PLATE 2 Range of odontocete audiograms superimposed on the background noise levels. The Wenz curves describe relative levels of marine ambient noise from weather, wind, geologic activity, and commercial shipping. The audiograms are pressure spectral levels with units of dB re 1 µPa, whereas the Wenz noise curves are those of pressure spectral density having units of dB re 1 µPa²/Hz. (Actually, Wenz collected ambient noise spectra for various frequency bandwidths and converted their levels to a 1-Hz [^“1-cps^”] bandwidth [Wenz, 1962].) The Wenz curves can be converted into spectral levels for a frequency band of interest by integrating the spectral density levels across that frequency band of interest, after first converting from logarithmic to linear units of µPa²/Hz. The comparison of spectral and spectral density levels shown assumes the bandwidth of integration is 1 Hz; the spectral density level is equivalent to the spectral level for a 1-Hz-wide bandwidth. From a biological perspective, the appropriate frequency band over which to integrate noise spectral densities is determined by the frequency discrimination capabilities of the animals’ hearing. This figure illustrates the similarity in the frequency dependence of naturally occurring wind noise and marine mammal hearing capability and indicates how the frequency content of other noise sources (e.g., shipping) relate to this hearing capability. (Unpublished abstract JASA, 2001; adapted from Wenz [1962] and presented at Acoustical Society of America, December 2001.)