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ing, migration, and physiological impacts. Nevertheless,
we take the quasi, steady-state din and multiple-source qual- ity of the road traffic noise field, and, to a limited extent, the impacts that it presents to humans, as interesting if not some- what useful analogies to consider in the context of underwa- ter ambient noise.
Comparison: Ambient sound levels in air and underwater
Acoustically, the atmosphere and the ocean are two very different environments and this is reflected in differences in the nature of the ambient noise. The absorption attenuation of sound in air is far greater than in water, typically hundreds of times greater at the same frequency, and thus for frequencies in the 100-1000 Hz range, absorption is more of a significant limitation on sound propagation in air than in water. The cap on the underwater environment provided by the air–water interface, besides causing a dramatic change in acoustic impedance, can change geometrical spreading losses from spherical (factor of 4 reduction in intensity per doubling of dis- tance, or 6 dB reduction) to cylindrical spreading (factor of 2 reduction, or 3 dB reduction) for ranges roughly greater than the water depth. However, this depends on the acoustical char- acteristics of the bottom and also acoustic frequency; a highly reflective bottom will maintain the energy in the water column while an absorptive one will remove it. On the other hand the usual situation in daytime air is upward refraction and there is nothing to return the ray paths back to the ground, which
tends to limit the area of sources contributing to airborne noise. (Important exceptions include weather dependent and nighttime temperature inversions, wind, and other features of propagation of sound in the atmosphere—see Volume 2 of
49 Acoustics Today for discussion. )
These differences in propagation mean that we can expect contributions from sources over a much wider area in the case of underwater noise than for airborne noise. Consequently, the spatial correlation scales for the intensity of underwater sound are expected to be greater than for sound in air. For example, the noise environments at the I-5 locality and the city residence, separated by a few kilometers, are man- ifestly different, whereas one can expect to find large expans- es of the North Atlantic presenting very similar noise envi- ronments. Shipping traffic spread over the entire ocean can contribute significantly to the ambient noise at any position, whereas high noise environments on land such as around busy highways and airports are much more localized.
Nevertheless, is there a convenient and physically correct way to compare measurements of sound in air and water? This question has caused a number of interesting discussions involv- ing air and underwater acousticians, journalists, and the gener- al public. From the previous discussion, intensity can be com- puted for plane waves in air and water by taking into account the rms pressure of the wave and the acoustic impedance of the media (see also ref. 50). From a physical point of view, a com- parison of intensities, rather than pressures or measures of par-
Fig. 5. Intensity spectral density of noise for different environments in air and underwater. Solid and dashed gray curves represent nominal high-level and low-level under- water ambient noise, respectively, as originally discussed in context of Fig. 2. Note: each tick mark on the ordinate is equivalent to a change of 20 dB (air or water).
30 Acoustics Today, January 2007