Measurement metrics related to inception and development of cavitation of a low-frequeacy flextensional transducer

The objective is to establish cavitation limits for an operational transmit array. Measurements on a single 1-kHz Sanders model 30 flextensional transducer cover an input power range of 140 to 4860 W at pulse lengths of 20 ms and 2 s. Acoustic tank and lake measurements, supplemented by underwater television video recordings, indicate: (i) near-field pressure waveform distortion is the earliest indicator of localized bubble formation or “soft” cavitation (500 W); (ii) the steady reduction of electrical drive impedance (175 to 75 Ω) and a steady increase in near-field pressure harmonic distortion (0% to 50%) are directly related to the growth of the cavitation field as electrical drive power is increased; (iii) near-field pressure broadband energy increases with cavitation development; and (iv) at the highest drive levels, the expected bifurcation of the fundamental frequency is observed indicating the presence of “hard” or transient cavitation. Near-field acoustic holography data provide surface pressure distributions and correlation of cavitation inception predictions with the video data. The relationships of these metrics to far-field source levels and beampatterns will be discussed in the context of defining an overall operational transmit system drive levels.