Instrumentation & Measurement Magazine 26-2 - 34

Fig. 3. The process of intercepting effective signal fragments from the original audio.
smoothing vectors can achieve the optimization effect,
and in practical application, the vector is selected according
to the specific performance of signal processing.
Preprocessing of Signal Measurement
Data
A hydrophone placed 1.5 m underwater is used for sampling.
The sampling rate is 48 kHz, the sampling bit width is 16 bits
and the effective signal can be collected for 5 min every day.
The original measurement data contains a multiplicity of
acoustic signals and strong background noise; thus, it needs to
be intercepted to obtain valid measurement acoustic signals.
The process of intercepting the signal at one time is as follows:
◗ In Fig. 3a, by distinguishing the " click " sound and observing
the time-domain waveform of the measurement data,
it is determined that an acoustic signal is collected once in
the 213~217 s part of the original measurement data.
◗ Intercept this segment and do STFT (window length of
256) for it, and get its time-frequency spectrum, as shown
in Fig. 3b. Due to the existence of full-time low-frequency
noise, the energy of the acoustic signal region is weaker
than the energy of the low-frequency region.
◗ To highlight the energy of the acoustic signal region, the
signal is processed by high-pass filtering with a threshold
of 2 kHz. Then we repeat step 2, and the spectrum is
shown in Fig. 3d.
34
◗ Observe the most prominent energy region in Fig. 3d,
and intercept the signal data within the pink narrow
box, which is the 216.213~216.233 s part of the original
signal, to obtain the effective acoustic signal s1
(t). Its timedomain
waveform is shown in Fig. 3c.
The measurement data continues to be processed, and three
representative effective acoustic signal data are obtained, as
shown in Fig. 4. To avoid the contingency of the single signal
analysis result, the simultaneous analysis of these three signals
is used for the feature extraction of the acoustic signal of
Penaeus vannamei.
Results and Discussion of Signal
Detection
Effect of Smoothing Treatment
The HHT and the optimized HHT are used for the signal.
Fig. 5a is the HHT time-frequency spectrum of s1
K1=[1,1,1,1,1]T
and K2=[1,2,4,2,1]T
(t), while
are used as smoothing vectors,
and the smoothed spectrums are shown in Fig. 5b and
Fig. 5c. The areas where the frequency components with relative
intensity >0.5 are outlined by pink boxes. For quantitative
analysis of the visual prominent effect of smoothing on the
strong frequency components of the spectrum, count the frequency
points with relative intensity >0.5 in 2.5~8.5 ms as
effective information points, as shown in Fig. 5d.
IEEE Instrumentation & Measurement Magazine
April 2023

Instrumentation & Measurement Magazine 26-2

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 26-2

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