J. Biomedical Science and Engineering, 2010, 3, 861-867
doi:10.4236/jbise.2010.39116 Published Online September 2010 (http://www.SciRP.org/journal/jbise/ JBiSE
Published Online September 2010 in SciRes. http:// www.scirp.org/journal/jbise
Using active echo cancellation to minimize stimulus
reverberations during hearing studies conducted
with the auditory brain response (ABR) technique
Mark R. Patterson1, Andrij Z. Horodysky1, Bruce W. Deffenbaugh2, Richard W. Brill3
1Virginia Institute of Marine Science, College of William & Mary, Gloucester Point, USA;
2Biomechatronics Group, Media Laboratory, Massachusetts Institute of Technology, Cambridge, USA;
3Northeast Fisheries Science Center, National Marine Fisheries Service, NOAA, Woods Hole, USA.
Email: rbrill@vims.edu
Received 26 March 2010; revised 11 March 2010; accepted 15 March 2010.
Because of the physical properties of water as sound
conducting medium and the proximity of tank walls,
creating an anechoic environment underwater is both
technically difficult and expensive to implement. Con-
ducting hearing studies of aquatic animals can there-
fore be challenging due to stimulus reverberations.
To address this issue, we developed MATLAB scripts
capable of pre-compensating acoustic stimuli result-
ing in location-specific echo cancellation. Our proce-
dures are specifically designed for hearing studies
conducted with the auditory brain response (ABR)
technique. Broadband white noise is used to charact-
erize the system response and the digitized acoustic
signal subsequently used to generate an acoustic inv-
erse file capable of cancelling reverberations. Echo
cancellation is nearly perfect, although location-spe-
cific. The effectiveness of echo cancellation dimin-
ishes with distance from test subject and hydrophone
(or microphone) used to create the pre-compensated
signal. This distance must be minimized and should
preferably be less than 5 cm. The spectral composi-
tion of the sound signal is not greatly affected, how-
ever. We have successfully used the procedure during
hearing studies of sev eral fish species, includ ing yellow-
fin tuna (Thunnus albacares). ABR experiments on
the latter were done at sea aboard an oceanographic
research vessel, a highly echoic environment.
Keywords: Acoustic; Aquatic; FFT; Fish; Noise; Tank
In studies of hearing in terrestrial organisms, anechoic
chambers are often used so that the response of the sub-
ject to the source stimulus, and not echoes, can be accu-
rately assessed [1]. Creating an anechoic environment
underwater is more problematic, however, because of the
physical properties of water as sound conducting med-
ium [2,3] and the proximity of tank walls. Similar to the
situation in air, it is possible to create anechoic chambers
using acoustic wedges [4], compliant materials that rap-
idly dampen wave propagation [5], piezoelectric ce-
ramic with embedded shunted resistors [6], or active
techniques involving coatings [7]. New smart materials
are also available to dampen echoes and can yield a
17-24 dB reduction [8], but they are hard to obtain.
Moreover, all the above implementations are expensive,
especially for hearing studies of aquatic organisms.
One cost estimate for an anechoic flow-through un-
derwater chamber was US$ 330K [9]. As a result, re-
searchers interested in either the hearing abilities or the
effects of sound on the behavior and health of aquatic
organisms [10] are often forced to choose less expen-
sive measure s such as placemen t of the test aquariu m in
an anechoic room [11]. Even this solution, however,
does not eliminate echoes within the tank itself. The
generation of microbubble curtains is effective for ul-
trasonic frequencies [12], but many aquatic organisms
hear only much low er frequen cies [13].
As a low-cost alternative, we developed a dig ital signal proc-
essing technique, implemented in MATLAB (Math Works,
Natick, Massachusetts, USA), capable of building stimulus
waveforms that cancel reverberant echoes. Thus, only a win-
dowed tone burst is perceived by the test subject. Originally
developed for hearing experiments in fishes, our technique is
also applicable in air. The system is readily implemented with an
omnidirectional hydrophone (or microphone) connected to an
analog-to-digital converter with anti-aliasing low-pass filter, a
sound source connected to a digital-to-analog converter with
impedance matched amplifier, and software for recording
M. R. Patterson et al. / J. Biomedical Science and Engineering 3 (2010) 861-867
Copyright © 2010 SciRes. JBiSE
waveforms from the hydrophone and playing signals
back through a sound emitter. We have successfully im-
plemented our active echo-cancellation procedure with a
signal-processing system (model: System II) from Tuck-
er Davis Technologies (Alachus, Florida, USA).
Conducting hearing studies within a specialized anec-
hoic environment also generally presupposes working
with species that can be transported to the laboratory.
The ability to access hearing abilities of a broader range
of aquatic animals than fit this criteria is becoming in cr-
easingly important because of the need to understand
and predict the d isruptive ef fects of anthropog enic noise,
especially in the marine environment [14,15]. For exam-
ple, the proposed expansion of wind-generated electrical
power (i.e., “wind farms”) to continental shelf areas
appears likely to increase ambient noise to levels that
could diminish significantly the range of effective com-
munication in soni ferous fish es [ 16 ].
With the auditory brain response (ABR) technique,
neural activity is monitored electrically in response to a
series of specified exposures to sounds of unique single
frequency, duration, and amplitude in order to construct
a hearing response curve. Our active echo cancellation
procedure makes ABR experiments more readily doable
outside of an anechoic environment. It therefore permits
a wider range of organisms to be investigated, including
aquatic organisms that are difficult (or impossible) to
transport to shore side tanks, maintain in captivity, or
Every chamber or aquarium used for hearing studies has
a unique system response to acoustic perturbations that
must be characterized prior to generating a stimulus that
will be perceived at a particular location as echo-free.
Sounds emitted by the stimulus source will reflect from
the walls and floor. Thus, at a particular location in the
chamber, the sound heard will consist of the sum of the
emitted waveform plus reflections. Reflections that
involve multiple surfaces are sometimes termed reverb er a -
tio n, but for the purposes of this paper (and in the litera-
ture, generally), these reflected waveforms are called
Echo-cancellation was invented at Bell Labs in the
1960’s. In its most basic form, a time-delayed sample of
the emitted signal is subtracted from the received signal.
The Least Mean Squares [17] and the Normalized Least
Mean Squares algorithms that followed are now imple-
mented in software or hardware in a host of modern
devices including faxes and telephones to reduce echoes
generating by reflections from impedance mismatching
in the communications channel, and other effects. The
net result of echo-cancellation is an improvement in the
signal-to-noise ratio of the received signal. More recen-
tly, nonlinear echo cancellation [18] can adaptively can-
cel noise in a dynamic environment where the echoes are
constantly changing. For example, these algorithms are
used in noise-canceling headphones used in noisy envi-
ronments. Although there is some evidence that marine
mammals like dolphins have neural circuitry able to cancel
noise from unwanted reflections from their own emitted
sound signals [19], most organisms will perceive the
sum of all arriving waveform s with thei r sensory appatus.
Most hearing studies are conducted over a range of
frequencies; thus it is appropriate to characterize the sys-
tem response of the test chamber using a white-noise
source. By measuring the system response to white noise
at a specific location in the chamber, it is possible to
then create an acoustic inverse waveform that nulls
reverberations at that location through destructive inter-
ference. This “inverse wave” is convolved with a desired
stimulus wave (in frequency space), to produce a pre-
compensated stimulus waveform. When transformed in-
to a time domain signal and played into the chamber, an
echo-free stimulus wave will be perceived at the location
where the acoustic system response was measured.
A set of four interconnected MATLAB scripts: BBTEST.m,
GMENT.m are used to produce a pre-compensated stim-
ulus for location-specific echo cancellation. Each script
is described below in chronological order of operation.
Copies of the scripts are available for download at:
as “MATLAB scripts.zip”.
3.1. BBTEST.m
The BBTEST.m script makes a broadband noise file in
signed 16 bit integer format that is then played through
the sound emitter to measure the system acoustics (i.e.,
the acoustic transfer function of the tank and the room).
This script must be run prior to running the MAKES-
TIMULUSFILE.m script described immediately below.
The parameters settable by the user, are: the system fre-
quency (in Hz, given by sampersec/2), the lowest and
highest frequencies of the “broadband” stimulus to be
used for a given set of experiments (lowfreqcps and
hifreqcps, respectively, in Hz), the length of the test
noise (samlensec, in seconds), and the peak amplitude of
the generated signal in volts DC (peakvolts), where –10
to +10 volts DC is assumed to correspond to values of
32,768 to 32,767 in the 16 bit generated noise file, which
is named “bb.16”. The BBTEST.m script checks for
violations of the Nyquist sampling criterion (sampling
M. R. Patterson et al. / J. Biomedical Science and Engineering 3 (2010) 861-867
Copyright © 2010 SciRes. JBiSE
must be at least twice the desired highest frequency for
testing) when setting the lowest and highest frequencies,
and the system frequency, and advises the user appropri-
ately. It also checks that the peak voltage amplitude is
set between 0 and 10 volts DC.
The generated noise file has all frequencies repre-
sented with equal amplitude. The script achieves this by
generating a flat spectrum (in frequency space), with
frequencies outside the desired window truncated to zero,
with zero DC offset. The resulting spectrum is translated
back to the time domain using an inverse discrete Fou-
rier transform, implemented using a fast Fourier trans-
form, and the real part is scaled appropriately by both
the peak voltage amplitude, followed by the signed six-
teen bit integer format.
The MAKESTIMULUSFILE.m script writes the acoust-
ically pre-compensated stimulu s files. This script creates
the following three subdirectories and a parent file direc-
tory labeled “director y”:
1) “bbplayfile” a subdirectory that holds the stochastic
differential broadband noise file used to characterize the
system acoustics. The BBTEST.m script (see above) is used
to produce a broadband noise file in 16 bit signed integer
format that is stored in this subdirectory, under the name
2) “bbrecvfile” a subdirectory that contains the
received signal caused by playing “bbplayfile”. It is
equal in length to the file “bbplayfile”, and is stored un-
der the name “bb.txt”,
3) “stimulus” a subdirectory where the generated
stimulus file is placed along with two other files: “ones
0.16” and “ones1.16”, which are files filled with -1 and
+1 values, respectively. These two files are used to inv-
ert the generated sound stimulus signals during presenta-
tion to the test subject during an experiment, when the
inverted pre-compensated stimulus file is sent to the
sound emitter. (Their purpose is to allow mechanical
artifacts arising from the sound stimuli themselves to be
later removed from the ABR.)
The script allows the user to set the following:
1) system frequency (sampersec/2, in Hz),
2) stimulus duration (stimdurm s, in m s),
3) a vector containing the stimulus frequencies to be
generated (stimfreqs, in Hz),
4) the phase at the center of the stimulus window (sti-
mcentphasedeg, in degrees),
5) the type of window functio n to be app lied to the sti-
mulus [Blackman, Hann, Hamming, None, Rectangular
(also called “Rectangle”) windows are available], and
6) whether the resulting stimulus file will be acousticall y
pre-compensated based on the acoustic characterization
of the system contained in the file “bbrecvfile”.
The ACOUSTICINV.m script does not directly interact
with the user, unless the transmitted and received broad
noise files are not of the same length, in which case the
user is notified of the error.The ACOUSTICINV.m
scr ipt creates the acoustic inverse response of the system
at the location of the test subject. This script opens the
file of signed 16 bit integers (“playfile”), the broadband
noise file previously played through the sound emitter,
as well as a file of equal length that was received at the
hydrophone (“recvfile”). The script then reads the data
into two signal matrices, and scales the received signal
such that its maximum value is equal to that in the pla-
yed signal. The discrete Fourier transform of the played
signal is then computed using a fast Fourier transform.
The resulting transform is subsequently masked (thro-
ugh convolution) to deal with the zero es that result from
the band-limiting of the played signal. The mask has
ones where the discrete transform component was not
too small (normalized coefficient > 1/200). A mask is
also applied to the received signal to zero out any fre-
quency components that were weakly received (normal-
ized coefficient < 1/50). The generated inverse signal
has a DC component equal to zero, is symmetric in time
around the origin, and is inverted. If this pre-compensated
signal were played at the same time that the original
sound source was played, no sound would be detected at
the hydrophone (or microphone). This waveform is thus
noise-canceling via destructive interference for the sys-
tem at the location of the hydrophone (or microphone).
After making an acoustic inverse file using the AC-
script checks that the length of the desired stimulus is
not longer in time than the acoustic pre-compensation
signal used to characterize the system. It then runs the
SEGMENT.m script described below. Note that the SEG-
MENT.m script is called automatically and uses para-
meters previously supplied by the user in the MAKE-
STIMULUSFILE.m script, such as the frequency of the
stimulus to be generated, the length in ms, etc.
3.4. SEGMENT.m
The SEGMENT.m script generates sine wave signals
that are windowed by various apodization functions.
This script can handle signal lengths that are even or odd
in size and produces waves either with no windowing; or
with Rectangular, Hann, Hamming, or Blackman win-
dowing. It only notif ies the user if an unknown windo w-
ing argument is used in the MAKESTIMULUSFILE.m
script. The SEGMENT.m script scales the signal appro-
M. R. Patterson et al. / J. Biomedical Science and Engineering 3 (2010) 861-867
Copyright © 2010 SciRes. JBiSE
priately for a 16 bit D/A converter with a r ange of -10 to
10 volts DC.
After the windowed sine wave is created, its fast Fou-
rier transform is computed and convolved with the aco-
ustic inverse file. The resulting waveform is inverse Fou-
urier transformed. The real part is taken and stored as the
pre-compensated stimulus waveform in signed 16 bit int-
eger form as a binary file. Finally, the script writes “ones
0.16” and “ones1.1 6” files, which are files filled with -1
and +1 values, respectively, into the same directory as
the pre-compensated stimulus files. These files are used
to create in-phase and out-of-phase stimulus signals to
allow for elimination of electrode movement artifacts
caused by the sound stimulus itself through subtraction.
To summarize, the scripts must be executed in the
following order:
1) Use the BBTEST.m script to produce a broadband
noise file in .16 binary format with the user-supplied ba-
ndpass, sample rate, sound duration, and sound ampli-
tude. The file produced is called “bb.16” and resides in a
user-specified directory.
2) Play the “bb.16” file into the test chamber by send-
ing the data within file to the sound emitter, taking care
to set the D/A output rate to the same sample rate that
was used to create “bb.16”. (Note by the Nyquist sampl-
ing criterion, the sample rate needs to be twice the high-
est frequency to be presented, and preferably at least ten
times. For example, a sample rate of 20,000 Hz gives a
system maximum frequency of 10,000 Hz, so it would
be prudent to outpu t a maximum frequency of 1,000 Hz.)
Record the received wave form concurrently from the
hydrophone and name it “bb.txt”. Place it in the same
directory as “bb.16”.
3) Run the MAKESTIMULUSFILE.m script, specify-
ing the directory where the “bb.txt” and “bb.16” are now
located, the stimulus duration, the type of windowing,
the peak stimulus amplitude, the set of discrete frequen-
cies desired in the stimulus files created (each tone will
be in its own file), the sample rate to be output, and the
phase in degrees at the center of the stimulus. Note that
this script also makes two waveforms, each equal in len-
gth to the stimulus file, filled eith er with -1’s or 1’s. The
stimulus waveform can be multiplied by these files to
create waveforms that are 180 degrees out of phase (i.e.,
of opposite polarity) which can be successively pre-
sented to the test subject. This procedure can be used to
eliminate electrode movement artifacts caused by the
sound stimuli. These files are created solely for conven-
ience of the experimenter and are not used in creating
the pre-compensated stimulus waveforms.
Our procedures are easily executed by end-users with
only modest familiarity with MATLAB, once the system
is fully implemented. Alternatively, script execution can
be automated, thereby requiring only minimal input from
the end-user. The software also allows for the use of
Blackman, Hanning, Hamming, and Rectangular win-
dow functions to be applied to the stimulus. These win-
dow functions have differing effects on the minimum
stop-band attenuation (dB) and the width of the transi-
tion bandwidth window (Hz). For example the Rectan-
gular window has the poorest minimum stopband at-
tenuation (21 dB), but the best (smallest) transition
bandwidth based on order of filtering, approximately
1.84 (system frequency in Hz) / (order of the filter). By
contrast, the Blackman filter has the best attenuation
(74dB) and for the same order filter and system fre-
quency, a transition bandwidth 605% greater than the
Rectangular window (11.13 x system frequency/filter
order). (The Rectangular window is also known as a
Rectangle window and a Dirichlet window.) All of the
included window functions in the code presented are
examples of high to moderate resolution windows, with
lower dynamic range than some other filters such as the
Nuttall or Blackman-Harris windows. This flexibility in
windowing the acoustic stimulus allows users to tailor
the stimulus to the sensory ability of th e organism under
study [20], and has the advantage that both spectral
sidelobes and speaker transients are reduced [21].
The effectiveness of the active echo cancellation pro-
cedure (using the Tucker Davis Technologies model Sys-
tem II described in [13]) is illustrated in Figure 1. In the
absence of an anechoic chamber, windowed electronic
sinusoidal waveforms delivered to the speaker (A) result
in highly distorted acoustic waveforms at the hydroro-
phone (B) due to reverberations. In contrast, when pre-
compensated electronic waveforms are delivered to the
speaker (C), the resultant acoustic waveforms received
by the hydrophone (D) are excellent representations of
the desired windowed sinusoidal waveforms (A).
The sound pressure and particle acceleration thresholds
of six sciaenid fishes commonly found in Chesapeake Bay
(eastern USA) have been successfully assessed using the
ABR and the system described above [13]. Likewise, the
system has been used successfully at sea aboard a large
oceanographic research ship (the NOAA research vessel
“Oscar Elton Sette”) to assess hearing in yellowfin tuna
(Thunnus albacares) with the ABR technique (R. Brill,
unpublished). Although the steel superstructure of the ship
presented a highly echoic environment, the active echo
cancellation procedures successfully produced clear sound
pulse stimuli required by the AB R technique, allowi ng the
M. R. Patterson et al. / J. Biomedical Science and Engineering 3 (2010) 861-867
Copyright © 2010 SciRes. JBiSE
Figure 1. Waveforms generated by the scripts, resulting in
echo-cancellation at an omnidirectional hydrophone submerged
in a small test tank used to study fish hearing. A = windowed
(Blackman filter) sinusoid electronic signal delivered to the
speaker. B = windowed non-pre-compensated acoustic signal
received at the hydrophone. Note that although the tone burst
was set to10 ms, echoes conti nue well past the end of the signa l
and the fundamental frequency is corrupted by additive ef fects of
reverberations. C = pre-compensated electronic signal delivered
to the speaker for echo cancellation (the x-axis is 120 ms, instead
of 30 ms, because the source sounds produced long after the tone
burst are required to cancel tank reverberations). D.
Echo-cancelled acoustic sig nal received at the h ydrophone.
delineation of hearing threshold at specific frequencies
(Figure 2).
There are two limitations to the active echo cancellation
technique. The first is th at the juxtaposition of the sound
source and all echoic surfaces must remain fixed once the
pre-compensated audio files have been generated. Echo-
ic surfaces typically include th e water surface (for exper-
iments with aquatic organisms in tanks), major hard sur-
faces, personnel (if they were near the sound source or
test subject when the “bb.16” broadband noise file was
played), and the test subject itself. Any changes in the jux-
taposition of reflective surfaces will cause the precom-
pensated waveform file to fail to produce an echocance-
lled signal. Failure of active echo cancellation is easily
detected however, if the hydrophone or microphone used
t o create the pre-compensated waveforms is left in place and
the acoustic stimulus waveforms observed during their
presentation to the test subject. Should such deviations
occur, the sequence of files described above can be
quickly re-run. In principle, the pre-compensated files
could be created, and the subject introduced into the ex-
per iment al set -up at the s ame pos ition as the hydrophone
or microphone. In practice, however, this is impractical
because of the likelihood of affecting the juxtaposition of
the echoic surfaces. We therefore recommend that compe-
nsation be done with subject and hydrophone (or micro-
phone) in their final positions. In our experience, this re-
quirement is not problematic as subjects generally anesthe-
tized, paralyzed, and restrained for ABR studies [13,20].
The second limitation of the pro cedu r e is th at the most
effective echo cancellation occurs at the position of the
hydrophone or microphone used to receive the original
broadband signal. The effectiveness of echo cancellation
diminishes with distance from this position (Figure 3).
Therefore, the distance between test subject and hydro-
phone (or microphone) used to create the pre-compensated
signal must be minimized and should preferably be less
than 5 cm. The spectral composition of the sound signal
Figure 2. Auditory brain response (ABR) recorded
from a yellowfin tuna (Thunnus albacares) to a 500
Hz pre-compensated tone burst using a signal pro-
cessing system (model System II) from Tucker Davis
Technologies (Alachua, Florida, USA), and pro-
cedures described in [13]. Experiments were con-
ducted at sea, in an interior compartment of a large
steel-hulled oceanographic vessel (a highly echoic
environment). Sound pressure levels were attenuated
in 5 dB steps, and repeated twice at each sound
pressure level. Congruence of the ABRs disappears at
a sound pressu re of less than 108 dB (re ferenced to 1
µPa at 1 m), indicating that this is minimal detectable
sound pressure level.
M. R. Patterson et al. / J. Biomedical Science and Engineering 3 (2010) 861-867
Copyright © 2010 SciRes. JBiSE
Figure 3. The degradation of the active-echo acoustic signals
recorded in the same situation as those in Figure 1, but with
the hydrophone displaced from its original position. These
traces demonstrate how the distance between the hydrophone
(or microphone) used during creation of the pre-compensated
audio files and the test subject can influence the acoustic signal
received by the latter. A = hydrophone in the original position
when pre-compensation acoustic files were created. B =
hydrophone moved 1 cm from its original position. C = hydro-
phone moved 5 cm from its original position. D = hydrophone
moved 10 cm from its original position.
Figure 4. The frequency composition of the active-echo acous-
tic signals recorded in the same situation as those in Figure 1,
but with the hydrophone displaced from its original position.
The frequency composition of the stimulus signal is only mod-
erately influenced by distance from the hydrophone (or micro-
phone) used to create pre-compensated stimulus files, even up
to distances of 10 cm. A = hydrophone in the original position
when pre-compensation acoustic files were created. B = hyd-
rophone moved 1 cm from its original position. C = hydrophone
moved 5 cm from its original position. D = hydrophone moved
10 cm from its original position.
is not greatly affected however (Figure 4). The decrease
in effectiveness of active echo can cellation with distance
from the hydrophone or microphone used during the
initial developmen t of the pre-compensated au dio files is
an important limitation that must be taken into consid-
eration when this procedure is employed. It is possible
that with large fishes the acoustic signal received
directly by the otolithic organs and signals resulting
from vibrations of the swim bladder [22,23] could be
different because the signal would be most effectively
echo-cancelled at the structure nearest to the hydrophon e
used when making the pre-compensated files. Thus our
technique, although highly effective at minimizing sig-
nal reverberations, should be carefully applied and in-
vestigators fully cognizant of its limitations.
This work was funded in part by the Pacific Islands and Northeast
Fisheries Science Centers (National Marine Fisheries Service, NOAA),
and the Pelagic Fisheries Research Program at the University of Ha-
waii. AZH was supported by the funds from the Virginia Marine Re-
sources Commission and the International Women’s Fishing Associa-
tion. MRP thanks J. Svavarsson, University of Iceland, for a place to
write during sabbatical. This is Contribution No.3075 of the Virginia
Institute of Marine Science, The College of William and Mary.
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