 For all sonar systems the limiting factor deciding whether a returning echo is detected is the echo-to-noise ratio (ENR).The ENR is given by the emitted source level (SL) plus the target strength, minus the two-way transmission loss (absorption and spreading) and the received noise.Animals will adapt either to maximize range under noise-limited conditions (increase source level) or to reduce noise clutter in a shallow and/or littered habitat (decrease source level).