Tuesday, December 9, 2008

If a Tree Falls

An old philosophical riddle asks, “If a tree falls in a forest and no one is around to hear it, does it make a sound?” The debate over this riddle mainly relates to the difference between human perception and the existence (or non-existence) of reality existing outside human perception and awareness. Philosophers have debated similar questions for centuries.

With respect to events triggered by a tree falling in a forest, discoveries since the scientific revolution have enabled researchers to measure, describe, and understand in great detail the physical processes which occur in sound production and transmission. These include the cause of sounds, formation of air compressions and rarefactions, speed of sound, how it is reflected and absorbed, and numerous other events taking place in the sound transmission medium.

In Leviticus 26:36, the Old Testament warned that “the sound of a wind blown leaf” would throw the disobedient Israelites into a panic. In our last post we spoke of a much louder sound, that produced by human vocal cords in normal conversation. Several hundred compression waves--regions of slightly more densely-packed molecules--strike the eardrums of listeners each second. One may wonder how much pressure increase there is in one of these compressions compared with normal, undisturbed air. The answer is a startling, mere one millionth greater pressure than in normal air carrying no sound. That is a 0.0001 % increase, causing our eardrums to be displaced a mere billionth of a centimeter. If our ears were very much more sensitive, we may even be able to hear air molecules in a sound-free room vibrating with their normal kinetic (motion) energy.

The “If a Tree Falls” riddle does not distinguish between the physical events taking place in air and the subjective experience of the listener. The answer depends on whether we are describing physical events in air or subjective experiences in the listener’s mind, or perhaps, both. The physical events could be described as “sound.” Resulting subjective experience is described as “hearing.” The answer also depends on whether we are present in the forest, or located at a great distance where the physical sound cannot reach us. Physical sound is studied by the physical scientist. The subjective effect of sound on the listener is of intense interest to the physiologist or psychologist. But the natural curiosity and sense of wonder of “just ordinary folks” concerning sound and hearing are wonderfully supported by discoveries in both fields.