Download OTOACOUSTIC EMISSIONS AT 0.5 kHz: PROPERTIES AND

OTOACOUSTIC EMISSIONS AT 0.5 kHz: PROPERTIES AND
APPLICATIONS
W. Wiktor Jedrzejczak, Edyta Pilka, Krzysztof Kochanek, Henryk Skarzynski
World Hearing Center, Institute of Physiology and Pathology of Hearing, Warsaw/Kajetany,
Poland
Otoacoustic emissions (OAEs) are low level sounds originating in the cochlea that can be
measured by sensitive microphone in the ear canal. In humans they are used in diagnostic
purposes. They are particularly efficient in determining the hearing status in 1-4 kHz
frequency range. In the present study OAEs evoked by tone bursts at 0.5 kHz are
investigated. Time-frequency properties of the signals are discussed and some clinical
applications are introduced.
1.
Introduction
Otoacoustic emissions (OAEs) were first described by Kemp [1] and are now well established in
audiological practice e.g. [2,3]. The most commonly used OAEs are click-evoked OAEs (CEOAEs)
and distortion product OAEs (DPOAEs). OAEs can also be evoked by tone bursts, which are short
stimuli centered at a certain frequency. Some studies e.g. [4] have shown that the spectral content of
a sum of tone burst-evoked OAEs (TBOAEs) centered on the 1–4 kHz region corresponds to that in
a CEOAE. The present study investigates TBOAEs centered at 0.5 kHz. The advantage of this
particular TBOAE is that it may contain components not present in the CEOAE. The study
overviews results from previous studies of 0.5 kHz TBOAEs [5-8].
2.
Material and method
The results from normally hearing adult subjects as well as from subjects with some hearing
impairment were studied. OAEs were measured in low-noise ambient conditions. Standard click
stimuli and 0.5 kHz tone bursts (average amplitude 80±3 peak dB SPL, nonlinear averaging
protocol) were used to evoke 260 OAE responses. The tone bursts were two cycles long with equal
rise/fall times and no plateau. The initial part of the response was windowed automatically by the
system to minimize stimuli artifacts. Window onset was 2.5 ms for clicks and 5 ms for 0.5 kHz tone
bursts, and all recordings used an acquisition window of 20 ms. Half-octave-band values of OAE
response levels and signal-to-noise ratios (SNRs) were used for analysis.
For all parameters the statistical significance of the mean difference between groups was
evaluated using the Wilcoxon rank sum test. As a criterion of significance, a 95% confidence level
(p < 0.05) was chosen.
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The 22nd International Congress on Sound and Vibration
2.1 Matching pursuit
Time–frequency (TF) analysis of the recorded signals was done by decomposing the signals into
their basic waveforms. The method of high-resolution adaptive approximation was used, a
technique based on the matching pursuit (MP) algorithm [9]. A slight modification was used to
account for the asymmetrical character of some components [10]. The modified MP method
allowed the CEOAE and TBOAE signals to be decomposed into waveforms of defined frequency,
latency, duration, and amplitude. The latency was taken to be the time taken from onset of the
stimulus to the maximum point in the waveform envelope. Note that the presence of spontaneous
OAEs (SOAEs) can, when using some methods, cause a false shift in evoked OAE latency, whereas
MP with an asymmetric dictionary provides estimates that are less prone to SOAEs [10].
3.
Results
Figure 1 shows average half-octave band values response levels and SNRs of OAEs evoked by
clicks and 0.5 kHz tone bursts. CEOAE magnitudes were greatest over the range 1–4 kHz,
decreasing substantially below 1 kHz. Responses from the 0.5 kHz TBOAEs were complementary
in that the main components occurred between 0.5 and 1.4 kHz. TBOAEs were larger than
CEOAEs (on average more than 5 dB larger) at 0.5–1 kHz (the difference was significant at
p<0.05). At 1.4 kHz there was no statistically significant difference between CEAOEs and
TBOAEs. As expected, at higher frequencies TBOAE response levels and SNRs declined, while
CEOAEs were significantly higher.
Figure 1. Average half-octave response levels and SNRs CEOAEs and 0.5 kHz TBOAEs.
Figure 2 shows TF properties of CEOAEs and 0.5 kHz TBOAEs, and a clear dependence of
latency on frequency can be seen. For CEOAEs, latencies extend from around 5 ms for the 4 kHz
band to 10 ms for the 1 kHz band. From 4 kHz to 1 kHz the dependence of latency on frequency is,
almost a straight line. For lower frequencies CEOAEs were weak and nearly absent. For TBOAEs
latency is only shown for the 0.5–2 kHz range since at higher frequencies there was practically no
response (see Figure 1).
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The 22nd International Congress on Sound and Vibration
Figure 2. Average latency of CEOAEs and 0.5 kHz TBOAEs.
In Figure 3 examples of SNRs for CEOAEs and 0.5 kHz TBOAEs are shown. This subject had
hearing thresholds better than 20 dB HL for frequencies up to 0.5 kHz and higher than 50 dB HL in
the remaining frequencies. It may be seen that SNRs for CEAOEs did not exceed 0 dB. On the other
hand, 0.5 kHz SNR was better than 3 dB for broad-band analysis and even higher than 8 dB for
band-limited values.
Figure 3. Examples of SNR values for CEOAE and 0.5 kHz TBOAE, for a partial deafness subject (normal
hearing for frequencies up to 0.5 kHz and severe hearing loss above 0.5 kHz). Global values as well as halfoctave band values around 0.5 kHz are shown.
4.
Discussion
The properties of OAE parameters such as reproducibility, response level, and signal to noise
ratio in relation to different frequency bands have been investigated in several studies e.g. [11,12].
Usually it was the case that CEOAEs were not efficient at 0.5 kHz. The same property was
reproduced in the present study. On the other hand, 0.5 kHz TBOAEs provide additional
information to CEOAEs about the frequency range below 1 kHz.
The apparent imperfection of 0.5 kHz TBOAE measured here is that they have maximum around
1 kHz and not close to 0.5 kHz. This is probably because of spectral splatter from the short tone
burst stimulus and from rapidly falling responses of the cochlea and the recording system at low
frequencies. From the previous works that used 4 cycles burst it might be concluded that it is better
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The 22nd International Congress on Sound and Vibration
in evoking response that has narrower spread in frequency and has maximum closer to 0.5 kHz.
However such option is not available in current commercial equipment used in this study.
0.5 kHz TBOAE is more reliable than the CEOAE in the case of activity at low frequencies. The
0.5 kHz TBOAE is also a promising tool for the detection of emissions in patients with deep, highfrequency hearing loss when click stimuli do not produce OAEs.
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