Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02385812 2002-09-30
1
A method for controlling the directionality of the sound
receiving characteristic of a hearing aid and a hearing
aid for carrying out the method.
s The present invention relates to a method for
controlling the directionality of the sound receiving
characteristic of a hearing aid comprising spaced apart
first and second sound receiving microphones.
Hearing aids having a directional sound receiving
io characteristic are useful to improve speech perception
in noisy environments, where human speech may be
received simultaneously from different directions, as is
the case e.g. in the noise environment frequently
referred to as cocktail party noise.
i5 With a directional sound receiving characteristic,
e.g. in the shape of a cardioid or super cardioid
characteristic, the speech perception in a hearing aid
is improved by reduced reception of sound coming from
the back of the user, while maintaining the level of
2o sound coming from the area in front of the user.
On the other hand, in environments with only a low
noise level or no significant speech signals, the
hearing aid user will normally prefer an omnidirectional
or spherical sound receiving characteristic, offering
2s the same perception of sound irrespective of the
direction, from which it arrives.
As will be further explained in the following, a
prior art hearing 'aid of the kind defined above,
offering the possibility of changing the sound receiving
3o characteristic between an omnidirectional characteristic
and a directional characteristic of varying shape, has
been disclosed in US-A-5,757,933.
CA 02385812 2002-09-30
2
With this prior art hearing aid operating with an
omnidirectional characteristic only the signal from the
first microphone facing the area in front of the user is
supplied to the signal processor. By manual operation of
s a switch a signal derived from the second microphone
facing the rear of the user and subjected to inversion
followed by adjustable phase delay and adjustable
attenuation is combined via a summing node with the
signal derived from the first microphone. When the
io sound receiving characteristic in a hearing~~aid of this
type is changed or changes from the omnidirectional to
a directional shape, the arrival time of the sound
changes during the transition. This change of phase or
time delay may become confusing in a binaural hearing
is aid system using a pair of separate hearing aids
operating with independent and automatic change of the
sound receiving characteristic. When phase or arrival
times change differently in the two hearing aids this
will degrade or deteriorate the users ability to locate
2o the various sound sources in the surrounding space and
the advantage of a binaural hearing aid system will be
degraded.
Furthermore, the phase and time relationship in a
hearing aid degrades the quality of the sound perceived
2s by the user. It may sound like the result of a Doppler-
ef f ect .
At the same time, in hearing aids of this type also
the amplitude characteristic will change during
transition between the omnidirectional and a directional
so characteristic, e.g. from a flat response to a response
in which the amplitudes of higher frequencies will be
increased. This increase may be in the area of 6
dB/octave. This results in the serious problem, that
CA 02385812 2002-09-30
3
hearing aids of this type can not be perfectly fitted with an optimum
transfer characteristic for both the omnidirection,al and the
directional characteristic.
On this background, it is the object of the present invention to
provide a method of the kind defined, in which the deficiencies of
the prior art hearing aid are remedied by effecting a smooth change-
over between the omnidirection,al characteristic and any directional
characteristic substantially without changing the phase relationship
or time delay and the amplitude characteristic of the signals . The
change-over between the omnidirectional characteristic and a
directional characteristic and vice versa may be controllable or even
automatic.
According to a first aspect of the present invention, there is
provided a method for controlling the directionality of the sound
receiving characteristic of a hearing aid comprising spaced apart
first and second sound receiving microphone means, a signal processor
for processing signals supplied by said microphone means and an output
transducer for emission of sound signals in response to output signals
from the signal processor, said method comprising the steps of
changing over said sound receiving characteristic between an
omnidirectional characteristic and a directional characteristic and,
while operating the hearing aid with said directional characteristic,
canbini.ng the signals supplied by said first and second microphone
means into an overall combined signal, an adjustable time or phase
delay being imposed on at least one signal, wherein said change over
of the sound receiving characteristic from the omnidirectional
characteristic to the directional characteristic and vice versa is
effected by controlling the attenuation and the time or phase delay
of signals derived from both of the signals (Xf~, X~~) from the
first and second microphone means before forming said overall combined
signal (Y), according to an adjustable attenuation control parameter
(c~razi) and a delay (T) , whereby said overall combined signal (Y) is
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4
determined by Y = Xf~,t * (1 - omni * ev'~') + X,~~ * (omni - e-~'~') . to
change over the hearing aid betvaeen said omnidirectional
characteristic and any desired form of said directional characteristic
as a smooth change over substantially without effecting phase
relationship, time delay and a~litude characteristic of the hearing
aid.
The present invention, in a second aspect, also relates to a
hearing aid with controllable directionality of its sound receiving
characteristic, ccxr~rising spaced apart first and second sound
receiving microphones means, a signal processor for pnxessing signals
supplied by said microphone means and an output transducer for
emission of sound signals in response to output signals fr<xn the
signal processor, and further comprising change-over control means for
change over of the sound receiving characteristic between an
omnidirectional characteristic and a directional characteristic and
combining means for combining of the signals from the first and
second microphone means to provide an overall combined signal supplied
to the signal processor, while operating the hearing aid with said
directional characteristic, and adjustable time or phase delay means
for producing a phase-delayed modification of at least one signal,
wherein said change-over control means comprises controllable
attenuation means and controllable time or phase delay means acting
on signals derived from the signals (X~~, X~~) from both of the
first and second miczrophone means, respectively, said attenuation and
phase delay means (1-3; 14, 15, 18, 19) being controlled for forming
said overall combined signal (Y) according to an adjustable
attenuation control parameter (omni) and a delay (T), whereby said
overall combined signal (Y) is determined by Y = Xfx~,t * (1 - omni
er°'T) + X~~ * (omni - erg') , to change aver the hearing aid between
said omnidirectional characteristic and any desired form of said
directional characteristic as a smooth change over substantially
CA 02385812 2004-09-16
without affecting phase relationship, time delay and amplitude
characteristic of the hearing aid.
The present invention, also discloses a directiona7_
controller for a hearing aid for processing input signals from
5 at least two spaced apart microphones for producing a combined
output signal for further processing in the hearing aid, the
directional controller comprising adjustable time delay means
for producing a time-delayed modification of at least one
signal, and change-over control means for effecting a change--
to over between an omnidirectional mode and a directional mode,
and further comprising a first controllable attenuator for
processing a signal derived from the signal (Xfronc) from the
first microphone to output a first processed signal, a second
controllable attenuator for processing a signal derived from
the signal (Xba~k) from the second microphone to output a
second processed signal, time delay means for delaying signals
derived from the signals from both of the first and thE:
second microphones and combining means connected to combine
the processed and delayed signals with the output signal from
2o the first microphone to generate the combined output signal,
the change-over control means being connected to control the
controllable attenuator.
In the following the invention will be further explained
with reference to the accompanying drawings, in which
Fig. 1 is a schematic block diagram of the prior art:
hearing aid of US-A-5,757,933,
Figs. 2 to 5 are graphic representations illustrating
variation of the sound receiving characteristic of the hearing
aid in fig. 1 between the omnidirectional characteristic and
different directional shapes and concurrent variation of:
amplitude characteristics of the front and back microphones
used therein.
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6
Fig. 6 shows a schematic arrangement of the front end of
a first embodiment of a hearing aid according to the present
invention,
Figs. 7 to 10 are graphic representations corresponding
to the representations in figs . 2 to 5 with respect to the
hearing aid shown in Fig. 6,
Fig. 11 shows a schematic arrangement of a second
embodiment,
Fig. 12 shows a similar schematic arrangement of a
to directional controller,
Fig. 13 schematically shows a further improvement of the
arrangement shown in fig. 6, and
Fig. 14 shows a still further development of a hearing
aid according to the invention.
In the prior art hearing aid shown in fig. 1 two non-
directional microphone circuits including a front microphone
MICE and a back microphone MICB. Whereas the output signal
from the front microphone MICE is supplied directly to the
hearing aid signal processor via a summing node SN, the signal
from the back microphone is supplied to the summing node SN
via an inverter, an adjustable phase delay circuit and an
attenuator with adjustable gain only by closure of a manually
operated switch SW, whereby the sound receiving characteristic
of the hearing is changed from the omnidirectional
characteristic of front microphone MICF to a directional
characteristic of varying shape.
The combined signal Y formed at the summing node SN with
switch SW closed and supplied to the signal processor will
thus be related to the signals Xfront and Xback from front and
3o back microphones MICF and MICB, respectively, by the relation
Y = Xfront - Xback * omni * e-'"T, where the adj ustable parameter
omni represents the adjustable gain of the attenuator, whereas
T represents the adjustable time delay corresponding to the
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7
difference in arrival time for sound signals received by the
front and back microphones MICF and MICB, respectively.
The graphic representations in figs. 2 and 3 illustrate
the variation of the sound receiving characteristic of the
hearing aid in figure 1 from the omnidirectional shape ND and
various directional shapes D1 to D10 ranging from weak
cardioid to super cardioid form for values of the adjustable
parameter omni ranging from 0 to l, measured at lkHz and 100
Hz, respectively, whereas the graphic representations in figs.
l0 4 and 5 show the variation in the amplitude characteristics
of the signals received from the areas in front and back of
the hearing aid, respectively, for correspondingly varying
values of the parameter omni.
As will appear from these representations the change
over between the omnidirectional characteristic and the
various shapes of directional characteristic results in this
prior art hearing aid not only in the desired gardual
reduction in gain or amplitude response for the signals
received from the area behind the user, but is accompanied
also by a significant change in gain or amplitude response for
the signals received from the area in front of the user. In
consequence thereof an adjustment or fitting of the hearing
to compensate for a users specific hearing impairment for
listening in quiet surroundings, where use of the
omnidirectional characteristic is preferred, will not provide
an optimum compensation, when a change-over is made to a
directional characteristic, e.g. for use of the hearing aid.
in a more noisy sound environment, such as a party.
Fig. 6, shows, in principle, the front end of a hearing
aid including a change-over controller for controlling change of
the directionality of sound receiving characteristic of the
hearing aid from the omnidirectional characteristic to a
directional charac
CA 02385812 2002-09-30
teristic and vice versa. This change may be effected as
a switch-over or as a gradual and smooth change-over.
The front end of the hearing aid comprises at least
two microphone circuits, i.e. a front microphone Fmic
s and a back microphone Bmic and possibly optional
preprocessing circuits for the electrical output signals
from the microphones. The distance between the two
microphones may be as small as 1 mm or as wide as a few
cm.
io The front end further contains at' least two
controllable amplifiers or attenuators 1 and 2, at least
one time or phase delay device 3 aid at least three
combining circuits 4, 5 and 6. It is to be understood
that the combining ;circuits may contain positive as well
i5 as negative input terminals, so as to form adding or
subtraction operations or combinations thereof.
In the structure, the back microphone Bmic is
connected to the controllable amplifier or attenuator 1
and to a first adding circuit 4.
2o The front microphone Fmic is connected directly to
the controllable amplifier or attenuator 2 and to a
second adding circuit 6. The output of the controllable
amplifier or attenuator 2 is further connected directly
to a second input of the first adding circuit 4, whereas
25 the output of the controllable amplifier 1 is directly
connected to a positive input of a subtraction circuit
5.
Between the output of the f first adding circuit 4
and the negative input of the subtraction circuit 5 a
3o preferable controllable delay device 3 is included.
In the following description the adding and
subtracting circuits will generally be referred to as
combining circuits.
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9
In operation, sounds from the environment of the
hearing aid is picked up both by the front microphone
Fmic and the back microphone Bmic. The distance between
the two microphones may be as small as 1 mm and as wide
s as a few cm.
The output signal of the front microphone Fmic is
supplied to the combining circuit 6. The output signal
of the back microphone Bmic is supplied to the
controllable attenuator or controllable amplifier 1, the
io gain of which may be controllably changed from zero to
one, i.e. from no amplification to full amplification.
This change-over may be effected as a switch-over or as
a controlled gradual change. This means that any
amplification between zero and one may be controllably
i5 achieved.
The output signal, if any, of the front microphone
Fmic is also supplied to a controllable attenuator or
amplifier 2, the amplification of which may controllably
be changed from zero to one, i.e. from no amplification
2o to full amplification. Also in this case the change-over
may be effected as a switch-over or as a gradual
controlled change. This means that any amplification
between zero and one may be achieved.
The output signal, if any, of the controllable
2s attenuator or amplifier 2 is supplied to a second input
of the combining circuit 4. The output signal, if any,
of combining circuit 4 is supplied to the controllable
delay device 3, the delay of which may be controlled
from as small as 1 ~,s up to 1000 us or more.
3o The output signal, if any, of delay device 3 is
supplied to the negative input of combining circuit 5,
the output of which is supplied to the second input of
the combining circuit 6.
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1
Thereby, the output signal of the front microphone
Fmic may be attenuated in attenuator or controllable
amplifier 2 before it is added to the undelayed output
signal of the back microphone Bmic in the combining
s circuit 4, the output signal of which is then delayed in
delay device 3 before being supplied to the combining
circuit 5. The controllable delay of delay device 3 will
usually have the same value as the acoustical delay
between the arrival times of sounds at the front
io microphone Fmic and at the back microphone Bmic.
Preferably this delay is also adjustable and/or
controllable.
Additionally, the output signal of the attenuator
or controllable amplifier 1 is supplied to the positive
is input of the combining circuit 5. In this combining
circuit the delayed output signal of delay device 3 is
subtracted from the attenuated output signal of
amplifier or attenuator 1. The output signal of the
combining circuit 5 is supplied, as a processed signal
2o to the combining circuit 6. The output signal of the
combining circuit 6 is then used as an input signal for
further processing in the remaining components of the
hearing aid such as the signal processor, which need not
to be described here.
2s The remaining parts of the hearing aid may, as
known in the art, comprise more than one signal
processing channel having either a common change-over
controller or a separate controller for each channel.
As further known in the art, the output signals of
3o both microphones Fmic and Bmic may advantageously be
converted into a digital representation before being
supplied to the change-over controller with its
components 1 to 6.
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11
The function of the circuit in fig. 6 is as
follows:
For the directional mode of operation the signal
transfer of the controllable attenuators 1 and 2 is set
s at zero, i.e. no signal is transferred.
The output signal of the front microphone Fmic is
directly supplied to the second adding circuit 6. The
output signal of the back microphone Bmic is supplied
via the first adding circuit 4 and delay device 3 to the
io negative input of the subtraction circuit 5~, where the
signal changes its polarity. The output signal of the
subtraction circuit 5 is then supplied to a second input
of the second adding circuit 6. Thus, the delayed signal
from the back microphone Bmic is subtracted from the
i5 undelayed output signal of the front microphone Fmic.
The directional front characteristic may then be
created by adjusting the delay T of the delay device to
be the same as the acoustical delay A between the back
microphone Bmic and the front microphone Fmic. With this
2o delay the signals, that are first received at the back
microphone Bmic and are later received at the front
microphone Fmic, are then suppressed in the adding
circuit 6, where the delayed signal of the back
microphone is subtracted from the output signal of the
25 front microphone.
This mode of operation results in an output signal
from adding circuit 6, which is the result of the
subtraction of the delayed output signal of the back
microphone Bmic from the output signal of the front
so microphone Fmic, thus canceling sound coming directly
from the back of the user.
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12
By adjusting T < A, sound coming partly from the
side of the user is cancelled, the direction of the
canceling effect is controlled by the ratio of T/A.
For the omnidirectional mode of operation both
s attenuators 1 and 2 are set for a full signal transfer.
The output signals from the front microphone Fmic
and the back microphone Bmic are supplied to the first
adding circuit 4, where they are combined and supplied
via delay device 3 to the subtraction circuit 5, where
io the combined and delayed signal is subtracted from the
output signal of the back microphone Bmic.
The output signal of the subtraction circuit 5 is
then supplied to the second adding circuit 6, where it
is combined with the undelayed output signal of the
is front microphone Fmic. The addition of these signals
creates the omnidirectional characteristic. This mode of
operation results in an output signal from the adding
circuit 6, which is generated by the addition of the
signals from the front and back microphones from which
2o the delayed front and back microphone signals are
subtracted.
The sound signals received at the two microphones
differ with respect to their arrival time at the
respective microphones from a source, the distance of
2s which is different for the two respective microphones.
This difference is the acoustical delay A and the
relationship between the sound signals Xfront and Xback
received at the front and back microphones,
respectively, may be generally expressed as
* -jU7A
Xback - Xftont
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13
where e-jwa is the acoustical delay for the actual direc-
tion to the sound source.
The combined signal Y from adding circuit 6 is
s Y = Xfront * ( 1 - omni * e-'~'T ) + Xback * C omni - e-j'~'' )
where omni is an adjustable parameter controlling
attenuators 1 and 2 and having preferably a value in the
range from 0 to 1, i.e. the lower limit omni = 0 means
io no signal transfer through attenuators 1, and 2, whereas
the upper limit omni = 1 means maximum signal transfer
through attenuators 1 and 2.
Although the invention is not limited thereto the
parameter omni should preferably be substantially the
i5 same for both attenuators 1 and 2.
If the full directional mode of operation is chosen
with omni = 0, then the combined signal Y becomes
Y = x * ~1-e-~wca+T>)
front
If the delay T is selected equal to the delay A
directly from the back microphone to the front
microphone in the directional mode of operation, then
the part of the sound signal X coming directly from the
back of the user is suppressed to the maximum extent and
a directional characteristic known as a cardioid charac-
teristic is achieved.
The signal process described so far is preferably
performed as a digital process in the time or frequency
3o domain. If processing in the frequency domain is
employed, it is advantageous to use microphone circuits,
which are capable of generating a delayed microphone
output signal in combination with a non-delayed
CA 02385812 2002-09-30
14
microphone output signal.
Figs. 7 to 10 are graphic representations of sound receiving
characteristics and amplitude response of a hearing aid embodying the
front end part shown in fig.l and corresponding to the representations
in figs. 2 to 5 and using the same reference designations as in these
figures, as will appear from figs. 7 and 8, the part of the sound
receiving characteristic representing the area in front of the user
is unaffected by the change over between the omnidirectional
characteristic 1~ and the various directional shapes D1 to D10. As
illustrated by fig. 10 the amplitude response of signals received from
the area in front of the user is unaffected by the change over and
remains the same irrespective of the change of the sound receiving
characteristic to suppress sounds coming from the area behind the
user. Thereby, the adjustment or fitting of the hearing aid to
compensate for the users hearing impairment in quiet surroundings,
where the omnidirectional characteristic is used, will provide optimum
listening performance also when the hearing aid is used in a more
noisy environment using a directional shape of the sound receiving
characteristic.
The circuit in fig. 11 is similar to the circuit in fig. 6 and
includes a change-over controller with components 1 to 6. Similar
components have been assigned the same reference numerals.
Additionally, signal processing units 7 and 8 are
placed at the outputs of the at least two
microphones, i.e. the front microphone Fmic and the back
microphone Bmic. The processed output signals of the two signal
CA 02385812 2004-09-16
processing units 7 and 8 are then supplied to the change-over
controller with components 1 to 6. The signal processing
units 7 and 8 may perform an equalizing function on the two
output signals of the two microphones and/or may contain
5 various filters, e.g. band pass filters. With the use of
band pass filters the microphone signals may be split up onto
several bands, each equipped with its own change-over
controller. The respective output signals from the adding
circuits 6 in the various bands or channels may then be
to combined into a composite combined signal to be further
processed in the remaining stages of the hearing aid.
Fig. 12 shows a similar circuit diagram as in Fig. 11,
wherein for the same components, the same reference numerals
are used. In this circuit the time delay for the output
15 signals of the two microphones Fmic and Bmic is effected in
separate delay units 3a and 3b representing the delay device
3. Otherwise, the function is similar to the function of the
circuits of figs. 6 and 11. Furthermore, a control unit 9 is
shown, which may control the attenuation of the controllable
attenuators 1 and 2 as well as the delays of delay units 3a
and 3b. This embodiment of the invention is of special
advantage in combination with microphone input circuits, which
are capable of supplying a delayed microphone signal together
with an undelayed microphone signal for a hearing aid.
As has been stated previously, it is of great importance
that, in the change-over controller, the amplitude response
as well as the time and phase of the
CA 02385812 2002-09-30
16
audio signals are not changed when their directivity
changes.
Fig. 13 schematically shows a further improvement
of the front end circuit of a hearing aid including a
s change-over controller as described so far with
reference to fig. 6. Similar components have been
designated with the same reference numerals as before.
Because of the technique used in combining the
output signals of the two microphones Fmic and Bmic, the
io resulting amplitude response of the output signals of
the adding means 6 will - of course - in the relevant
frequency range - rise with 6 dB per octave compared to
the amplitude response of a single microphone.
This behavior: may be observed in all systems, in
i5 which a delayed version of the output signal from the
back microphone is subtracted from the undelayed output
signal from the front microphone, while achieving a
directional effect.
However, in most cases, it is desirable to
2o compensate for this change in the amplitude response by
adding a filter at the output of the front end of the
hearing aid, i.e. at the adding circuit 6. Such a
filter, of course, means a reduction of 6 dB per octave
in the relevant frequency range. The drawback of such a
25 solution is that more circuit components, time and power
would be required, all of which are of very crucial
importance in modern hearing aid technology.
However, the change-over controller of the present
invention could also be adapted to perform this
3o compensation filtering. Therefore there will be no need
to add a filter at the output of the adding circuit 6.
For this purpose an additional subtraction circuit
is arranged between the adding circuit 4 and the
CA 02385812 2002-09-30
17
delay device 3, an the output signal of the adding
circuit 6 is directly supplied to the negative input of
adding means 10 in a feedback loop.
This new arrangement has already the desired
s effect.
It may be preferable to include in the feedback
loop a controllable amplifier or attenuator 11.
Thus, the output signal of the change-over
controller is fed back from the adding circuit 6 via the
io controllable attenuator 11 to the negative input of
subtraction circuit 10. Thus, the output signal of
attenuator 11 is subtracted in the subtraction circuit
from the output signal of adding circuit 4.
The resulting; output signal of subtraction circuit
i5 10 is supplied to the delay device 3 and hence to the
negative input of the subtraction circuit 5, the
positive input of which is connected to the output of
the controllable attenuator 1.
In principle, in the embodiments in figs. 6 and 11
2o to 13 subtraction circuit 5 and adding circuit 6 could
also be combined into a single combining circuit,
provided this has, in every respect, the same properties
as the two separate adding means 5 and 6.
Ideally, the gain factor of attenuator 11 should be
2s one or unity for the filtering being able to perform the
6 dB per octave fall at very low frequencies. However,
this would probably result in a loop gain of unity so
that the circuit might become unstable. Therefore, it is
preferred to have the gain of the amplifier or
3o attenuator 11 set to a little less than one or unity.
In fig. 14 a further development of a hearing aid
embodying the invention is shown, in which the
controllable attenuation and phase delay operations, to
CA 02385812 2002-09-30
18
which the signals from the front and back microphones
Fmic and Bmic are subjected before forming the overall
combined signal as represented by the relationship
stated in the foregoing, i.e.
s
-jwT
= Xfront * ( 1 - omni * a ) + Xback * ( omni - a j
are implemented by a different circuit structure.
In this case, the change-over means comprises a
first adding circuit 12 connected with the front and
io back microphones Fmic and Bmic and a first~subtraction
circuit 13 having a positive input connected with the
front microphone Fmic and a negative input connected
with the back microphone Bmic. First and second phase
delay devices 14 and 15 are connected with the first
is subtraction and adding circuits 13 and 12, respectively.
A second adding circuit 16 is connected with the first
subtraction circuit 13 and the first phase delay device
14 and a second subtracting circuit 17 has its positive
input connected with the first adding circuit 12 and its
2o negative input connected with second phase delay device
15. A first controllable attenuator 18 acts on the
signal from the second adding circuit 16 for attenuation
of this signal by a factor (1 - omni) /2 and a second
controllable attenuator 19 acts on the signal from the
25 second subtraction circuit 17 for attenuation of this
signal by a factor (1 + omni)/2, whereas a third adding
circuit 20 is connected with the first and second
attenuators 18 and 19 for addition of the signals
therefrom to provide the overall combined signal to be
3o supplied to the signal processor.
The microphones used in the described embodiments
are preferably omnidirectional microphones.
CA 02385812 2002-09-30
19
When two microphones are used in the omni-
directional mode of operation, both microphones generate
an electrical noise signal N. These two noise signals
have a similar power:
Nback ~ - ~ Nfront ~ i
where Nback is the noise signal from the back microphone
Bmic, and Nfr~r,r is the noise signal from the front
to microphone Fmic.
The noise signals N are random signals. Therefore,
the resulting signal amplitude is less than twice the
single amplitude. Thus, a 3 dB-noise reduction results.
The total noise signal can be calculated as:
~ N 12= ~ Nfront ~ 2* ~ 1-omni*e-j'~T, 2+ I Nback ~ 2* I 1-OlfIIl1*e-j~T ~ 2
N ~ = I Nfront I *20.5 ~ 1-OInIll.*e-j~~' I
ao It has been shown that with the new front end of a
hearing aid comprising a change-over controller in
accordance with the invention a great variety of
directional characteristics patterns may be controllably
realized.
