Ocean Noise and Marine Mammals (2003) / Chapter Skim
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Appendix E: Glossary of Terms
Appendix E: Glossary of Terms
Pages 173-188
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From page 173... ...
In underwater acoustics, the standard unit of acoustic pressure is the micro Pascal (,uPa) , or one-millionth of a Pascal.
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From page 174... ...
This factor of 10 applies to quantities that are second order in the acoustic variables; for example, are proportional to the square of pressure, squared particle velocity amplitude, or the product of pressure and particle velocity. Examples of such quantities are acoustic energy density, magnitude of vector acoustic intensity, and acoustic power (see the "Physics of Sound" section of the Glossary)
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From page 175... ...
This effort has been accomplished successfully in situations for naturally occurring discrete sources that can be modeled as simple points in space such as individual vocalizing animals. However, estimating the source levels of spatially diffuse naturally occurring sources chorusing fish schools, snapping shrimp colonies, ocean surface breaking waves, oscillating bubble clouds is a topic of research.
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From page 176... ...
Ambient noise is not necessarily that from distant sources, as sometime stated, since the collection of breaking waves directiv above a receiver are not "distant" nor is thermal agitation. , ~ T 1 1 - · 1 · · · 1 · 1 · 1 11 n aualtlon, ambient noise in this report does not imply naturally occurring since ocean traffic noise has long been considered part of the ambient.
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From page 177... ...
The mean (a statistical measure of central tendency) of acoustic pressure and of acoustic particle velocity is equal to zero, by their definitions as variations about an equilibrium state.
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From page 178... ...
Similarly, the acoustic kinetic energy density is proportional to the mean squared particle velocity amplitude. The standard units of energy are joules, so that acoustic energy density (either potential or kinetic, or the sum of the two)
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From page 179... ...
Acoustic particle velocity, along with acoustic pressure is a quantity that is first order in the acoustic field variables. Acoustic Power the integral over a well-defined surface area of the component of active vector acoustic intensity perpendicular to the surface.
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From page 180... ...
The other acoustic field quantities presented in this section acoustic energy density, acoustic intensity, and acoustic power are second-order field quantities. Adiabatic Incompressibility the change in pressure (acoustic pressure plus ambient pressure)
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From page 181... ...
1 ~ Fluid Ambient Density a property of a fluid that is equal to its mass per unit volume in the absence of sound. The term ambient as used here signifies the fluid's equilibrium state and should be distinguished from its use in the term ambient noise.
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From page 182... ...
Window functions often are used to taper the ends of a segment of time series prior to performing the transform in order to reduce the possibility of the spectral levels in one frequency band contaminating the levels in a significantly different frequency band ("spectral leakage"; resulting from the spectral sidelobes of the window function; see Harris, 1978~. In order to numerically calibrate the FFT output to obtain an auto spectrum or autospectral density, the square of the Fourier transform amplitudes is normalized by the following quantities: ..
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From page 183... ...
There exist several theorems that relate the properties of signals in the time domain and their corresponding Fourier transforms. These theorems can have important applications to the topic of this report.
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From page 184... ...
of the original time series over a given time interval equals the integral of the spectral density for that time period across the whole frequency band. The decibel unit for the pressure spectral density in underwater acoustics is dB re 1 ,uPa2/Hz and those for the particle velocity spectral density are dB re 1 (mls)
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From page 185... ...
For example, the potential energy density spectrum is equal to the pressure spectrum normalized by twice the adiabatic incompressibility, the kinetic energy density spectrum equals half the fluid ambient density times the particle velocity amplitude spectrum, and the acoustic intensity spectrum equals the cross-spectrum between the pressure and particle velocity (D'Spain et al., 1991~. The acoustic power spectrum is the acoustic intensity spectrum integrated over a specified surface area.
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From page 186... ...
period seismic air-guns and arrays all those under Transient pile driving pingers and AHDs Frequency Domain repetition rate spectral density or spectrum of each transient Continuous in time, Time Domain broadband ship cavitation Aperiodic mean squared amplitude dredging rms amplitude icebreaking O-pk amplitude pk-pk amplitude Frequency Domain spectral density7
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From page 187... ...
Conversely, the spectrum level of signals whose frequency content varies continuously with frequency ("continuous spectra") also varies with FFT length, since the FFT length determines the bandwidth over which the signal energy is integrated (see the "Spectral Analysis and the Frequency Domain" section of the Glossary)
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From page 188... ...
Oppositely propagating waves give rise to a standing wave pattern that radiates sound with twice the frequency of that of the interacting surface waves. Microseisms are the dominant natural noise source in the space- and time-averaged ocean noise spectra below 5-10 Hz.
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Key Terms
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