Technical Information

Acoustic Theory

Theory of Acoustic Guidance

For fish to be repelled by a sound, a number of criteria must be met:

  1. The fish must be able to detect the frequencies used to compose the deterrent signal.
  2. The sound signal composition must be of a type that is repellent to fish (note: some sounds attract, others have no effect);
  3. The level of the sound must be high enough to elicit a reaction, taking account of the background noise.

The background noise issue is important, especially where acoustic systems are deployed near to underwater machinery such as pumps and turbines. In such cases it may be necessary to measure the underwater noise spectra under typical operating conditions before a system is designed.

Underwater noise may be repellent to fish if:

  1. Noise of any type having frequencies that lie within the fish hearing range is emitted at very high audio levels (but this is very cost-inefficient and may impact on other biota);
  2. The characteristics of the noise have any special biological meaning to the fish (e.g. mimicking the approach of a predator);
  3. The noise is designed by experimentation to cause particularly strong avoidance.

The biological approach (b) may offer good possibilities for individual species but the empirical approach (c) has yielded a number of signal types that are effective against a wide range of species. It is type (c) signals that are used within FGS acoustic deterrent systems.

The signal types that have proved most effective in all applications are based on artificially generated waveforms that rapidly cycle in amplitude and frequency content, thus reducing habituation. A human equivalent would be being made to stand near to a wailing police or ambulance siren. It simply gets uncomfortable, so you move away!

For further information on acoustic issues associated with installing a fish deflection system please contact FGS.