JUNGLE ACOUSTICS I: TRANSMISSION AND AUDIBILITY OF SOUNDS IN THE JUNGLE.
Research rept. no. 7, 1965-1966,
ARMY TROPIC TEST CENTER FORT CLAYTON CANAL ZONE
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The US Army Tropic Test Center compared the penetrability and audibility of five sound frequencies at several jungle sites in the Panama Canal Zone during the 1965 and 1966 rainy seasons. A transmission study determined excess acoustic loss at five horizontal distances from a fixed sound source over 2,300 transmissions were made. A hearing study determined minimal audible sound intensities at the same distances from the sound source 3,000 auditory thresholds were determined for 50 EM from a TO and E unit. Jungle vegetation, distance, and sound frequency had significant main and joint effects on excess signal loss. Time of transmission day vs. night had no effect. The jungle served as a low-pass filter for audible frequencies. Excess signal loss increased as sound frequency increased at distances greater than 800 feet. At shorter distances significant reversals occurred. The decay curves for different frequencies showed different slopes. The usefulness of dbft type ratios in extrapolating jungle acoustic loss is questioned. The 63 cps signal penetrated the jungle better than all others--better than the law of inverse squares predicts--and was most audible at longer distances. Ambient sound spectrum and transmission loss are moderately reliable predictors of human hearing in jungles. The ambient sound shifts the point of maximum human sensitivity down the frequency scale to 1,000 cps at distances less than 400 feet, and down to 63 cps at 400 feet and beyond. At 1,000 cps and below, signals were heard better at night. Above 1,000 cps, signals were heard better during the day. Author
- Anatomy and Physiology