Absorption of Supersonic Waves in Water near One Megacycle Available to Purchase

Several observers have found that the absorption coefficient for supersonic radiation in water is inversely proportional to the square of the frequency, in the range 3 to 80 megacycles. This agrees with classical theory except in the size of the proportionality constant, but when extrapolated to lower frequencies disagrees sharply with earlier measurements which suggested a strong absorption peak near one megacycle. An experiment is described here which employed a small microphone detector and attempted to measure absorption in a divergent supersonic beam by comparison of the measured and theoretical acoustic pressure curves along the beam axis. The results, between 1.09 and 1.30 mc, are negative in that the computed absorption coefficients fall below the experimental error. However, a fairly reliable upper limit can be set on the coefficient. This limit is consistent with the value predicted by extrapolating the frequency‐squared relationship down to one megacycle. There seems little chance of a strong absorption peak in the near neighborhood of one megacycle.