Application to biomedical signals - Practical Biomedical Signal Analysis Using MATLAB - page 224

The usefulness of MP for PCG analysis was also confirmed by a study by Wang
et al. [Wang et al., 2001], who reported that MP energy distribution provides better
time-frequency representation of PCG than spectrogram. The authors reported good
results of classification of mitral valve abnormality based on MP decomposition of
first heart sound. Figure 4.60 shows spectrograms and MP time-frequency maps of
first heart sound for normal and pathological case.
From the evidence gathered by the authors who applied different kinds of methods
to PCG it follows that the time-frequency methods such as wavelets and MP, and
also parametric methods based on the autoregressive approach are suitable for PCG
analysis and the choice of the method depends on the problem to be solved.

4.5.2 Otoacoustic emissions
Otoacoustic emissions (OAEs) are weak acoustic signals generated by the in-
ner ear in response to the stimulation and sometimes also spontaneously. OAE are
measured by a sensitive microphone placed in the ear canal. Their typical frequen-
cies range from 0.5 to 6 kHz [Probst et al., 1991]. Responses that are evoked by
short acoustic stimuli are referred to as transiently evoked otoacoustic emissions
(TEOAEs). They are usually measured in 20 ms window from stimulus onset. Long
lasting OAEs recorded in 80 ms windows in response to broadband click stimulus
are called synchronized spontaneous OAEs (SSOAEs). They are considered to rep-
resent spontaneous otoacoustic emissions (SOAEs). Distortion product otoacoustic
emissions (DPOAEs) are the emissions which are generated by two tones f 1 and f 2 ,
where f 1

/ f 2 is typically around 1 . 2 f 1 . The emitted otoacoustic signal has the largest

amplitude at 2 f 1 f 2 frequency.

OAEs were first registered by Kemp [Kemp, 1978] and very quickly became an
important tool in the diagnosis of hearing impairment. However, the mechanisms of
their generation, which are closely related to the hearing process are still a matter
of debate between two theories, namely traveling wave [Zweig and Shera, 1995,
Shera and Guinan, 1999, Talmadge et al., 2000] and resonance theory [Sisto and
Moleti, 1999, Bell, 2002, Bell and Fletcher, 2004]. OAEs are especially useful as a
screening hearing test in neonates and small children. The review of OAE techniques
and clinical applications may be found in [Robinette and Glattke, 2002].
During the recording procedure consecutive OAEs are stored in two buffers and
the correlation between the averages from these two buffers serves as an index of the
reproducibility and hence, the quality of the signal. Usually the discrimination be-
tween TEOAE responses from normal-hearing subjects and patients with sensorineu-
ral hearing losses is based on the reproducibility (or correlation) of the TEOAE re-
sponses obtained from two buffers, the signal to noise ratio (S/N) and overall TEOAE
response level. Sometimes the frequency characteristics of the signal are considered;
OAE measurement devices usually allow to calculate Fourier spectra.
More recently other TEOAE parameters have proven to be good signal descriptors
and among these are the latencies of the TEOAE components. It was demonstrated in
several papers that there are differences in latencies of OAEs between subjects with
normal hearing and subjects exposed to noise, e.g., [Tognola et al., 1997, Sisto and

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