FIELD OF THE INVENTION
[0001] The invention relates to a hearing instrument, in particular a hearing aid, and to
a method for operating a hearing instrument.
BACKGROUND OF THE INVENTION
[0002] State of the art hearing instruments are usually either behind-the-ear (BTE) hearing
instruments, in-the-ear (ITE) hearing instruments, in-the-canal (ITC) hearing devices
or completely-in-the-canal (CIC) hearing instruments. ITE, and especially ITC and
CIC hearing instruments are less visible than BTE hearing instruments and are therefore
preferred by many users. However, in these devices the space in the ear canal has
to be used efficiently, and the ear canal essentially has to be closed by the device
so as to minimise acoustic feedback due to the proximity of the sound outlet of the
receiver and the sound inlet of the microphone. This plugging of the ear canal may
cause undesirable effects, known as occlusion effect which has an impact on the perception
of the wearer's own voice and on the wearing comfort. The occlusion effect may also
occur when BTE hearing instruments are used, since also BTE hearing instruments comprise
a piece ("earpiece") placed in the ear canal, which is used for holding sound conduction
tube(s) and/or other elements.
[0003] In this text, the term "earpiece" or "in-the-ear-canal component" is used to denote
any device or device part of a hearing instrument that is meant to be placed at least
partially in the ear canal of the user. It may for example be a ITE, ITC, or CIC hearing
instrument. It may as an alternative be an earpiece (or otoplastic) of a hearing instrument
which also comprises an outside-the-ear-canal component, for example a behind-the-ear
component of a BTE. In the case of a BTE with an open fitting, the in-the-ear-canal
component may merely be a fixation means for at least one sound tube. Concerning different
types of in-the-ear-canal components and ways to connect them to a possible outside-the-ear-canal
component, it is referred to the
European patent application publication EP 1 681 904.
[0004] In order to reduce the occlusion effect, in-the-ear-canal components comprising a
"vent" - a duct through the in-the-ear-canal component - are used. Hearing instruments
with large vents are especially popular, since the open fitting is perceived as very
comfortable by the user. One of the reasons for this is that the occlusion effect
is greatly reduced, and the own voice is perceived more naturally. However, large
vents also have disadvantages.
- a. Strong direct sound through the vent, which may not be controlled by the hearing
instrument and which, due to delay differences, may interfere with the sound produced
by the hearing instrument receiver.
- b. Especially in ITE, ITC, and CIC hearing instruments, enhanced tendency for feedback,
since the sound produced by the hearing instruments gets through the vent back to
the microphone without substantial attenuation.
- c. Reduced space: The space used up by the vent diminishes the design degrees of freedom,
for example concerning the placement of a receiver in the instrument.
[0005] There are several proposals for dealing with sound conduction through ducts. The
active control in ducts was proposed, for example in
US 4,473,906, to reduce noise carried through heating and ventilating ducts in factories and the
like. Such systems rely on a first microphone placed along the duct which detects
a noise signal, and a loudspeaker arranged downstream of the microphone that produces
a compensation signal. In more advanced embodiments, there may be a second microphone
for detecting a remaining signal to be minimised. Such active control systems, however,
are as such not suitable for hearing instruments, since in hearing instruments the
sound conduction tube is extremely short, and in hearing instruments sound conduction
not only in one but in both directions is an issue.
[0006] Active direct sound compensation systems aim at the canceling of direct sound transmitted
towards the eardrum, especially as active ear protection devices. Such active direct
sound compensation systems have been described in the publications
EP 1 499 159,
US 6,445,799,
US 5,740,258, and
WO 2005/052911.
EP 1 499 159,
US 6,445,799, and
US 5,740,258 describe combinations of a microphone with a loudspeaker that rely on the principle
of evaluating, from the microphone signal, an inverse noise and radiating the inverse
noise by the loudspeaker downstream of the microphone.
WO 2005/052911 discloses a further example of such a feed forward noise canceller, which is suitable
for attenuating sound signals bypassing the hearing instrument in situations where
an auxiliary signal is received, for example through a wireless or wired connection,
such as a telecoil. The wanted signal is fed to a receiver from an electrical input
- such as a telecoil - whereas the signal of a microphone is fed to a noise cancellation
part, which supplies a canceling signal to a receiver in the ear canal. An error microphone
is used for adjusting the attenuating signal.
[0007] There are known disadvantages of such approaches. Firstly, no receiver is available
with sufficient power for providing the compensation signal (inverse noise). Secondly,
in practice it is difficult to separate between the wanted signal and the disturbing
signal to be reduced. Thirdly, the electro-acoustic transfer function in the excitation
path (often mentioned as error path) is unfavorable with respect to the needed compensation
filter. Finally, these approaches do not provide a solution to the problem of enhanced
feedback due to there being a vent.
[0008] A further category of active systems for hearing instruments, therefore, deals with
hearing instruments with no vent or only a small vent and with an active reduction
of the occlusion effect. The disclosures of
WO 2004/021740 and
US 6,937,738 are examples of such systems.
[0009] Yet a further category of systems deals with an active canceling of feedback through
a vent. In
US 5,033,090 a microphone signal in the vent is used to improve the feedback tendency by way of
suitable subtraction from the input signal. The vent microphone features the advantage
that the unknown feedback path from the receiver to the vent microphone is taken into
account. The remaining path from the vent microphone to the input microphone, however,
is hard to estimate with sufficient accuracy, since the sound radiation from the vent
to the concha scatters strongly from individual to individual. Thus, in practice,
the estimation is difficult. Therefore, one may as well estimate the whole feedback
path from the receiver to the input microphone. This is done in modern feedback cancellers.
Such feedback cancellers, however, today have reached their limits in terms of impact
and artefacts.
[0010] The
European patent publication EP 1 499 159 describes, next to the above-mentioned cancellation of direct sound by means of a
microphone and a receiver also the canceling of sound from the opposite direction.
To this end, the sound is recorded by the microphone in the vent and is, via a compensation
device, radiated at a different position in the vent. This disclosure thus relies
on the same principle as above-described, and entails the same disadvantages. It features
the additional disadvantage that two microphones have to be placed adjacent the vent.
SUMMARY OF THE INVENTION
[0011] It is an object of the invention to provide a hearing instrument comprising a vent
that includes means for overcoming disadvantages of prior art hearing instruments
and by which the acoustic feedback path is influenced and improved in an active way.
[0012] This object is achieved by the invention as defined in the claims.
[0013] A method of operating a hearing instrument in accordance with the invention comprises
the steps of
- Obtaining an electrical input signal,
- Processing, on a signal path, the electrical input signal into an electrical output
signal,
- Converting the electrical output signal into an output acoustic signal and emitting
the output acoustic signal into a remaining volume between the in-the-ear-canal component
and the user's eardrum,
- Taking a signal from the signal path, processing the signal into an electrical compensation
signal, converting the electrical compensation signal into an acoustic compensation
signal and emitting the acoustic compensation signal into a duct between the remaining
volume and a surrounding atmosphere,
- Detecting a duct acoustic signal, and
- Adapting, based on the detected duct acoustic signal, at least one of processing parameters
and of a processing structure for said processing the signal into an electrical compensation
signal.
[0014] The acoustic compensation signal at least partially compensates by an active noise
control process (also known as noise cancellation, or active noise reduction (ANR))
the signal in the duct and thus of the signal transmitted through the duct to the
outside. In this way, the tendency for feedback is reduced. An additional effect may
- depending on the chosen implementation - be that the direct sound that goes through
the vent in the forward direction is also reduced, so that the wanted signal in the
ear canal is easier to control.
[0015] The electrical input signal may be obtained by conversion, by a microphone or a plurality
of microphones, of an incident acoustic signal. It may as an alternative be obtained
by a receiving unit for signals transmitted to the hearing instrument wirelessly or
by wire.
[0016] In the method according to the invention, the detected acoustic signal may be viewed
as serving as an error signal in a closed-loop controller. The system under control
is the entity that processes the signal taken from the signal path (also called the
input signal of the compensation controller in the following) into the electrical
compensation signal and transfers the latter into the acoustic compensation signal.
The output of the system under control is the feedback canceling signal, and the reference
value is the (inverse of) the acoustic signal that would be present in the duct if
no compensation signal was emitted into the duct. The error signal in this closed-loop
controller is determined acoustically and is recorded by a microphone (also called
the 'second microphone' or 'error microphone' in the following) in operative communication
with the duct.
[0017] A signal processing device for carrying out the method according to the invention
comprises an input for receiving an electrical input signal and an output connectable
to an electrical-to-acoustical converter (first receiver), and further comprising
a compensation output connectable to a second electrical-to-acoustical converter,
and an interface for receiving a duct acoustic signal. The signal processing device
- it may comprise one or several digital and/or analog signal processing elements
- is programmed to carry out the method according to the invention.
[0018] A hearing instrument in accordance with the invention comprises an in-the-ear-canal
component to be worn at least partially in the ear of a user. When the hearing instrument
is worn, a remaining volume between the in-the-ear-canal component and the user's
eardrum is defined. The hearing instrument includes at least one first microphone
operable to convert an acoustic input signal incident on the hearing instrument into
an electrical input signal, signal processing means operable to convert the electrical
input signal into an electrical output signal on a signal path, and an electrical-to-acoustic
converter (a receiver or a plurality of receivers with potentially different frequency
characteristics). Between the remaining volume and surrounding atmosphere, a duct
(such as a vent) is defined. A second receiver is in operative communication with
the duct, i.e. an output directly opens out into the duct or to a sound conductor
opening out into the duct. The hearing instrument further comprises a second microphone
in operative communication with said duct and operable to record an error signal in
said duct. The hearing instrument further may comprise means for carrying out the
method of operating a hearing instrument as defined above.
[0019] The hearing instrument preferably includes a compensation controller that is in operative
connection with said signal path and is operable to convert a signal from the signal
path into a compensation signal fed to the second receiver. An output of said second
microphone is in operative connection with an auxiliary input of said compensation
controller. The compensation controller is, in terms of hardware, not limited to a
particular class of hardware. Thus it may comprise digital signal processing means,
but may also be a passive filter the parameters of which are controllable. The compensation
controller may be integrated with the signal processing means in a common unit - such
as a digital signal processor, potentially including analog signal processing and/or
amplifying means. It may as an alternative be a separate element such as a separate
signal processor or comprise separate elements.
[0020] The second microphone may serve as an error microphone. Signal processing parameters
- such as filter coefficients of an adaptive filter of the compensation controller
- and/or a signal processing structure may be adjusted based on the signal recorded
by the error microphone. As examples of different signal processing structures, the
compensation signal may be switched off if the wanted signal is below a certain level,
or different filtering methods may be chosen depending on the nature and/or dynamics
of the incident acoustic signal.
[0021] The hearing instrument can be viewed as comprising a feed-forward compensation signal
generator to obtain a compensation signal from the wanted signal or from a signal
, and a closed loop adaptation of the compensation signal generator. In the adaptation
loop, the acoustic signal incident on an error microphone is the error signal to be
minimized, and the compensation signal generator processing parameters and/or processing
structure are adapted.
[0022] The invention, thus, follows the principle that a compensation sound is generated
based on the processed input signal itself - and not, as in some prior art systems,
based on a signal recorded upstream or downstream in the duct. The error microphone,
which is operable to record a duct signal, is merely used to influence processing
parameters and/or a processing structure for establishing the compensation signal.
The compensation controller may for example include the functionality of an adaptive
filter, where the filter parameters are corrected based on the error microphone signal.
[0023] The approach according to the invention has the substantial advantage over prior
art systems that there is no acoustical feedback path from the compensation receiver
to an input of the compensation controller (since the second microphone merely detects
an error signal to be minimised).
[0024] The output of the compensation controller may (apart from possible delays) be a monotonous
function of the signal taken from the signal path. If the signal on the signal path
is zero, the compensation receiver does not emit any signal, into the duct.
[0025] A compensation controller has at least two inputs: the first input from the signal
path itself, processed in a real time, feed-forward manner into the compensation signal,
and the auxiliary input from the error microphone by which the effect of the acoustic
compensation signal may be surveyed and if necessary processing parameters may be
adjusted. The processing of the duct acoustic signals into the processing parameters
and/or processing structure can be done with a frequency corresponding to the sampling
frequency or being of the same order of magnitude as the sampling frequency. It can
as an alternative be done with a lower time-constant than the processing of the signal
into the compensation signal. Then, the frequency with which the processing parameters
are updated is comparably low, for example lower than the sampling rate of the acoustic
input signal by an order of magnitude or more. This allows the error signal recorded
by the error microphone (or a quantity dependent on it) to be integrated before processing,
or the processing parameters to be (lowpass) filtered and/or integrated before being
applied.
[0026] Due to the fact that the compensation signal is obtained from the signal path and
not from the second microphone, and that the second microphone serves as an error
microphone, it is not necessary that the second microphone is arranged upstream of
the compensation receiver. The error signal may, in accordance with a preferred embodiment
of the invention, be recorded in the same longitudinal position in which the acoustic
compensation signal is radiated. This means that the compensation receiver and the
error microphone are arranged at essentially the same position along the duct, i.e.
the path of an acoustic signal from the compensation receiver intersects the duct
at the same depth in the ear canal as the acoustic signal path to the error microphone.
The possibility of arranging the compensation receiver and the error microphone at
the same longitudinal position along the duct constitutes an important distinction
from the prior art, where a microphone along the duct has to be place upstream of
the compensation receiver. If the error microphone is at the same longitudinal position
as the compensation receiver, the error signal recorded by the error microphone is
a very good indicator of the efficiency with which the signal in the duct is cancelled
by the compensation receiver.
[0027] It is, however, not a requirement that the compensation receiver and the error microphone
(or the corresponding intersections of the acoustic signal paths with the duct) are
at the same longitudinal positions. If they are not, an additional delay will result.
In any case will the position of the error microphone be the place at which the acoustic
signal in the duct is, at least for some frequencies, minimised.
[0028] The at least one error microphone and/or the at least one compensation receiver may
be placed in the in-the-ear-canal component. As an alternative, compensation receiver(s)
and/or error microphone(s) may be or in communication with in-the-ear-canal component
by way of sound at least one sound conducting tube opening out into the duct.
[0029] Preferably, exactly one error microphone and exactly one compensation receiver is
present.
[0030] The location of the error microphone (and thus preferably also of the compensation
receiver) in the duct is chosen not to be too close to either end of it, i.e. preferably
it is about in the middle of the duct. The position along the duct - in this text
also called 'longitudinal position' - is defined to be the component parallel to an
axis of the duct. Such an axis of the duct is approximately parallel to an axis of
the ear canal. The duct need not be straight but may - as is well-known for vents
- comprise curves etc., in which case the axis, along which the longitudinal position
is measured is also curved. In the case of open fittings, the duct is defined by the
ear canal, and the relevant quantity regarding positioning is a ratio between is a
longitudinal distance /
1 between an outer end of the duct (here: ear canal) and a position of the error microphone
(or a junction between the duct and a sound conductor connecting the duct with the
error microphone) on the one hand, and a longitudinal distance
l2 between said position of the error microphone (or junction between the duct and a
sound conductor connecting the duct with the error microphone) and an outlet into
the remaining volume for sound produced by the first receiver(s). Also this ratio
should not be too far away from 1, for example 0.05
l1<
l2<20l1, preferably 0.1
l1< l2<10
l1, especially preferred 0.25
l1<
l2<4
l1. As an alternative measure, if the duct is a vent, the distance of the error microphone
(or junction) from either end of the vent corresponds to preferably at least one or
at least two or at least three vent diameters.
[0031] In practice, the quantities
l1,
l2 may be the result of an optimization process the result of which depends on the particulars
of the hearing instrument.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032]
- Fig. 1 shows a diagram of components of a hearing instrument according to a first
embodiment of the invention;
- Fig. 2 shows a diagram of components of a hearing instrument according to a second
embodiment of the invention;
- Fig. 3 depicts a diagram of components of a hearing instrument with details about
the adaptive filtering;
- Fig. 4 depicts a scheme of feedback suppression in the case of an open fitting; and
- Fig. 5 shows a definition of longitudinal distances in case of a not straight duct.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0033] The hearing instrument represented in
Fig. 1 comprises an input microphone 1. In practice, often more than one input microphones
are used, and/or in addition to the input microphone further receiving means for receiving
signals may be present, such as a telecoil receiver, a receiving unit including an
antenna for receiving wirelessly transmitted signals, etc. The electrical input signal
SI obtained from the at least one input microphones is processed by a signal processing
unit 3 to obtain an electrical output signal
SO. The electrical output signal is converted into an acoustic output signal by at least
one receiver 5 and is emitted into a remaining volume 7 between the user's eardrum
and the in-the-ear-canal-component of the hearing instrument. Between the remaining
volume 7 and the surrounding atmosphere, a duct is present. The duct may be a vent
8 of the in-the-ear-canal-component, or it may be formed by the ear canal itself in
the case of an open fitting. The hearing instrument comprises a second receiver 9
(or compensation receiver), which is operable to emit a compensation signal into the
vent 8. The hearing instrument further comprises an error microphone 11 operable to
convert an acoustic signal in the portion of the vent 8, which is irradiated acoustically
by the compensation receiver, into an electrical error signal.
[0034] In the shown embodiment, thus, the (longitudinal) position along the length of the
vent - approximately corresponding to the depth in the ear canal - of the microphone
and of the receiver are approximately equal. In case the microphone and/or the receiver
is/are not placed adjacent the vent but connected to the vent by way of sound conductors,
the corresponding condition is that the longitudinal position of the junctions of
the sound conductor(s) with the vent are equal, or are equal to the position of the
microphone or receiver, respectively.
[0035] An other parameter to be considered is the longitudinal position itself of the positions
of the compensation receiver and of the error microphone (or the respective longitudinal
positions of the mentioned junctions). In this text, this position is represented
by the lengths /
1 and
l2 of the two sections of the vent outward from the longitudinal position, and inside
from the longitudinal position, respectively.
[0036] More in general,
l1 is the longitudinal distance (i.e. the component of the distance parallel to the
duct) between an outer end of the duct and the position of the error microphone or
an inlet into the duct for guided to the error microphone. /
2 then denotes the longitudinal distance between said position (or inlet) and an outlet
into the remaining volume 7 for sound produced by the first receiver(s) 5.
[0037] It has been found that if the place where the sound level is to be minimised (the
place of the error microphone) is too far outside in the vent (
l1 <<
l2) the efficiency becomes small, since the sound is almost short-circuited, and energy
is wasted for sound emitted away from the user. If the place is too far towards the
inner side (/
1 >> /
2), then the wanted signal in the remaining volume is compensated away, i.e. erased
by the compensation receiver.
[0038] Thus it is advantageous if
l1 ≈
l2. More concretely, it is for example 0.05/
1<
l2<20
l1, preferably 0.1/
1< /
2<10/
1, especially preferred 0.25
l1<
l2<4
l1, for example 0.5/
1<
l2<2l1. The ratio
l1/
l2 may be used for optimizing the range in which the canceling is effective. Below a
threshold frequency depending on /
1/
l2, the wanted signal of the first receiver may be reduced by the canceling system.
Therefore, in any embodiment of the invention the compensation signal may be restricted
to pre-defined frequency ranges, for example frequencies above a certain frequency
threshold which depends on /
1//
2.
[0039] The compensation signal (or feedback canceling signal)
SFC fed to the compensation receiver 9 is obtained from a compensation controller 13
which calculates the compensation signal from the electrical output signal
SO. The electrical error signal
SE obtained from the error microphone is fed to an auxiliary input of the compensation
controller and serves for adjusting processing parameters for the signal procession
by the compensation controller.
[0040] Thus, the real input signal of the compensation controller is not obtained from an
additional microphone in the ear canal but is obtained directly as an electrical (digital
or possibly analog) signal from the hearing instrument receiver input.
[0041] As is illustrated in Fig. 2, instead of from the receiver input, the input signal
of the compensation controller may also be tapped from different positions on the
signal processing flow, thus on the signal path. The signal processing unit is illustrated
as comprising a first stage 3.1 and a second stage 3.2, and in each stage one or a
plurality of signal processing steps are carried out. In accordance with yet another
variant, the signal may be tapped from the signal processing unit input. Preferred
versions of the hearing instrument according to the invention, however, include obtaining
the signal from the signal processing unit's output or from close thereto, so that
signal processing steps carried out by the same are automatically taken into account
when the compensation signal is determined.
[0042] Whereas in the drawings, the compensation controller is illustrated as a part separate
from the signal processing unit, in practice the compensation controller may be integrated
in one signal processing element, such as a digital signal processor or a signal processor
that includes digital and analog elements. More in general, the hearing instrument
may comprise one or a plurality of signal processing elements on which the functionality
of the signal processing unit and of the compensation controller and potentially further
functionalities are implemented.
[0043] Figure 3 shows a potential realization of a compensation controller. The realization
is based on a typical application of a filtered-x LMS algorithm as adaptive controller.
The input signal of the adaptive filter 21 is the hearing instrument's wanted signal,
namely the electrical output signal So. This signal is also filtered with a simulation
of the error path E and is used, together with the error signal
SE of the error microphone 11 as an input for the adaptation of the filter coefficients.
Other realizations of the compensation controller - based on adaptive filtering or
on other principles - are possible.
[0044] Figure 4 shows an embodiment of the invention for an "open fitting" hearing instrument, i.e.
for a hearing instrument that does not close off the ear canal. Instead of the vent,
the duct is constituted by the open (except for tubing and (not shown) holders of
the tubes) ear canal. Also in this case, the longitudinal positioning of the compensation
receiver (and the corresponding error microphone) is a quantity to be considered preferably.
The ratio
l1/
l2 also in this case is preferably between 0.25 and 4. This is achieved by choosing
appropriate lengths of sound conducting tubes 31, 32, 33 for the first receiver(s)
5 and for the compensation receiver 9 and the error microphone 11.
[0045] Figure 5 schematically shows, referring to the example of the distance
l1, how longitudinal distances between objects are defined in the case of a duct 41
that is not straight, namely as the distance between the intersections with the axis
42 of planes through the objects perpendicular to the axis. Even in cases where the
duct does not have a constant cross section (such as if the duct is the ear canal
itself), nevertheless a longitudinal axis can be defined.
[0046] In Figures 1-3, all of the first receiver(s), the compensation receiver and the error
microphone are shown to be placed in the in-the-ear-canal component. The first microphone(s)
may be placed within or outside of the in-the-ear-canal component. In Figure 4, all
of these elements are shown to be placed outside of the ear canal (for example in
a behind-the-ear component of the BTE) and connected by sound conduction tubes to
the respective outlets. Of course, also compromises between these variants are possible.
[0047] A special variant is to place the error microphone in the ear canal directly adjacent
the vent of an in-the-ear-component and to place both, the first receiver(s) and the
compensation receiver outside of the ear canal. This is advantageous if the space
in the in-the-ear-canal component is especially scarce, since receivers tend to be
comparably large-sized.
[0048] More in general, if the hearing instrument comprises, next to the in-the-ear-canal
component, also an outside-the-ear-canal component, the following may hold: All of
the first microphone(s), the first receiver(s), the compensation receiver, the error
microphone, signal processing means and further components (such as battery compartments,
communication modules for communication with further devices etc.), may be placed
either in an outside-the-ear-canal component or in an in-the-ear-canal component.
Arbitrary combinations are possible. However, preferably at least the signal processing
means, a battery compartment and the first microphone(s) are placed in the outside-the-ear-canal
component. The transducers (receivers, microphones) placed in the outside-the-ear-canal
component are connected to the ear canal by means of sound conductors, such as tubes.
The transducers are also connected to the signal processing means by data and/or power
transmission means, such as wires, conductor paths of a printed circuit board, and/or
non-contact signal transmission means.
1. A method of operating a hearing instrument, the hearing instrument comprising an in-the-ear-canal
component to be worn at least partially in the ear of a user, the method comprising
the steps of
- Obtaining an electrical input signal,
- Processing, on a signal path, the electrical input signal into an electrical output
signal,
- Converting the electrical output signal into an output acoustic signal and emitting
the output acoustic signal into a remaining volume (7) between the in-the-ear-canal
component and the user's eardrum,
- Tapping a signal from the signal path, processing the signal into an electrical
compensation signal, converting the electrical compensation signal into an acoustic
compensation signal and emitting the acoustic compensation signal into a duct (8)
between the remaining volume (7) and a surrounding atmosphere,
- Detecting a duct acoustic signal, and
- Adapting, based on the detected duct acoustic signal, at least one of processing
parameters and of a processing structure for said processing the signal into an electrical
compensation signal.
2. The method according to claim 1, wherein the steps of emitting the acoustic compensation
signal into the duct (8) and of detecting the duct acoustic signal are both carried
out at a same position along the duct.
3. The method according to claim 1 or 2, wherein, if l1 denotes a longitudinal distance between an outer end of the duct (8) and a position
at which the duct acoustic signal is detected, and if /2 denotes a longitudinal distance between said position and a place at which the output
acoustic signal is emitted into the remaining volume, then the inequality 0.1l1 < l2<10l1 holds.
4. The method according to any one of claims 1-3, wherein, if l1 denotes a longitudinal distance between an outer end of the duct (8) and a position
at which the duct acoustic signal is detected, and if l2 denotes a longitudinal distance between said position and a place at which the output
acoustic signal is emitted into the remaining volume, then both, /1 and l2 each are greater than or equal to a diameter of the duct.
5. The method according to any one of claims 1-4, wherein the detected duct acoustic
signal is used as an error signal in said adapting at least one of processing parameters
and of a processing structure for said processing the signal into an electrical compensation
signal.
6. The method according to claim 5, wherein the step of processing the signal into an
electrical compensation signal includes adaptive filtering, and wherein filter parameters
for said adaptive filtering are adjusted based on said error signal.
7. A signal processing device for a hearing instrument, the signal processing device
comprising an input for receiving an electrical input signal and an output connectable
to an electrical-to-acoustical converter (5), and further comprising a compensation
output connectable to a second electrical-to-acoustical converter (11), and an interface
for receiving a duct acoustic signal, the signal processing device being programmed
to carry out the method according to any one of the previous claims.
8. A hearing instrument comprising an in-the-ear-canal component to be worn at least
partially in the ear of a user, and at least one first microphone (1) operable to
convert an acoustic input signal incident on the hearing instrument into an electrical
input signal, signal processing means operable to convert the electrical input signal
into an electrical output signal on a signal path, and an electrical-to-acoustic converter
for converting the electrical output signal into an acoustic output signal, the electrical-to-acoustic
converter comprising at least one first receiver (5) in operative communication with
a remaining volume (7) in front of the user's ear drum, the hearing instrument further
defining a duct between the remaining volume and a surrounding atmosphere, and comprising
a second microphone (9) in operative communication with said duct (8) and operable
to record an acoustic signal in said duct,
characterised by
a second receiver (11) in operative communication with said duct (8), wherein a path
of an acoustic signal produced by said second receiver (11) intersects the duct at
the same position along said duct as an acoustic signal path to said second microphone
(9).
9. The hearing instrument according to claim 8, comprising a compensation controller
(13), wherein an input of said compensation controller is in operative connection
with said signal path, and wherein an output of said second microphone (9) is in operative
connection with a further input of said compensation controller.
10. The hearing instrument according to claim 9 wherein the signal processing means and
the compensation controller (13) each comprise a digital signal processing stage,
and wherein the digital signal processing stages of the signal processing means and
of the compensation controller are both formed by a common digital signal processor.
11. The hearing instrument according to claim 9 or 10, wherein the compensation controller
(13) is operable to convert a signal from the signal path into a compensation signal
fed to the second receiver (11), and wherein the compensation controller is further
operable to adapt, based on an error signal input from the second microphone, at least
one of processing parameters and of a processing structure for said processing the
signal into an electrical compensation signal.
12. The hearing instrument according to any one of claims 9-11, wherein said compensation
controller comprises an adaptive filter (21), and is operable to adapt, based on an
error signal input from the second microphone (9), filter constants of said adaptive
filter.
13. The hearing instrument according to any one of claims 8-12, wherein, if l1 denotes a longitudinal distance between an outer end of the duct (8) and a position
at which the second microphone records the acoustic signal, and if l2 denotes a longitudinal distance between said position and an inner end of the duct,
then both, l1 and l2 each are greater than or equal to a diameter of the duct.