Non-linear effects in a 200-kHz sound beam and the consequences for target-strength measurement

When estimating target strength (TS), sound pressure in the beam where the target is located has to be measured accurately. Sound pressure is normally calculated from the source level, and transmission loss is based on geometric spreading and absorption loss. Additional losses caused by non-linear acoustic propagation may be important, especially in the case of high-power, high-frequency, and highly directive sources. ‘Non-linear loss’ from the fundamental frequency is due to energy from the fundamental harmonic being transferred into higher harmonics. This loss affects the beam pattern in ways that will depend on both power and range, since the non-linear loss depends, in turn, on sound pressure. We present the results of sound-pressure measurements and simulations from a 200-kHz transducer with a beam width of 7°. Sound pressure was measured at different ranges and power levels using a broadband hydrophone to detect some of the higher harmonic frequencies that occur in non-linear acoustic propagation. The TS of a solid copper sphere was measured using a standard echosounder with no correction for non-linear loss. Our study illustrates that this can be significant and that a corresponding correction needs to be considered when estimating TS.