Underwater acoustics

Underwater acoustics runs on the same physics as airborne acoustics but on a different scale and a different reference: levels are expressed re 1 µPa (not 20 µPa), exposure re 1 µPa²·s, and the medium itself, with its depth-dependent sound speed, refracts sound into channels that carry it for kilometres. This section covers the discipline in two halves that mirror the source-path split of the rest of the library.

The source half, in Underwater acoustics: radiated noise and pile driving, sets up the ISO 18405 terminology (SPL, SEL and peak levels and their references) and applies it to two regulated measurement cases: ships, with the radiated noise level of ISO 17208-1 and the equivalent monopole source level of ISO 17208-2 via the Lloyd’s-mirror surface correction, and percussive pile driving, with the single-strike, peak and cumulative sound exposure of ISO 18406.

The path half, in Underwater sound propagation, predicts what the sea does to that sound. It layers up from closed forms to full numerics: geometrical spreading plus volume absorption (Francois-Garrison, Ainslie-McColm or Thorp), the speed of sound in sea water by three formulations, the passive and active sonar equation, Rayleigh seabed reflection loss, the Wenz ambient-noise spectrum with JOMOPANS-ECHO ship traffic, and the normal-mode, ray-tracing and parabolic-equation numerical solvers for range-dependent problems.

Read the pages in that order: the reference levels come first because every propagation result is expressed in them. Unusually for this site, the theory for both pages lives inline with the guides rather than in the theory reference.

Pages in this section

• Underwater acoustics: radiated noise and pile driving: ISO 18405 reference levels, ISO 17208 ship radiated noise and monopole source level, and ISO 18406 pile-driving sound exposure.
• Underwater sound propagation: transmission loss, sound speed, the sonar equation, seabed reflection, ocean ambient noise and the numerical propagation solvers.