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Claims  |
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What is claimed is:
1. A hearing aid apparatus comprising:
an omnidirectional microphone for converting sound waves to electrical
signals:
a directional microphone of at least the first order for converting sound
waves into electrical signals, said electrical signals of said directional
microphone having low, mid, and high frequency components;
an equalization amplifier accepting said electrical signals from said
directional microphone for at least partially equalizing the amplitude of
said low frequency electrical signal components of said electrical signal
of said directional microphone with the amplitude of said mid and high
frequency electrical signal components of said electrical signals of said
directional microphone, said equalization amplifier having an equalized
electrical signal output;
a hearing aid amplifier for amplifying electrical signals received at an
input thereof; and
switch means including automatic means for automatically switching between
first and second switching states in response to sensed ambient noise
levels, said switch means connecting said electrical signal from said
omnidirectional microphone to said input of said hearing aid amplifier
when said switch means is in said first switching state, said .switch
means connecting said equalized electrical signal from said equalization
amplifier to said input of said hearing aid amplifier when said switching
means is in said second switching state, said automatic means comprising:
noise sensing means for sensing ambient noise and generating an output
signal indicative of the level of said ambient noise;
a comparator for comparing the amplitude of said output signal of said
noise sensing means with the amplitude of a reference signal, said
reference signal being indicative of a reference ambient noise level at
which said switch means is to switch between said first and second switch
states, said comparator having an output signal indicative of whether said
ambient noise level is above or below said reference ambient noise level;
a first switch disposed between said electrical signal of said
omnidirectional microphone and said hearing aid, said first switch
responsive to said output signal of said comparator to through-connect
said electrical signal to said hearing aid amplifier when said ambient
noise level falls to a level below said reference ambient noise level,
said first switch responsive to said output signal of said comparator to
disconnect said electrical signal from said hearing aid amplifier when
said ambient noise level rises to a level above said reference ambient
noise level; and
a second switch disposed between said equalized electrical signal of said
equalizer and said hearing aid, said second switch responsive to said
output signal of said comparator to through-connect said equalized
electrical signal to said hearing aid amplifier when said ambient noise
level rises to a level above said reference ambient noise level, said
second switch responsive to said output signal of said comparator to
disconnect said equalized electrical signal from said hearing aid
amplifier when said ambient noise level falls to a level below said
reference ambient noise level.
2. A hearing aid apparatus as claimed in claim 1 wherein said first switch
comprises:
at least one series pass FET connected between said electrical signal and
said hearing aid amplifier;
an inverter having an input connected to receive said output signal of said
comparator and an output connected to control the resistance of said at
least one series pass FET.
3. A hearing aid apparatus as claimed in claim 1 wherein said second switch
comprises at least one series pass FET connected between said equalized
electrical signal and said hearing aid amplifier.
4. A hearing aid apparatus comprising:
an omnidirectional microphone for converting sound waves to electrical
signals;
a directional microphone of at least the first order for converting sound
waves into electrical signals, said electrical signals of said directional
microphone having low, mid, and high frequency components;
an equalization amplifier accepting said electrical signals from said
directional microphone for at least partially equalizing the amplitude of
said low frequency electrical signal components of said electrical signal
of said directional microphone with the amplitude of said mid and high
.frequency electrical signal components of said electrical signals of said
directional microphone, said equalization amplifier having an equalized
electrical signal output;
a hearing aid amplifier for amplifying electrical signals received at an
input thereof; and
switch means including automatic means for automatically switching between
first and second switching states in response to sensed ambient noise
levels, said switch means connecting said electrical signal from said
omnidirectional microphone to said input of said hearing aid amplifier
when said switch means is in said first switching state, said switch means
connecting said equalized electrical signal from said equalization
amplifier to said input of said hearing aid amplifier when said switching
means is in said second switching state, said automatic means comprising:.
noise sensing means for sensing ambient noise and generating an output
signal indicative of the level of said ambient noise; and
fader means responsive to said output signal of said noise sensing means
for gradually decreasing the relative amplitude of said equalized signal
supplied to said hearing aid amplifier from said equalizer while gradually
increasing the relative amplitude of said electrical signal supplied to
said hearing aid amplifier from said omnidirectional microphone as said
switching means transitions from said first switching state toward said
second switching state, and for gradually increasing the relative
amplitude of said equalized signal supplied to said hearing aid amplifier
from said equalizer while gradually decreasing the relative amplitude of
said electrical signal supplied to said hearing aid amplifier from said
omnidirectional microphone as said switching means transitions from said
second switching state toward said first switching state, said switching
means transitioning from said first switching state toward said second
switching state as the level of said sensed ambient noise increases and
transitioning from said second switching state toward said first switching
state as said sensed ambient noise decreases.
5. A hearing aid apparatus as claimed in claim 4 wherein the voltage of the
signal supplied to the input of said hearing aid is a monotonic function
of the sound pressure level at said microphones.
6. A hearing aid apparatus as claimed in claim 4 wherein said noise sensing
means comprises:
an amplifier connected to amplify said electrical signal from said
omnidirectional microphone; and
a logarithmic rectifier for logarithmically rectifying said amplified
electrical signal of said amplifier to generate a logarithmically
rectified signal.
7. A hearing aid apparatus as claimed in claim 6 wherein said fader means
comprises:
a first series pass FET connected between said equalized electrical signal
and said hearing aid amplifier;
an inverting amplifier for inverting said logarithmically rectified signal
to generate an inverted logarithmically rectified signal output, said
first series pass FET responsive to said inverted logarithmically
rectified signal to control the resistance of said first series pass FET;
a second series pass FET connected between said electrical signal of said
omnidirectional microphone and said hearing aid amplifier, said second
series pass FET responsive to said logarithmically rectified signal to
control the resistance of said second series pass FET.
8. A hearing aid apparatus comprising:
an omnidirectional microphone for converting sound waves to electrical
signals;
a directional microphone of at least the first order for converting sound
waves into electrical signals, said electrical signals of said directional
microphone having low, mid, and high frequency components;
an equalization amplifier accepting said electrical signals from said
directional microphone for at least partially equalizing the amplitude of
said low frequency electrical signal components of said electrical signal
of said directional microphone with the amplitude of said mid and high
frequency electrical signal components of said electrical signals of said
directional microphone, said equalization amplifier having an equalized
electrical signal output;
a hearing aid amplifier for amplifying electrical signals received at an
input thereof;
a noise sensing circuit for sensing ambient noise and generating an output
signal indicative of the level of said ambient noise;
a fader circuit responsive to said output signal of said noise sensing
circuit for gradually decreasing the relative amplitude of said equalized
signal supplied to said hearing aid amplifier from said equalizer while
gradually increasing the relative amplitude of said electrical signal
supplied to said hearing aid amplifier from said omnidirectional
microphone as the level of said ambient noise decreases, and for gradually
increasing the relative amplitude of said equalized signal supplied to
said hearing aid amplifier from said equalizer while gradually decreasing
the relative amplitude of said electrical signal supplied to said hearing
aid amplifier from said omnidirectional microphone as the level of said
ambient noise increases.
9. A hearing aid apparatus as claimed in claim 8 wherein the voltage of the
signal supplied to the input of said hearing aid is a monotonic function
of the sound pressure level at said microphones.
10. A hearing aid apparatus as claimed in claim 8 wherein said noise
sensing circuit comprises:
an amplifier connected to amplify said electrical signal from said
omnidirectional microphone; and
a logarithmic rectifier for logarithmically rectifying said amplified
electrical signal of said amplifier to generate a logarithmically
rectified signal.
11. A hearing aid apparatus as claimed in claim 10 wherein said fader
circuit comprises:
a first series pass FET connected between said equalized electrical signal
and said hearing aid amplifier;
an inverting amplifier for inverting said logarithmically rectified signal
to generate an inverted logarithmically rectified signal output, said
first series pass FET responsive to said inverted logarithmically
rectified signal to control the resistance of said first series pass FET;
a second series pass FET connected between said electrical signal of said
omnidirectional microphone and said hearing aid amplifier, said second
series pass FET responsive to said logarithmically rectified signal to
control the resistance of said second series pass FET.
12. A hearing aid apparatus as claimed in claim 8 wherein said directional
microphone is a second order directional microphone.
13. A hearing aid apparatus as claimed in claim 12 wherein said second
order directional microphone comprises:
a first order directional gradient microphone having first and second
spaced apart sound ports, sound waves received at said first and second
sound ports being converted to an electrical signal output;
a further first order directional gradient microphone having first and
second spaced apart sound ports, sound waves received at said first and
second sound ports being converted to an electrical signal output, said
further first order directional microphone being disposed adjacent said
first order directional microphone;
a subtracter circuit for electrically subtracting said electrical signal of
said first order directional microphone from said electrical signal of
said further first order directional microphone to generate said
electrical signal of said second order directional microphone.
14. A hearing aid apparatus as claimed in claim 13 and further comprising a
face plate, said first order directional gradient microphone and said
further first order directional gradient microphone being disposed on said
face plate so that all of said sound ports are generally collinear.
15. A hearing aid apparatus as claimed in claim 14 and further comprising:
a first diffraction scoop disposed on said face plate at said first sound
port of said first order directional gradient microphone; and
a second diffraction scoop disposed on said face plate at said second sound
port of said further first order directional microphone.
16. A hearing aid apparatus as claimed in claim 15 and further comprising a
wind screen disposed over said face plate and said diffraction scoops.
17. A hearing aid apparatus as claimed in claim 16 wherein said wind screen
is in the form of a porous screen.
18. A hearing aid apparatus as claimed in claim 16 wherein said wind screen
is in the form of a multiply porous housing.
19. A hearing aid apparatus as claimed in claim 13 wherein said second
sound port of said first order directional microphone and said first sound
port of said further first order microphone are joined together to form a
common sound port.
20. A method of operating a hearing aid apparatus comprising the steps of:
providing said hearing aid apparatus with an omnidirectional microphone for
converting sound waves to an electrical signal;
providing said hearing aid apparatus with a directional microphone of at
least a first order for converting sound waves into an electrical signal,
said electrical signal of said directional microphone having low, mid, and
high frequency components;
at least partially equalizing the amplitude of said low frequency
electrical signal components of said electrical signal of said directional
microphone with said mid and high frequency electrical signal components
of said electrical signals of said directional microphone to generate an
equalized electrical signal;
sensing the ambient noise level;
connecting said electrical signal of said omnidirectional microphone for
supply to an input of a hearing aid amplifier;
connecting said equalized electrical signal for supply to said input of
said hearing aid amplifier;
gradually decreasing the relative amplitude of said equalized signal
supplied to said input of said hearing aid amplifier while gradually
increasing the relative amplitude of said electrical signal supplied to
said input of said hearing aid amplifier from said omnidirectional
microphone as the level of said ambient noise decreases;
gradually increasing the relative amplitude of said equalized signal
supplied to said input of said hearing aid amplifier from said equalizer
while gradually decreasing the relative amplitude of said electrical
signal supplied to said input of said hearing aid amplifier from said
omnidirectional microphone as the level of said ambient noise increases.
21. A method of operating a hearing aid apparatus as claimed in claim 38
wherein said step of providing said hearing aid apparatus with a
directional microphone is further defined by providing said hearing aid
apparatus with a second order directional microphone for converting sound
waves to said electrical signal. |
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Claims |
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Description |
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FIELD OF THE INVENTION
This invention relates to improvements in the use of directional
microphones for hearing aids that are used in circumstances where the
background noise renders verbal communication difficult. More
particularly, the present invention relates to a microphone switching
system for such a hearing aid.
BACKGROUND OF THE INVENTION
Individuals with impaired hearing often experience difficulty understanding
conversational speech in background noise. What has not heretofore been
well understood is that the majority of daily conversations occur in
background noise of one form or another. In some cases, the background
noise may be more intense than the target speech, resulting in a severe
signal-to-noise ratio problem. In a study of this signal-to-noise problem,
Preasons et al, "Speech levels in various environments," Bolt Beranek and
Newman report No. 3281, Washington, D.C., October 1976, placed a head-worn
microphone and tape recorder on several individuals and sent them about
their daily lives, obtaining data in homes, automobiles, trains,
hospitals, department stores, and airplanes. They found that nearly 1/4 of
the recorded conversations took place in background noise levels of 60 dB
sound pressure level (SPL) or greater, and that nearly all of the latter
took place with a signal-to-noise ratio between -5 dB and +5 dB. (A
signal-to-noise ratio of -5 dB means the target speech is 5 dB less
intense than the background noise.) As discussed in a review by Mead
Killion, "The Noise Problem: There's hope," Hearing Instruments Vol. 36,
No. 11, 26-32 (1985), people with normal hearing can carry on a
conversation with a -5 dB signal-to-noise ratio, but those with hearing
impairment generally require something like +10 dB. Hearing impaired
individuals are thus excluded from many everyday conversations unless the
talker raises his or her voice to an unnatural level. Moreover, the
evidence of Carhart and Tillman, "Interaction of competing speech signals
with hearing losses," Archives of Otolaryngology, Vol. 91, 273-9 (1970),
indicates that hearing aids made the problem even worse. More recent
studies by Hawkins and Yacullo, "Signal-to-noise ratio advantage of
binaural hearing aids and directional microphones under different levels
of reverberation," J. Speech and Hearing Disorders, Vol. 49, 278-86
(1984), have shown that hearing aids can now help, but still leave the
typical hearing aid wearer with a deficit of 10-15 dB relative to a
normal-hearing person's ability to hear in noise.
One approach to the problem is the use of digital signal processors such as
described in separate papers by Harry Levitt and Birger Kollmeier at the
15th Danavox Symposium "Recent development in hearing instrument
technology," Scanticon, Kolding, Denmark, March 30 through Apr. 2, 1993
(to be published as the Proceedings of the 15th Danavox Symposium). This
approach, using multiple microphones and high-speed digital processors,
provide a few dB improvement in signal-to-noise ratio. The approach,
however, requires very large research expenditures, and, at present, large
energy expenditures. It is estimated that the processor described by
Levitt would require 40,000 hearing aid batteries per week to keep it
powered up. One of the approaches described by Kollmeier operated at 400
times slower than real time, indicating 400 SPARC processors operating
simultaneously would be required to obtain real-time operation, for an
estimated expenditure of 60,000 hearing aid batteries per hour. Such
digital signal processing schemes therefore hold little immediate hope for
the hearing aid user.
First-order directional microphones have been used in behind-the-ear
hearing aids to improve the signal-to-noise ratio by rejecting a portion
of the noise coming from the sides and behind the listener. Carlson and
Killion, "Subminiature directional microphones", J. Audio Engineering
Society, Vol. 22, 92-6 (1974), describe the construction and application
of such a subminiature microphone suitable for use in behind-the-ear
hearing aids. Hawkins and Yacullo (see above) found that such a microphone
could improve the effective signal-to-noise ratio by 3-4 dB.
First-order directional microphones, however, are not without their
drawbacks when utilized in the in-the-ear hearing aids employed by some
75% of hearing aid wearers. The experimental sensitivity of a first-order
directional microphone is typically 6-8 dB less when mounted in an
in-the-ear hearing aid compared to its sensitivity in a behind-the-ear
mounting. These results come about because of the shortened distance
available inside the ear and the effect of sound diffraction about the
head and ear. An additional problem with directional microphones in
head-worn applications is that the improvement they provide over the
normal omni-directional microphone is less than occurs in free-field
applications because the head and pinna of the ear provide substantial
directionality at high frequencies. Thus in both behind-the-ear and
in-the-ear applications, the directivity index (ratio of sensitivity to
sound from the front to the average sensitivity to sounds from all
directions) might be 4.8 dB for a first-order directional microphone
tested in isolation and 0 dB for an omnidirectional microphone tested in
isolation. When mounted on the head, however, the omnidirectional
microphone might have a directivity index of 3 dB at high frequencies and
the directional microphone perhaps 5.5 dB. As a result, the improvement in
the head-mounted case is 2.5 dB.
An approach exploiting microphone directional sensitivity was pursued by
Wim Soede. That approach utilizes 5-microphone directional arrays suitable
for head-worn applications. The array and its theoretical description are
described in his Ph.D. dissertation "Development and evaluation of a new
directional hearing instrument based on array technology," Gebotekst
Zoetermeet/1990, Delft University of Technology, Delft, The Netherlands.
The array provided a directivity index of 10 dB or greater. The problem
with this array approach is that the Soede array is 10 cm long, requiring
eyeglass-size hearing aids. It is certainly not practical for the
in-the-ear hearing aids most often used in the United States. While there
may be many individuals whose loss is so severe that the improved
signal-to-noise obtained with such a head-worn array would make it
attractive, a majority of hearing aid wearers would find the size of the
array unattractive.
Second-order directional microphones are more directionally sensitive than
their first order counterparts. Second-order directional microphones,
however, have always been considered impractical because their sensitivity
is so low. The frequency response of a first-order directional microphone
falls off at 6 dB/octave below about 2 kHz. The frequency response of a
second-order directional microphone falls off at 12 dB/octave below about
2 kHz. At 200 Hz, therefore, the response of a second-order directional
microphone is 40 dB below that of it's comparable omni-directional
microphone. If electrical equalization is used to restore the
low-frequency response, the amplified microphone noise will be 40 dB
higher. The steady hiss of such amplified microphone noise is
objectionable in a quiet room, and hearing aids with equivalent noise
levels more than about 10-15 dB greater than that obtained with an
omni-directional microphone have been found unacceptable in the
marketplace. For similar reasons, first order microphones have likewise
not gained wide acceptance for use in hearing aids.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved speech
intelligibility in noise to the wearer of a small in-the-ear hearing aid.
It is a further object of the present invention to provide the necessary
mechanical and electrical components to permit practical and economical
second-order directional microphone constructions to be used in head-worn
hearing aids.
It is a still further object of the present invention to provide a
switchable noise-reduction feature for a hearing aid whereby the user may
switch to an omni-directional microphone for listening in quiet or to
music concerts, and then switch to a highly-directional microphone in
noisy situations where understanding of conversational speech or other
signals would otherwise be difficult or impossible.
It is a still further object of the present invention to provide an
automatic switching function which, when activated, will automatically
switch from the omni-directional microphone to a directional microphone
whenever the ambient noise level rises above a certain predetermined
value, such switching function taking the form of a "fader" which smoothly
attenuates one microphone and brings up the sensitivity on the other over
a range of overall sound levels so that no click or pop is heard.
These and other objects of the invention are obtained in a hearing aid
apparatus that employs both an omnidirectional microphone and at least one
directional microphone of at least the first order. The electrical signals
output from the directional microphone are supplied to an equalization
amplifier which at least partially equalizes the amplitude of the low
frequency electrical signal components with the amplitude of the mid and
high frequency electrical signal components of the directional microphone.
A switching circuit accepts the signals output from both the
omnidirectional microphone and the directional microphone. The switching
circuit connects the signal from the omnidirectional microphone to an
input of a hearing aid amplifier when the switching circuit is in a first
switching state, and connects the output of the equalization circuit to
the hearing aid amplifier input when the switching circuit is in a second
switching state.
Several switching circuit embodiments are set forth. In one embodiment, the
switching circuit is manually actuatable by a wearer of the hearing aid.
In a further embodiment, the switching circuit is operated automatically
in response to the level of sensed ambient noise to switch directly
between the first and second switching states. In a still further
embodiment, the switching circuit is operated automatically as a fader
circuit in response to the level of sensed ambient noise to gradually
switch between the first and second states thereby providing a gradual
transition between the microphones.
In a further embodiment of the invention three different types of
microphones are employed: an omnidirectional microphone, a first order
microphone, and a second order microphone. The microphone outputs are
gradually switched to the input of the hearing aid amplifier in response
to the sensed level of ambient noise.
In one embodiment of the invention, the directional microphone is of the
second order. The second order microphone is constructed from two first
order gradient microphones that have their output signals subtracted in a
subtracter circuit. The output of the subtracter circuit provides a second
order directional response. Optionally, diffraction scoops may be disposed
over the sound ports of the first order gradient microphones to increase
their sensitivity. Hearing aid performance may be further increased by
employing a windscreen in addition to the diffraction scoops.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and features of the present invention may be further
understood by reference to the following detailed description of the
preferred embodiment of the invention taken in conjunction with the
accompanying drawings, on which:
FIG. 1 is a schematic block diagram of one embodiment of a hearing aid
apparatus constructed in accordance with the teachings of the invention;
FIG. 2 is a polar chart showing the directional response of an
omnidirectional microphone;
FIG. 3 is a graph of the frequency response of an omnidirectional
microphone, a first order directional microphone, and a second order
directional microphone;
FIG. 4 is a polar chart showing a directional response of one type of first
order directional microphone having cardioid directivity;
FIG. 5 is a polar chart showing a directional response of one type of a
second order directional microphone;
FIG. 6 is a schematic block diagram of a hearing aid apparatus of the
invention that utilizes two first order directional microphones to produce
a second order directional response;
FIG. 7 is a more detailed circuit diagram of the circuit of FIG. 6;
FIG. 8 is a schematic diagram of a hearing aid apparatus having automatic
ambient-noise-level dependent switching between microphones;
FIG. 9 is a schematic diagram of a hearing aid apparatus having automatic
ambient-noise-level dependent switching between microphones wherein the
switching is performed by a fader circuit;
FIGS. 10-12 are graphs showing various signals of the circuit of FIG. 9 as
a function of sound pressure level;
FIGS. 13-15 are schematic block diagrams of various constructions of a
hearing aid apparatus and its associated components employing automatic
switching between an omnidirectional microphone, a first order directional
microphone, and a second order directional microphone;
FIGS. 16 and 17 are cross sectional views showing the mechanical
construction of various microphones suitable for use in the various
hearing aid embodiments set forth herein;
FIG. 18 is a perspective view of a hearing aid constructed in accordance
with the invention as inserted into an ear;
FIG. 19 is a cross sectional view showing certain mechanical structures of
one embodiment of a hearing aid in accordance with the invention;
FIG. 20 is a perspective view showing an alternate mechanical construction
of the second order microphone shown in FIG. 19; and
FIG. 21 is a front view of the diffraction scoop used in FIG. 19.
It will be understood that the drawings are not necessarily to scale. In
certain instances, details which are not necessary for understanding
various aspects of the present invention have been omitted for clarity.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A hearing aid apparatus constructed in accordance with one embodiment of
the invention is shown generally at 10 of FIG. 1. As illustrated, the
hearing aid apparatus 10 utilizes both an omnidirectional microphone 15
and a directional microphone 20 of at least the first order. Each of the
microphones 15,20 is used to convert sound waves into electrical output
signals corresponding to the sound waves.
The free space directional response of a typical omnidirectional microphone
is shown by line 21 in FIG. 2 while the corresponding frequency response
of such a microphone is shown by line 25 of FIG. 3. The directional and
frequency response of a typical omnidirectional microphone make it quite
suitable for use in low noise environments when it is desirable to hear
sound from all directions. Such an omnidirectional microphone is
particularly suited for listening to a music concert or the like.
The free space directional response of one type of a first order
directional microphone is set forth by line 26 in FIG. 4 and the
corresponding frequency response is shown by line 30 of FIG. 2. As
illustrated, the first order directional microphone tends to reject sound
coming from the side and rear of the hearing aid wearer. As such, the
directivity of a first-order directional microphone may be used to improve
the signal-to-noise ratio of the hearing aid since it rejects a portion of
the noise coming from the sides and behind the hearing aid wearer. The
first order directional microphone, however, experiences decreased
sensitivity to low frequency sound waves, sensitivity dropping off at a
rate of 6 dB per octave below approximately 2 KHz.
The free space directional response of one type of a second order
directional microphone is set forth by line 31 in FIG. 5 and the
corresponding frequency response is shown by line 35 of FIG. 2. As
illustrated, the second order directional microphone is even more
directional than the first order microphone and, as such, tends to improve
the signal-to-noise ratio of the hearing aid to an even greater degree
than the first order microphone. The second order directional microphone,
however, is even less sensitive to low frequency sound waves than its
first order counterpart, sensitivity dropping off at a rate of 12 dB per
octave below approximately 2 KHz.
Referring again to FIG. 1, the output of the directional microphone 20 is
AC coupled to the input of an equalizer circuit 40 through capacitor 45.
The equalizer circuit 40 at least partially equalizes the amplitude of the
low frequency components of the electrical signal output from the
directional microphone 20 with the amplitude of the mid and high frequency
components of the electrical signal output. This equalization serves to
compensate for the decreased sensitivity that the directional microphone
provides at lower frequencies. The equalizer circuit 40 provides the
equalized signal at output line 50.
As explained above, the equalizer circuit 40 raises the noise level of the
hearing aid system. The noise level is significantly raised when a second
order microphone is equalized. This noise is quite noticeable to the
hearing aid wearer when the hearing aid is used in low ambient noise
situations, but tends to become masked in high ambient noise level
situations. It is in high ambient noise level situations that the
directionality of the directional microphone is most useful for increasing
the signal to noise ratio of the hearing aid system. Accordingly, the
equalized electrical signal output from the equalizer circuit 40 and the
electrical signal output from the omnidirectional microphone 15 are
supplied to opposite terminals of a SPDT switch 55 that has its pole
terminal connected to the input of a hearing aid amplifier 60. The
electrical signal output from omnidirectional microphone 15 is AC coupled
through capacitor 62. The hearing aid amplifier 60 may be of the type
shown and described in U.S. Pat. No. 5,131,046, to Killion et al, the
teachings of which are hereby incorporated by reference.
The SPDT switch 55 has at least two switching states. In a first-switching
state, the electrical signal from the omnidirectional microphone 15 is
connected to the input of the hearing aid amplifier 60 to the exclusion of
the equalized signal from the equalizer circuit 40. In a second switching
state, the equalized electrical signal from the equalizer circuit 40 is
connected to the input of the hearing aid amplifier 60 to the exclusion of
the electrical signal from the omnidirectional microphone 15. Microphone
selection, such as is disclosed herein, allows optimization of the
signal-to-noise ratio of the hearing aid system dependent on the ambient
noise conditions. As will be set forth in more detail below, such
selection can be done either manually or automatically.
FIG. 6 shows another embodiment of a hearing aid system 10. The hearing aid
system 10 employs two first-order directional microphones 65 and 70. The
electrical signal output of directional microphone 70 is AC coupled to the
positive input of a summing circuit 75 while the electrical signal output
of directional microphone 65 is AC coupled to the negative input of the
summing circuit 75. The directional microphones 65,70 have matched
characteristics. The resultant electrical signal output on line 80 of the
summing circuit 75 has second order directional and frequency response
characteristics and is supplied to the input of the equalizer circuit 40.
A more detailed schematic diagram of the system shown in FIG. 6 is given in
FIG. 7. As illustrated, the electrical signal output of first order
directional microphone 65 is AC coupled through capacitor 85 to the input
of an inverting circuit, shown generally at 90. The inverting circuit 90
includes an inverting amplifier 95, resistors 100 and 105, and balance
resistor 110. The electrical signal output of first order microphone 70 is
AC coupled through capacitor 115 to resistor 120 which, in turn, is
connected to supply the electrical signal output to summing junction 80.
The signal at summing junction 80 is supplied to the input of the equalizer
circuit 40. The equalizer circuit 40 includes inverting amplifier 125,
resistors 130 and 135, and capacitor 140. The equalized electrical signal
output from the equalizer circuit 40 is supplied to switch 55 on line 145.
The components of the embodiment shown in FIG. 7 may have the following
values and be of the following component types:
______________________________________
Component Description
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100, 105 27K
85, 115 .027MF
110 25K.sub.variable
120 15K
130 100K
135 1M
140 560pf
95, 125 LX 509
of Gennum Corp.
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In an alternative embodiment of the switching system, the SPDT switch 55
can be replaced by an automatic switching system that switches between the
directional microphone and the omnidirectional microphone dependent on
sensed ambient noise levels. Such alternative embodiments are shown in
FIGS. 8 and 9.
The embodiment of FIG. 8 includes a directional microphone 20 of at least
the first order and an omnidirectional microphone 15. The output of
directional microphone 20 is supplied to the input of equalizer circuit 40
through capacitor 45. The equalized output signal from the equalizer is
supplied on output line 50 to an FET switch 150. The output signal from
omnidirectional microphone 15 is supplied through capacitor 62 to a
further FET switch 155.
Each FET switch 150 and 155 includes two complementary FETs 160 and 165
arranged as series pass devices. Where the DC signal level at the input of
hearing aid amplifier 60 is 0 V (such as with the hearing aid amplifier
design set forth in the above-noted U.S. Pat. No. 5,131,046), only a
single FET (i.e., an N-channel FET) need be employed. The FET switches 150
and 155 receive respective control signals from a noise comparison
circuit, shown generally at 170, to control their respective series pass
resistances.
The noise comparison circuit 170 includes a noise sensing circuit portion
and a control circuit portion. The noise sensing circuit portion includes
an amplifier 175 that accepts the electrical output signal from
omnidirectional microphone 15. The amplified output signal is supplied to
the input of a rectifier circuit 180 which rectifies the amplified signal
to provide a DC signal output on line 185 that is indicative of the
ambient noise level detected by omnidirectional microphone 15.
The control circuit portion includes comparator 190 and logic inverter 195.
The DC signal output from the rectifier circuit is supplied to the
positive input of comparator 190 for comparison to a reference signal
V.sub.REF that is supplied to the negative input of the comparator 190.
The output of comparator 190 is a binary signal and is supplied as a
control signal to FET switch 150. The output of the comparator is also
supplied to the input of logic inverter 195, the output of which is
supplied as a control signal to FET switch 155.
In operation, the signal V.sub.REF is set to a magnitude representative of
a reference ambient noise level at which the hearing aid apparatus is to
switch between the directional and omnidirectional microphones 20 and 15.
For example, the signal V.sub.REF can be set to a level representative of
a 65 dB ambient noise level. When the sensed ambient noise level thus
rises above 65 dB, FET switch 150 will have a low series pass resistance
level and will connect the equalized output signal at line 50 to the input
of the hearing aid amplifier 60 while FET switch 155 will have a high
series pass resistance and will effectively disconnect the electrical
signal output of omnidirectional microphone 15 from the input of the
hearing aid amplifier 60. When the ambient noise level drops below 65 dB,
FET switch 155 will have a low series pass resistance level and will
connect the electrical signal output of microphone 15 at line 200 to the
input of the hearing aid amplifier 60 while FET switch 150 will have a
high series pass resistance and will effectively disconnect the equalized
signal output on line 50 from the input of the hearing aid amplifier 60.
To avoid excessive switching at ambient noise levels near 65 dB, the
comparator 190 may be designed to have a certain degree of hysteresis.
The reference signal V.sub.REF may be variable and may be set to a level
that is optimized for the particular hearing aid wearer. To this end,
reference signal V.sub.REF may be supplied from a voltage divider having a
trimmer pot as one of its resistive components (not shown). The trimmer
pot may be adjusted to set the optimal V.sub.REF value.
A further embodiment of a hearing aid apparatus that employs automatic
switching is set forth in FIG. 9. The circuit of FIG. 9 is the same as
that shown in FIG. 8 except that the noise comparison circuit 170 is
replaced with a fader circuit, shown generally at 205.
The fader circuit 205 includes an amplifier 210 connected to receive the
electrical signal output of omnidirectional microphone 15 through
capacitor 62. The amplified signal is supplied to the input of a
logarithmic rectifier 215 such as is shown and described in the
aforementioned U.S. Pat. No. 5,131,046, but with reversed output polarity.
The output of the logarithmic rectifier 215 is supplied as a control
signal VC1 to FET switch 155 and is also supplied to the input of an
inverting amplifier circuit 220 having a gain of 1. Where the output range
of the logarithmic rectifier is insufficient to drive FET switch 155, an
amplifier may be used the output of which would be supplied as the control
signal VC1 and to the input of inverting amplifier circuit 220. The output
of inverting amplifier 220 is supplied as a control signal VC2 to FET
switch 150.
FIG. 10 is a graph of the control voltages VC1 and VC2 as a function of
sound pressure level. As the ambient noise level increases there is an
increase in the sound pressure level at omnidirectional microphone 15.
This causes an increase of the level of control voltage VC1 while
resulting in a corresponding decrease of the level of control voltage VC2.
Similarly, as ambient noise level decreases there is a decrease in the
sound pressure level at omnidirectional microphone 15. This causes an
increase of the level of control voltage VC2 while resulting in a
corresponding decrease of the level of control voltage VC1.
FIG. 11 is a graph of the resistances RS1 and RS2 respectively of FET
switches 155 and 150 as a function of sound pressure level. As the ambient
noise level and, thus, the sound pressure level, increases, there is a
corresponding increase in the series resistance RS1 of FET switch 155 and
a decrease in the series resistance RS2 of FET switch 150. At the input to
the hearing aid amplifier 60, there is thus an increase in the relative
level of the signal received from directional microphone 20 and a decrease
in the relative level of the signal received from the omnidirectional
microphone 15. As the ambient noise level and, thus, the sound pressure
level decreases, there is a corresponding increase in the series
resistance RS2 of FET switch 150 and a decrease in the series resistance
RS1 of FET switch 155. At the input to the hearing aid amplifier 60, there
is thus a decrease in the relative level of the signal received | | |
