Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Technology Corporation
recording system and techniques had evolved over six years of making such
recordings from small boats.
The analysis was performed using a Hewlett-Packard Vectra computer system
(compatible with the IBM PC-AT). The technique was based on the weighted,
overlapped segment averaging technique of Carter and Nuttall (1980). the
signals were played back through a Krohn-Hite model 3342 filter to prevent
aliasing. A 12-bit Metrabyte model DASH 16 analog-to-digital converter was
used to digitize sections of signal each 16.5 s in duration. The sample rate
was 8192 samples/second. The 16.5 s sections were analyzed separately and the
results saved for comparison and statistical analyses. They were taken every
16 s. The results were sound pressure spectra with calibrated levels from 10
to 4000 Hz.
Each 16.5 s section was further divided into one-second segments for
Fourier analysis using a fast Fourier transform routine. Segments were
overlapped by 50% to permit extracting information from samples at the ends of
each segment attenuated by "windowing" (Harris 1978); we used the Blackman-
Harris minimum three-term window. The magnitudes squared (the ttpowers") in
each transform cell, or bin, were computed. The results of analyzing each
segment were averaged to obtain our estimates of the sound power spectrum for
a 16.5 s section of sound.
The effective bandwidth of each spectrum analysis cell was 1.7 Hz,
although the cells were spaced 1 Hz apart. The powers in the cells were added
to obtain the sound power in selected frequency bands, in particular, the
standard third-octave bands widely used in acoustical sound and noise
measurements. All levels, both spectrum levels and band levels, were saved
for statistical analysis, printing, and plotting.
There were two statistical analysis techniques. In one, each of the
analysis cells (frequency bins) in the 53 resulting spectra were sorted from
smallest to largest. Then, the minimum, fifth percentile, fiftieth percentile
(median), ninety-fifth percentile and maximum levels for that bin were
identified and saved until five statistical spectra were generated,
corresponding to those levels. The five statistical spectra were plotted.