When a tree falls and there is no one there to hear it, does it make a sound?

It depends on what you mean by sound. We think we know what it means because our perception of sound is second in importance only to sight.

From the perspective of physics, there are three factors involved in any sound: a source of energy, a medium to transport the energy and a receiver to capture the vibrations.

Sound energy is transmitted in the form of waves. Natural objects vibrate over a range of frequencies when supplied with energy.

Manufactured objects may vibrate at a single frequency. The simplest example is the tuning fork, which vibrates at a single specific frequency, which we hear as a pure musical tone. Musical instruments produce vibrations with various characteristic combinations of frequencies called overtones. Combine two different musical tones and you get harmony, either consonant or dissonant.

White noise contains all of the frequencies in the range audible to humans, from 20 to 20,000 hertz. A hertz is one cycle per second, the standard measure of frequency. We hear white noise in the roar of a waterfall, tumbling surf or the crash of a falling tree.

Pure white noise has equal power per hertz distributed through all frequencies. Pink noise is similar except that the power distribution is equal power per octave. Pink noise sounds typically flat and lifeless compared with the crisp sound of white noise.

A medium is any material that has elasticity. The most common medium for sound is air. Sound travels through air as pressure waves that force air molecules to oscillate forward and backward as sound energy passes through it.

The pressure exerted by sound waves is extremely small. The acoustic threshold of human hearing is the smallest pressure that creates a sensation. It is roughly the sound of a mosquito flying 10 feet away, about one ten-billionth of atmospheric pressure.

The displacement of air molecules also is extremely small. It varies from the size of air molecules at the threshold pressure to the size of a human hair at the pain threshold of 140 decibels.

A sound level of 85 decibels, which is the level known to cause hearing loss over time, is equivalent to nearly 20,000 times the acoustic threshold. The pain threshold 0f 140 decibels is 10 million times the acoustic threshold.

Sound travels through all materials, solid, liquid or gas. It travels faster in more dense materials, much faster in water than in air, and much faster in steel than in water.

On the receiving end, the human ear has a remarkable range of sensitivity, in terms of both intensity and frequency. Within this tremendous range of intensities, we can hear approximately 10 octaves, two octaves more than the keys on a grand piano can play.

This range gets smaller as we get older and lose sensitivity to higher frequencies.

Does the falling tree make a sound? According to the physical definition of sound the answer is a resounding yes.

Richard Brill is a professor of science at Honolulu Community College. His column runs of the first and third Friday of the month. Email questions and comments to brill@hawaii.edu.