Background noise analysis variables
The mean Sv (or corresponding NASC or ABC) due to a defined level of background noise can be calculated in Echoview and exported in terms of the three analysis variables Sv_noise, NASC_noise and ABC_noise. The background noise variables can be used to correct integration variables for background noise by subtraction, but this correction must be done outside of Echoview.
The following parameters are needed for calculating the background noise:
On the Calibration page of the Variable Properties dialog box:
AbsorptionCoefficient 

TvgRangeCorrection 

On the Analysis page of the Variable Properties dialog box:
BN0  background noise Sv at 1 m from the transducer face (dB re 1 m1)
The background noise function BN in terms of r, the range from the transducer is:
BN(r) = BN0 + 20 log (r) + 2α (r1)
Where:
r = range or corrected range^{1} from the transducer face (m)
α = absorption coefficient (dB/m)
The background noise for a region, cell or selection is calculated as follows:
 An Sv_noise value is calculated for each available sample using the formula above and the sample's range. The sample Sv_noise values are added in the linear domain and divided by the number of good samples. The final value is converted to the dB domain. Note: No_data and 999 dB samples are excluded from calculations.
 These noise values are integrated over the region, cell or selection in the same way as Sv when calculating the Sv_mean for a region, cell or selection. Then NASC_noise and ABC_noise are calculated from mean Sv_noise in the same way NASC and ABC are calculated from Sv_mean.
^{1}From Echoview 4.20 onwards, if a TVG range correction is applied on the Calibration 2 page (from Echoview 5.0 aka Calibration page) of the Variable Properties dialog box, then this correction is also used in the background noise analysis variable calculations. Previously, Echoview did not account for TVG range correction in such calculations. As a consequence there may be small but significant errors in the TVT data. The errors are of the order of 0.5dB (at 2m) to 0.05 dB (at 17m).
See also
ABC and NASC
Background noise in Echoview
Logged draft warning
Simrad Time Varied Gain Range correction
About time varied gain