AN EXPERIMENTAL INVESTIGATION OF THE NONLINEAR PRESSURE FIELD PRODUCED BY A PLANE CIRCULAR PISTON

An experimental investigation of the nonlinearly generated harmonics in the sound beam of a plane circular piston in water has been carried out. Special attention was paid to the higher harmonic sidelobes called fingers. All experiments were performed using a high precision, computer-controlled tank facility where movement resolution is better than 50 mum. Harmonic beam patterns generated by a 1-MHz, 12-mm radius plane circular piston driven with a face sound pressure level of 223 dB (re: 1 muPa) were measured. Beam patterns of the first ten harmonics (out to the first sidelobe of the fundamental beam pattern) are shown. The first four harmonic beam pattern data are compared with results produced by a numerical solution of the Khokhlov-Zabolotskaya-Kuznetzov (KZK) equation. Agreement between experiment and theory is excellent. Finally, careful measurements of the decay of the fingers were made. The decay of the first finger in the second harmonic beam pattern generated by a 2.2-MHz, 3-mm radius circular piston [217 dB (re: 1 muPa)] out to 30 Rayleigh distances was measured. As predicted by Naze Tjotta and Tjotta [J. Acoust. Soc. Am. 83, 487-495 (1988)], the finger decays as 1/z (neglecting absorption).