Abstract
A continuous 2.3-MHz signal of amplitude 0.09 atm was generated in water by a ceramic transducer. After passage through distances up to 0.7 m, the signal was detected by a second transducer and fed to a spectrum analyzer. The noise in the medium was provided by a mechanical noisemaker striking the metallic wall of the water-filled tank. The noise spectrum lay between 2–8 kHz, with component amplitudes up to 0.01 atm. The analyzed spectrum of the perturbed signal contained sidebands, shifted from the original signal frequency by 2–8 kHz, down approximately 70 dB from the primary signal level. Steps were taken to avoid interference from direct pickup of the audio noise signal by the transducer, by inserting a high-pass filter between the receiving transducer and the spectrum analyzer. Measurements of the sideband level were made as a function of the length of the interaction region, and an increase in the level was noted with increase in the path length. [Work supported by ONR.]
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