[en] It is shown that the effective volume for a single hydrophone or coherent ensemble of hydrophones is independent of the gain or signal-to-noise ratio and is of the order of 0.3 km³. The event rate, however, depends on the gain and S/N which determine the range of detectable neutrino energy. The latter has a practical lower limit of 10³ to 10⁴ TeV. Atmospheric fluxes (including the prompt flux) are probably not detectable in any practical acoustic array, but the acoustic technique may still be well matched to a study of the ultrahigh-energy extraterrestrial flux. The 90% confidence level upper-limit flux which is measurable acoustically above these energies is estimated to be 2.5 x 10^-15 cm^-2 s^-1 for 100 modules in one year's running. Since there are no existing data on such fluxes, such a measurement is of some astrophysical value and would be comparatively inexpensive. 2 figures, 2 tables