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Long Distance Sound

Bio-Acoustics 106

To get sound to go a long way, the recipe is low frequencies, great intensity, a hard surface, no wind, and a layered atmosphere.

At Queen Victoria's funeral in 1901, great batteries of cannon were fired and the sound was heard in parts of Germany but skipped over the 500 kilometers of France and Belgium. A layered atmosphere with warm air aloft is ideal for such long-distance reflection of krieg-sounds.

Sounds of a much lower frequency than elephants are capable of (0.001 to 0.1 Hz) can travel hemispheric distances and they are little attenuated by the atmosphere. Attenuation here means that scattering does not diffuse the sound. Sound intensity declines with the square of the distance just like light and gravity.

Because sonic booms, chemical and nuclear explosions produce lots of such infrasound, our arms-control agreements with the former Soviets included an international system of listening posts to monitor such sounds. It has also picked up sounds of unknown pedigree. The candidate sources of such ultra-low frequency sounds are severe storms, intense atmospheric shear of winds flowing across obstacles, and perhaps the winds on the sea surface. The winds have their own voice.

For those of us who have shunned high-dB hard rock, our ears are tuned to the frequency spectrum 16 to 20,000 Hz. Sounds below the range of human hearing we can term infrasound. Either some people have capabilities of hearing sounds

Marshall Islanders call these "sounds" Lowa.

The Bushmen of the Kalahari say these sounds are like a person humming to the windward. Such into-the-wind-humming would quickly and progressively be stripped of all its higher frequencies until they become sub-aural and only the keenest of eared would be privy to them.

As storms generate such just sub-aural sounds, it is a reasonable hypothesis that the proud owner of such ears could predict the weather. He would have sort of cochlear-bunions!

This probably explains the existence of bull-roarers. The most famous bull-roarer is that of the aborigines of Australia. They make a pointed oval airfoil to one end of which they attach a string. When whirled around their heads they make a helicopter-like low frequency sound which they call the "master of thunder."

In West Africa, such infra sounds are called the voice of Oro. The frontal bone of the human skull can be used in similar fashion and is used in rainmaking work by the Apache, Navajo, Zuni, Ute and Kwakiutl people. Such oval pendants are found in Paleolithic occupation sites. We might consider a thunder hearing test to identify those among us with this capacity to hear the wind.



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