier to produce sound, detecting one of an impedance and admittance of the loudspeaker across a predetermined audio frequency range based on a detected voice coil current and voice coil voltage and determining a fundamental resonance frequency of the loudspeaker based on the detected impedance or admittance. The determined the fundamental resonance frequency of the loudspeaker is compared with a nominal fundamental resonance frequency of the loudspeaker representing a nominal acoustic operating condition of the electrodynamic loudspeaker and a change of acoustic operating condition of the electrodynamic loudspeaker is detected based on a frequency deviation between the determined fundamental resonance frequency and a nominal fundamental resonance frequency of the electrodynamic loudspeaker. The level of the audio signal may be attenuated in response to the frequency deviation meets a predetermined frequency error criterion.
The present invention relates to a method of controlling sound reproduction of an enclosure mounted electrodynamic loudspeaker of a portable communication device and a corresponding assembly for controlling sound reproduction of the enclosure mounted electrodynamic loudspeaker. The methodology preferably includes comparing a dynamically determined fundamental resonance frequency of the loudspeaker with a nominal fundamental resonance frequency of the loudspeaker representing a nominal acoustic operating condition of the electrodynamic loudspeaker. A change of acoustic operating condition of the electrodynamic loudspeaker is detected based on a frequency deviation between the determined fundamental resonance frequency and the nominal fundamental resonance frequency of the electrodynamic loudspeaker. The detection of the change of acoustic operating condition of the electrodynamic loudspeaker is highly useful in portable communication device for various purposes such as detection of acoustic leakage of a sealed enclosure in which the electrodynamic loudspeaker of the portable communication device is mounted. The electrodynamic loudspeaker is utilized for sound reproduction purposes in the portable communication device, e.g. as a receiver for producing sound by acoustic coupling to the user's ear, or as a loudspeaker for playing recorded music or for voice reproduction in teleconferencing applications. The change of acoustic operating condition of the electrodynamic loudspeaker can also be exploited to detect an acoustically blocked operating condition of the loudspeaker of the portable communication device and in response thereto adapt the sound reproduction of the loudspeaker in various ways as described below in additional detail.
Furthermore, it is of significant interest and value to provide a relatively simple method for detecting the change of acoustic operating condition of the electrodynamic loudspeaker and the underlying cause, e.g. enclosure leakage, to avoid excessive expenditure of computational resources of an application microprocessor of the portable communication device and/or other computational hardware resources.
SUMMARY OF THE INVENTION
A first aspect of the invention relates to a method of controlling sound reproduction of an enclosure mounted electrodynamic loudspeaker of a portable communication device, the method comprising steps of: applying an audio signal to a voice coil of the electrodynamic loudspeaker through an output amplifier to produce sound, detecting a voice coil current flowing into the voice coil, detecting a voice coil voltage across the voice coil, detecting one of an impedance and admittance of the loudspeaker across a predetermined audio frequency range based on the detected voice coil current and voice coil voltage, determining a fundamental resonance frequency of the loudspeaker based on the detected impedance and/or admittance, comparing the determined the fundamental resonance frequency of the loudspeaker with a nominal fundamental resonance frequency of the loudspeaker representing a nominal acoustic operating condition of the electrodynamic loudspeaker, detecting a change of acoustic operating condition of the electrodynamic loudspeaker based on a frequency deviation between the determined fundamental resonance frequency and the nominal fundamental resonance frequency of the electrodynamic loudspeaker. The level of the audio signal applied to the voice coil is preferably attenuated in response to the frequency deviation meets a predetermined frequency error criterion.
The detection of the voice coil voltage may be accomplished by a direct measurement e.g. by an A/D converter coupled to the voice coil voltage or by indirect determination where the voice coil voltage is determined or estimated from a known level of the audio signal, e.g. digitally represented, and a known DC supply voltage of the output amplifier.
The audio signal applied to the loudspeaker during normal operation may comprise speech and/or music supplied from a suitable audio source such as radio, CD player, network player, MP3 player. The audio source may also comprise a microphone generating a real-time microphone signal in response to incoming sound. The skilled person will appreciate that each of the audio signal, the voice coil voltage, and the voice coil current may be represented by an analog signal for example as a voltage, current, charge etc. or alternatively be represented by a digital signal, e.g. sampled and coded in binary format at a suitable sampling rate and resolution.
The output amplifier preferably comprises a switching or class D amplifier for example a Pulse Density Modulation (PDM) or Pulse Width Modulation (PWM) output amplifier which both possess high power conversion efficiency. This is a particularly advantageous feature for use in battery powered portable communication devices. In the alternative, the output amplifier may comprise traditional non-switched power amplifier topologies like class A or class AB.
The skilled person will understand that the predetermined frequency error criterion may comprise different types of frequency domain criteria such as preset frequency range, a frequency limit, a preset percentage or absolute deviation between the nominal and detected fundamental resonance frequencies, etc. which indicates the existence of a particular acoustic operating condition. The preset frequency range, frequency limit or percentage deviation may be determined a priori by conducting suitable experiments or simulations of the effect a particular acoustic operating condition has on the fundamental resonance frequency of the enclosure mounted loudspeaker and/or the impedance or admittance of the voice coil at the fundamental resonance frequency of the enclosure mounted loudspeaker.
According to one embodiment of the invention, the predetermined frequency error criterion represents acoustic leakage of the enclosure of the electrodynamic loudspeaker. In another embodiment of the invention, the predetermined frequency error criterion represents acoustic blocking of a frontal side of the electrodynamic loudspeaker.
In both of these embodiments, the present methodology exploits a leakage induced or blockage induced shift or change of the fundamental resonance frequency of the enclosure mounted loudspeaker to detect the change of acoustic operating condition of the loudspeaker. This change of fundamental resonance frequency of the electrodynamic loudspeaker is preferably detected in real-time during normal operation of the electrodynamic loudspeaker to allow appropriate audio signal level attenuation or excursion limitation measures to be applied substantially instantaneously in response to the change of acoustic operating condition. Hence, the risk of forcing the movable diaphragm assembly to excessive excursion, i.e. above a maximum allowable excursion limit, due to enclosure leakage is minimized and so is the accompanying risk of imparting mechanical damage of the loudspeaker. The mechanical damage may be caused by collision between movable loudspeaker components, such as the voice coil, diaphragm or voice coil bobbin, and a stationary component such as the magnetic circuit.
In the present context, the fundamental resonance frequency of the electrodynamic loudspeaker is the resonance frequency determined or set by total compliance acting on the movable diaphragm assembly and the total moving mass of the electrodynamic loudspeaker. The total compliance acting on the movable diaphragm assembly will typically comprise a parallel connection of a compliance of an edge suspension of the loudspeaker and a compliance caused by the trapped air inside the sealed enclosure. The fundamental resonance frequency of the enclosure mounted electrodynamic loudspeaker can typically readily be identified by inspection of its low-frequency peak electrical impedance.
The nominal fundamental resonance frequency of the loudspeaker preferably represents an estimated or measured fundamental resonance frequency of the electrodynamic loudspeaker mounted in the relevant enclosure of the portable communication device when the enclosure is appropriately acoustically sealed and the frontal side of the loudspeaker unblocked, e.g. radiating into a substantially free field. The nominal fundamental resonance frequency can accordingly be determined or set in various ways. According to one embodiment of the invention, the nominal fundamental resonance frequency is based on the speaker manufacturer's data sheet for the actual combination of sealed enclosure volume and the electrodynamic loudspeaker model in question. In this case, the nominal fundamental resonance frequency may represent an average, or any other suitable statistical measure of the resonance frequency value for the particular type of electrodynamic loudspeaker in question. This embodiment may be used to test or verify correct sealed mounting of the loudspeaker in the enclosure or chamber during manufacturing. This test or verification may be accomplished by measuring the fundamental resonance frequency of the loudspeaker after enclosure mounting and compare the measured fundamental resonance frequency with the nominal fundamental resonance frequency. If the measured value of the fundamental resonance frequency falls below a preset frequency threshold frequency or outside certain a predetermined frequency band or range around the nominal fundamental resonance frequency, the enclosure may be flagged as leaking. This flag may be used to inspect and possibly repair the enclosure and/or the mounting of the loudspeaker therein during the manufacturing process and hence avoid expensive and annoying field returns of for example a portable communication device housing the enclosure mounted loudspeaker.
The above outlined expectation based determination of the nominal fundamental resonance frequency of the loudspeaker may be less accurate than desired in certain situations due to sample-to-sample manufacturing spread on the fundamental resonance frequency of the type electrodynamic loudspeaker in question. Hence, in other embodiments, the nominal fundamental resonance frequency may be represented by a measured fundamental resonance frequency of the electrodynamic loudspeaker in question as determined from an operational measurement on the electrodynamic loudspeaker when mounted in the enclosure in the sealed and unblock state. Under this operational measurement, the enclosure is accordingly in a known appropriately sealed condition. The measurement of the fundamental resonance frequency may be accomplished during manufacturing of the portable communication device in which the electrodynamic loudspeaker and associated enclosure is integrated. As before, the measured fundamental resonance frequency may be compared with the nominal fundamental resonance frequency to verify correct operation of the loudspeaker itself and correct mounting of the loudspeaker in the enclosure. In both of these embodiments, the set value of the nominal fundamental resonance frequency may be stored in digital format in an electronic memory of the portable communication device such as a non-volatile memory area.
A rapid and reliable detection of acoustic leakage of the loudspeaker enclosure is highly useful in numerous sound reproduction applications and equipment. It is important to rapidly and reliably detect enclosure leakage because of the associated loss of mechanical stiffness or compliance of the trapped air mass inside the sealed enclosure behind the loudspeaker diaphragm. The loss of stiffness leads to markedly increased diaphragm excursion for a given voice coil voltage, i.e. for a given level of the audio signal. The increase of diaphragm excursion is likely to force the diaphragm and voice coil assembly of the loudspeaker beyond its maximum allowable peak excursion leading to various kinds of irreversible mechanical damage to the loudspeaker. The user will typically notice this kind of irreversible mechanical damage of the loudspeaker due to a grossly distorted sound quality of the loudspeaker or a complete absence of audible sound. While this problem is of significant importance in numerous areas of loudspeaker technology, it is particularly important in miniature loudspeakers for portable communication devices such as mobile phones, smartphones, audio enabled tablets etc. In the latter type of devices, a miniature electrodynamic loudspeaker is often mounted in a small sealed enclosure or chamber for example having a volume of about 1 cm3. The way users handle mobile phones and smartphones makes it unavoidable that these occasionally are dropped. These accidental drops may, depending on the impact surface and drop height, lead to severe impact blows on the phone housing or casing. Experience shows that these impacts often are sufficiently large to break a small hole of crack in the small sealed enclosure of the miniature loudspeaker leading to the undesired acoustic leakage. While the costs of a replacement miniature electrodynamic loudspeaker itself are quite modest, the costs of handling the entire repair service procedure are high. This is caused by the multitude of operational activities which typically includes various transportation and order tracking activities, disassembling of the communication device, removal of the defective miniature speaker, mounting of a new miniature speaker, testing, re-assembling and returning etc. In addition, the user is left without an often vital communication tool for the duration of the repair procedure. Hence, it is of considerable value to rapidly and reliably detect enclosure leakage and apply proper precautionary measures in the portable communication device to prevent damage to the miniature electrodynamic loudspeaker by limiting the diaphragm excursion to a value below its maximum allowable peak excursion.
The enclosure leakage detection methodology may be applied to a wide range of applications of sealed enclosure mounted electrodynamic loudspeakers such as large diameter woofers or broad-band loudspeakers for High Fidelity, automotive or Public Address applications as well as to miniature electrodynamic loudspeakers for portable communication devices and/or music players. In the latter case, the electrodynamic loudspeaker may be integrated in a mobile phones or smartphone and mounted in a sealed enclosure with a volume between 0.5 and 2.0 cm3 such as about 1 cm3. The enclosure mounted electrodynamic loudspeaker may produce useful sound pressure throughout an audio frequency range from about 100 Hz and up to 15 kHz, or even up to 20 kHz.
If the enclosure suddenly leaks, the fundamental resonance frequency of the electrodynamic loudspeaker decreases in direction of a free-air fundamental resonance frequency of the electrodynamic loudspeaker because of increasing compliance (or decreasing stiffness) of the trapped air in the enclosure as illustrated below in connection with the appended drawings. The attenuation of the audio signal level in response to the detection of enclosure leakage or acoustic blocking preferably limits a maximum sound pressure level of the audio signal in at least a sub-band or sub-range of the audio frequency range of the audio signal. The attenuation of the audio signal level in response to the detection of enclosure leakage is preferably adapted such that the diaphragm displacement or excursion of the electrodynamic loudspeaker is limited to prevent various kinds of mechanical damage to the loudspeaker as discussed above. This may be accomplished by limiting a maximum sound pressure level of the audio signal in at least a sub-band of the audio signal such as a low-frequency range of the audio signal such as a range at and below the nominal fundamental resonance frequency of the electrodynamic loudspeaker as these .frequencies are more likely to drive the loudspeaker above its maximum excursion limit. Alternatively, the level attenuation may be carried out by attenuation of a relatively narrow frequency band within the low-frequency range such as a 1⁄3 octave band, or by broad band attenuation of the entire frequency range of the audio signal. The maximum sound pressure level of the loudspeaker may be limited by an amplitude limiter without affecting a normal level of the audio signal.
The attenuation of the audio signal level may be accomplished by attenuating a level of the voice coil voltage or voice coil current. The predetermined frequency error criterion may comprise a maximum frequency deviation between the determined fundamental resonance frequency and the nominal fundamental resonance frequency of the loudspeaker. The maximum frequency deviation may have a preset value of e.g. 200 Hz or larger for typical sealed enclosure mounted miniature loudspeakers of portable communication terminals. Hence, the limitation of the diaphragm excursion of the loudspeaker may be invoked if the measured or detected fundamental resonance frequency drops more than the pre-set value, e.g. 200 Hz, 300 Hz or 400 Hz, below the nominal fundamental resonance frequency. Another embodiment of the predetermined frequency error criterion is based on a simple threshold criterion where the setting of the threshold frequency may be derived from the known nominal fundamental resonance frequency of the loudspeaker. The threshold frequency is set to an absolute value, such as 500 Hz, 600 Hz etc. which preferably lies below a normal range of variation or spread of the nominal fundamental resonance frequency. Hence, if the determined fundamental resonance frequency falls below the threshold frequency, it can safely be assumed that enclosure leakage has occurred and the excursion limiting measures are to be invoked.
Another advantageous embodiment of the present methodology of detecting enclosure leakage includes increased robustness against temporary abnormal operating conditions of the portable communication device in which the loudspeaker is integrated for sound reproduction purposes. This embodiment comprises steps of detecting a failure time during which the determined fundamental resonance frequency meets or matches the predetermined frequency error criterion, comparing the detected failure time with a predetermined failure time period, limiting diaphragm excursion in response to the detected failure time exceeds the predetermined failure time period. According to the latter embodiment, the methodology may ignore a temporary compliance with or match to the predetermined frequency error if the compliance is of shorter duration than the predetermined failure time period. Alternatively, the diaphragm excursion limitation may be immediately activated in response to compliance with the predetermined frequency error criterion and subsequently cancelled once the fundamental resonance frequency again fails to meet the predetermined frequency error criterion. This embodiment is particularly helpful in allowing the leakage detection methodology to ignore certain acceptable and temporary handling events of the device in which the loudspeaker is integrated.
The use of the previously discussed acoustic blockage induced shift or change of the fundamental resonance frequency of the enclosure mounted loudspeaker to detect acoustic blocking of a frontal side of the electrodynamic loudspeaker has numerous useful applications in the portable communication device. In one such embodiment, acoustic blocking of the frontal side of the electrodynamic loudspeaker may form part of a user interface of the communication device replacing the function of traditional control knobs, buttons and touch-screens. In this embodiment, a processor, e.g. a microprocessor or DSP of the communication device, may be configured to attenuate sound reproduction of the device, e.g. completely interrupt sound, in response to the detection of acoustic blocking of the frontal side of the electrodynamic loudspeaker. Hence, the user of the communication device can deliberately turn-off or attenuate the sound reproduction by pressing his/hers finger, or any suitable object, against a sound opening of the housing of the communication device above the loudspeaker. The turn-off or attenuation of the sound may last for a predetermined time period or last until another predefined user interface event takes place to restore normal sound reproduction, i.e. removal of the previously applied attenuation of the level of the audio signal. The user can also conveniently turn-off or attenuate sound by pressing the sound opening of the housing of the communication device against a suitable blocking surface, e.g. a table surface or book cover etc. Turning-off or attenuating sound reproduction reduces power consumption of the power amplifier which is highly desirable feature in battery powered portable devices.
Another advantageous embodiment of the present methodology of controlling sound reproduction may lead to increased discrimination between the above-discussed three different types of acoustic operating conditions, e.g. temporary abnormal acoustic operating conditions, enclosure leakage and frontal side acoustic blocking by additionally monitoring the impedance or admittance of the loudspeaker at the determined fundamental resonance frequency. Under certain acoustic operating conditions or circumstances, the change of measured fundamental resonance frequency may be rather small and appear to be caused by acoustic leakage unless a further error criterion is evaluated or examined by the signal processor as described below in further detail in connection with the appended drawings. The addition of the further impedance criterion may advantageously comprise steps of: monitoring one of the impedance or admittance of the loudspeaker at the determined fundamental resonance frequency, detecting the change of acoustic operating condition of the electrodynamic loudspeaker based on a deviation between the impedance or admittance at the determined fundamental resonance frequency and a nominal impedance or admittance at the fundamental resonance frequency at the nominal acoustic operating condition of the electrodynamic loudspeaker. The deviation between the determined and nominal impedances or admittances at the determined and nominal fundamental resonance frequencies may be compared to a predetermined impedance error criterion. The latter may comprise upper and/or lower impedance/admittance threshold(s) at a certain frequency such as the measured fundamental resonance frequency or an impedance range around the measured fundamental resonance frequency.
A further embodiment of the present methodology of controlling sound reproduction applies a rate of change measure to the determined fundamental resonance frequency to further improve the discrimination between the above-discussed three different types of acoustic operating conditions. In this embodiment the predetermined frequency error criterion comprises a rate of change over time of the fundamental resonance frequency and the methodology comprises an additional step of: monitoring and determining a rate of change over time of the fundamental resonance frequency. This is useful to discriminate between a slowly progressing change of the fundamental resonance frequency of the loudspeaker for example caused by ageing of certain loudspeaker materials and acoustic leakage or acoustic blocking which typically takes place in a considerable more abrupt manner, e.g. on a time scale of seconds.
The skilled person will appreciate that the detection of the impedance or admittance of the loudspeaker across the predetermined audio frequency range may be carried by a number of different schemes. According to one embodiment, corresponding values of the voice coil current and voice coil voltage are measured one or more frequency bands in the predetermined audio frequency range such that a ratio between these quantities directly reflects the impedance or admittance per band. According to one such embodiment, the method comprises steps of: filtering the voice coil current by a plurality of adjacently arranged bandpass filters across the predetermined audio frequency range to produce a plurality of bandpass filtered voice coil current components, filtering the voice coil voltage by a plurality of adjacently arranged bandpass filters across the predetermined audio frequency range to produce a plurality of bandpass filtered voice coil voltage components, determining one of the voice coil impedance and admittance within a pass band of each bandpass filter based on the voice coil current component and voice coil voltage component. The plurality of adjacently arranged bandpass filters may comprise a time-domain filter bank and/or a frequency domain filter bank. The frequency domain filter bank may for example comprise a Fourier Transform based filter bank such as an FFT filter bank with a suitable frequency resolution at and below the nominal fundamental resonance frequency such as a bin spacing somewhere between 25 Hz and 100 Hz. In a number of alternative embodiments the time-domain filter bank comprises traditional octave spaced filters for example a plurality of 1⁄6 or 1⁄3 octave spaced bandpass filters. The plurality of bandpass filters are preferably implemented as digital filters for example IIR digital filters.
Another advantageous embodiment of the invention utilizes a model based methodology or approach to compute the fundamental resonance frequency of the loudspeaker, and optionally the impedance value at the computed fundamental resonance frequency. This methodology comprises steps of: applying the detected voice coil current and the detected voice coil voltage to an adaptive digital model of the loudspeaker, said adaptive digital model comprising a plurality of adaptable model parameters, computing the fundamental resonance frequency of the loudspeaker based one or more of the adaptable parameters of the adaptive digital model of the loudspeaker.
The adaptive digital model of the loudspeaker preferably comprises an adaptive digital filter, for example an adaptive IIR filter of second or higher order, which models a time varying and frequency dependent impedance of the loudspeaker across a predetermined audio frequency range, for example between 10 Hz and 10 kHz. The detected voice coil current and detected voice coil voltage are preferably represented by a digital voice coil current signal and a digital voice coil voltage, respectively, as explained in additional detail below with reference to the appended drawings.
To assist proper adaptation of the adaptive digital model of the loudspeaker the latter preferably comprises at least one fixed parameter such as a total moving mass of the loudspeaker in addition to the one or more adaptable or free model parameters.
A second aspect of the invention relates to a sound reproduction assembly for an enclosure mounted electrodynamic loudspeaker. The sound reproduction assembly comprising: an audio signal input for receipt of an audio input signal supplied by an audio signal source, an output amplifier configured to receive the audio input signal and generate a corresponding voice coil audio voltage at a pair of output terminals connectable to a voice coil of an electrodynamic loudspeaker, a current detector configured for detecting a voice coil current flowing into the electrodynamic loudspeaker in response to the application of the voice coil voltage; and a signal processor configured to: detecting one of an impedance and an admittance of the loudspeaker across a predetermined audio frequency range based on the detected voice coil current and voice coil voltage, determining a fundamental resonance frequency of the loudspeaker based on the detected impedance or admittance, comparing the determined the fundamental resonance frequency of the loudspeaker with a nominal fundamental resonance frequency of the loudspeaker representing a nominal acoustic operating condition of the electrodynamic loudspeaker, detecting a change of acoustic operating condition of the electrodynamic loudspeaker based on a frequency deviation between the determined fundamental resonance frequency and the nominal fundamental resonance frequency of the electrodynamic loudspeaker. The signal processor is preferably configured to attenuating a level of the voice coil audio voltage in response to the frequency deviation meets a predetermined frequency error criterion.
The properties of the output amplifier have been disclosed in detail above in connection with the corresponding excursion detection methodology. The Class D output amplifier may comprises a half-bridge driver stage with a single output coupled to the electrodynamic loudspeaker or a full-bridge/H-bridge driver stage with the pair of output terminals coupled to respective sides or terminals of the electrodynamic loudspeaker.
The audio input signal may comprise a real-time digital audio signal supplied from an external digital audio source such as a digital microphone. The real-time digital audio signal may be formatted according to a standardized serial data communication protocol such as IIC or SPI, or formatted according to a digital audio protocol such as I2S, SPDIF etc.
The nominal fundamental resonance frequency may be stored in digital format in a suitable data memory location of a data memory device of the leakage detector assembly implementing the present leakage detection methodology. The data memory device may be integrated on the signal processor. The skilled person will appreciate that the signal processor preferably comprises a software programmable processor such as a microprocessor or DSP integrated on, or operatively coupled to, the leakage detector assembly. The software programmable microprocessor or DSP is controlled by an application program of executable program instructions stored in a program memory such that the above steps or operations of the signal processor are executed when the application program is executed as described below in additional detail. In some embodiments of the invention the signal processor may be an integral part an application processor of the portable communication device while the signal processor may be a separate microprocessor or DSP in other embodiments of the invention.
The skilled person will appreciate that the current detector may comprise various types of current sensors for example a current mirror connected to an output transistor of the output amplifier or a small sense resistor coupled in series with the loudspeaker voice coil. The voice coil current may accordingly be represented by a proportional/scaled sense voltage. The latter sense voltage may be sampled by an A/D converter to allow processing of the voice coil current in the digital domain. Preferably, both the voice coil current and voice coil voltage are processed in the digital domain such that a preferred embodiment of the leakage detection assembly comprises a first A/D converter configured to sample and digitize the voice coil current to supply a digital voice coil current signal; and a second A/D converter configured to sample and digitize the voice coil voltage to supply a digital voice coil voltage signal.
One embodiment of the leakage detection assembly utilizes the previously described model based methodology or approach to compute the fundamental resonance frequency of the loudspeaker. According to this embodiment, the application program comprises a first set of executable instructions providing, when executed, an adaptive digital model of the loudspeaker comprising a plurality of adaptable model parameters. A second set of executable instructions provides, when executed, steps of: reading the digital voice coil current signal, reading a digital voice coil voltage signal, applying the digital voice coil current signal and the digital voice coil voltage signal to the adaptive digital model of the loudspeaker, computing updated values of the plurality of adaptable model parameters, computing the fundamental resonance frequency of the loudspeaker from one or more of the adaptable model parameters. The features and advantages of the adaptive digital model of the loudspeaker have previously been discussed in detail above. An alternative embodiment of the leakage detection assembly utilizes the previously described ratio between the measured voice coil current and voice coil voltage to compute the fundamental resonance frequency during operation. According to the latter embodiment, the application program comprises: a first set of executable instructions configured to, when executed, providing steps of: filtering the digital voice coil voltage signal by a plurality of adjacently arranged bandpass filters across the predetermined audio frequency range to produce a plurality of bandpass filtered voice coil voltage components, filtering the digital voice coil current signal by a plurality of adjacently arranged bandpass filters across the predetermined audio frequency range to produce a plurality of bandpass filtered voice coil current components, determining one of the voice coil impedance and admittance within a pass band of each bandpass filter based on the voice coil current component and voice coil voltage component.
A third aspect of the invention relates to a semiconductor substrate or die on which a leakage detection assembly according to any of the above-described embodiments is integrated. The semiconductor substrate may be fabricated in a suitable CMOS or DMOS semiconductor process.
A fourth aspect of the invention relates to a sound reproduction system for an enclosure mounted electrodynamic loudspeakers, comprising: an electrodynamic loudspeaker comprising a movable diaphragm assembly for generating audible sound in response to actuation of the diaphragm assembly, sound reproduction assembly according to any of the above-discussed embodiments thereof electrically coupled to the movable diaphragm assembly, an audio signal source operatively coupled to the audio signal input of the leakage detection assembly.
One advantage of the present sound reproduction control system is that it may be configured as a self-contained sound delivery system comprising the above-discussed functions such as loudspeaker enclosure leakage detection and excursion limitation and user interface features. The sound reproduction control system can operate independently of an application processor of the portable communication device or terminal in which it is integrated to provide reliable and convenient protection against excursion induced mechanical damage of the electrodynamic loudspeaker.



BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will be described in more detail in connection with the appended drawings, in which:
FIG. 1A) is a schematic cross-sectional view of a miniature electrodynamic loudspeaker for various portable sound reproducing applications for use in the present invention,
FIG. 1B) is a schematic cross-sectional view of the miniature electrodynamic loudspeaker mounted in an enclosure with acoustic leakage,
FIG. 2 shows a schematic block diagram of a sound control assembly for sealed enclosure mounted electrodynamic loudspeakers in accordance with a first embodiment of the invention,
FIG. 3 is a graph of experimentally measured average loudspeaker impedance versus frequency curves for a set of miniature electrodynamic loudspeakers,
FIG. 4 is graph of experimentally measured average diaphragm excursions versus frequency curves for the set of miniature electrodynamic loudspeakers,
FIG. 5 is graph of four experimentally measured loudspeaker voice coil impedance versus frequency curves for a single enclosure mounted miniature electrodynamic loudspeaker under four different acoustic operating conditions; and
FIG. 6 shows an adaptive IIR filter based model of the miniature electrodynamic loudspeaker for fundamental loudspeaker resonance monitoring and detection.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1A) is a schematic cross-sectional illustration of a typical miniature electrodynamic loudspeaker 1 for sealed box mounting and use in portable audio applications such as mobile phones and smartphones where the loudspeaker 1 provides sound reproduction for various types of applications such as speaker phone and music playback. The skilled person will appreciate that electrodynamic loudspeakers exist in numerous shapes and sizes depending on the intended application. The electrodynamic loudspeaker 1 used in the below described methodologies of detecting enclosure leakage and the corresponding assemblies for detecting enclosure leakage has a rectangular shape with maximum outer dimension, D, of approximately 15 mm and an outer dimension in transversal direction of about 11 mm. However, the skilled person will appreciate that the present methodologies for leakage detection and corresponding detection assemblies for enclosure mounted electrodynamic loudspeakers are applicable to virtually all types of enclosure or box mounted electrodynamic loudspeakers.
The miniature electrodynamic loudspeaker 1 comprises a diaphragm 10 fastened to an upper edge surface of a voice coil. The diaphragm 10 is also mechanically coupled to a speaker frame 22 through a resilient edge or outer suspension 12. An annular permanent magnet structure 18 generates a magnetic flux which is conducted through a magnetically permeable structure 16 having a circular air gap 24 arranged therein. A circular ventilation duct 14 is arranged in the frame structure 22 and may be used to conduct heat away from an otherwise sealed chamber structure formed d beneath the diaphragm 10. The resilient edge suspension 12 provides a relatively well-defined compliance of the movable diaphragm assembly (voice coil 20 and diaphragm 10). The compliance of the resilient edge suspension 12 and a moving mass of the diaphragm 10 determines the free-air fundamental resonance frequency of the miniature loudspeaker. The resilient edge suspension 12 may be constructed to limit maximum excursion or maximum displacement of the movable diaphragm assembly.
During operation of the miniature loudspeaker 1, a voice coil voltage or drive voltage is applied to the voice coil 20 of the loudspeaker 100 thorough a pair of speaker terminals (not shown) electrically connected to a suitable output amplifier or power amplifier. A corresponding voice coil current flows in response through the voice coil 20 leading to essentially uniform vibratory motion, in a piston range of the loudspeaker, of the diaphragm assembly in the direction indicated by the velocity arrow V. Thereby, a corresponding sound pressure is generated by the loudspeaker 1. The vibratory motion of the voice coil 20 and diaphragm 10 in response to the flow of voice coil current is caused by the presence of a radially-oriented magnetic field in the air gap 24. The applied voice coil current and voltage lead to power dissipation in the voice coil 20 which heats the voice coil 20 during operation. Hence, prolonged application of too high drive voltage and current may lead to overheating of the voice coil 20 which is another common cause of failure in electrodynamic loudspeakers.
The application of excessively large voice coil currents which force the movable diaphragm assembly beyond its maximum allowable excursion limit is another common fault mechanism in electrodynamic loudspeakers leading to various kinds of irreversible mechanical damage. One type of mechanical damage may for example be caused by collision between the lowermost edge of the voice coil 20 and an annular facing portion 17 of the magnetically permeable structure 16.
FIG. 1B) is a schematic cross-sectional illustration of the miniature electrodynamic loudspeaker 1 mounted in an enclosure, box or chamber 31 having a predetermined interior volume 30. The enclosure or chamber 31 is arranged below the diaphragm 10 of the loudspeaker 1. An outer peripheral wall of the frame structure 22 of the loudspeaker 1 is firmly attached to a mating wall surface of the sealed box 31 to form a substantially air tight coupling acoustically isolating the trapped air inside volume 30 from the surrounding environment. The enclosed volume 30 may be between 0.5 and 2.0 cm3 such as about 1 cm3 for typical portable communication device or terminal applications like mobile phones and smartphones. The mounting of the loudspeaker 1 in the sealed enclosure 30 leads to a higher fundamental resonance frequency of the miniature loudspeaker than its free-air fundamental resonance frequency discussed above due to a compliance of the trapped air inside the chamber 30. The compliance of the trapped air inside the chamber 30 works in parallel with the compliance of the resilient edge suspension 12 to decrease the total compliance (i.e. increase the stiffness) acting on the moving mass of the loudspeaker. Therefore, the fundamental resonance frequency of the enclosure mounted loudspeaker 1 is higher than the free air resonance. The amount of increase of fundamental resonance frequency depends on the volume of the enclosure 30. The wall structure surrounding the sealed enclosure 31 may be a formed by a molded elastomeric compound with limited impact strength. An undesired small hole or crack 35 in the wall structure 31 of the enclosure 30 has been schematically illustrated and the associated acoustic leakage of sound pressure to the surrounding environment indicated by the arrow 37. The acoustic leakage through the small hole or crack 35 leads to an undesired leaky state of the enclosure 30 and to a decrease of the fundamental resonance frequency of the loudspeaker 1 as discussed above. This change of the fundamental resonance frequency caused by the small hole or crack 35 is detected by monitoring the associated change of an electrical impedance of the loudspeaker 1 as described in further detail below.
FIG. 2 is a simplified schematic block diagram of a sound reproduction assembly 200 for enclosure mounted electrodynamic loudspeakers of portable communication devices. The sound reproduction assembly may for example be used to control sound reproduction of the miniature loudspeaker 1 illustrated on FIG. 1B) above. The sound reproduction assembly 200 is coupled to the miniature electrodynamic loudspeaker 1 through a pair of externally accessible speaker terminals 211a, 211b. A pulse modulated Class D output amplifier comprises a composite up-sampler and modulator 204 coupled to an H-bridge output stage 206 which is further connected to the speaker terminals 211a, 211b. The class D output amplifier receives a processed digital audio signal at input 203, derived from a digital audio signal supplied at digital audio signal input 201 of a programmable Digital Signal Processor (DSP) 202. The Class D output amplifier generates a corresponding PWM or PDM modulated voice coil voltage that is supplied to the voice coil of the miniature electrodynamic loudspeaker 1 through suitable speaker terminals. In the present embodiment, the sound reproduction assembly 200 operates primarily in the digital domain, but other embodiments thereof may instead use analog signals or a mixture of analog and digital signals. The digital audio signal input 201 of the sound reproduction assembly 200 receives the previously discussed digital audio signal supplied by an external digital audio source such as an application processor of a portable communication device in which the present sound reproduction assembly 200 is integrated. The externally generated digital audio signal may be formatted according to a standardized serial data communication protocol such as IIC or SPI, or formatted according to a digital audio protocol such as IIS, SPDIF etc.
The sound reproduction assembly 200 is supplied with operating power from a positive power supply voltage VDD. Ground (not shown) or a negative DC voltage may form a negative supply voltage for the loudspeaker excursion detector 200. The DC voltage of VDD may vary considerably depending on the particular application of the sound reproduction assembly 200 and may typically be set to a voltage between 1.5 Volt and 100 Volt. A master clock input, f:clk_1, sets a master clock frequency of the DSP 202.
The sound reproduction assembly 200 comprises at least one A/D converter 208 that is configured to sample and digitize the instantaneous voice coil voltage across the speaker terminals 211a, 211b. The A/D converter 208 furthermore comprises a second input that is configured to sample and digitize an analog voice coil current signal delivered at a second input, Icoil, of the converter 208. The skilled person will appreciate that the least one A/D converter 208 may comprise a multiplexed type of converter alternatingly sampling the voice coil voltage and analog voice coil current signal. Alternatively, the least one A/D converter 208 may comprise two separate A/D converters fixedly coupled to the voice coil voltage and the voice coil current signal, respectively. The skilled person will appreciate that the voice current signal may be generated by various types of current sensors that generate a voltage, current or charge signal proportional to the instantaneous voice coil current flowing the voice coil. Exemplary current sensors include a current mirror connected to an output transistor of the H-bridge 206 and a small sense resistor coupled in series with the voice coil of the loudspeaker 1. The at least one A/D converter 208 is clocked by an external sample clock, f_clk2, that may have a frequency between 8 kHz and 96 kHz for non-oversampled types of A/D converters and a frequency between 1 MHz and 10 MHz for oversampled types of A/D converters such as sigma-delta converters.
The at least one A/D converter 208 has a first output supplying a digital voice coil current signal Im[n] to a first input of an adaptive digital model 210 of the loudspeaker 1 wherein the model 210 comprises a plurality of adaptable model parameters as discussed in further detail below. The at least one A/D converter 208 furthermore comprises a second output supplying a digital voice coil voltage Vm[n] to a second input of the adaptive digital model 210. The adaptive digital model 210 of the loudspeaker preferably comprises an adaptive filter which models the frequency dependent impedance of the loudspeaker across a predetermined audio frequency range, for example between 10 Hz and 10 kHz, based on the detected or measured voice coil current and voice coil voltage as represented by the digital voice coil current signal Im[n] and the digital voice coil voltage Vm[n]. The operation of the adaptive digital model 210 is discussed in further detail below. The adaptive digital model 210 is configured to computing or determining a fundamental resonance frequency of the enclosure mounted miniature loudspeaker 1. The output of the adaptive digital model 210 comprises the determined fundamental resonance frequency f0 which is supplied to the DSP 202 in digital format for example via a data bus and a data communication port of the DSP 202.
The DSP 202 is configured to continuously or discontinuously read a current value of f0 and compare the latter with a nominal fundamental resonance frequency of the miniature loudspeaker 1 representing the fundamental resonance frequency in a sealed state of the enclosure. Hence, the nominal fundamental resonance frequency represents the nominal or desired acoustic operating condition of the electrodynamic loudspeaker 1. The value of the nominal fundamental resonance frequency of the miniature loudspeaker 1 is preferably stored in a predetermined data memory address of a data memory accessible to the DSP 202. The nominal fundamental resonance frequency of the miniature loudspeaker 1 may have been obtained in numerous ways. In one embodiment, the nominal fundamental resonance frequency is determined directly from the speaker manufacturer's data sheet for actual volume of the sealed enclosure 31. In this case, the nominal fundamental resonance frequency may represent an average enclosure mounted resonance frequency for the particular type of miniature loudspeaker 1. This embodiment may be used to verify correct sealed mounting of the miniature loudspeaker 1 in the enclosure or chamber 31 during manufacturing. This verification may be accomplished by measuring the fundamental resonance frequency f0 of the miniature loudspeaker 1 after enclosure mounting and compare the measured f0 with the nominal fundamental resonance frequency. If the measured value of the fundamental resonance frequency f0 falls outside certain a predetermined frequency band or range around the nominal fundamental resonance frequency, the enclosure is flagged as leaking. This may be used to repair the enclosure and/or the mounting of the miniature loudspeaker 1 therein during the manufacturing process and hence avoid expensive and annoying field returns of the portable communication device housing the enclosure mounted miniature loudspeaker 1.
In other embodiments, the above outlined average resonance frequency value determination may be less accurate than desired because the moving mass and diaphragm suspension compliance of the miniature loudspeaker 1 tend to vary due to production and material tolerances. Hence, the nominal fundamental resonance frequency of the miniature loudspeaker 1 is determined from an actual measurement on the of the miniature loudspeaker 1 after mounting in the sealed enclosure 31. This may be accomplished during manufacturing of the mobile terminal if the enclosure 31 is known to be appropriately sealed and the miniature speaker 1 in proper working condition.
During operation of the sound reproduction assembly 200, the DSP 202 regularly determines a current f0 of the miniature loudspeaker 1 and compares the determined value of f0 with the nominal fundamental resonance frequency. The DSP 202 detects a change of acoustic operating condition, in particular acoustic leakage of the enclosure, of the electrodynamic loudspeaker based on a frequency deviation between the determined current f0 and the nominal fundamental resonance frequency of the electrodynamic loudspeaker. The deviation is preferably expressed by a predetermined frequency error criterion which indicates or represents the occurrence of acoustic leakage of the enclosure. The skilled person will appreciate that one or more additional predetermined frequency error criteria may be applied by the DSP 202 representing one or more additional acoustic operating conditions of interest such as acoustic blocking of a frontal side of the electrodynamic loudspeaker as discussed in additional detail below with reference to FIG. 5. The predetermined frequency error criterion may in each condition comprise a certain frequency limit or range of the determined fundamental resonance frequency. The frequency error criterion may comprise a certain frequency difference between the determined fundamental resonance frequency and the nominal fundamental resonance frequency. If the DSP 202 determines that the current f0 meets or complies with the predetermined frequency error criteria, the DSP 202 preferably proceeds to attenuate the attenuating the level of the audio signal applied to the voice coil of the miniature loudspeaker 1 based on the assumption that the enclosure has become acoustically leaking due to a hole or crack. In this situation, a continued unrestrained or unmodified application of drive voltage to the loudspeaker through the class D output amplifier is likely to cause the previously discussed excessive diaphragm excursion or displacement that may irreversibly damage the loudspeaker. The DSP 202 is preferably configured or programmed to attenuate the level of the audio signal such that the excursion of the diaphragm of the miniature loudspeaker 1 is limited. This may be accomplished in various ways for example by attenuating a level of the processed digital input signal to the class D output amplifier. This could be accomplished by selectively attenuating low-frequency range or components of the processed digital input signal (which are more likely to drive the loudspeaker above its maximum allowable excursion limit). The low-frequency range may comprise all frequencies below a certain threshold frequency such as 800 Hz or 500 Hz or only a single low-frequency band such as one-third octave band around a center frequency such as 400 Hz or 300 Hz in the low-frequency range. Alternatively, the DSP 202 may be configured to apply broad band attenuation of the entire frequency spectrum of the processed digital input signal to limit the diaphragm excursion.
Generally, the DSP 202 may be configured to respond to an event where the predetermined frequency error criterion or criteria have been met in at least two different ways. According to one set of embodiments, the DSP 202 is configured to respond immediately to non-compliance with the predetermined frequency error criterion and apply the previously discussed limitation of diaphragm excursion or displacement. These embodiments have the advantage that the time period during which potentially dangerous levels of voice coil voltage is applied to the miniature loudspeaker is minimized. However, according to another embodiment, the DSP 202 is configured to on purpose delay the limiting of the diaphragm excursion. According to the latter embodiment, the DSP 202 is configured to detect a failure time during which the determined fundamental resonance frequency meets the predetermined frequency error criterion. Only when, and if, the detected failure time exceeds a predetermined failure time period, the DSP 202 proceeds to limit diaphragm excursion in the way explained above. The failure time may for example be detected by a counter in the DSP 202 which is initialized or started instantly in response to compliance with the predetermined frequency error criterion. A significant advantage of the later embodiment is its robustness against short term abnormal acoustic operating conditions or signal glitches.
The skilled person will appreciate that the adaptive digital model 210 of the loudspeaker 1 may be implemented by a software programmable microprocessor or DSP core controlled by executable program instructions such that each signal processing function may be implemented by a particular set of executable program instructions. In certain embodiments, the adaptive digital model 210 may be fully or partially integrated with the programmable DSP 202. In the latter embodiments, the adaptive digital model 210 may be implemented by a dedicated set of executable program instructions and a plurality of memory locations holding a plurality of adaptable model parameters of the speaker model 210. Hence, the adaptive modelling of the miniature loudspeaker, the above-discussed monitoring of f0 of the miniature loudspeaker 1 and the associated signal attenuation (with the preferred accompanying diaphragm excursion limitation) may all be carried out by the programmable DSP 202 through one or more suitable program routines of application programs. The skilled person will understand that the programmable DSP 202 may be integrated together with the previously discussed application processor of the portable communication terminal or be implemented as a separate programmable or hard-wired DSP dedicated to perform the above-described sound reproduction control methodologies. In the latter embodiment, the adaptive digital model 210 may be implemented as a separate hard-wired digital logic circuit comprising appropriately configured sequential and combinatorial digital logic instead of a set of executable program instructions associated with the software implementation on the programmable embodiment. The hard-wired digital logic circuit may be integrated on an Application Specific Integrated Circuit (ASIC) or configured by programmable logic or any combination thereof.
To illustrate how the fundamental resonance frequency f0, of the miniature loudspeaker 1 changes when the normally sealed enclosure (30 of FIG. 1B)) is broken and becomes acoustically leaking, the graph 300 of FIG. 3 shows experimentally measured average loudspeaker impedance versus frequency curves for a set of miniature electrodynamic loudspeakers of the same type as the above-discussed miniature loudspeaker 1. The x-axis of graph 300 depicts measurement frequency on a logarithmic scale across a frequency range from 5 Hz to about 5 kHz and the y-axis shows the measured electrical impedance magnitude on a linear scale from approximately 6Ω to 15Ω. A first impedance curve 301 shows the average measured magnitude of the impedance of the miniature loudspeakers when mounted in an unbroken or sealed enclosure, i.e. the intended sealed operation of the loudspeaker and its enclosure. The average fundamental resonance frequency of the measured loudspeakers is approximately 900 Hz and average peak impedance about 14Ω. A second impedance curve 303 shows the average measured impedance when the miniature loudspeakers are mounted in a broken or unsealed enclosure, i.e. the error or failure condition of the loudspeaker and its associated enclosure. As illustrated, the average fundamental resonance frequency of the measured loudspeakers has been lowered markedly to approximately 550 Hz and the average peak impedance lowered to about 13Ω. The average cross-sectional area of the apertures or holes in enclosure was about 0.75 mm2 which the inventors have found representative for typical broken loudspeaker enclosures after numerous field studies.
The pronounced variation of the average fundamental resonance frequency in the sealed and broken conditions of the enclosure makes the present leakage detection methodology very robust against unavoidable production spread of the fundamental loudspeaker resonance frequency. It may for example be possible to choose a threshold frequency criterion for the fundamental resonance frequency such that the leakage detection flags a leakage error if the measured fundamental resonance frequency falls below a predetermined threshold frequency such as 750 Hz for the depicted embodiment. The skilled person will appreciate that the threshold frequency criterion in the alternative to absolute frequency could be expressed as a certain frequency deviation from the nominal fundamental resonance frequency for example 250 Hz, or 1⁄3 octave etc.
The effect of the broken or leaking loudspeaker enclosure on the loudspeaker excursion or displacement is illustrated on the graph 400 of FIG. 4. The depicted excursion curves 401 and 403 correspond to the average impedance curves 301 and 303, respectively, depicted on graph 300. The x-axis of graph 400 depicts measurement frequency on a logarithmic scale across the frequency range 5 Hz to about 5 kHz while the y-axis shows the measured excursion in mm per Volt (voice coil voltage) on a linear scale from approximately 0.0 mm to 0.25 mm. The depicted diaphragm excursion values were measured by a laser interferometer. A marked increase of average loudspeaker diaphragm excursion is evident from the first excursion curve 401 to the second excursion curve 403 for the fixed voice coil voltage condition applied. The average diaphragm excursion increases markedly throughout the entire low frequency audio range from 20 Hz to 500 Hz when there is acoustic leakage of the enclosures. The average diaphragm excursion at 50 Hz when the miniature loudspeakers are mounted in sealed loudspeaker enclosures is about 0.05 mm/V and this value increases to about 0.13 mm/V when the miniature loudspeakers instead are mounted in the leaky or unsealed loudspeaker enclosures. Since the majority of signal energy or power of normal speech and music signals is concentrated in the low frequency portion of the audio frequency range, the pronounced increase of diaphragm excursion in this frequency range can lead to irreversible mechanical damage of the speaker unless proper precautionary actions are taken to limiting the maximum excursion. The maximum excursion of a particular type of electrodynamic loudspeaker depends on its dimensions and construction details. For the above-discussed miniature loudspeaker 1 with outer dimensions of approximately 11 mm×15 mm, the maximum allowable diaphragm excursion is typically about +/−0.45 mm.
FIG. 5 comprises a graph 500 of experimentally measured loudspeaker impedance versus frequency curves for a single miniature electrodynamic loudspeaker sample arranged in four different acoustic loading conditions, i.e. loaded by different acoustic loads. The miniature electrodynamic loudspeaker sample is similar to the miniature loudspeakers discussed above in connection with the previous impedance and excursion measurements. The x-axis of graph 500 depicts measurement frequency on a logarithmic scale across a frequency range from 300 Hz to about 3 kHz and the y-axis shows the measured electrical impedance magnitude of the miniature speaker on a linear scale spanning from approximately 7Ω to 16Ω. A first impedance curve 501 shows the measured impedance magnitude when the miniature loudspeaker is mounted in an unbroken or sealed enclosure, i.e. the intended or normal sealed acoustic operating condition of the loudspeaker and its enclosure. Furthermore, the frontal side, i.e. the side of the diaphragm facing away from the enclosure, of the loudspeaker is unblocked corresponding to sound emission under essentially free field conditions. The measured fundamental resonance frequency of the loudspeaker sample is 838 Hz and the accompanying peak impedance is about 15Ω.
A second impedance curve 503 shows the measured impedance magnitude or impedance when the miniature loudspeaker is mounted in a typical acoustically leaking or unsealed enclosure with unblocked frontal side, i.e. the error or failure operating condition of the loudspeaker and its associated enclosure. As illustrated, the measured fundamental resonance frequency of the miniature loudspeaker sample drops markedly from 838 Hz to approximately 382 Hz. Additionally, the impedance at the fundamental resonance frequency drops from about 15Ω in the sealed operating condition to about 13Ω. A third impedance curve 505 shows the measured impedance magnitude of the miniature loudspeaker when mounted in a sealed or non-leaking enclosure as represented by frequency curve 501, but now with tight acoustic blocking of the frontal side of the electrodynamic loudspeaker. The acoustic blocking is performed by blocking a small frontal cavity above the loudspeaker diaphragm. The tightly blocked acoustic operating condition was achieved by firmly pressing the frontal side of the miniature loudspeaker sample against a paper stack. As illustrated by impedance curve 505, the measured fundamental resonance frequency of the miniature loudspeaker sample increases markedly from 838 Hz under a normal non-leaking operating condition to 1676 Hz with the tightly blocked frontal cavity. The impedance magnitude at the measured fundamental resonance frequency decreases from about 15Ω to about 10Ω. The increase of the fundamental resonance frequency is caused by an increase of the mechanical stiffness of the trapped air mass at the front side of the miniature loudspeaker inside the frontal cavity. Finally, a fourth impedance curve 507 shows the measured impedance magnitude of the miniature loudspeaker when mounted in a sealed or non-leaking chamber as represented by frequency curve 501, but now with a loosely blocked frontal cavity above the loudspeaker. The loosely blocked acoustic loading or operating condition was achieved by resting, rather than actively forcing as in the tightly blocked condition discussed above, the frontal side of the miniature loudspeaker sample against the paper stack. As illustrated by curve 507, the measured fundamental resonance frequency of the miniature loudspeaker sample decreases from 838 Hz under a normal non-leaking operating condition to 763 Hz with loosely blocked frontal cavity. The impedance magnitude at the measured fundamental resonance frequency decreases from about 15Ω to about 12Ω.
The variation of the fundamental resonance frequency between the sealed enclosure and unblocked operating condition and the tightly blocked or loosely blocked frontal cavity makes the present sound control methodology able to additionally detect whether a particular change of the measured fundamental loudspeaker resonance frequency of the miniature loudspeaker is caused by acoustic leakage of the loudspeaker enclosure or by acoustic blocking of the frontal side of the loudspeaker. The skilled person will appreciate that detection or discrimination efficiency between these different operating conditions may be improved by monitoring and measuring the impedance or admittance of the loudspeaker at the fundamental resonance frequency in addition to detecting the change of fundamental resonance frequency of the miniature loudspeaker. The determined or measured impedance or admittance at the determined fundamental resonance frequency may for example be compared with a nominal impedance or admittance at the nominal fundamental resonance frequency. A deviation between these impedances may be compared to a certain impedance error criterion.
According to one embodiment of the invention, the detection of the above-discussed tightly blocked or loosely blocked frontal cavity operating conditions of the miniature loudspeaker is used to temporarily interrupt the audio or drive signal to the loudspeaker and thereby interrupt sound reproduction. This reduces power consumption of the power amplifier and loudspeaker. Sound reproduction is preferably resumed once normal acoustic operating conditions of the miniature loudspeaker are re-established, i.e. once the measured fundamental resonance frequency of the loudspeaker no longer meets the predetermined frequency error criterion and/or impedance error criterion. Furthermore, if enclosure leakage is detected the DSP 202 may be configured to permanently, i.e. until the enclosure has been repaired, attenuate the level of the audio signal applied to the voice coil of the miniature loudspeaker to prevent damage as discussed above.
FIG. 6 is a detailed view of interior components of the previously discussed adaptive digital model 210 of the loudspeaker 1. The adaptive digital model 210 comprises an adaptive IIR filter 510 which adaptively tracks or models the impedance of the voice coil of the miniature electrodynamic loudspeaker 1 for fundamental resonance frequency tracking and detection. The previously discussed digital voice coil current signal Im[n] is applied to a first input of the adaptive digital model 210 and the digital voice coil voltage Vm[n] is applied to a second input of the adaptive digital model 210. The output (not shown) of the digital model 210 is the estimated fundamental resonance frequency f0, of the miniature loudspeaker 1. This output is not expressly depicted on FIG. 5, but can be computed directly from the model parameters of the adaptive IIR filter 510 as discussed below in further detail.
The adaptive digital model 210 comprises the following model parameters:
Ve [n]: Estimate of voice coil voltage or drive voltage;
RDC: DC electrical resistance of voice coil;
BI: Force factor of loudspeaker (B·I product);
MMS: Total mechanical moving mass (including acoustic loading);
KMS: Total mechanical stiffness;
RMS: Total mechanical damping;
The adaptive IIR filter 510 is a second order filter and for convenience preferably expressed by its mechanical mobility transfer function Ym(s) in the z-domain as illustrated by the lower mobility equation. The overall operation of the adaptive digital model 210 of the loudspeaker 1 is that a parameter tracking algorithm tries to predict the voice coil voltage Ve [n] based upon a measurement of the voice coil current Im[n] and an impedance model of the miniature loudspeaker. An error signal VERR[n] is obtained from a difference between the measured, actual, voice coil voltage Vm[n] and the estimate of the same produced by the model Ve [n]. The skilled person will understand that various adaptive filtering methods may be used to adapt free model parameters in the chosen loudspeaker model to minimise the error signal VERR[n]. The free model parameters are preferably continuously transmitted to the DSP 202 and when the error signal becomes sufficiently small, e.g. comply with a predetermined error criterion, the adapted model parameters are assumed to be correct. The DSP 202 is configured to make the computation of the current fundamental resonance frequency f0 of the miniature loudspeaker 1 from the received model parameters. In the alternative, the adaptive digital model 210 may include appropriate computing power to perform the computation of f0 and transmit the latter to the DSP 202. By keeping fixed one of the four parameters BI, MMS, KMS and RMS depicted in FIG. 5 the residual three parameters can be determined by identifying the relationship between Im[n] and u[n]. Mathematically, it is unimportant which one of these four parameters that is fixed but the total moving mass MMS is the typically the most stable of these parameters in terms of manufacturing spread and variation over time and temperature. Therefore, it is preferred to keep the total moving mass MMS as a fixed parameter in the present embodiment of the invention.
The skilled person will appreciate that f0, can be calculated analytically from the free parameters a1 and a2 leading initially to:




  
    
      
        
          ω
          z
        
        =
        
        ⁢
        
          
            
              
                ln
                2
              
              ⁡
              
                (
                
                  
                    a
                    2
                  
                
                )
              
            
            +
            
              
                arctan
                2
              
              (
              
                -
                
                  
                    
                      
                        -
                        
                          a
                          1
                          2
                        
                      
                      +
                      
                        4
                        ⁢
                        
                          a
                          2
                        
                      
                    
                  
                  
                    a
                    1
                  
                
              
              )
            
          
        
      
    
  
  
    
      
        =
        
        ⁢
        
          
            ω
            0
          
          /
          
            F
            s
          
        
      
    
  




Hence, ω0 can be found by multiplying ωz, with the sampling frequency, Fs, of the digital model signals and f0, finally computed by:

f0=ω0/2n.

## Claims

1. A method of controlling sound reproduction of a loudspeaker, the method comprising:
applying an audio signal to a coil of the loudspeaker;
detecting a current flowing through the coil and a voltage across the coil;
applying information about the detected current and detected voltage to an adaptive digital model of the loudspeaker to determine a resonance frequency of the loudspeaker, the determined resonance frequency of the loudspeaker based on one or more adaptive parameters of the adaptive digital model of the loudspeaker;
calculating a frequency deviation by comparing the determined resonance frequency with a nominal resonance frequency of the loudspeaker representing a nominal acoustic operating condition of the loudspeaker; and
attenuating a level of the audio signal applied to the coil based on the calculated frequency deviation.

2. The method of claim 1, wherein the attenuation the level of the audio signal includes when the calculated frequency deviation meets a specified error criterion.

3. The method of claim 1, wherein the adaptive parameters of the adaptive digital model of the loudspeaker include one or more of a resistance of the coil, a force factor of the loudspeaker, a mechanical stiffness of the loudspeaker, or a mechanical damping of the loudspeaker.

4. The method of claim 2, wherein the specified error criterion represents an acoustic blocking of a frontal side of the loudspeaker.

5. The method of claim 2, wherein the specified error criterion represents an acoustic leakage of the enclosure of the loudspeaker.

6. The method of claim 4, further comprising:
subsequent to the attenuating the level of the audio signal, monitoring and determining the resonance frequency of the loudspeaker over time;
detecting a removal of the acoustic blocking of the frontal side of the loudspeaker; and
restoring the level of the audio signal in response to the removal of the acoustic blocking.

7. The method of claim 1, comprising determining:
based on the detected current and the detected voltage, one of an impedance or an admittance of the loudspeaker at the determined resonance frequency; and
calculating the frequency deviation based on a deviation between the impedance or the admittance at the determined resonance frequency and a nominal impedance or a nominal admittance at the nominal resonance frequency of the loudspeaker.

8. The method of claim 1, wherein the specified error criterion comprises a rate of change over time of the resonance frequency and the method further comprising:
monitoring and determining the rate of change over time of the resonance frequency.

9. The method of claim 1, further comprising:
filtering the current by a plurality of adjacently arranged bandpass filters across a predetermined audio frequency range to produce a plurality of current components;
filtering the voltage by a plurality of adjacently arranged bandpass filters across the predetermined audio frequency range to produce a plurality of voltage components; and
determining one of an impedance and an admittance of the loudspeaker within a pass band of each bandpass filter based on the corresponding current component and voltage component.

10. The method of claim 9, wherein the plurality of adjacently arranged bandpass filters comprises one of a time-domain filter bank and a frequency-domain filter bank.

11. The method of claim 10, wherein the frequency-domain filter bank comprises a Fourier Transform based filter bank.

12. The method of claim 10, wherein the time-domain filter bank comprises a plurality of 1⁄3 octave bandpass filters.

13. The method of claim 1, further comprising:
computing an impedance or an admittance of the loudspeaker at the determined resonance frequency from one or more of the adaptive parameters, and determining the resonance frequency of the loudspeaker based on the computed impedance or admittance of the loudspeaker.

14. The method of claim 1, wherein the adaptive digital model of the loudspeaker comprises an adaptive HR filter of second or higher order.

15. The method of claim 1, wherein the adaptive digital model of the loudspeaker comprises at least one fixed parameter such as a total moving mass of the loudspeaker.

16. The method of claim 1, wherein the attenuating the level of the audio signal comprises one of selectively attenuating one of a sub-band of the audio signal and a broad-band of the audio signal.

17. The method of claim 1, wherein the attenuating the level of the audio signal comprises limiting a maximum sound pressure level of the audio signal in at least a sub-band of the audio signal.

18. A sound reproduction assembly for a loudspeaker, comprising:
an input for receiving an audio signal supplied by a source;
an amplifier configured to receive the audio signal and generate a corresponding voltage at a pair of output terminals connectable to a coil of the loudspeaker;
a detector configured to detect the voltage and a current flowing into the coil in response to the application of the voltage; and
a processor configured to:
apply information about the detected voltage and current to an adaptive digital model of the loudspeaker to determine a resonance frequency of the loudspeaker, the determined resonance frequency of the loudspeaker based on one or more adaptive parameters of the adaptive digital model of the loudspeaker, calculate a frequency deviation by comparing the determined resonance frequency with a nominal resonance frequency of the loudspeaker representing a nominal acoustic operating condition of the loudspeaker, and attenuate a level of the voltage based on the calculated frequency deviation.

19. The sound reproduction assembly of claim 18, wherein the detector comprises a first converter configured to sample and digitize the current to supply a digital current signal, and a second converter configured to sample and digitize the voltage to supply a digital voltage signal.

20. The sound reproduction assembly of claim 18, wherein the processor comprises a microprocessor controllable by an application program of executable program instructions stored in a program memory.

21. The sound reproduction assembly of claim 20, wherein the application program comprises:
a first set of executable program instructions providing, when executed, the adaptive digital model of the loudspeaker comprising the one or more parameters; and
a second set of executable program instructions providing, when executed, steps of:
reading the digital current signal and the digital voltage signal, applying the digital current signal and the digital voltage signal to the adaptive digital model of the loudspeaker, computing updated values of the plurality of parameters, and computing the resonance frequency of the loudspeaker from one or more of the updated values of the plurality of parameters.

22. The sound reproduction assembly of claim 20, wherein the application program comprises:
a first set of executable instructions providing, when executed, steps of:
filtering the digital voltage signal by a plurality of adjacently arranged bandpass filters across the predetermined audio frequency range to produce a plurality of voltage components, filtering the digital current signal by another plurality of adjacently arranged bandpass filters across the predetermined audio frequency range to produce a plurality of current components, and determining one of an impedance and an admittance of the loudspeaker within a pass band of each bandpass filter based on the corresponding current component and voltage component.

23. The sound reproduction assembly of claim 18, wherein the amplifier comprises a class D power stage configured to supply a pulse modulated voltage to the loudspeaker.

24. A semiconductor substrate having a sound reproduction assembly of claim 18 integrated thereon.

25. A sound reproduction system comprising:
a loudspeaker comprising a movable diaphragm assembly for generating audible sound in response to actuation of the diaphragm assembly;
the sound reproduction assembly of claim 18 electrically coupled to the movable diaphragm assembly; and
a source operatively coupled to the input of the sound reproduction assembly.

26. A portable communication device comprising the sound reproduction system of claim 25.

27. The sound reproduction assembly of claim 18, wherein the adaptive parameters of the adaptive digital model of the loudspeaker include one or more of a resistance of the coil, a force factor of the loudspeaker, a mechanical stiffness of the loudspeaker, or a mechanical damping of the loudspeaker.