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: 2f k c • The component of the wave number in the direction of the array is 2f 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: 2f 2f 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 2f 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.
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