Decay of Sound

Two years ago the University of California's Dr. Vern O. Knudsen, president of the Acoustical Society of America, communicated to the Society's Journal some studies which indicated that sound is absorbed in air ten to 25 times more rapidly than had been previously calcu lated, and that absorption is fastest in dry air, a familiar fact to those who know how well sound carries on a foggy day. Since then Dr. Knudsen has not ceased to experiment with the "decay" of sound under various conditions. To the same Journal he has now reported new findings.

The sound whose decay Dr. Knudsen studies is a pure tone produced electrically and fed through a loudspeaker into a reverberation chamber filled with gas. Some of the sound is absorbed by the walls instead of by the gas, but this is calculated and discounted. The sound is picked up by a microphone, amplified, converted into electric current which causes a bulb to glow. If the sound has decayed beyond a certain level the current produced is insufficient to light the bulb. The time is measured between the cessation of tone production and the point in decay at which there is just enough current to cause the glowing of the bulb.

Germany's Dr. H. O. Kneser has suggested that a large part of the absorption in air is due to collisions between oxygen molecules and water vapor molecules. Dr. Knudsen's experiments with air and its two major components, oxygen and nitrogen, weigh heavily in favor of this suggestion. There was no appreciable difference in the decay rates in moist nitrogen and dry nitrogen. But the decay rate in moist air was only one-fifth the rate in moist oxygen, and oxygen is one-fifth of air.

This new importance of humidity in acoustics led Dr. Knudsen to a new and important conclusion: "In large auditoriums, the reverberation of the high frequency [high-pitched] components of speech and music is affected more by the condition of the air in the room than it is by the nature of the materials which form the boundaries of the room."

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