Effects of sphere suspension on echosounder calibrations

Abstract

Abstract Echosounders may be calibrated by suspending a solid metal sphere in the acoustic beam and adjusting the system gain so that the measured target strength (TS) equals the theoretical value for the sphere. The suspension apparatus (i.e. net bag or loop of line glued into a hole, and control lines) may appreciably scatter sound, potentially affecting the sphere-echo intensity and phase, and hence the calibration uncertainty. Here, we characterize the effects of conventional suspension materials and configurations on measurements of sphere TS and recommend approaches to mitigate the observed effects on echosounder calibrations. We show that the effects of suspension lines on both calibration accuracy and precision change with line type and increase with line diameter and acoustic frequencies above ∼100 kHz. Compared to commonly used polyamide (nylon) line, ultra-high-molecular-weight polyethylene (PE) line has an acoustic impedance closer to that of water and therefore has a lessened effect on measurements of sphere TS. The effects may be mitigated in a practical manner using a 0.38-mm diameter, multifilament, PE line for the suspension.