Underwater Acoustics

In most cases, the measurement of underwater sound is made electrically, so that the electrical properties of the transducers are important. Equally, many sound sources are electrically driven, and their transmission can be related to the electrical signal used.

Hydrophone sensitivity is the ratio of its output voltage to the sound pressure in the fluid surrounding it. The receive sensitivity is usually given in decibels, although reputable manufacturers usually also give the linear value of the voltage sensitivity, " Mv":

The decibel value thus has a reference level of units "Volt/Pascal", or in underwater convention "V /μPa". A typical hydrophone may have a value of Mv peaking around 50 /μV/Pa, but the corresponding decibel level then becomes -206 dB re 1 V /μPa. These extremely negative decibel levels can cause confusion, which is ironic given that the choice of 1 /μPa as the pressure reference level was partly to avoid negative levels. Note, in particular, that a level of, say, -200 dB re 1 VμPa or 100 /μV/Pa is a increased sensitivity.

The same format can be used for a directional hydrophone if the energy flow is unidirectional. However, the response will then change with its orientation, and it is more likely that the pressure field is affected by the presence of the hydrophone.

The transmit response of an underwater transducer is the ratio of the response to the applied voltage or current. The transmit voltage response (or "TVR") "Sv" has physical units of Pascal·metres/Volt:

However, the TVR values quoted are seldom given this way, being more usually given as decibel level referred to 1 μPa/V at 1m. This presentation is convenient when the measurements are made in a test tank with the hydrophone at a 1m range, but can be a source of confusion for large directional arrays, where the near field is complex.

As with other large sources, the measurement of source output or source level must be made in the far field. If in doubt, this should be checked by looking to see if the P_h· r product is independent of range.

In some cases, the response is more usefully linked to the current, by the transmit current response ("TCR") or "Si":