On dB echograms the following data value has special significance:
| -999 dB |
This value is considered to represent 0 in the linear domain. That is a signal strength of 0 volts, which has no valid representation in the dB domain is given the nominal value of -999 dB. Datapoints which have a value below the specified data threshold will also be considered to have this value. An important consequence of this, is that certain operators will not necessarily behave as expected for this value. For example, the plus and minus operators will yield results as follows: -50 dB + -30 dB = -80 dB but notably: -50 dB + -999 dB = -999 dB This is because adding -999 dB is the equivalent of multiplying the underlying linear value by 0, and subtracting -999 dB is the equivalent of dividing the underlying linear value by 0 (which has no solution but Echoview will return the maximum valid dB value of 999 dB). |