Noise Level Calculation

Many types of noise can be eliminated during data collection or post-processing.  Noise removal during data collection is preferable.  Noise that cannot be removed will increase S_v measurements and bias in situ TS measurements, leading to errors in fish density or biomass estimates.

Some instruments output noise levels at 1 m depth and this should be recorded.  We recommend that authors report the noise levels or the SNR at the deepest depth included in the analyses. Remember that noise levels at 1 m are different if expressed as TSu or S_v data.  This is because S_v values include a term for the equivalent beam angle and sampling volume whereas uncompensated target strength (TSu) values do not (compare Equations 3 and 6).  Therefore, the difference between TSu and S_v data is depth dependent.  Note that TS refer to the target strength of the fish and TSu refer to the measure of echo level with a 40 log R TVG function, also sometimes called non-adjusted TS.  This distinction is important and not clear in some existing software. From combining equation 6 and 12, we get:

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A noise level measured in the S_v domain as -120 dB at 1 m depth (with a 120 kHz, 7.8º transducer and therefore a Ψ of –20.4 dB, a sound speed of 1450 m•s^-1, and a pulse duration of 0.4 ms) is -146 dB in the TS domain at 1m (20 log (1) = 0)