Initial Analysis of Acoustic Data from the Narcine Array

Initial Analysis of Acoustic Data
from the Narcine Array
Paul White
Institute of Sound and Vibration Research (ISVR),
University of Southampton
Outline
• Array design
• Introduction to the acoustic data
• Spectral Analysis
– Short-term
– Long-term
• Frequency-wavenumber analysis
• Beamformer results
Array Design
Sampling frequency = 10 kHz,
Bit depth = 24
Array design frequency ~ 1.7 kHz (Frequency such that d=0.45=l/2)
Data collected for 40 mins (most of the analysis here is based on 30 mins)
Time Series
• Example time series
Channels 1 and 16 unusual.
Channels 5 and 10 are typical of the
12 channels not shown.
Spectra (next slide)
• Computed using the standard segment averaging approach
(Welch’s method) with ~5 Hz resolution (2048 point FFT)
• Green lines: show the average of all the spectra across all the
channels
• Blue lines: show the spectra of each channel
???
Tonal Noise
Effect of “rare” electrical
impulses
Median Spectra
• Computed as in a typical power spectra, with the difference
that the median of the segment’s FFTs are computed instead
of the mean.
• This means transient components do not significantly affect
the spectral estimates.
• (The for the median estimates are not computed to represent
a PSD – the values on the y-axis are not comparable to those
on the last slide).
Long-term Spectra
PSDs of 5 s blocks from channel 8 shown over 30 mins.
Electrical noise
Frequency-Wavenumber Analysis
• The wavenumber vector (k) defines spatial frequency.
• Temporal frequency, f, and the magnitude of the wavenumber
are related to each other via the sound speed c:
2f
k 
c
• The component of the wave number in the direction of the
array is
2f sin  q 
karray 
c
where q is the angle of arrive of the acoustic wave.
Constraints in the f-k Analysis
• For any given frequency the wavenumber must satisfy:
2f
2f
 karray 
c
c
• Energy outside this region cannot be due to acoustic waves in
the medium.

– e.g. flow noise.
• Energy inside the region are from wave propagating at a
speed of c or greater.
Tonal electrical noise
PSD
Alias of own “ship”
noise
f-k spectrum
karray
Own “ship” noise
2f

c
Low frequency
non-acoustic noise
Energy-Bearing Plots
Beamformer outputs.
Energy measured over 10 s segment
Tonal electrical noise
Example of Beamoutputs
Long-Term Spectrograms of Beam Outputs
-90°
0°
45°
90°
Closing comments
• Some noise issues on the array:
– Low frequency noise, below 250 Hz (flow noise?)
– Some narrowband electrical noise.
• Lowest tone just below 1.5 kHz
• Narrowband filters may be adequate to remove this, if necessary.
– Impulsive noise.
• Seems to emanate from the autonaut.
• Effects can be reduced, in some circumstances, using median processing.
– Beamformers can be used to mitigate these noise sources.
• Some evidence of ambient noise changes can be seen in this
dataset.