Acoustic sensor assembly having improved frequency response

An acoustic sensor assembly includes a housing having an external-device interface and a sound port to an interior of the housing. An electro-acoustic transducer and an electrical circuit are disposed within the housing. The electro-acoustic transducer separates the interior into a front volume and a back volume, where the sound port acoustically couples the front volume to an exterior of the housing. The back volume includes a first portion and a second portion. The electrical circuit is electrically coupled to the electro-acoustic transducer and to the external-device interface. One or more apertures acoustically couple the first and second portions of the back volume and are structured to shape a frequency response of the acoustic sensor assembly.

TECHNICAL FIELD

The present disclosure relates generally to acoustic sensor assemblies and more particularly to acoustic sensor assemblies, for example, microelectromechanical systems (MEMS) microphones, having an improved frequency response.

BACKGROUND

Microelectromechanical systems (MEMS) microphones are widely used in various devices including hearing aids, mobile phones, smart speakers, personal computers among other devices and equipment for their low cost, small size, high sensitivity, and high signal to noise ratio (SNR). A MEMS microphone generally comprises a MEMS motor and an integrated circuit disposed in a housing formed by a metal can or shielded cover mounted on a base configured for integration with a host device. The MEMS motor converts sound entering the housing via a port to an electrical signal conditioned by a downstream integrated circuit. The conditioned electrical signal is output on a host-device interface of the microphone for use by the host device. The performance of a microphone can be characterized by its sensitivity over a range of frequencies (referred to herein as a “frequency response”). Sensitivity is a ratio of an analog or digital output signal to a reference input sound pressure level (SPL), typically 1 Pascal at 1 kHz. However, the sensitivity of a microphone is not uniform across all frequencies of interest and may not meet an aspired performance specification.

The various aspects, features and advantages of the present disclosure will become more fully apparent to those having ordinary skill in the art upon consideration of the following Detailed Description and the accompanying drawings described below.

DETAILED DESCRIPTION

According to one aspect of the disclosure, an acoustic sensor assembly (e.g., a MEMS microphone assembly) comprises an electro-acoustic transducer (e.g., a MEMS motor) and an electrical circuit disposed within a housing. An electrical circuit is also disposed within the housing and electrically coupled to the electro-acoustic transducer and to electrical contacts on an external-device interface of the housing. The transducer separates the interior into a front volume and a back volume. A sound port acoustically couples the front volume to an exterior of the housing, and the back volume includes a first portion and a second portion. One or more apertures acoustically couple the first and second portions of the back volume and are structured to shape a frequency response of the acoustic sensor assembly. The acoustic sensor assembly can be implemented as a microphone or vibration sensor among other sensors and combinations thereof.

In some embodiments, the housing comprises a cover disposed on a surface of a base, the electro-acoustic transducer is mounted on the base, and the sound port is located on the cover. The first back volume portion is located between the cover and the base. The second back volume portion is located at least partially in the base. In one implementation, a portion of the second back volume is located in the base. In another implementation, the entire second back volume portion is located in the base. The one or more apertures are disposed partially or fully through a portion of the base that separates the first and second portions of the back volume, depending on whether the second back volume portion is fully or partially located in the base. In certain embodiments, a second cover is disposed on a surface of the base opposite the cover, wherein the one or more apertures extend fully through the base and at least a portion of the back volume is located in the second cover. In the various acoustic sensor assemblies described herein, the one or more apertures can be a part of a screen or other damping member separating the first and second portions of the back volume. For example, the screen can be disposed over an aperture in the base connecting to the back volume. The screen separates the first and second portions of the back volume. Alternatively, the one or more apertures can be one or more openings partially or fully through the base without a screen or other damping member.

In various embodiments, characteristics of the one or more apertures between the first and second portions of the back volume shape the frequency response of the acoustic sensor assembly. These characteristics include acoustic impedance, e.g., resistance, inertance, or compliance or any combination thereof. The frequency response can be characterized by a plot of sensor sensitivity versus frequency. The one or more apertures between the first and second portions of the back volume can be structured to increase or decrease sensitivity of the sensor at certain frequencies.

FIGS.1and2show perspective views of an acoustic sensor assembly100comprising a housing102having a lid or cover104mounted on a base108. The cover includes a sound port106through which sound can enter the housing. The cover can be configured to shield the interior of the housing from electromagnetic interference. The base includes a top surface109(on which the cover is mounted) and a bottom surface110(shown only inFIG.2). The bottom surface includes an external-device interface with electrical contacts111-113(e.g., supply voltage, ground, clock, data, etc.). InFIG.2, the ground plane is shown as a ring-shaped contact114. In other embodiments, the ground contact has a different shape. In one implementation, the external-device interface is a surface-mount interface suitable for integrating the sensor assembly to a host device, for example by reflow or wave soldering or some other known or future surface-mount technology. Alternatively, the sensor assembly can include through-hole pins for integration with the host. The microphone assembly ofFIG.1shows a top port device with the sound port on the cover.

In one implementation, generally, a top port sensor assembly comprises an enhanced back volume formed internally in the sensor assembly. The back volume is configured to shape the frequency response of the sensor assembly as described further herein. InFIGS.3,6and9, the base includes sidewalls302,304and end walls306,308. InFIGS.4,5,7,8,10and11, an electro-acoustic transducer402and an electrical circuit502(e.g., an integrated circuit) are disposed in an interior of the housing. The electrical circuit is electrically coupled to the electro-acoustic transducer and to the electrical contacts on the external-device interface. The electro-acoustic transducer can be any suitable type including a capacitive, piezoelectric, or optical transduction device among others implemented with electret materials, microelectromechanical systems (MEMS) technology or other known or future technology. The electro-acoustic transducer is configured to convert sound into an electrical signal. Once converted, the electrical circuit conditions the electrical signal before providing the conditioned signal at the external-device interface. Such conditioning may include buffering, amplification, filtering, analog-to-digital (A/D) conversion for digital devices, and signal protocol formatting among other conditioning or processing.

Generally, the electro-acoustic transducer separates the interior of the housing into a front volume and a back volume. The sound port acoustically couples the front volume to an exterior of the housing. The back volume comprises a first back volume portion acoustically coupled to a second back volume portion by one or more apertures, wherein the aperture is structured to shape a frequency response of the sensor.

InFIGS.4,5,7,8,10and11, the electro-acoustic transducer separates the interior of the housing into a front volume404and a back volume406. The sound port acoustically couples the front volume to an exterior of the housing. The back volume includes a first portion407and a second portion408. The first portion of the back volume is located at least partially between the electro-acoustic transducer and the base. The second portion of the back volume is located entirely in the base and defined in part by the sidewalls and the end walls of the base. InFIGS.4,5,7,8,10and11, the second portion of the back volume408is a cavity fully formed in the base. InFIGS.16and17, the second portion of the back volume is a cavity fully partially in the base. The portion of the back volume located in the based can be formed by drilling, milling, or molding, among other fabrication techniques.

InFIGS.4,5,7,8,10and11, one or more apertures410acoustically connect the first and second portions of the back volume.FIGS.12-14show multiple apertures having different configurations. InFIG.12, the configuration includes two arrayed apertures corresponding to the plurality of apertures ofFIGS.3-5. InFIG.13, the configuration includes four arrayed apertures corresponding to the plurality of apertures used inFIGS.6-8. InFIG.13, the configuration includes six arrayed apertures corresponding to the plurality of apertures used inFIGS.9-11. Alternatively, a single aperture can connect the first and second portions of the back volume.

FIGS.16-18illustrate another embodiment of a sensor assembly100wherein a portion of the back volume is formed by a second cover1602fastened to a portion of the base108opposite the cover104. In this manner, the enclosed volume created by the second cover constitutes a portion of the second portion of the back volume. One or more vents1604extending through the base acoustically connect the first and second portions of the back volume. The one or more apertures can be part of a screen, mesh or other panel1606disposed over a portion1608of the one or vents.

The acoustic performance of the sensor assembly can be shaped or modified by structurally configuring the one or more apertures coupling the first and second portions of the back volume. For example, an acoustic impedance of the one or more apertures can be configured by selectively sizing an aperture between the first and second portions of the back volume. Alternatively, the acoustic impedance can be configured by increasing or decreasing the number of apertures between the first and second portions of the back volume. The acoustic impedance can also be configured by impeding or enhancing the propagation of sound through the one or more apertures via introduction or removal of a mechanical obstruction medium (e.g., a screen, barrier, etc.) in or over the one or more apertures.

Generally, lower acoustic impedance of the one or more apertures between the first and second portions of the back volume increases sensor sensitivity at higher frequencies and vice-versa. In one implementation, the one or more apertures are structured to increase sensitivity at frequencies above 11 kHz.FIG.15shows the frequency responses for different configurations of the one or more apertures connecting the first and second portions of the back volume. Plot1502corresponds to the frequency response of a sensor assembly having the two-aperture array configuration ofFIG.12. Plot1504corresponds to the frequency response of a sensor assembly having the four-aperture array configuration ofFIG.13. Plot1506corresponds to the frequency response of a sensor assembly having the six-aperture array ofFIG.14. The four-aperture array has less acoustic impedance than the two-aperture array. Similarly, the six-aperture array has less acoustic impedance than the four-aperture array. InFIG.16, plot1504shows that the sensor assembly having the four-aperture array has greater sensitivity at higher frequencies than the sensor assembly having a two-aperture array acoustically coupling the first and second portions of the back volume. Plot1506shows that the sensor assembly having the six-aperture array has greater sensitivity at higher frequencies than the sensor assembly with the four-aperture array. Alternatively, the increase in sensitivity can be obtained by reducing the acoustic impedance of a single aperture between the first and second portions of the back volume. Conversely, sensitivity at higher frequencies of the frequency response can be reduced by increasing the acoustic impedance of one or more aperture between the first and second portions of the back volume.

Among other advantages, employing a plurality of apertures to enhance the back volume of a microphone or another sensor can serve to create more desirable frequency responses in the microphone. Other benefits will be recognized by those of ordinary skill in the art.

While the present disclosure and what is presently considered to be the best mode thereof has been described in a manner that establishes possession by the inventors and that enables those of ordinary skill in the art to make and use the same, it will be understood and appreciated that there are many equivalents to the exemplary embodiments disclosed herein and that myriad modifications and variations may be made thereto without departing from the scope and spirit of the disclosure, which is to be limited not by the exemplary embodiments but by the appended claims.