Dredging Sound Event Characteristics

ERDC TN-DOER-E14

August 2001

Segments of audio sessions containing dredge cycle event sequences representative of as wide a

selection of ranges and distances as possible were ordered by increasing range and distance and

analyzed. Individual cycles from sessions 6, 8, 11, 14, and 15 were chosen based on the nearest

match to the representative range. Each of these cycles was edited from the respective session WAV

file, and a new 16-bit mono WAV file with a 44.1-kHz sampling rate was created for each of the

individual cycles for the representative range. The WAV files for these individual cycles were

reviewed and the sound pressure level (SPL) and frequency relative to the six-event sequence

summarized (Tables 2 and 3, respectively).

Dredging Sound Event Characteristics: Cycle WAV files were partitioned into event WAV

files. A spectral analysis of each event WAV file was conducted for the bottom contact, digging,

bucket closing, and winch in/up event categories using Sound Technologies SpectraLab audio

spectral analysis/fast Fourier transform (FFT) software. An FFT block size of 32,768 was used,

which produced a frequency resolution of 1.346 Hz. A Hanning window was used to reduce leakage

during the FFT analysis, and a 50 percent overlap was used to increase the time resolution for the

FFT analysis. Peak and infinite average FFT analyses were conducted for each of the event WAV

files and a spectral plot produced.

For each event spectral plot, peak measurements of sound pressure levels (dB rms) are displayed at

closest and farthest ranges. A linear trend line was overlaid for each of these plots. Plots indicating

peak event noise compared with ambient conditions were also produced. All amplitudes are

reported in relative dB units. The SpectraLab software is calibrated to display the relative power

levels in dB. Calibration tone data were generated that allow the relative sound signal dB levels to

be compared to referenced values (dB re 1 Pa-m). Each plot was produced for a linear frequency

range of 3 Hz to 20 kHz. However, due to recording limitations inherent in the equipment,

frequencies below 20 Hz were excluded from further analysis.

Because of variation in sound event duration, the number of samples used for FFT transformation

analysis varied among specific sound events (e.g., bottom impact versus grab) and among cycles.

Three "snaps" as the bucket closed during session 15 (cycles 5, 15, and 16) could not be analyzed

because their durations were less than the 32,768 minimum sample size required for FFT analysis.

RESULTS

Ambient Noise: Ambient noise includes a variety of background sources. In Cook Inlet, these

included, but were not limited to, wind- and wave-driven turbulence, hydrodynamic noise associated

with variable tidal flow conditions, and precipitation. Although not considered as natural back-

ground noise, "traffic noise" generated from commercial shipping and recreational or commercial

fishing vessels contributes to ambient noise primarily at frequencies <1 kHz (Richardson et al.

1995). Ambient noise may vary with changes in season, location, and time of day. Ambient noise

measurements were taken in a location away from the influence of dredging activities. Five 1-min

segments were selected and analyzed for inclusion in this technical note. Peak ambient noise

occurred at 73.2 dB re 1 Pa-m at a peak frequency of 57.8 Hz. Generally ambient noise averaged

60 dB re 1 Pa-m at frequencies less than 100 Hz and around the mid-50 dB re 1 Pa-m level at

frequencies ranging from 100 Hz to 1 kHz.