Abstract (Invited)

Phase of Shock Wave Harmonics in Focused and Unfocused Ultrasonic Sound Beams

H.Hobaek, R.A.Kippersund (Department of Physics, University of Bergen, Norway)

e-mail: halvor.hobak@fi.uib.no

The asymmetric waveform observed in shock waves i n high amplitude sound beams is strongly related to the phase between the individual harmonic components. We present experimental results from 2 plane and 2 focusing sound sources with frequencies near 1 MHz in water. The phase angle of each harmonic component, n, can be approximated by A(n)=a n + B, where "a" simply tells the location of the shock front within one period of the signal, and the phase parameter "B" is a constant. The numerical value of B, determined by least squares fit (neglecting fundamental frequency), is found to depend on the range from the source, and also slightly on the amplitude. Moreover, this dependance is different for plane sources and focusing ones. For plane sources: after small fluctuations within the nearfield the phase parameter flattens out at a value near -pi/2, reaching a slightly more negative value with higher source amplitudes. With the focusing sources the phase parameter decreases more rapidly with range, approaching values near 1. They are more sensitive to the source amplitude than with the plane sources. At long ranges the waveform approaches a sawtooth with plane sources, and a U-formed shock with focusing ones.

Section : 1