[0001] The present invention relates to a method to determine a feedback threshold in a
hearing device, to use of the method as well as to a hearing device.
[0002] Hearing devices are electronic devices in which sound is recorded by a microphone,
is processed or amplified, respectively, in a signal processing unit, and is transmitted
into the ear canal of a hearing device user over a loudspeaker which is also called
receiver.' The amplified or processed sounds which are emitted by the receiver are
partially recorded by the microphone. In other words, it must be dealt with a closed
loop comprising a hearing device with an output signal and an input signal. It must
be noted that the path of the sound energy is not limited to acoustic energy, but
also comprises, as the case may be, a mechanical transmission from the output to the
input, as e.g. over the housing of the hearing device (so-called body sound). Furthermore,
one has realized that over a vent, which is actually used for pressure equalization
between the inner ear of the hearing device user and the surrounding, or over electrical
paths in the hearing device, signal feedback can occur. It has been shown that of
all these possible components, the acoustic signal feedback contributes the largest
part.
[0003] The mentioned effects can result in a squealing for hearing devices, which squealing
is very uncomfortable for the hearing device user and finally renders the hearing
device unusable during the occurrence of the squealing. Although there exists the
possibility to keep the gain in the hearing device so small that no buildup and therefore
no squealing, which is a result of signal feedback, occurs. Therewith, the use of
a hearing device is compromised, to be precise in particular for those applications,
by which a large hearing loss must be compensated as it occurs for a person who is
hard of hearing, because for such patients a comparatively large gain in the hearing
device must be adjusted in order to obtain an adequate compensation.
[0004] In order that all gain settings, in particular the maximum possible gain setting,
for a hearing device can be used in its full extent, it is absolutely necessary to
determine the feedback threshold, which means to know the maximum gain setting for
a hearing device, for which maximum gain setting there occurs only just no signal
feedback.
[0005] Methods to determine the feedback threshold in a hearing device are already known.
In US-6 134 329, such a method is described with the aid of which the transfer function
of the hearing device is estimated from measurements which are made with a hearing
device inserted into the ear of a user. Thereby, the overall transfer function is
calculated with different gain values without that the closed loop is being opened.
Therewith, so-called optimal Wiener filter models are being used. The transfer function
in the forward path and the one in the backward path are being calculated together
in the following. From the transfer function in the forward path, the possible instable
frequencies and the maximum gain settings can be determined in the hearing device.
Furthermore, it is also disclosed how the transfer function in the forward path and
the one in the backward path can be calculated from the measurements of the overall
transfer function. For these measurements, an additional microphone is inserted into
the ear canal of the hearing device user, the insertion being done into the hearing
canal preferably through the vent.
[0006] It is obvious that these known methods ask for a large processing power in order
to obtain the desired information. Furthermore, an additional microphone is being
used for this variant, which is based on an in-situ measurement, by which the acoustical
but also the mechanical characteristics of the overall system is being changed in
a disadvantageous manner, such that, as a consequence thereof, errors will occur in
the further calculations to determine the feedback threshold.
[0007] Furthermore, reference is made to US-6 128 392 from which the use of a hearing device
with a compensation filter in its feedback path in the form of a FIR-(Finite Impulse
Response) filter is known. Acoustical and mechanical signal feedback shall be compensated,
an impulse at the output of the hearing device being applied in order to determine
the filter coefficients of the compensation filter. At the input of the hearing device,
the impulse response is measured and the values for the coefficients are being determined
for the compensation filter therefrom. It is an integrated signal feedback damping
which has an influence on the overall transfer function of the hearing device partly
in an undesirable manner because signal components of the desired signal are being
damped at the same time.
[0008] For the sake of completeness, reference is made to a method to determine the signal
feedback threshold, which method is applied in practice. The method consists therein
that the gain in the hearing device will be increased step by step until signal feedback
occurs. As a result, the corresponding value for the amplification, for which only
just no signal feedback occurs, corresponds to the signal feedback threshold. This
simple method has the great disadvantage that the hearing device user is exposed to
high sound levels. Furthermore, the hearing device must produce a high power during
the determination of the feedback threshold.
[0009] Therefore, it is an object of the present invention to provide a method which does
not incorporate the disadvantages mentioned above.
[0010] This object is solved by the features of claim 1. Further embodiments, uses of the
method as well as a hearing device are given in further claims.
[0011] The present invention uses the fact that the gain during feedback in the forward
path of a compressive system, as it is the case for a hearing device used to compensate
a hearing loss, after having reached its steady state in "closed loop" operation,
is equal to the feedback threshold gain. The steady state is reached soon after having
applied a low input signal level to the hearing device, which input signal level is
below 55 dB SPL (Sound Pressure Level), for example, and would result, for the open
loop compressive system, in a larger gain than the feedback threshold gain of the
closed loop system, respectively, would result in the maximum possible hearing device
gain if maximum possible hearing device gain is below feedback threshold gain. The
signal feedback gain is assessed in this steady state.
[0012] In one embodiment of the present invention, a maximum gain is adjusted below the
determined feedback threshold gain in the hearing device. By limiting the gain in
the forward path to the determined maximum gain, feedback cannot occur in this system.
[0013] In case signal feedback does not occur for the presented input signal level, i.e.
if the gain applied is too small to result in signal feedback, the maximum gain is
set to the maximum gain applied during the test.
[0014] The step of assessing the feedback threshold gain can be performed in different ways
assuming the steady state, as mentioned above, is reached:
[0015] First, the feedback threshold gain can be read out of the internal memory of a digital
hearing device.
[0016] Second, the feedback threshold gain in the forward path can be determined by assessing,
for example via a measurement, the levels of the input and the output signals of the
hearing device, be it implemented using analog or digital technology.
[0017] Third, the damping in the backward path can be determined via measuring the levels
of the input and the output signals of the hearing device, be it implemented as analog
or digital hearing devices, the feedback threshold gain in the forward path being
equal the damping in the backward path.
[0018] Fourth, the feedback threshold gain can be determined via the input signal provided
by the microphone of the hearing device in combination with the gain model applied
to the input signal.
[0019] It has already been pointed out that knowledge of the feedback threshold gain is
of great importance. This is in particular true if the hearing device disposes over
no efficient feedback canceling. But also in the case where a feedback canceling is
available, knowledge of the feedback threshold is of great value. Thus, by the present
invention, a possibility is given to improve the quality of the hearing device and/or
the quality of the hearing, in particular for an in-the-ear device (ITE).
[0020] Furthermore, the present invention has at least one of the following advantages:
- The forward path does not have to be opened up to determine the feedback threshold
gain; the assessment of the feedback threshold gain is carried out in closed-loop
operation of the hearing device, and while the hearing device is inserted into the
ear of the hearing device user.
- At the microphone input of the hearing device, no signal-to-noise ratio is necessary,
i.e. for a given maximum sound pressure P at the ear and for a surrounding noise S,
a maximum feedback threshold gain Vmax can be determined up to:
The known method needs a signal-to-noise distance DS at the microphone such that the
feedback threshold gain can be determined up to a value of
- For a given surround noise and for the same sound pressure at the ear during the determination
of the feedback threshold gain, a higher maximum gain can be reached by the present
invention;
- The method according to the present invention can be realized without or with only
little additional expenditure with existing signal processing possibilities which
are used in modern hearing devices.
[0021] In a further embodiment of the present invention, it is intended to carry out the
assessment of the feedback threshold gain in different frequency bands in that a feedback
threshold gain is determined in each frequency band.
[0022] In yet another embodiment of the present invention, the frequency bands correspond
to the so-called critical frequency bands which are given by the structure of the
human hearing. Critical frequency bands are frequency regions within which the ear
groups together sounds of different frequency. Sounds spaced apart more than a critical
band can be separately recognized by the brain, at least for normal-hearing people.
[0023] The present invention will be further described by referring to exemplified embodiments
represented in drawings.
- Fig. 1
- is a block diagram of a known system having forward and backward paths;
- Fig. 2
- is a block diagram of a hearing device with a backward path which represents all possible
signal feedback for a hearing device;
- Fig. 3
- represents a course of a gain for which the gain is drawn in function of an input
level of a hearing device in double logarithmic representation; and
- Fig. 4
- is a further embodiment for a gain course as analogously represented in Fig. 3.
[0024] Fig. 1 shows a block diagram for a feedback system as it is generally known. By 100,
a processing unit having a transfer function G, and, by 200, a feedback unit having
a transfer function K are identified. An input signal I is fed to one of the two inputs
of an addition unit 10 of which the only output is fed to the processing unit 100.
In the processing unit 100, an output signal O is generated that is fed to the second
input of the addition unit 10 via the feedback unit 200, besides the circumstance
that the output signal O is fed to the outside.
[0025] Having identified the transfer function in the forward and in the backward path by
G and K, respectively, the following overall transfer function for the system according
to Fig. 1 can be obtained as follows:
[0026] Fig. 2 schematically shows a block diagram of a hearing device 1, comprising a processing
unit 100 with a transfer function G. Seen from a propagation direction of signals
in the hearing device, a loudspeaker 30, which is also called receiver in the technical
field of hearing devices, is positioned after and connected to the processing unit
100, and a microphone 20 is positioned before and connected to the processing unit
100. The output signal of the hearing device 1, respectively of the receiver 30, is
fed via a feedback unit 200 to an addition unit 10, to which also an input signal
I is being fed. An output signal is generated in the addition unit 10, which output
signal is fed to the microphone 20.
[0027] It is emphasized that Fig. 2 only represents a simplified structure of a hearing
device in that only a microphone 20, a signal processing unit 100 and a receiver 30
are shown. In fact, other functional units - as e.g. other microphones, an analog-to-digital
converter, observation units for observation of power supply, a digital-to-analog
converter, memory units, etc. - might be provided. Such additional units do not have
an impact on the concept of the present invention.
[0028] The feedback unit 200 having a transfer function K is the actual equivalent circuit
for the effects mentioned above, of which the acoustic signal feedback contributes
the largest part. In this connection, reference is made to the already said and to
the general explanations in US-6 134 329.
[0029] Apart from additional influences to the overall transfer function on the basis of
specific transfer function characteristics of the microphone 20 and the receiver 30,
the overall transfer function of the block diagram according to Fig. 2 is equal to
the one according to Fig. 1.
[0030] Fig. 3 shows, in a schematic view, a course for the gain of a compressive system,
as it is used in a hearing device to compensate a hearing loss. While on the horizontal
axis the level of the input signal I is drawn using a logarithmic scale and the unit
decibel (dB), on the vertical axis the gain V is drawn also by using a logarithmic
representation. The course of the gain in function of the input signal level has a
negative slope which is characteristic for a compressive system.
[0031] In case a compressive system is being used in the forward path, as it can be seen
from Fig. 3 for the gain course as a function of the input signal level, and in case
an input signal level I
A results in a larger gain V
A than a supposed, i.e. not yet known feedback threshold gain V
KRIT, the system will adjust to a steady state in which the gain in the forward path will
be equal to the damping in the backward path. As already mentioned, the gain in the
forward path will be equal to the feedback threshold gain V
KRIT. Therewith, the feedback threshold gain V
KRIT can be assessed, according to the present invention, by assessing the gain in the
forward path or the damping in the backward path, e.g. in one of the following ways:
- the feedback threshold gain VKRIT is assessed by reading out an internal memory unit of the hearing device representing
the gain in the forward path;
- for an analog device, the feedback threshold gain VKRIT is assessed by measuring a steering parameter representing the gain in the forward
path of the hearing device;
- the feedback threshold gain VKRIT in the forward path can be determined by assessing the levels of the input and the
output signals of the hearing device;
- the damping in the backward path can be determined via measuring the levels of the
input and the output signals of the hearing device, be it implemented as analog or
digital hearing devices, the feedback threshold gain VKRIT in the forward path being equal the damping in the backward path;
- the feedback threshold gain VKRIT can be determined via the input signal provided by the microphone of the hearing
device in combination with the gain model applied to the input signal.
[0032] Having determined the feedback threshold gain V
KRIT by one of the methods mentioned above, a maximum gain V
max is adjusted that is below the feedback threshold gain V
KRIT. Thereby, a signal feedback is prevented. The gain difference between the feedback
threshold gain V
KRIT and the maximum gain V
max is selected as small as possible in order to obtain a maximum gain range for the
hearing device user. On the other hand, it must be taken into account that other factors
may influence the signal feedback occurrence. In particular for applications in which
feedback threshold gains V
KRIT are determined in different frequency bands, it should be assured that an overall
gain applied in a particular frequency band is less than V
KRIT, the overall gain being determined by a superposition of a gain applied in the frequency
band as well as all additional gain components resulting from overlapping of neighboring
gain functions. Especially in the case where no feedback canceling is available, it
is possible that signal feedback occurs due to dynamic changes in the feedback path,
although the adjusted maximum gain V
max has not been surpassed. In these situations, the maximum gain must be further reduced
in relation to the feedback threshold gain V
KRIT to account for the dynamic changes in the feedback path, reductions of V
max typically between 4dB and 8dB below V
KRIT may be applied.
[0033] In case signal feedback does not occur for the presented input signal level, i.e.
if the gain applied is too small to result in signal feedback, the maximum gain V
max is set to the maximum gain applied during the test.
[0034] In a further embodiment of the present invention it is provided to fix the slope
of the course of gain V to -1 in a first phase in order to reach the steady state
very fast which in turn results in obtaining the feedback threshold gain V
KRIT very quickly. In a later second phase, a flatter slope - which means a slope which
is less than -1 - is selected for the course of the gain. As a result thereof, a higher
exactness for the feedback threshold gain V
KRIT is obtained.
[0035] In a still further embodiment of the present invention, it is intended to split the
range of human hearing into different frequency bands in each of which a feedback
threshold gain V
KRIT is determined by applying one of the methods mentioned above. Thereby, it is feasible
to determine feedback threshold gains V
KRIT in one or several as well as in all frequency bands. In a preferred embodiment of
the present invention, so-called critical frequency bands are used which are given
by the structure of the human ear.
[0036] The invention will be further described by referring to Fig. 4 in which a gain course
V is represented of a hearing device 1 using the same scaling as in Fig. 3. The gain
course V corresponds to the one which is adjusted after the assessment of the feedback
threshold gain V
KRIT according to one of the above-mentioned methods, whereby four regions I, II, III
and IV dividing the horizontal axis can be identified.
[0037] Region III is the compressive region in which a slope for a gain course is applied
that is dependent on a specific hearing loss of a hearing device user. In order to
prevent any feedback of the kind mentioned above, the gain course is essentially horizontal
in region II at a gain level equal to the maximum gain V
max which is below the feedback threshold gain V
KRIT that has been determined in the manner described above. The level of the input signal
I at the transition between region III and II is therefore derived from the feedback
threshold gain V
KRIT and the maximum gain V
max, respectively.
[0038] In region I, the gain course decreases towards lower levels of the input signal I
in order to prevent noise from being amplified. The level of the input signal I at
the transition between region I and II is set to a level at which noise influence
increases.
[0039] In region IV, the gain course decreases towards higher levels of the input signal
I in order to prevent very loud sound from being amplified. The level of the input
signal I at the transition between region III and IV is set accordingly.
[0040] It is noted that while the level of the input signal I at the transition between
region II and III is determined according to the procedures described above, all other
levels of transitions are adjusted more heuristically.
[0041] According to the present invention, the gain course V is limited in region II with
the aid of a limiting unit provided in the hearing device in order to limit the gain
to the maximum gain V
max, thereby preventing signal feedback.
[0042] The present invention opens up a number of applications or uses, some of which have
already been discussed above. In addition, or as a repetition, these are the following,
for example:
- A maximum gain is adjusted below the determined feedback threshold gain in the hearing
device. By limiting the gain in the forward path to the determined maximum gain, feedback
cannot occur in this system.
- The assessed feedback threshold gain is used as parameter for steering an active feedback
canceling unit, wherein the feedback unit is generally known in the art.
- The assessed feedback threshold gain is used to estimate other acoustical coupling
parameters related to the feedback threshold while the hearing device is inserted
into an ear of a hearing device user. In particular, the assessed feedback threshold
is used to improve an estimation of the real-ear-to-coupler difference.
1. A method to determine a maximum gain in a hearing device comprising a forward path
between an input and an output of the hearing device, a compressive gain model being
applied in the forward path, the method comprising the steps of
- exposing the hearing device to a low input signal level, preferably below 55 dB
SPL, while the hearing device is inserted into an ear of a hearing device user, and
- assessing a feedback threshold gain after a steady state has been reached in the
hearing device.
2. The method of claim 1, characterized by the step of assessing the feedback threshold gain is carried out by determining a
current gain in the forward path.
3. The method of claim 2, characterized in that the current gain is read out of a memory unit contained in the hearing device.
4. The method of claim 2, characterized in that the current gain is determined by one ore more steering parameters of the hearing
device allowing to assess or directly representing the gain in the forward path of
the hearing device.
5. The method of claim 1, characterized in that the step of assessing the feedback threshold gain is carried out by assessing input
and output signal levels, and by dividing the output signal level by the input signal
level.
6. The method of claim 1, characterized in that the step of assessing the feedback threshold gain is carried out by measuring a gain
in the forward path of the hearing device.
7. The method of claim 1, characterized in that the step of assessing the feedback threshold gain is carried out by measuring a damping
between an output and an input signal level of the hearing device.
8. The method of claim 1, characterized in taht the step of assessing the feedback threshold gain is carried out by measuring
an input signal level and by applying the gain model of the hearing device to the
input signal level.
9. The method of one of the claims 1 to 8, characterized by further comprising the step of adjusting a course of a gain as a function of a level
of the input signal in the hearing device as a slope steeper than -0.7 for the course
of gain represented double-logarithmically.
10. The method of one of the claims 1 to 9,
characterized in further comprising the steps of adjusting a course of a gain as a function of a level
of the input signal in the hearing device as follows:
- choosing a slope steeper than -0.7 for the course of gain represented double-logarithmically
in a first phase, and
- choosing a slope less than -1 for the course of gain represented double-logarithmically
in a second phase.
11. The method of one of the claims 1 to 10, further comprising the step of assessing
feedback threshold gains in at least two frequency bands.
12. A use of the method of one of the claims 1 to 11 in a hearing device comprising a
forward path between an input and an output of the hearing device, a gain model being
applied in the forward path, characterized in that a maximum gain is applied in the forward path, said maximum gain is adjusted below
the feedback threshold gain.
13. A use of the method of one of the claims 1 to 11 in a hearing device comprising a
forward path between an input and an output of the hearing device, a gain model being
applied in the forward path, characterized in that the assessed feedback threshold gain is used as parameter for steering a feedback
canceling unit.
14. A use of the method of one of the claims 1 to 11 in a hearing device, characterized in that the assessed feedback threshold gain is used to estimate other acoustical coupling
parameters related to the feedback threshold while the hearing device is inserted
into an ear of a hearing device user.
15. The use of claim 14 wherein the feedback threshold is used to improve an estimation
of the real-ear-to-coupler difference.
16. A Hearing device comprising
- a forward path between an input and an output and a gain model being applied in
the forward path, said gain model having a maximum gain,
- means for assessing a feedback threshold gain after a steady state has been reached
in the hearing device, and
- means for adjusting the maximum gain below the feedback threshold gain.