Spheres to Calibrate Echo Sounders at Any Frequency.

Abstract

The calibration of transducers of quantitative echo sounders is indispensable. Recently, metal spheres have been used to calibrate acoustic systems. The suitability of three kinds of material is examined for calibration spheres, and the following conclusions are derived at: (1) although highcarbon steel spheres are machined in accordance with Japanese Industrial Standards (JIS) with regard to hardness and size, the uniformity of the acoustic parameters is rather questionable; (2)copper spheres are good for calibration at lower frequencies, but it is difficult to use them for rather high frequencies; and (3) tungsten carbide with 6% cobalt binder is a superior material for all frequencies. Therefore, if any accurate size can be provided, the tungsten carbide is suitable for calibration spheres at any frequency. A group of tungsten carbide spheres is proposed for calibration spheres, whose diameters are 35.0mm, 38.1mm, 41.7mm, 45.0mm, 48.8mm, 50.0mm, 52.7mm, and 54.1mm. They can be used for echo-sounders operating at any frequency in the range of 10kHz-200kHz in the environmental conditions likely to be encountered.