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ticle motion, is appealing. From the point
of view of the sensitivity of mammals to
noise in the two environments, it may
also be the most appropriate compari-
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Terrestrial mammals have evolved a
complicated auditory system that match-
es the impedance of the air to that of the
liquid filled cochlea, the sensing part of
the inner ear. Marine mammals have a
similar adaptation to their environment.
The impedance matching allows the opti-
mal transfer of energy from the sound
wave in the medium into the cochlea,
irrespective of the relative pressures. For
the same rate of flow of energy (per unit
area) into the cochlea, the pressure in the
medium will be about 35 dB higher in water than in air (this results from the ratio of impedances from Eq. 1 because inten- sity equals the rate of flow of energy per unit area). Let us now proceed with the comparison of ambient noise environments in air and water by revisiting some of the results in Fig. 2, con- cerning the pressure spectral density of underwater noise, and in Fig. 3, concerning the one-third octave spectrum of back- ground noise measurements made in air, viewing each on the same plot with ordinate having a dimension of W/m2/Hz. This view is an intensity spectral density.
son.
For this, both kinds of data must be stripped of their decibel notation, converted to pressure spectral density in terms of Pa2/Hz, then divided by their respective acoustic impedances. For the third-octave measurements of airborne noise, the aforementioned measurement bandwidth that increases with frequency must also be taken into account. In any case the conversion is straightforward, and the results allow comparisons among the I-5 traffic, quiet residential, and Grand Canyon noise environments with our nominal high and low underwater noise levels, plus the case of extremely low level underwater noise measured under smooth Antarctic ice (Fig. 5).
It is somewhat remarkable that the intensity spectral density of noise representing a quiet residential environment can exceed that of nominal high-level underwater ambient noise conditions. However, the residential environment is only 4 km from a busy highway and it is difficult to find an underwater environment to match that in terms of density of mechanical sources. Perhaps a site at a similar distance from a constant stream of boats or ferries would be appropriate, but even that is unlikely to match the number of vehicles per hour on the highway. To be sure, we showed that pressure spectral density of underwater noise from waters close to busy commercial shipping harbors readily exceed the high level curve by 10 to 20 dB. Thus the intensity spectral densi- ty from the harbor environment is nominally similar to that from the quiet residential environment at frequencies near 100 Hz and exceeds the residential case for frequencies greater than about 1000 Hz. Note that our labeling of the res- idential environment as “quiet” is somewhat subjective. It appears to be quiet to us because much of the energy is fil- tered out by the response of the human auditory system, as
“It is somewhat remarkable that the intensity spectral density of noise representing a quiet residential environment can exceed that of nominal high-level underwater ambient noise conditions.”
suggested by the A-weighted level being about 14 dB below the unweighted level. The ambient noise in the Grand Canyon could be considered to be repre- sentative of very low level noise, possibly among the lowest in an open environ- ment in air. The case of low shipping plus sea state 0 could be considered to be rep- resentative of the lowest noise in the open ocean and is remarkably similar to the noise in the Grand Canyon, considering the differences in the acoustics of the two environments. The under-ice noise levels are even lower, but are less typical of
ambient sea noise.
It is tempting to infer from Fig. 5 that
the dynamic range of quasi, steady-state airborne ambient noise, insofar it can be represented by the difference between the I-5 and Grand Canyon environments, is greater than that for underwater ambient noise. For example, to reach the inten- sity spectral density levels as measured along I-5, the nominal high-level underwater noise curve must be increased by more than 4 orders of magnitude (40 dB). We cannot find examples of time-averaged, underwater ambient noise possessing an intensity spectral density of such magnitude. On the other hand, it would be quite rare to find a similar spatial concentra- tion of high energy human sources in the ocean. Even busy harbors do not have a similar density of sources. The closest comparison seems to be marine dredging, which produces sustained noise that may sometimes reach intensity levels in
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Underwater ambient noise is a complex subject of keen interest to a diverse set of scientists in underwater acoustics, biology, and oceanography, and public stakeholders in the area of marine mammal biology and conservation. Here we have only touched upon the subject, with particular focus on the approximate magnitude and frequency dependence of under- water ambient noise and a partial inventory of its primary sources. A comparison of typical underwater ambient noise lev- els with some examples of airborne noise has been made possi- ble by recasting results in each environment in terms of an intensity spectral density, providing useful context for the on- going discussion concerning the relation between anthro- pogenic underwater noise and the ecology of marine mammals.
In spite of the substantially different acoustic environ- ments in air and water, examples of ambient noise spectra representing a range of air and water environments are simi- lar in spectral shape, and the spectral intensity levels are broadly similar when both environments are far removed from human activities. Levels vary over a wide range (in excess of 30 dB) in both environments as conditions change. Underwater ambient noise originates from a much larger spatial distribution of sources than ambient noise in air because of the lower transmission loss in water, and thus underwater ambient noise associated with human activities is more wide spread than it is in air. AT
the 20–1000-Hz band that are comparable to those near I-5.
Concluding remarks
Underwater Ambient Noise 31