Bubble sources of the Knudsen sea noise spectra

Single coherent bubble contributions to the incoherent underwater noise of spilling breakers have been studied in an anechoic laboratory facility. The waves are generated by a plunger, they propagate 17 m along a 1.2×1.2-m water waveguide, and ‘‘spill’’ and create bubbles at the surface of a 3×3×3-m anechoic cube of water. Several species of bubbles have been identified. In general, they act as transient dipoles of duration from 2 to several milliseconds, with peak axial source strength of the order of tenths of pascals, at 1 m. The noise is emitted when the bubble is within hundreds of micrometers or a few millimeters of the surface. Bubbles were observed in the 2 decades of frequency from 500 to 50 000 Hz. The average of the individual bubble events yielded a spectrum that slopes at about 5 dB/oct from 1 to 20 kHz, the same as the Knudsen wind noise spectra at sea. The magnitude of the laboratory breaker noise during continual wave-breaking events was approximately 80 dB re: 1 μ Pa2/Hz at 1 kHz, which is essentially the same as observed during the continual bubble production that occurs with very high winds at sea. The reasons for this agreement are discussed.