Sound Propagation in Homogeneous Deciduous and Evergreen Woods

Abstract

The sound attenuation due to trees has been measured (under atmospheric conditions requiring no corrections) at about 20 locations, representing four selected types of woods. 9000 readings of sound-pressure level allow good accuracy in the means and standard deviations of results even when broken down by frequency, species of tree, and distance from the source—instrumental errors are eliminated from the values of excess attenuation by the use of the same instruments and operators at both ends of each propagation path. A constant excess attenuation of 7 dB/100 ft was found at frequencies below 2000 cps, in disagreement with previous workers. With the sound source in the open at the edge of the woods, effects due to change of medium were observed, viz., an increase of sound-pressure level in the woods and a distinctive S-shaped curve of attenuation vs frequency. This is suggestive of resonant absorption, although a theory developed according to such a hypothesis, while having the correct frequency dependence, fails to predict the magnitude of the observed results by a factor of 103. Transverse modes of vibration of the branches were shown to be the oscillators active in the frequency range of interest, 250 cps in the lower branches to about 1100 cps near the top of the tree; indications were that trunk resonances might also be important.