Estimation of Cross-Talk Compensation Filter using
Bone Conduction Ear Microphone
IRWANSYAH+,∗ ; Tsuyoshi USAGAWA++ Kumamoto University, Japan

ABSTRACT
Having a bone conduction (BC) transducer anywhere on the head will result in a BC sound reaching the cochlea inboth ears. This “cross-talk” phenomenon may limit a listener’s ability with a pair of BC transducer to sense sounddirection. In this paper, we discuss a way to minimize cross-talks in BC sound reproduction using a method called“cross-talk cancellation.” Ideally, the method requires transfer functions (TFs) from each of the BC transducers toeach of the listener’s cochleae to accurately synthesize cross-talk compensation (CTC) filters; however, a direct
measurement of the TFs at the cochlea is not possible. Since the ear canal is the closest to the cochlea, we thus
hypothesize that the TFs measured at the ear canal might be used to achieve cross-talk cancellation at the cochlea
in the inner ear. Therefore, we utilized a BC ear microphone to capture the vibration of the bony ear-canal causedby vibrating transducers on the mastoid. The filtered-x least mean square (FxLMS) algorithm was then used toestimate the CTC filter. Experiments with and without cross-talk cancellation were done to determine the effective
frequency range that could achieve cancellation in the ear canal of three normal-hearing participants.
Keywords: Cross-Talk Cancellation, Bone Conduction Ear Microphone, FxLMS

1. INTRODUCTION
Binaural hearing refers to our ability to perceive a sound with two ears. It enables us to use the relative
differences in the sound as it is transmitted by air conduction (AC) to both ears as cues to sense the sounddirection. The AC-sound-arrival time and intensity differences between the two ears, known as binaural cues,
provide us robust information regarding the sound direction. Besides, when a target speech source has been
localized and is in a different direction from interferers or noise, the speech can be more easily understoodeven in the presence of multiple interferers (1). Such benefits are usually found in normal individuals whohear with air conduction. However, the benefits of binaural hearing for sound heard through bone conduction(BC) are less than for that heard through air conduction (2); such as in the case of patients with conductive
hearing loss who may require bone conduction hearing aids/devices to perceive sound.
Conductive hearing loss (CHL) is a decrease in perceived sound due to a problem in the outer or middle
ear that prevents the sound from reaching the cochlea in the inner ear. For individuals with permanent CHL,
a bone conduction device is an efficient alternative to regular AC-based hearing aids. It works by deliveringsound directly to the cochlea in the inner ear by bone vibration; however, unlike AC sound, BC sound froma single BC transducer can reach the cochlea in both ears as illustrated in Fig. 1. As a result, binaural cues
tend to be distorted by the BC pathway. This “cross-talk” or “cross-hearing” phenomenon is then consideredby researchers as one of the factors responsible for limiting the benefits of binaural processing for patients
fitted bilaterally with BC devices (2, 3, 4).

Figure 1 – (a) Air conduction hearing; (b) Vibrating transducers on the mastoid transmitting BC sounds.
∗ irwansyah@ieee.org

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Figure 2 – Cross-talk in binaural sound reproduction through (a) a two-loudspeaker system analogous to thatusing (b) a pair of BC transducers.
Sound from a BC transducer is transmitted not only to the ipsilateral cochlea but also to the contralateralcochlea as “cross-talk” sound. For this reason, researchers sometimes argue whether the binaural applicationof BC devices is beneficial or not. See Fig. 2(b); if the greater the intensity difference between the transfer
functions of HRR (z) and HRL (z) (from the right BC transducer to the ipsilateral and contralateral cochleae),
the more beneficial the binaural hearing will be. Over the past three decades, many attempts have been madeto study this “cross-talk” issue. Nolan and Lyon (1981) (5) and Stenfelt (2012) (6) used subjective measures
to investigate the transcranial attenuation, which is defined as the difference in sensitivity between BC sounds
transmitted from one BC transducer to the ipsilateral and contralateral cochleae. Stenfelt found the mediantranscranial attenuation to be 3–5 dB at frequency up to 500 Hz with BC stimulation at the mastoid; it isabout 0 dB at 0.5–1.8 kHz, and close to 10 dB at the frequency between 3 and 5 kHz (6). Even though theattenuation is rather small, depending on the individual, but it can also indicate that listeners with two BC
devices might be able to preserve binaural cues to some degree in the presence of cross-talk sound.
It is well known that binaural cues play an important role not only in sound localization but also in theseparation of speech in noise. When target speech and noise have distinct binaural cues or they are spatiallyseparated, normal-hearing individuals can understand speech better at a low signal-to-noise ratio (SNR) (7).
Stenfelt and Zeitooni (2013) showed in their speech-based tests that the binaural benefit with BC stimulation
for normal-hearing participants was only about half of the benefit with AC stimulation in terms of SNR (8).
On the other hand, localization tests conducted with three experienced bone-anchored hearing aid (BAHA)
users in (3) and with twelve normal-hearing individuals in (4) revealed that listeners show relatively good
localization accuracy only for angles less than 45◦ . Although binaural benefits are present as reported in(3, 4, 8), improvements are still necessary in order to assist listeners in better localizing and segregating adesired target speech from the interfering speech and/or noise.
Cross-talk that occurs between a two-loudspeaker system and the listener’s ears as illustrated in Fig. 2 isbasically analogous to that in the human head during BC sound reproduction. So, a “cross-talk cancellation”
method which is a solution to minimize cross-talks in reproduction of binaural sounds via loudspeakers (9),might also be applied to BC sound reproduction. Liao (2010) showed that the cross-talk cancellation method
can be applied to a dry human skull, achieved by placing accelerometers at the locations of the cochleae toestimate BC transfer functions (TFs) (10). In the skull, it works at the frequency between 0.25 and 1.5 kHzwith the cross-talk attenuation of up to 12 dB, depending on frequency (10). However, applying the method
to humans is an still issue since it is not possible to directly observe BC sound at the cochlea. Therefore,
a non-invasive way to estimate TFs is required and preferable.
The closest and easily accessible position that might help to estimate BC sounds at the cochlea in live
humans is in the ear canal. Reinfeldt (2013) reported that the vibration of the cochlea due to BC stimulation
at the mastoid is relatively similar to the vibration of the bony ear-canal (11). We thus hypothesize that the
TFs measured from each of the BC transducers to each of the listener’s ear canals might be used to achievecross-talk cancellation at the cochlea in the inner ear. We then utilized a BC ear microphone to capture thevibration of the bony ear-canal caused by BC transducers on the mastoid; the time-stretched pulse (TSP) isused as a stimulus to estimate the TFs (12). The filtered-x least mean square (FxLMS) algorithm was thenused to estimate a cross-talk compensation (CTC) filter, which is used for generating a cancellation signal.
Experiments with and without cancellation were also done to determine the effective frequency range that
could achieve cancellation in the ear canal of three normal-hearing participants.

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Figure 3 – (a) Image of the right side view of a participant wearing a helmet headband with attached BC
transducers. (b) Illustration of the unilateral cross-talk cancellation for the left ear. (c) Block diagram of theunilateral cross-talk cancellation. (d) Block diagram of the cross-talk compensation (CTC) filter estimation
using the FxLMS algorithm. (e) Diagram of the measurement system setup.

2. UNILATERAL CROSS-TALK CANCELLATIONIn this paper, we mainly focus on implementing a one-sided or unilateral cross-talk cancellation system
in order to confirm whether or not BC sounds observed in the ear canal can be used to achieve cancellation
at the cochlea in the inner ear. To do this, participants required to wear a helmet headband with attached BC
transducers to help maintain a stable acoustic coupling between the transducers and the head, see Fig. 3(a).
BC transducers were placed on the left and right mastoid, and a BC ear microphone was inserted only into
the left ear canal; the left and right sides are referred to as the ipsilateral and contralateral sides, respectively.
Figure 3(b) illustrates the unilateral cross-talk cancellation system (for the case of the left ear) in whicha cross-talk sound from a contralateral BC transducer is canceled by an anti-sound from the ipsilateral one.
To achieve such cancellation, the cross-talk compensation (CTC) filter WRL (z) is introduced between the rightear input signal sr (n) and the ipsilateral (left) BC transducer as shown in Fig. 3(c). The input signal sr (n)is then passed to the CTC filter to create the anti-sound (or cancellation signal). Details about how the CTC
filter, WRL (z), is estimated are explained in the sub-section below.

2.1 Cross-Talk Compensation Filter
In this paper, the CTC filter is a linear filter whose coefficients are adjusted based on the FxLMS algorithm to achieve destructive interference in the target ear. Figure 3(c) presents the diagram of the unilateralcross-talk cancellation for the left ear, where WRL (z) is the CTC filter. Because it is not possible to directly
observe a BC sound at the cochlea, we then define HRL (z) and HLL (z) as the transfer functions (TFs) fromthe contralateral and ipsilateral BC transducers to the BC ear microphone, respectively. In our case, the CTC
filter wRL (n) is not estimated in real time, but we first measure the impulse responses hRL (n) and hLL (n)– HRL (z) and HLL (z) in the frequency domain – using TSP sound. Then, the obtained impulse responses areused to estimate the CTC filter by computer simulation.
The diagram of the CTC filter estimation using the FxLMS algorithm is shown in Fig. 3(d). The weightvector of the CTC filter at time n is defined as wRL (n) = [wRL0 (n), wRL1 (n), · · ·, wRLNw −1 (n)]T where Nw is
the filter tap-length. Since the CTC filter is estimated by simulation, a generated white noise is used as thereference signal sr (n) and then filtered by the obtained impulse response hRL (n) to simulate the cross-talksound s0r (n). On the other hand, the cancellation signal anti s0r (n) is obtained by filtering the white noise sr (n)with the CTC filter wRL (n).
0Tanti sr (n) = sr (n) wRL (n)(1)

where sr (n) = [sr (n), sr (n − 1), · · · , sr (n − Nw + 1)]T . The error signal e(n) observed by the BC microphone in
the left ear canal is defined as(2)e(n) = s0r (n) + anti s0T
r (n) hLL (n)
0
0
0
T
T
where anti s0T
r (n) = [ anti sr (n), anti sr (n − 1), · · · , anti sr (n − NLL + 1)] , and hLL (n) = [hLL0 , hLL1 , · · · , hLLNLL −1 ]with a length of NLL . The error signal e(n) needs to be minimized by updating the weight vector of the
CTC filter wRL (n) using the FxLMS algorithm to achieve the cross-talk cancellation at the location of the
BC ear microphone. The update algorithm is defined as follows

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wRL (n + 1) = wRL (n) − µ x(n) e(n)(3)+ 1)]T , and µ denotes the step size that determines the convergence

where x(n) = [x(n), x(n − 1), · · · , x(n − NLLof the algorithm. x(n) is the filtered version of sr (n) defined asx(n) = sTr (n) c(n)(4)

where c(n) = hLL (n). Once the error signal e(n) converges, the final CTC filter coefficients wRL will be used
to estimate cancellation signals in order to confirm whether or not the cross-talk sound can be canceled in
the left ear canal and its attenuation can also be felt by participants.

2.2 Instrumentation
The measurement system setup using BC ear microphone is shown in Fig. 3(e). For observing BC sounds,
the BC ear microphone – Vibraaudio EM-21 (13) – was used. Sound stimuli were presented by Cynaps BC
transducers (14). A 3W stereo amplifier, PAM8403, with power bank was used to drive the BC transducers.
Playback and recording were done through an external sound card, Sound Blaster Omni Surround 5.1, which
has a USB computer connection; and recorded signals were stored in a laptop.

3. Measurements
3.1 Participants
Three self-reported normal hearing individuals participated in the experiments. The age range is between
23 and 29 years old. They were wearing a helmet headband with attached BC transducers as illustrated inFig. 3(a); and had the BC ear microphone in their left ear canal. The participant sat in a quiet room.
3.2 Parameters
16-bit playback and recording were used at the sampling rate of 16 kHz. The length of the TSP signal
used for estimating HRL (z) and HLL (z) was 16383 samples (equal to about 1 s); see examples of a recorded
TSP signal in Figs. 4(a) and (b). Both estimated BC transfer functions – see Fig. 4(c) – were then used toestimate the CTC filter by computer simulation. Estimation of the CTC filter was made by minimizing theerror signal e(n) – see Fig. 4(d) – using the FxLMS algorithm in which a 15-s white noise was used as the
input signal. The filter tap-length was considered to be 256 samples (or 16 ms) such as shown in Fig. 4(e).
Then, a 14-s pink noise as test signal for all participants was filtered by their own CTC filter, and its result
was considered to be the “individualized” cancellation signal; see Fig. 4(f) as an example.

Figure 4 – Spectrograms of the TSP signal produced by (a) ipsilateral or (b) contralateral BC transducer andrecorded by the BC ear microphone in the left ear of the participant 1. (c) Frequency responses of HLL (z)and HRL (z). (d) Error signal e(n) converging in 15 seconds. (e) The estimated CTC filter coefficients wRL (n).
(f) Pink noise (for contralateral BC transducer) and its estimated cancellation signal (for ipsilateral one).

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Figure 5 – (a) Frequency responses of ipsilateral BC transfer function HLL (z) (blue) and contralateral BC
transfer function HRL (z) (red); means are shown as solid lines (—) while standard deviations are provided
as band around the lines. (b) Impulse response of estimated 10 CTC filters shown as the mean (µ) with its
standard deviation (σ ). (c) Attenuation of the cross-talk sound observed by the BC ear microphone in theleft ear canal of each participant.

3.3 Procedure
Single TSP signal (12) was presented alternately to the left and right BC transducers and simultaneously
recorded by the BC ear microphone in the left ear canal of each participants in order to estimate HLL (z) andHRL (z). Once they were obtained, both were used to run an “offline” FxLMS algorithm (a simulation) with
15-s white noise as input to estimate the CTC filter which is unique for each participant. A 14-s pink pinknoise as a test signal was then filtered by the CTC filter to estimate the “individualized” cancellation signal.
After that, the pink noise and its cancellation signal were provided to the right and left BC transducers,
respectively, and at the same time, the BC ear microphone picked up the interference of the two signals to
confirm whether or not destructive interference occurs in the left ear canal. The picked-up signals with andwithout cross-talk cancellation were analyzed in each one-third octave band to obtain the attenuation of thecross-talk signal. All test sounds presented to the BC transducers were at a level corresponding to 60 dB
sound pressure level (SPL) adjusted based on a loudness balancing technique (15). This measurement was
repeated ten times in which the BC ear microphone was being reinserted into the left ear canal between
repeated measures for each participant.
3.4 Result and Discussion
We hypothesized that the BC transfer function measured from the ipsilateral or contralateral BC transducer to the left ear canal might be used to achieve cross-talk cancellation at the cochlea in the inner ear.
We thus estimated the BC transfer functions using a TSP sound presented alternately to the ipsilateral andcontralateral BC transducers. Figure 5(a) shows the frequency responses of ipsilateral BC transfer function
HLL (z) (blue) and contralateral BC transfer function HRL (z) (red) where means are shown as solid lines andstandard deviations are provided as band around the lines. As expected, because the ipsilateral BC transducer
is close to the BC ear microphone (see Fig. 3(b)), HLL (z) for all participants looks very stable, indicated bysmall changes in standard deviation across frequency. On the other hand, since the contralateral BC transducer is on the opposite side of the BC ear microphone (see Fig. 3(b)), HRL (z) at high frequencies were
mostly attenuated by the participant head, and therefore, has large standard deviations. But, both HLL (z) andHRL (z) at low frequency below about 2 kHz were almost at the same level, which means that BC cross-talks
in the three participant heads were quite dominant for the low-frequency range.

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Using the transfer functions HLL (z) and HRL (z), see Fig. 5(a), estimated by TSP sounds, we then utilized
a computer simulation to estimate the cross-talk compensation (CTC) filter based on the block diagram of
the FxLMS algorithm shown in Fig. 3(d). Because we also intend to check how stable the estimated CTC
filter is when the BC ear microphone was being reinserted between repeated measures, we therefore madeten times measurements (or ten times CTC filter estimations) to the three participants. Figure 5(b) presents
the impulse response of estimated 10 CTC filters shown as the mean (µ) where the standard deviation (σ ) isprovided as gray band around the mean line. From the figure, we can see clearly that when the positions of
the BC transducers remained fixed, small changes in the position of the BC ear microphone in the left ear
canal have little influence on the impulse response of the CTC filter, indicated by small standard deviations
for all participants. This means we may not need to frequently update the coefficient values of the CTC filter
except when the positions of BC transducers change.
Estimation of the CTC filter was done ten times for each participant, and at the same time, using theestimated filter we also created ten “individualized” cancellation signals with a 14-s generated pink noise as
the filter input signal. To perform unilateral cross-talk cancellation experiments, the pink noise was presented
to the contralateral BC transducer (on the right-ear side) and recorded by the BC ear microphone in theleft ear canal as the cross-talk. On the other hand, the cancellation signal was presented to the ipsilateral
BC transducer (on the same side as the BC ear microphone). In this paper, the difference in level between
the recorded BC sounds with and without cancellation signal is defined as the attenuation of the cross-talksound. Figure 5(c) shows the attenuation result of the cross-talk sound observed by the BC ear microphonein the left ear canal of each participant. From the figure, it can be seen that BC cross-talks can be attenuatedin the left ear canal of all participants. The attenuation mostly occurs in the low-frequency range (below
about 2 kHz), but it looks ineffective for frequency below about 800 Hz. We suspect that this ineffectivenessmight be related to the BC transducer’s frequency response, which is 300 Hz to 1.9 kHz (14). Although
the cancellation presented in Fig. 5(c) occurred in the ear canal, all participants reported they could feela decrease in the perceived loudness due to the cross-talk cancellation. However, further experiment is stillnecessary in order to determine the effective frequency range that could achieve cancellation in both the ear
canal and inner ear perceived by participants; such as using tone reception thresholds (TRT) (16).

4. CONCLUSION
This paper discusses the use of bone conduction (BC) ear microphone for estimating a cross-talk compensation (CTC) filter by which a cancellation signal with a similar amplitude but with inverted phase to a BC
cross-talk sound can be estimated to achieve destructive interference in the target ear. We have demonstrated
that it is potentially possible to achieve the cross-talk cancellation at the cochlea by canceling the cross-talksound in the ear canal since it is the closest position to the cochlea. To estimate the CTC filter, we assumed
that the vibration of the bony ear canal observed by BC ear microphone in the ear canal is relative to thevibration of the cochlea during BC stimulation. As a result, using a TSP sound, BC transfer functions can be
estimated, and hence, the CTC filter can also be estimated using the filtered-x least mean square (FxLMS)
algorithm. Experiment results show that low-frequency attenuation was observed by the BC ear microphonewhen a pink noise and its cancellation signal were presented through BC transducers. Also, all participants
reported that they could feel a decrease in the perceived loudness. Because the BC ear microphone wasplaced in the ear canal, not exactly at the cochlea, further investigation is still necessary to determine theeffective frequency range that could achieve cancellation in both the ear canal and inner ear perceived byparticipants; such as using tone reception thresholds (TRT) which remains as our future work.

ACKNOWLEDGEMENTS
The authors thank Kamelia Rosiana, Fajar Purnama, Muhammad Bagus Andra, Zheng Chenyang and
Akito Kawamura for their helpful contributions during measurements.

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