System for testing hearing assistance devices using a planar waveguide

A system and method for testing and measuring hearing assistance devices using a plane wave tube is provided. According to an embodiment, a hearing assistance device is mounted proximal to an acoustic waveguide having a soundfield with acoustic waves propagating down the waveguide. A microphone of the hearing assistance device is placed in the soundfield of the acoustic waveguide to increase a direct acoustic component and to reduce reflected acoustic components and scattered acoustic components of sound sensed by the microphone. Sound is generated using a sound generator to propagate sound of desired frequencies down the waveguide.

FIELD OF THE INVENTION

The present subject matter relates generally to hearing assistance devices, and in particular to a method and apparatus for testing and measuring hearing assistance devices.

BACKGROUND

Hearing assistance devices, or hearing aids, are electronic instruments worn in or around the ear that compensate for hearing losses by amplifying sound. Because hearing loss in most patients occurs non-uniformly over the audio frequency range, hearing aids are usually designed to compensate for the hearing deficit by amplifying received sound in a frequency-specific manner. The clarity, noise reduction, and overall quality of the performance of these devices require that the frequency response of the devices be properly calibrated and tested during and after the production process. Testing of the electro-acoustic performance of hearing aids is important to verify that an instrument is functioning both according to the manufacturer's specifications and according to the auditory needs of the wearer.

Conventional testing of hearing assistance devices can be performed in a test box, which provides the acoustical environment, or the acoustical conditions under which the device under test (DUT) is measured. The total acoustical signal Ptsensed by microphone(s) of the DUT typically consists of three components: a direct component Pdfrom the loudspeaker, scattered components Psfrom reflections and diffraction off of the DUT and its fixtures and features, and the boundary reflections Prof the acoustical environment. Mathematically,
Pt=Pd+Ps+Pr.

Therefore, the measured response of the DUT is dependent upon the relative magnitude and temporal contributions of the direct component, scattered components and reflected components from the test box boundaries. The scattered components and reflected components can inhibit the ability to properly test and calibrate the DUT. Thus, there is a need in the art for a method and apparatus for imparting sound to a hearing assistance device to reduce the occurrence of these indirect components and hence provide improved calibration and testing of hearing assistance devices.

SUMMARY

The present system provides a method and apparatus to address the foregoing needs and additional needs not stated herein. In one embodiment, the system provides a method and apparatus for testing and measuring a hearing assistance device. According to an embodiment, the hearing assistance device is mounted proximal to an acoustic waveguide having a soundfield with acoustic waves propagating down the waveguide. A microphone of the hearing assistance device is placed in the soundfield of the acoustic waveguide to increase a direct acoustic component and to reduce reflected acoustic components and scattered acoustic components of sound sensed by the microphone. Sound is generated using a sound generator to propagate sound of desired frequencies down the waveguide.

Another aspect of this disclosure relates to an apparatus for imparting sound to a hearing assistance device. According to one embodiment, the apparatus includes an acoustic waveguide having a soundfield with acoustic waves propagating down the waveguide. The apparatus also includes a mount fixedly receiving the hearing assistance device and adapted to place a microphone of the hearing assistance device in the soundfield of the acoustic waveguide, the mount adapted to place the microphone to increase a direct acoustic component and to reduce reflected acoustic components and scattered acoustic components of sound sensed by the microphone. The apparatus further includes a sound generator to propagate sound of desired frequencies down the waveguide. According to various embodiments, the apparatus is adapted to impart sound to a hearing assistance device having more than one microphone.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that the embodiments may be combined, or that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description provides examples, and the scope of the present invention is defined by the appended claims and their equivalents.

It should be noted that references to “an”, “one”, or “various” embodiments in this disclosure are not necessarily to the same embodiment, and such references contemplate more than one embodiment.

Disclosed herein is a testing system and method for hearing assistance devices. The disclosed acoustic testing system provides a planar waveguide, or plane wave tube, in which planar acoustic waves propagate over the microphone inlets of a hearing assistance device. The system reduces reflected and scattered components of the acoustic wave, improving the reliability and accuracy of testing of hearing assistance devices. Further advantages of the system include: convenient and accurate placement of the hearing aids; repeatable measurement with negligible system error; excellent sound and vibration isolation; and improved efficiency of compensation. The system is adaptable for testing both in-the ear (ITE) and behind-the-ear (BTE) hearing assistance devices.

FIG. 1is a diagram of a system200for testing a hearing assistance device208incorporating a planar waveguide, according to one embodiment of the present system. An acoustic waveguide202is shown having a soundfield with acoustic waves204propagating down the waveguide202. In this embodiment, a mount206for fixedly positioning the hearing assistance device208is adapted to place a microphone210of the hearing assistance device208in the soundfield of the acoustic waveguide. The mount206is adapted to place the microphone210to increase a direct acoustic component Pdand to reduce reflected acoustic components Prand scattered acoustic components (not shown) of sound sensed by the microphone210. A sound generator212, or moving-coil loudspeaker, is used to propagate sound of desired frequencies down the waveguide202. In this embodiment, loudspeaker212is a 1.5 inch diameter, closed-back woofer with ferrofluid damping. Other moving-coil, balanced-armature, or hybrid-type sound-generating devices could be substituted. Sound generator212is coupled to waveguide202through an air cavity205. Air cavity205is shaped to appropriately couple the mechanical impedance of sound generator212to the acoustical impedance of waveguide202. In this embodiment, the air cavity205is shaped like a tapered cylinder, though other shapes can be used depending on the properties of sound generator212.

The boundary207of air cavity205and waveguide202defines a relative reference point for planar wavefronts to envelope within waveguide202. Typically, for a waveguide having a circular cross-section, planar wavefronts develop approximately two waveguide diameters from boundary207. Therefore, it is recommended to position microphone210at least approximately two waveguide diameters from boundary207. If waveguide202has other cross-sectional shapes such as rectangular, or U-shaped, etc., the characteristic (largest) dimension should substitute as the defining criteria for planar wavefront development. It should also be noted that the internal cross section of the waveguide202may change subtly in the local region around device208, thereby causing minimal perturbation in the developing planar wavefront.

The acoustic waveguide202provides a fixed relative distance between the microphone210of the device208and the loudspeaker212, minimizes reflections from the boundaries of the test environment, and substantially eliminates the scattered component by positioning the microphone inlets within the test environment (waveguide) and positioning all other features and fixtures of the device outside the test environment. The waveguide202also provides an incident planar wavefront to the device at a known, repeatable angle and can provide simultaneously the same acoustical excitation (magnitude and phase) to multiple microphone ports on a device under test, when the ports are positioned along a line perpendicular to the axis of the waveguide.

In one embodiment of the system200, the acoustic waveguide202has a circular cross section and cutoff frequency, i.e., the highest frequency for planely propagating acoustic waves, of 10 kHz. If the plane wave cutoff frequency is 10 kHz, the characteristic dimension, or diameter, of the waveguide is approximately 0.68 inches. For a plane wave cutoff frequency of 8 kHz, the characteristic dimension of the waveguide is approximately 0.85 inches. In another embodiment, the acoustic waveguide202provides an acoustic field with minimal reflections and a relatively flat frequency response between 100 Hz and 8 kHz. In various embodiments, the acoustic waveguide202provides an acoustic field from 100 Hz to 8 kHz with a relative level less than 15 dB in range, provides repeatable measurement of the hearing assistance device208with test-retest placement error less than 1 dB and dual microphone acoustic excitation disparity less than 0.1 dB, and provides between 20 dB (lowest frequencies) and 40 dB (mid to high frequencies) of sound isolation.

FIG. 2is a diagram showing a cross-sectional side view of one embodiment of a system300for imparting sound to a hearing assistance device, according to one embodiment of the present system. An acoustic waveguide302, or plane wave tube, is shown having a soundfield with acoustic waves propagating down the waveguide. A mount304is provided for fixedly positioning the hearing assistance device. In this embodiment, the mount includes a holding fixture306with pins308for securing a faceplate312to the waveguide302. According to this embodiment, magnets310along the surface of the waveguide are used to hold the fixture in place. One of ordinary skill will appreciate that other mounting methods are equally appropriate. Several others will be described in more detail below with respect toFIGS. 6A through 8C.

According to various embodiments, the mount304is further adapted to prevent portions of the hearing assistance device, other than the microphone of the hearing assistance device, from being placed in the soundfield of the acoustic waveguide302.

In various embodiments of system300, the acoustic waveguide302contains at least one minimally-reflecting boundary to dissipate acoustic waves. According to one embodiment, the acoustic waveguide302includes a damping structure318along the boundary316opposite the sound generator314. The damping structure318may include a 0.25 inch thick layer of foam (100 ppi) or other acoustically absorptive material, which in an embodiment can be enclosed in a 20 foot long, 0.8 inch inner diameter, coiled, polyvinyl tube320stuffed loosely with fibrous, acoustically-absorptive material. Other sizes and types of tubes are within the scope of this disclosure. According to one embodiment, the acoustic waveguide302includes a boundary316opposite the sound generator314separated from the hearing assistance device by sufficient distance to dissipate boundary reflections.

A sound generator314or driver is used to propagate sound of desired frequencies down the waveguide. In one embodiment, the acoustic waveguide302includes an acoustic filter322adjacent the sound generator. The acoustic filter322may consist of a weaved fabric, metal etched screen, formed material of known acoustic resistance, or other acoustic filtering device. According to various embodiments, a damping filter324can be used at the cone section of the waveguide302to further improve acoustic filtering.

FIG. 3is a diagram showing a three-dimensional view of one embodiment of a system350for imparting sound to a hearing assistance device, according to one embodiment of the present system. An acoustic waveguide352is shown having a cutoff frequency that is higher than any frequencies of interest, the waveguide352having a soundfield with acoustic waves propagating down the waveguide352. In this embodiment, a mount356for fixedly receiving the hearing assistance device is adapted to place a first microphone and a second microphone of the hearing assistance device in the soundfield of the acoustic waveguide. The mount356is adapted to place the first microphone and the second microphone to increase a direct acoustic component Pdand to reduce reflected acoustic components Prand substantially eliminate scattered acoustic components Psof sound sensed by the microphones. Those of skill in the art will recognize that more than two microphones (a third, a fourth, an Nth) may be placed in the soundfield using the disclosed system. A sound generator362, or loudspeaker, is used to propagate sound of desired frequencies down the waveguide352.

FIG. 4is a diagram showing an acoustic field in a waveguide. The acoustic signal402is shown propagating in the Z-direction, and the dimensions of the waveguide (Lxand Ly) are such that Lx,y<λ/2 where λ is the signal's wavelength, i.e., the acoustic signal's frequency is f<c/(2Lx,y) where c is the sound speed. Under these conditions, planar pressure waves internal to the waveguide can be expressed mathematically as
P(z)=[Aejkz−Bejkz]e−jωt.
where j=−11/2, ω=2πf, and k=ω/c. If the boundary at the end of the waveguide is sufficiently absorptive thereby rendering reflections in the Z-direction negligible, i.e., B<<A, then forward propagating waves dominate and the expression becomes
P(z)=Aej(kz−ωt).
Under these conditions, the above expression indicates that both the pressure amplitude and phase are uniform over the waveguide's cross-section. Although the above expression suggests the pressure amplitude is constant along the Z-dimension, in practice there are small losses in the walls of the waveguide so that the planar wavefront is slightly attenuated as it propagates in the Z-direction away from the sound generator.

The general description above can be applied to waveguides having various cross-sectional areas. For example, instead of a waveguide with a rectangular cross-section of Lxand Ly, an ameba-shaped cross section could be used. The principle of planar wave propagation can be extended here by considering the characteristic dimension, i.e., the largest dimension in the ameba's cross section and substituting it into the above equations for Lx,y.

FIG. 5is a flow diagram of a method for testing a hearing assistance device, according to one embodiment of the present system. According to this embodiment of the method500, the hearing assistance device is mounted proximal to an acoustic waveguide having cutoff frequency that is higher than any frequencies of interest, the waveguide having a soundfield with acoustic waves propagating down the waveguide at502. At504, a microphone of the hearing assistance device is placed in the soundfield of the acoustic waveguide to increase a direct acoustic component and to reduce reflected acoustic components and scattered acoustic components of sound sensed by the microphone. At506, sound is generated using a sound generator to propagate sound of desired frequencies down the waveguide. In one embodiment, a magnetic fixture is used to hold the hearing assistance device proximal an acoustic waveguide.

According to various embodiments, the method further includes measuring a frequency response of the hearing assistance device. According to various embodiments, the method further includes rotating the mount with respect to the waveguide to measure a polar response of the hearing assistance device, or to measure microphone mismatch of hearing assistance devices having multiple microphones. These data can further be used with pre-measured head related transfer functions in order to predict three-dimensional directional performance of the assistance device, thereby simulating measurements that would occur at the ears of the wearer.

FIG. 6Ais a diagram showing a rotational fixture602for holding a hearing assistance device during testing, according to one embodiment of the present system. The rotational fixture602allows for rotating the mount with respect to the waveguide604to measure polar response of the hearing assistance device. Circular member606integrates with rotational fixture602to mount the hearing assistance device for testing.FIG. 6Bis a close up view of a portion ofFIG. 6A, according to one embodiment of the present system. In this view, the rotational fixture602is shown apart from the waveguide.

FIG. 7Ais a diagram showing a battery-door-aligning fixture702for holding a hearing assistance device704during testing, according to one embodiment of the present system. The battery-door-aligning fixture702has a diametrical member708which is designed and fabricated to receive and align the battery door710of the hearing assistance device704under test. The battery-door-aligning fixture702may be constructed of metal, such as aluminum. According to this embodiment, a sealing gasket706provides an acoustic seal exposing only the microphone of the hearing assistance device to the waveguide during testing. The sealing gasket may be a preformed die-cut of closed cell foam, according to various embodiments.

FIG. 7Bis a diagram showing the assembled fixture ofFIG. 7A, according to one embodiment of the present system. The battery-door-aligning fixture702is shown affixed to the hearing assistance device704. In this embodiment, the diametrical member708of the battery-door-aligning fixture702has oriented and located the battery door710of the hearing assistance device704under test. One of ordinary skill will appreciate that the described fixture can be designed and fabricated to accommodate all possible faceplates and battery-door configurations. In addition, the described mounting fixtures are adaptable for cased hearing aids.

FIG. 8Ais a diagram showing a silicone investment fixture for holding a hearing assistance device804during testing, according to one embodiment of the present system. The silicone investment, or putty802, seals the microphone portion808of the device804to the metal fixture806, which is subsequently placed into an opening of a planar waveguide. In one embodiment, the metal fixture806is constructed of aluminum, but those of skill in the art will appreciate that other materials may be used.

FIG. 8Bis a diagram showing the assembled fixture ofFIG. 8A, according to one embodiment of the present system. The silicone investment802has sealed the microphone portion808of the device to the metal fixture806. In various embodiments, the silicone investment is a vacuum-forming investment.FIG. 8Cis a diagram showing the use of putty, or fun-tack, in the fixture ofFIG. 8A, according to one embodiment of the present system. The diagram depicts the underside of the metal fixture806, showing the putty802sealing the device to the metal fixture806.

FIG. 9is a graphic diagram showing a comparison of measurement sensitivity of conventional systems and one embodiment of the present system. The diagram, which plots relative sensitivity of measurement (in dB), reveals that a testing system environment provided by an embodiment of the present system901approaches the environment of an anechoic chamber903, and is measurably different than two known environments, including a first Frye box905and a second Frye box907.

The present system has a number of potential applications for testing sound amplification equipment. The following examples, while not exhaustive, are illustrative of these applications.

Delay-and-sum Directional Test

Using conventional testing environments for dual omni directional systems, a delay-and-sum directional hearing assistance device has its polar pattern adjusted by positioning the device such that a wavefront impinged on the device at an angle of approximately 120 degrees relative to the directional axis. The level of a potentiometer or value of resistance, controlling the relative level of the rear omni microphone, is then adjusted until the device's total output is minimized thereby prescribing a polar pattern that resembles a hypercardioid or supercardioid. This process is an indirect way of matching the amplitudes of the two omni microphones. Performance variance for this process was wide when done in a conventional test box, due primarily to box reflections that allow acoustic wavefronts to impinge on the device at angles other than 120 degrees.

Using the present system with a planar waveguide, the device is housed in a rotational fixture that allows the device to be rotated such that the incident wavefront impinges on the device at a precisely defined angle with negligible reflections from the boundaries of the test environment.

Directional Compensation of Channel Mismatch

In directional digital devices, the polar pattern was designed under the presumption that electro-mechanical-acoustical mismatch between the front and rear channels of the devices was perfectly characterized. This characterization was performed by subjecting the front and rear microphone inlets of the device to the same magnitude and phase of an acoustic field, and by using a least mean-square (LMS) signal processing scheme to compute a filter. When this filter was convolved with the output of the rear channel, the resultant response would match the response of the front channel so that the two channels were matched when the filter was engaged.

The problem with this approach in a conventional test box is that the acoustic excitation between the two microphone inlets, separated by very small distance (e.g., 5 mm), can cause substantial anomalies in directional processing. These anomalies are due to the LMS filter mischaracterizing acoustic mismatch as channel mismatch. The present system uses a planar waveguide to minimize acoustic excitation disparity between front and rear microphone inlets, thereby allowing more precise characterization of these directional digital devices.

In more contemporary directional digital devices, the signal processing switches dynamically in a non-adaptive manner between an omni pattern and a fixed directional patter. The algorithm that facilitates the switching is based on background noise processing. In these devices, it is preferred that the frequency response of directional mode is closely matched to the frequency response of omni mode, in order to allow unbiased estimates of background noise and more repeatable switching conditions.

Using a conventional test box, a frequency response of a directional device can vary substantially at each frequency depending on the angle of impingement of the acoustic wavefront used to test the device. This effect can prevent proper estimates of background noise using a dynamic-switching algorithm. The planar waveguide of the present system ensures a fixed relationship between the device and the impinging wavefronts, which provides a tighter frequency response measurement and thus better estimates for making dynamic switching decisions.

It is often desirable to perform polar measurements on individual devices at the end of production for quality control. Using the present testing system with a planar waveguide, a device can be mounted in a rotational fixture that can be rotated at specific rates and angles. The output polar response can be measured accurately and rapidly, and then provided to a user on a data sheet. In addition, these polar measurements can be used to predict KEMAR (Knowles Electronics Mannequin for Acoustic Measurements) polar patterns through additional modeling, eliminating the need for actual mannequin testing. Three dimensional KEMAR polar patterns can be provided to the user on a data sheet or displayed on a website using a user-specific password or identification number.

Although the present system is discussed in terms of hearing aids, it is understood that many other applications in other hearing devices and audio devices are possible. It is to be understood that the above description is intended to be illustrative, and not restrictive. Other embodiments will be apparent to those of skill in the art upon reviewing and understanding the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.