Modelling of Underwater Noise and Vibration

Activities that generate high levels of noise in the marine environment usually require a Licence granted under the FEPA (Food and Environmental Protections Act). This normally requires that an assessment of the emitted underwater noise and vibration levels and its related effect to be made. The monitoring and assessment strategy is likely to require measurements of underwater noise to be taken and interpreted in a biologically significant way.

Transmission of sound in the underwater environment is highly variable from region to region and as a consequence, considerable errors can occur if local factors that can influence sound propagation are not taken into account.

Transmission Loss (i.e. the rate at which the sound energy decays with range) is measured by undertaking a series of sound measurements at incremental range from a consistent sound source, and then determining the sound level decay with range.

As underwater noise propagates, different frequency components decay at different rates depending upon the local environment.

For example, the propagation of low frequency sound is inefficient in shallow water since it typically travels in combined modes involving the simultaneous motion of the water and the seabed, in which losses can readily occur. By comparison, high frequency sound may propagate in the same environment with relatively low losses.

Consequently, at range a species with good high frequency hearing (typically marine mammals) will perceive the same noise at a very much higher level than a species with low frequency hearing sensitivity (typically fish).

The Subacoustech Acoustic model attempts to quantify these variables, but whenever possible the Transmission Loss characteristics for the region should be measured. The ideal approach is to back up initial acoustic modelling with a series of sound Transmission Loss measurements for the region, and then use this data to refine the model.

Where an activity is likely to generate high levels of underwater noise, and is therefore likely to have a behavioural impact over long range, the Transmission Loss characteristics of the local environment can have a considerable influence on the species impact range. In these cases, Subacoustech believe it is best practice to measure the overall and species specific Transmission Loss characteristics and thereby accurately predict the impact range for species of significance.

To discuss your specific needs please contact Subacoustech.