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 Sv 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 Sv data. This is because Sv 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 Sv 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:
A noise level measured in the Sv 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)