Measuring Noise Underwater

Units for Measuring Noise

Underwater sound is usually expressed using the logarithmic deciBel scale. All of the levels of sound presented in this report are expressed in decibels referenced to one micro-Pascal, that is, as dB re 1 µPa. If you are not familiar with the use of the dB scale, you can read a short review of the deciBel scale.

Parameters for Estimating Noise

In order to provide an objective and quantitative assessment of the degree of any environmental effect and the range within which it will occur, or to understand the quality and applicability of an underwater measurement of noise for this purpose, it is necessary to understand three parameters. These are:

1. The Source Level (i.e. level of sound) generated by the source
2. The Transmission Loss, the rate at which sound is attenuated as it propagates
3. The Effect Threshold, the level of sound at which a particular effect, such as death, injury or avoidance of a species occurs

The first two parameters allow the sound level at all points in the water to be specified, and in the current state of knowledge are best measured at sea, although it is in principle possible to estimate the transmission loss using numerical models. Usually, this data has to be extrapolated to situations other than those in which the noise was measured; the usual method of modelling the level is from the expression:

• SPL = SL - N log(R) - aR

Where:

• SPL = Sound Pressure Level
• SL = Source Level
• N = Transmission Loss
• R = Distance from the source
• a = Coefficient of absorption

An understanding of all three parameters allows an estimate of the range within which there will be an environmental effect to be estimated. If the population density of a particular species within the area is known, the number of individuals of the species likely to be effected can be estimated.