Measuring the acoustic backscattering form function from a spherical target under laboratory conditions

Spherical targets are routinely used as standard targets to calibrate active sonars. It is important to know the exact acoustic response of such targets if they are to be used for calibration. This paper presents two methods that can be used to acquire and/or validate the acoustic scattering response under laboratory conditions with a monostatic transducer system. The methods are distinguished by whether the free-water surface is available or not to act as a reference scatterer. To obtain the form function of a target, the backscattered signal is compared to a reference signal that is equal to the incident wave at the position of the target when it is not present. The reference signal can be measured in one of two ways: (1) as the far field reflection response from a free-water surface, and (2) as the first part of the echo that is free of interference from other signal paths from a large solid target. Backscattering spectra of example targets were rendered absolute by each of these procedures. Ultimately, this enabled determination of the overall frequency response function of two monostatic transducer systems. Illustrative results are presented wo frequency bands: 80-200 kHz and 300-600 kHz.