Hearing aid with in situ testing capability

A hearing aid has a built in or internal test tone generator for providing test tones and noise for diagnostic tests to a user through the receiver of the hearing aid. Alternatively an external test tone generator may be coupled to the hearing aid and selectively coupled to the receiver of the hearing aid for the diagnostic tests. A memory internal to the hearing aid may store real world sounds for diagnostic tests to simulate actual usage of the hearing aid.

RELATED APPLICATION 
The subject matter of this application is related to the subject matter of 
patent application Ser. No. 08/907,337 entitled "Digital Signal Processing 
Hearing Aid" filed on Oct. 10, 1995 by Eric Lindemann & John Melanson. 
FIELD OF THE INVENTION 
This invention relates to hearing aids, and more particularly to hearing 
aids having the capability of in situ testing. 
BACKGROUND OF THE INVENTION 
Hearing aid fitting is a process of adjusting the overall gain, the 
frequency response, and dynamic processing parameters of an electronic 
hearing instrument to best match the requirements of an individual user. 
The fitting process is generally carried out by a hearing professional, 
such as an audiologist, an ear, nose, and throat doctor, or a hearing aid 
dispenser. Hearing aid fitting is usually based on a number of diagnostic 
tests which are performed as part of, or prior to the fitting session. 
These diagnostic tests may include a threshold audiogram, and tests to 
establish the most comfortable (MCL) and uncomfortable (UCL) listening 
levels in different frequency bands. These tests are usually administered 
using standard audiometers which present pure test tones and bands of 
noise at different frequencies and different amplitudes. These sounds are 
presented to the test subject through headphones or in free space from 
loudspeakers. The test subject responds to the presentation sounds by 
indicating whether the sound is barely audible, as in the case of 
threshold tests, or with a judgment about the loudness of a sound, as in 
the case of the MCL and UCL tests. 
The result of these diagnostic tests is often a prescription for a hearing 
aid having an insertion gain (IG) which specifies the desired frequency 
dependent gain that a hearing aid delivers to provide maximum satisfaction 
for the hearing aid user. 
Some hearing aids provide dynamic range compression in which the gain 
applied in a given frequency band can be a function of the amount of power 
in that frequency band. This may be viewed as different insertion gains 
for different input power levels. Compressing hearing aids have a number 
of time constants which determine how quickly the insertion gain changes 
as a function of changes of input power level. A prescription for a 
compressing hearing aid may include multiple frequency dependent insertion 
gains or a formula for modifying a single frequency dependent insertion 
gain based on input power--compression ratios are a way to express 
this--and associated compression time constants. 
A number of formulae have been devised which receive as input the result of 
a set of diagnostic tests and produce as an output a hearing aid 
prescription. An example of this is the Australian National Acoustics 
Laboratory (NAL) formula for noncompressing aids. Systems for fitting 
compressing aids from loudness judgment test data are described in Fred 
Waldhauer et al., "Full Dynamic Range Multiband Compression In A Hearing 
Aid", The Hearing Journal, September 1988, at 1-4 and U.S. Pat. No. 
4,718,499. 
Given a hearing aid prescription, an important goal of the fitting session 
is to adjust the hearing aid to achieve the prescription. A limitation of 
performing this adjustment is that the frequency response and gain of a 
hearing aid can only really be determined when it is plugged into the ear. 
This is because the ear canal, eardrum, the degree to which the hearing 
aid seals the ear canal, and variations from one hearing aid device to 
another, all affect the frequency response of the aid. To overcome this 
limitation the hearing aid fitter often uses a probe microphone which is a 
microphone in the form of a very fine flexible tube which can be inserted 
into the ear canal with the tip of the tube placed near the eardrum while 
the hearing aid is in place. The probe microphone then measures the energy 
present at the eardrum. Another microphone is generally placed just 
outside the ear to determine the energy arriving at the input of the 
hearing aid. With these two measurements, the overall gain and frequency 
response characteristics of the hearing aid can be determined. 
The probe microphone measurement approach to hearing aid fitting is 
susceptible to various causes of measurement errors. These include 
pinching of the probe microphone tube, variability in placement of the tip 
of the tube in relation to the eardrum, and plugging of the tube with 
earwax, dirt or debris. These problems make probe tube measurements 
difficult and time consuming. 
Even if the hearing aid prescription has been successfully implemented, 
there is still no guarantee that the hearing aid has been adjusted for 
maximum satisfaction. This is because the formulae which have been used to 
determine the hearing aid prescription cannot account for the myriad 
subjective factors which govern hearing aid acceptance. As a result, after 
implementation of a hearing aid prescription, the fitting session may 
continue with the hearing aid fitter applying a number of artful manual 
readjustments of the hearing aid response. To aid in this process, the 
hearing aid fitter often presents a selection of real world sounds which 
the test subject listens to through loudspeakers in an attempt to simulate 
various listening environments. The hearing aid fitter then interrogates 
the subject about the quality of the sounds and uses the responses as a 
guide to further readjustment. 
A problem relating to this readjustment process is that presentation of 
sounds through loudspeakers must be done in a controlled and repeatable 
way so that, for example, sounds which are supposed to be perceived as 
being at conversational level are indeed presented at this level. This 
means that the placement of the loudspeakers, the amplification system, 
and the distance and orientation of the subject in relation to the 
loudspeakers all must be properly controlled. 
It is desired to have a hearing aid that alleviates the problems associated 
with traditional fitting using probe microphones and external 
loudspeakers. 
SUMMARY OF THE INVENTION 
In the present invention, a hearing aid generates the diagnostic test tones 
and the sounds to simulate a real listening environment. The hearing aid 
generates such tones and sounds in situ. 
The hearing aid includes a microphone, a hearing rehabilitator for 
processing an audio signal from the microphone, and including a receiver. 
A tone generator coupled to the receiver produces tones for diagnostic 
tests. The tone generator may vary gain and the frequency shaping of the 
test tones responsive to user selected commands. A switch selectively 
couples either the hearing rehabilitator or the tone generator to the 
receiver. A memory stores recordings of real world sounds, which are 
retrieved by a controller and provided to a digital-to-analog converter, 
which converts the recordings into an analog audio signal. The switch also 
selectively couples the digital-to-analog converter to the receiver. 
A hearing aid comprises a microphone for providing an electrical signal in 
response to sounds and comprises a receiver. A digital-to-analog converter 
receives a digital audio signal and provides an analog audio signal to the 
receiver in response to the digital audio signal. A programmable digital 
signal processor selectively executes either a hearing rehabilitation 
program to alter the electrical signal or a test tone generation program 
for producing tones for diagnostic tests. The digital signal processor 
provides the digital audio signal to the digital-to-analog converter in 
response to either the altered electrical signal or the tones. The 
programmable digital signal processor retrieves stored recordings of 
sounds from a memory and provides such stored recordings to the 
digital-to-analog converter. The programmable digital signal processor 
varies the gain and the frequency shaping of the test tones responsive to 
a control signal. 
A hearing aid comprises a microphone, a hearing rehabilitator for 
processing an audio signal from the microphone, and including an input 
port for receiving test tones for diagnostic tests from an external sound 
source. An amplifier amplifies the test tones. A receiver provides a sound 
signal in response to the amplified test tones. A switch selectively 
couples either the hearing rehabilitator or the input port to the 
amplifier to provide audio signals indicative of the sounds detected by 
the microphone or of the test tones generated externally. The test tones 
may be analog or digital. For digital test tones, the hearing aid further 
comprises an digital-to-analog converter for converting the digital test 
tones into an analog audio signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, there is shown a block diagram illustrating a hearing 
aid 100 in accordance with the present invention. The hearing aid 100 
provides test tones to a user for in situ testing and adjustments of the 
hearing aid. 
The hearing aid 100 comprises a microphone 102, a hearing rehabilitator 
104, a controller 105, a memory 106, a digital-to-analog converter 107, a 
test tone generator 108, an input port 110, a switch 112, a filter 114, an 
amplifier 116, a receiver 118, and a switch 120. In a normal hearing aid 
mode, the hearing aid wearer hears sounds from the external environment. 
In this mode, the microphone 102 receives sounds from the external 
environment and provides an analog audio signal indicative of the sounds 
to the hearing rehabilitator 104. The microphone 102 may be, for example, 
a conventional hearing aid microphone. The hearing rehabilitator 104 
filters, amplifies, and dynamic range compresses the audio signal. The 
hearing rehabilitator 104 may be, for example, a programmable master 
hearing aid model GP520 manufactured by Gennum Corporation of Burlington, 
Ontario, Canada. The hearing rehabilitator 104 provides the processed 
audio signal to the switch 112, which selectively provides the processed 
signal to the filter 114 to allow the hearing aid 100 to detect sounds 
from the external environment. 
The filter 114 provides a filtered audio signal to the amplifier 116 which 
amplifies the filtered audio signal. The characteristics of the filter 114 
may be dynamically controlled to alter the frequency content of the audio 
signal in response to control signals from the hearing aid fitter. The 
receiver 118 converts the amplified audio signal into sound which is 
provided to the user. The receiver 118 may be, for example, a conventional 
hearing aid receiver. The term "receiver" as used in the art of hearing 
aids refers to a hearing aid speaker. 
In a diagnostic test mode, the hearing aid fitter adjusts the operational 
characteristics of the hearing aid to match the particular need of the 
wearer. The hearing aid fitter may select from either an internal test 
mode or an external test mode. In the external test mode, the test tones 
and sounds are received through the input port 110. In the internal test 
mode, the test tones are generated by the test tone generator 108 in a 
test tone generation mode and the real world sounds are generated by 
retrieving the recorded sounds from the memory 106 in a sampled tone mode. 
In the diagnostic test mode, the switch 112 selectively couples the switch 
120 to the filter 114 to provide either internally or externally generated 
sounds to the user. 
In the test tone generation mode, the test tone generator 108 provides 
tones and noise for diagnostic tests of the hearing aid 100 to the switch 
120 and to the switch 112, which provides the tones and noise to the 
filter 114 for processing as described above. The tones are synthesized 
tones such as a sine wave having a single controlled frequency, a 
composite sinewave, band limited noise, or another audio signal. The test 
tone generator 108 may vary the gain and the frequency shaping of the test 
tones responsive to user selected signals. The noise may be narrow band 
noise. 
In the external test generation mode, the input port 110 receives tones and 
noise for diagnostic tests from an external source (not shown), such as an 
external test tone generator, audiometer, tape recorder, compact disk 
player, or other sound source, which are provided to the switches 120 and 
112. The input port 110 also may be used for receiving recordings of real 
world sounds from a tape, compact disk, or the like. In an alternate 
embodiment, the hearing aid 100 does not include the test tone generator 
108 and provides the test tones received through the input port 110. 
In the sampled tone mode, the memory 106 provides sampled real world sounds 
stored therein. The controller 105 sends addresses and control signals to 
the memory 106 to read the sampled real world sounds. In response, the 
memory 106 provides the read sampled real world sounds to the 
digital-to-analog converter 107, which converts the sampled real world 
sounds into an analog audio signal that is provided to the switch 120 and 
then to the switch 112. The memory 106 is preferably a nonvolatile memory. 
The memory 106 may be, for example, a conventional electrically erasable 
programmable read only memory (EEPROM). The sounds may be stored in the 
memory 106 in a compressed form. In an alternate embodiment, the hearing 
aid 100 does not include the memory 106 and receives the real world sounds 
through the input port 110. 
Having described the hearing aid 100, the diagnostic tests are now 
described. One diagnostic test is the pure tone threshold audiogram test. 
In this test, the hearing aid fitter asks the subject to determine at 
which amplitude level a set of pure tones of varying 
frequencies--approximately 100 Hz to 6000 Hz--become just barely audible. 
This establishes the frequency dependent threshold of hearing for the 
subject. The results of this test are plotted as an audiogram which 
displays the hearing loss relative to a normal non-impaired listener. The 
purpose of the audiogram in hearing aid fitting is that it permits the 
determination of an insertion gain. The insertion gain is the gain 
required to amplify tones at or somewhat above the normal threshold of 
hearing to a level which is at the threshold of the impaired listener. 
With conventional fitting practices, the insertion gain is first determined 
with audiometric equipment, then probe microphones are used to verify that 
the hearing aid is delivering the desired gain. For the hearing aid 100, 
an in situ testing approach is used, in which the hearing aid 100 directly 
generates pure tones, such as a sinewave. In particular, the test tone 
generator 103 generates such tones. To determine the actual frequency 
dependent gain of the hearing aid 100, i.e. the set of parameter 
adjustments needed so that the hearing aid implements the desired 
insertion gain, the gain of the hearing aid 100 is increased in various 
bands until the tones become barely audible. In particular, the gain of 
the amplifier 116 is adjusted in the various frequency bands. Thus, no 
probe microphone technology is needed. This results in a more reliable 
fitting and at a reduced cost to the hearing aid fitter because a probe 
microphone system is not required. 
A second diagnostic test is a loudness scaling approach to fitting. This 
test is similar to the pure tone threshold audiogram test. In this test, 
sounds--usually narrow bands of noise--are played at various frequencies 
and amplitudes and the hearing aid fitter asks the test subject to rate 
these sounds according to a loudness scale. The loudness scale may be, for 
example, very soft, soft, comfortable, loud, and very loud. Based on the 
subject's responses and the known responses of an averaged set of normal 
subjects, it is possible to determine a frequency and amplitude dependent 
gain map for the test subject, i.e., the gain at which the hearing 
impaired subject associates a test noise at a given frequency and power 
level with the same loudness category that the normal subject would. In 
other words, after application of the gain, soft sounds sound soft, loud 
sounds sound loud, and so forth. 
A conventional loudness test uses audiometric equipment to generate a gain 
map--a set of insertion gains or a single insertion gain and a set of 
compression ratios. The hearing aid then is adjusted to implement this 
gain. This adjustment is performed using probe microphone measurement 
techniques. 
In contrast, the hearing aid 100 directly generates the test noises using 
the test tone generator 108. The gain of the amplifier 116 is adjusted for 
each frequency band until the hearing impaired subject identifies the test 
noise as being in the correct loudness category. These gain adjustments 
are then applied to signals received during normal use of the hearing 
head. This simplifies the loudness test. Again no probe microphone 
equipment is necessary. 
A third diagnostic test is a manual readjustment of the insertion gain for 
real world sounds. In this test, the hearing aid fitter generally plays 
real world sounds through loudspeakers and adjusts the gain and frequency 
response of the hearing aid for maximum clarity and comfort as determined 
by the subjective responses of the hearing aid wearer. 
In the present invention, real world sounds can be played by the hearing 
aid 100 from signals received via the input port 108 from an external 
source (not shown) or from signals generated internally in the hearing aid 
100 by retrieving the real world sounds stored in the memory 106. This 
eliminates the need for loudspeakers and their inherent problems of 
positioning the loudspeakers in relation to the hearing aid wearer and 
controlling the calibration of the loudspeaker amplification system. 
Referring to FIG. 2, there is shown a block diagram illustrating a digital 
hearing aid 200 in accordance with the present invention. The hearing aid 
200 provides test tones to a user for in situ testing and adjustments of 
the hearing aid. The hearing aid 200 comprises a microphone 202, a 
programmable digital signal processor 204, a digital-to-analog converter 
206, a digital input port 208, an analog input port 209, analog-to-digital 
converters 210 and 211, a receiver 212, and a switch 222. In a normal 
hearing aid mode, the microphone 202 receives sounds from the external 
environment and provides an analog audio signal indicative of the sounds 
to the analog-to-digital converter 210, which converts the analog audio 
signal to a digital audio signal. The microphone 202 may be, for example, 
a conventional hearing aid microphone. The analog-to-digital converter 210 
provides the digital audio signal to a hearing rehabilitator 216 of the 
digital signal processor 204 for processing. 
The digital signal processor 204 executes software programs for normal 
operation of the hearing aid 200 and for diagnostic tests. The digital 
signal processor 204 comprises a test tone generator 214 and the hearing 
rehabilitator 216. The test tone generator 214 is a computer program that 
generates a synthesized tone signal that is either a sinewave having a 
controlled frequency, band limited noise, composite sine waves, or other 
audio signals, and provides such a signal to a controller 218 for 
diagnostic tests of the hearing aid 200 in a test tone generation mode. 
The test tone generator 214 also may vary the gain and the frequency 
shaping of the test tones responsive to user selected signals. The hearing 
rehabilitator 216 is a computer program for filtering, amplifying, and 
dynamic range compressing the audio signal. The hearing rehabilitator 216 
provides the processed audio signal to the controller 218. Such a hearing 
rehabilitator 216 is described in Fred Waldhauer et al., "Full Dynamic 
Range Multiband Compression In A Hearing Aid", The Hearing Journal, 
September 1988, at 1-4 and U.S. Pat. No. 4,718,499 for compression, 
described in U.S. patent application Ser. No. 08/123,503 entitled "Noise 
Reduction System for Binaural Hearing Aid" filed Sep. 17, 1993, inventors 
Lindemann et al., described in U.S. patent application Ser. No. 08/184,724 
entitled "Dynamic Intensity Beamforming System for Noise Reduction in a 
Binaural Hearing Aid", filed Apr. 20, 1994, inventors Lindemann et al., 
and described in U.S. patent application Ser. No. 08/907,337 entitled 
"Digital Signal Processing Hearing Aid", filed Oct. 10, 1995, inventors 
John Melanson and Eric Lindemann, the subject matter of all is 
incorporated herein by reference. 
In the diagnostic test mode, the controller 218 couples the switch 222 to 
the digital-to-analog converter 206. A memory 220 stores sampled real 
world sounds that are provided to the programmable switch 218. The digital 
signal processor 204 reads the sampled real world sounds from the memory 
220 and provides the sounds to the switch 222 in a sampled tone mode. The 
memory 220 is preferably a nonvolatile memory. The memory 220 may be, for 
example, an EEPROM. The sounds may be stored in the memory 220 in a 
compressed form. The controller 218 decompresses the data. In an alternate 
embodiment, the hearing aid 200 does not include the memory 220 and 
receives the real world sounds through the input ports 208, 209. 
In the external test generation mode, the switch 222 couples either the 
digital input port 208 or the analog-to-digital converter 211 to the 
controller 218. The digital input port 208 coupled to the digital signal 
processor 204 receives tones and noise in a digital format from an 
external source (not shown), such as an external test tone generator, 
audiometer, tape recorder, compact disk player, or other sound source. The 
digital input port 208 may be also used for receiving recordings of real 
world sounds from a tape, compact disk or the like. The digital data 
provided to the input port 208 may be a compressed digital audio stream. 
The digital signal processor 204 decompresses the digital audio stream. 
The analog input port 209 coupled to the analog-to-digital converter 211 
receives tones and noise in an analog format from an external source (not 
shown). The analog-to-digital converter 211 converts the analog tones and 
noise into a digital format and provides the digital tones and noise to 
the digital signal processor 204. In an alternate embodiment, the hearing 
aid 200 does not include a tone generator 214 and provides the test tones 
received through the input port 208. In another alternate embodiment, the 
hearing aid 200 does not include the input port 208 or the input port 209 
or both. 
In response to control signals from the hearing aid fitter, the controller 
218 selectively couples either the hearing rehabilitator 216 in the normal 
hearing aid mode or the switch 222 to the digital-to-analog converter 206 
for using the hearing aid 200 in a diagnostic test mode. In response to 
control signals from the hearing aid fitter in the diagnostic test mode, 
the controller 218 commands the switch 222 to selectively couple either 
the analog-to-digital converter 211, the test tone generator 214, the 
input port 208, or the memory 220 to the controller 218 for diagnostic 
testing of the hearing aid 200 as described above. The controller 218 
processes the digital audio signal by filtering the audio signal and 
adjustingthe amplitude of the audio signal as a function of frequency in 
response to control signals from the hearing aid fitter. The 
digital-to-analog converter 206 converts the digital audio signal into an 
analog audio signal, which is provided to the receiver 212 which then 
converts the analog audio signal into sound which is provided to the user. 
The receiver 212 may be, for example, a conventional hearing aid receiver. 
Having described the hearing aid 200, the diagnostic tests are now 
described. The hearing aid 200 executes a pure tone threshold audiogram 
test in a manner similar to that described above for the hearing aid 100. 
The hearing aid fitter determines an audiogram as described above except 
that the test tone generator 214 generates the pure tones digitally. 
Further, the adjustments to the insertion gain in various bands are also 
performed digitally. 
The hearing aid 200 executes a loudness scaling test in a manner similar to 
that described above for the hearing aid 100. The test tone generator 214 
generates the narrow bands of noise digitally by reading the tables of 
sampled values of the sine waves for various frequencies and digitally 
adjusting the amplitude. The digital signal processor 204 generates a 
frequency and amplitude dependent gain map which is applied to received 
signals from the microphone to thereby provide the user with a sounds that 
vary according the loudness of the received signal at the microphone. 
The hearing aid fitter performs manual readjustments to the hearing aid 200 
to reflect real word sounds in a manner similar to that of the hearing aid 
100. Such real world sounds may be received through the input port 208 or 
may be generated by the hearing aid 200. More specifically, the digital 
signal processor 204 reads from the table of sampled values of the real 
world sounds stored in the memory 220 and provides the sampled values to 
the digital-to-analog converter 206. The digital signal processor 204 
adjusts the gain and the frequency response to improve clarity and comfort 
as determined by the subjective response of the hearing aid wearer.