Hearing aid with permanently adjusted frequency response

A hearing aid is permanently adjusted to match prescribed amplification characteristics at predetermined frequencies. A single channel filter (24) includes three identical filter stages (60, 62, and 64) that are optionally cascaded in series. Each of the filter stages (60, 62, and 64) exhibits a frequency response curve having a first corner frequency (36) below which the magnitude of the response curve approaches a predetermined minimum value, a second corner frequency (38) above which the magnitude of the response curve approaches a maximum value, and a predetermined gain in magnitude between the minimum and maximum values.

TECHNICAL FIELD 
Our invention relates to hearing aids that are prescribed for matching 
patterns of hearing impairment with desired amplification characteristics. 
BACKGROUND 
Hearing aids help to compensate for a wide range of hearing impairments 
that vary in magnitude from mild to severe based on amounts of 
amplification required to meet hearing threshold levels at predetermined 
frequencies. Individual patterns of hearing impairment are plotted in 
audiograms that record a range of hearing threshold levels over a domain 
of audible frequencies. 
Detailed audiological assessments of hearing performance are made by 
measuring the hearing threshold levels throughout the range of audible 
frequencies presented as both pure tones and speech. Other measures 
include air and bone conduction, reflexes, tympanometry, most comfortable 
loudness level (MCL), loudness discomfort level (LDL), and real-ear 
unaided response (REUR)--the acoustical influence of the auditory canal 
and concha. 
Various combinations of these measures are used in conjunction with a 
variety of prescriptive formulae for selecting hearing aid amplification 
characteristics. For example, one prescriptive formula, known as the Byrne 
and Tonisson procedure, calculates amplification characteristics required 
to present important frequency components of speech with equal loudness. 
Another prescriptive formula, known as the Berger procedure, calculates 
amplification characteristics required to restore a fraction close to 
one-half of the measured hearing loss at each frequency. 
The prescriptive formulae specify exact amplification characteristics at 
predetermined frequencies, and these desired characteristics are often 
referred to as "target frequency responses". However, the ability to meet 
these target frequency responses with known hearing aids is limited, and 
procedures for selecting and adjusting hearing aids to approximate the 
target frequency responses are much less exacting. 
One approach allows physicians and audiologists to choose from a large 
array of electrical components such as microphones, amplifiers, filters, 
and receivers, each contributing to a total frequency response of the 
assembled hearing aid. The large number of available components, 
peculiarities of each component, and the interaction between components 
make the choice of a complete set of components very difficult and time 
consuming. The large number of different components also adds considerable 
inventory, design, and manufacturing costs. 
Another approach provides physicians and audiologists with a matrix of 
frequency responses from which to choose. The electrical components for 
achieving the target frequency responses are selected in advance by the 
hearing aid manufacturers. However, the limited choices for frequency 
response usually preclude a close match with the target frequency 
response, and the hearing aid performance is correspondingly reduced. 
Improperly matched hearing aids can also produce distorted or 
uncomfortable sounds and can obscure information important to the 
perception of speech. 
The ability to match target frequency responses with known hearing aids is 
also limited by the performance of filters within the hearing aid 
circuits. Attempts have been made to combine filters in both series and 
parallel circuits to more closely match target frequency responses. 
However, any frequencies that are attenuated by a first filter in a series 
circuit cannot be fully restored to a higher level of amplification. 
Filters arranged in parallel (i.e., in separate channels) for processing 
different portions of the audible spectrum produce individual phase shifts 
that interfere with recombining the two processed portions of the 
spectrum. 
Some hearing aids are also provided with potentiometers to provide a 
further adjustment to frequency response after manufacture. The 
potentiometers are used to control the performance of hearing aid 
components such as amplifiers and filters. The adjustments are often based 
on subjective responses of the hearing aid wearer and may produce results 
that are inappropriate for other sound environments. 
SUMMARY OF INVENTION 
Our invention simplifies procedures for fitting hearing aids to 
prescription requirements and provides for more closely matching a broad 
range of prescription requirements with a single set of components. The 
prescription requirements are fit during manufacture of our hearing aid. 
This allows physicians and audiologists to specify desired performance 
characteristics without regard for hearing aid manufacturing 
considerations. 
Our hearing aid incorporates conventional components including a 
microphone, an amplifier, and a receiver. These components, including a 
shell for in-the-ear (ITE) hearing aids, preferably exhibit a combined 
frequency response similar to a conventional prescription fit for no 
hearing loss. Within the hearing aid, the frequency response is 
represented by an audio signal with an amplitude that varies as a function 
of frequency. 
A single channel filter provides for varying the frequency response to 
match predetermined patterns of hearing loss. The filter exhibits a 
response curve having (a) a first corner frequency below which the 
magnitude of the audio signal approaches one of a predetermined minimum 
and maximum values, (b) a second corner frequency above which the 
magnitude of the audio signal approaches the other of the predetermined 
minimum and maximum values, and (c) a desired gain in magnitude between 
the minimum and maximum values. The first and second corner frequencies, 
along with the desired gain, are set to match the predetermined pattern of 
hearing loss. 
The filter is implemented with a plurality of identical filter stages that 
are optionally cascaded in series. Preferably, each of the filter stages 
is a biquadratic filter structure defined by a transfer function having 
two independently adjustable corner frequencies (i.e., "zero" and "pole" 
frequencies of the transfer function). A roll-off rate (i.e., a slope of 
the response curve) between the two corner frequencies is determined by 
the number of filter stages that are cascaded in series. 
Together, the conventional hearing aid components and the single channel 
filter exhibit a total frequency response that closely matches 
prescription requirements for the hearing aid. The single channel filter 
can also be adjusted to compensate for different prescriptive formulae and 
for variations in the frequency response of particular components.

DETAILED DESCRIPTION 
Our hearing aid is preferably constructed as an in-the-ear (ITE) hearing 
aid having components including a microelectronic chip mounted within a 
shell that is molded to fit within a patient's external ear or concha. The 
microelectronic chip forms the core of a hearing aid circuit shown as a 
block diagram in FIG. 1. 
A microphone 10 converts sound waves into an audio signal 12. A 
preamplifier 14 receives the audio signal 12 and outputs an audio signal 
16 that is increased in magnitude. Preferably, the audio signal 16 
reflects a gain of about 18 decibels over the audio signal 12. 
An automatic gain control circuit 18 receives the preamplified audio signal 
16 and outputs an audio signal 20 that is limited in magnitude by a 
threshold setting 22. Normally, the audio signal 20 reflects a 
predetermined gain (preferably about 14 decibels) over the input audio 
signal 16. However, the predetermined gain of the automatic gain control 
circuit 18 is reduced for input signals above the threshold setting 22. 
Still higher input levels are compressed to limit the magnitude of the 
audio signal 20 in accordance with a prescribed loudness discomfort level 
(LDL). 
A state variable filter 24 receives the gain-controlled audio signal 20 and 
outputs an audio signal 26 that is varied in magnitude as a function of 
frequency. FIG. 2 depicts a response curve of the state variable filter 24 
in a simplified form as a piecewise curve composed of three interconnected 
asymptotes 30, 32, and 34 of the actual response curve. The curve also 
includes two corner frequencies 36 and 38 formed by intersections of the 
three asymptotes. 
The filter response curve exhibits a flat frequency response below the 
corner frequency 36 approaching a predetermined minimum value (e.g., -30 
decibels), a sloped frequency response between the corner frequencies 36 
and 38, and a flat frequency response above the corner frequency 38 
approaching a predetermined maximum value (e.g., 0 decibels). A difference 
in gain "G" in decibels between the two corner frequencies is determined 
in accordance with the following equation: 
EQU G=20 log [(W.sub.z.sup.2 /W.sub.p.sup.2).sup.n ] 
where "W.sub.z " is the corner frequency 36, "W.sub.p " is the corner 
frequency 38, and "n" is an integer. The magnitude of the gain "G" is 
preferably limited to between -45 decibels and 9 decibels. The magnitude 
"W.sub.p " of corner frequency 38 is preferably limited to between 1250 
hertz and 2500 hertz. 
The shape of the response curve is controlled by three settings 40, 42, and 
44 corresponding to the respective magnitudes "W.sub.z " and "W.sub.p " of 
the corner frequencies 36 and 38 and the integer "n" to match patterns of 
hearing loss with a minimum of variables. The three variables "W.sub.z ", 
"W.sub.p ", and "n" control magnitudes of gain in two different portions 
of the acoustic spectrum, as well as roll-off rates (i.e., the slope of 
the response curve) between the two spectrum portions. 
Preferably, the magnitudes of "W.sub.p ", "G", and "n" are prescribed to 
match patterns of hearing loss. However, the magnitude of "W.sub.z " can 
be readily determined by rewriting the above equation for "G" as follows: 
##EQU1## 
The preferred limits on the magnitude "W.sub.p " of corner frequency 38 
and on the resulting difference in gain "G" allow the filter response 
curve to fit most patterns of hearing loss. 
An output buffer 46 receives the filtered audio signal 26 and outputs an 
audio signal 48 that is adjusted for volume control. A maximum gain of the 
audio signal 48 over the filtered audio signal 26 is controlled by a 
maximum volume setting 50. Preferably, the maximum gain is limited to 
between -25 decibels and 15 decibels. An externally adjustable volume 
control setting 52 provides for reducing the maximum gain of the audio 
signal 48 in accordance with user preference. 
The volume-adjusted audio signal 48 drives a receiver 54 that converts the 
audio signal 48 into sound waves. Preferably, the receiver 54, like the 
microphone 10, is of highest quality to accurately reproduce a wide range 
of frequency responses. 
The microphone 10, the amplifiers 14, 18, and 46, the receiver 54, and the 
shell (not shown) preferably produce, independently of the state variable 
filter 24, a bow-shaped frequency response curve such as the curve 56 
shown in FIG. 3. The curve 56 is shaped similar to the frequency response 
curve of a conventional prescription fit for no hearing loss. This allows 
the three settings 40, 42, and 44 of the state variable filter to match 
the overall frequency response of the hearing aid to most patterns of 
hearing loss. 
The state variable filter 24 is preferably constructed from a series of 
biquadratic filter stages that are optionally cascaded together within a 
single channel circuit. FIG. 4 illustrates a particular biquadratic filter 
stage 60, and FIG. 5 illustrates how this filter stage can be optionally 
cascaded together with two other identical biquadratic filter stages 62 
and 64 for producing a higher order filter. 
Each of the biquadratic filter stages exhibits a general transfer function 
"H(s)" as follows: 
##EQU2## 
where "s" is a complex frequency equal to j [2 pi f] (with "j" being an 
imaginary number equal to the square root of -1, with "pi" being the ratio 
of the circumference of a circle to its diameter, and with "f" being 
frequency measured in hertz); "W.sub.z " is the corner frequency 36, now 
representing a "zero" of the transfer function in angular measure; 
"W.sub.p " is the corner frequency 38, now representing a "pole" of the 
transfer function in angular measure; and "Q.sub.z " and "Q.sub.p " are 
referred to as "quality factors" or "inverse dampening factors" of the 
zero and pole, respectively. 
The illustrated biquadratic filter 60 includes seven operational 
transconductance amplifiers labeled, "g.sub.ml ", "g.sub.m2 ", "g.sub.m3 
", g.sub.m4 ", "g.sub.m5a ", "g.sub.m5b ", "g.sub.m5c ". Each 
transconductance amplifier includes two inputs that produce a differential 
voltage. The transconductance gain of each amplifier is multiplied by the 
differential voltage to produce an output current. Capacitors "C.sub.1 " 
and "C.sub.2 " continuously sum outputs of the transconductance 
amplifiers. 
The output of the filter circuit as a model of the transfer function H(s) 
is given below: 
##EQU3## 
where "V.sub.o " and "V.sub.i " are the respective output and input 
voltages shown in FIG. 4; "C.sub.1 " and "C.sub.2 " are the respective 
capacitances of the like-labeled capacitors; "g.sub.m1 ", "g.sub.m2 ", 
"g.sub.m3 ", and "g.sub.m4 " are the transconductance gains of the 
amplifiers labeled the same; and "g.sub.m5 " is the effective 
transconductance gain of the three amplifiers labeled "g.sub.m5a ", 
"g.sub.m5b ", "g.sub.m5c " according to the following relationship: 
##EQU4## 
Relating the particular transfer function of the circuit shown in FIG. 4 to 
the general transfer function of a biquadratic filter yields the following 
equations for the corner frequencies "W.sub.z " and "W.sub.p " and quality 
factors "Q.sub.z " and "Q.sub.p ": 
##EQU5## 
Preferably, all of the amplifiers are identical, and the values of the 
quality factors "Q.sub.z " and "Q.sub.p " are set by the capacitances 
"C.sub.1 " and "C.sub.2 " at nominal values of the corner frequencies 
"W.sub.z " and "W.sub.p ". The values of the quality factors "Q.sub.z " 
and "Q.sub.p " are both preferably set at approximately 0.707 to provide 
for a maximum change in curvature at the corner frequencies without 
producing any peaks. 
The corner frequencies "W.sub.z " and "W.sub.p " are set within an 
adjustment circuit by changing the respective values of resisters "R.sub.1 
" and "R.sub.2 ", which correspond to the settings 40 and 42. Conventional 
laser trimming techniques are preferably used for this purpose. The 
resister "R.sub.1 " varies the value of three identical currents "I.sub.1a 
", "I.sub.1b ", and "I.sub.1c " that are replicated by a current mirror 66 
for controlling the transconductance gains of respective amplifiers 
"g.sub.m5a", "g.sub.m5b ", and "g.sub.m4 ". Since the gain "g.sub.m5 " is 
determined in part by the product of the gains "g.sub.m5a " and "g.sub.m5b 
", the gain "g.sub.m5 " varies as the square of the variation in the gain 
"g.sub.m4 ". This allows variations in the resistance of resister "R.sub.1 
" to set the zero corner frequency "W.sub.z " at different values without 
changing the value of the quality factor "Q.sub.z ". 
The resister "R.sub.2 " is trimmed by the same technique to vary the value 
of four other identical currents "I.sub.2a ", "I.sub.2b ", "I.sub.2c ", 
and "I.sub.2d " that are replicated by a current mirror 68 The currents 
"I.sub.2a ", "I.sub.2b ", "I.sub.2c ", and "I.sub.2d " control the 
transconductance gains of respective amplifiers "g.sub.m5c ", "g.sub.m1 ", 
"g.sub.m2 ", and "g.sub.m3 ". The pole corner frequency "W.sub.p " is set 
to a desired value by varying the gains "g.sub.m1 " and "g.sub.m2 ".The 
quality factor "Q.sub.p " is unchanged because the product of the gains 
"g.sub.ml " and "g.sub.m2 " varies as the square of the gain "g.sub.m3 ". 
In addition, the variation in gain "g.sub.m5c " cancels the effect of the 
variation in the gain "g.sub.m2 " on the values of the zero corner 
frequency "W.sub.z " and the quality factor "Q.sub.z ". 
FIG. 5 illustrates the biquadratic filter stage 60 optionally cascaded in 
series with the identical biquadratic filter stages 62 and 64. The integer 
"n" is equal to the number of filter stages that are cascaded in series 
from zero to three. This selection is implemented within an adjustment 
circuit by a one-of-four selector switch 70 that is set to one of its four 
positions by optional cuts made across two conductor paths "Si" and 
"S.sub.2 " which correspond to the setting 44. 
In a first position corresponding to "n" equal to zero, line 72 bypasses 
all three filter stages 60, 62, and 64, thereby directly interconnecting 
the audio signals 20 and 26. In the three remaining positions 
corresponding to "n" equal to 1 through 3, a number of the filter stages 
corresponding to the integer "n" are connected in series for configuring 
the state variable filter 24 in different exponential orders. Each filter 
stage is identical, and the connection of filter stages raises the second 
order transfer function "H(s)" by the power of "n". 
For example, the second position of the selector switch 70, corresponding 
to the selection of "n" equal to one, opens line 74 and connects only the 
filter stage 60 between the audio signals 20 and 26 for configuring the 
state variable filter as a second order filter. In the third position, 
corresponding to the selection of "n" equal to two, filter stages 60 and 
62 are incorporated by line 76 in a fourth order state variable filter. 
Finally, the fourth position, corresponding to the selection of "n" equal 
to three, connects all three filter stages 60, 62, and 64 in series along 
line 78 forming a sixth order state variable filter. 
Since all three filter structures 60, 62, and 64 are identical, the corner 
frequencies "W.sub.z " and "W.sub.p " of the response curve of the state 
variable filter 24 are not affected by the changes in filter order. 
However, the selected filter order equal to twice "n" is effective for 
controlling the roll-off rate between the two corner frequencies. For 
example, the roll-off rate at "n" equal to one is 12 decibels per octave, 
whereas respective roll-off rates of 24 decibels per octave and 36 
decibels per octave are achieved at values of "n" equal to two and three. 
The three variables "W.sub.z " "W.sub.p " and "n" which vary the shape of 
the response curve of the state variable filter 24, are incorporated in a 
conventional "least squares"algorithm with the maximum volume setting 50 
for best fitting the overall frequency response of our hearing aid to 
prescription requirements. The choice of these variables permits most 
patterns of hearing loss to be closely matched to the prescription 
requirements with a single configuration of components. The single channel 
within which the prescription requirements are met supports a high 
fidelity response within both high and low ranges of the audible frequency 
spectrum. 
Procedures for fitting our new hearing aid to prescription requirements are 
greatly simplified. For example, physicians and audiologists can prescribe 
desired hearing aid amplification characteristics based on audiological 
assessments without choosing among large arrays of components for 
achieving these characteristics, or selecting among a limited array of 
available frequency responses. Instead, a best fit of our hearing aid to 
prescribed requirements is made during manufacture. 
Preferably, the dispensing physicians and audiologists are only required to 
provide audiological assessments of hearing impairments, and the 
manufacturer processes these assessments with a known prescription 
algorithm to determine desired amplification characteristics at 
predetermined frequencies. For example, a prescription algorithm (NAL-R) 
proposed by National Acoustic Laboratories of Chatswood, N.S.W., 
Australia, could be programmed for use by the manufacturer. 
However, prior to fitting individual prescriptions, amplification 
characteristics of the hearing aid components including the microphone 10 
and the receiver 54 are determined at the predetermined frequencies. A 
conventional curve fitting technique is used to match the response curve 
of the state variable filter 24 to a difference between the collective 
amplification characteristics of the hearing aid components and the 
desired amplification characteristics. Laser trimming techniques are used 
to permanently set the automatic gain control threshold 22, the two corner 
frequencies 40 and 42, the roll-off rate 44, and the maximum volume 50. 
Our hearing aid circuit is preferably implemented on a microelectronic chip 
that is sized to fit within a shell of an in-the-ear (ITE) hearing aid. 
The frequency response of the hearing aid is permanently set during 
manufacture, and only the volume control setting 52 is left for 
adjustment. The permanent settings are made by laser trimming techniques 
to allow precise, fast, and programmable adjustments during manufacture. 
An iterative procedure involving a least squares fit is used to calculate 
the settings required to best match the frequency response of our hearing 
aid to prescribed requirements. 
Our hearing aid circuit also preferably includes a noise suppression 
circuit that filters out low-frequency noise. A noise suppression circuit 
appropriate for this purpose is disclosed in copending application Ser. 
No. 861,301, filed Mar. 13, 1992, entitled "Aid to Hearing Speech in a 
Noisy Environment". The disclosure of this copending application is hereby 
incorporated by reference. 
Although only the corner frequencies "W.sub.z " and "W.sub.p " and the 
roll-off rate "n" of the filter transfer function are varied in the above 
example, it would also be possible to vary the quality factors "Q.sub.z " 
and "Q.sub.p " of the same or similar transfer function to match more 
unusual patterns of hearing loss. The required transfer function can also 
be implemented in a variety of other configurations.