If a tree falls in the woods and nobody is there to hear it does it make any sound? Does it make a noise? If you were there in the woods and heard the tree fall, what could you distinguish from those noises? It's an age old theory, but one that's important to examine.

Sound is a physical disturbance in a medium (air), and is a combination of two components, loudness and frequency. Speech, music, and most of the things we hear, such as the tree falling, are made up of complex sounds created by loudness and frequency. These are the two different things we would be able to distinguish if we were near the tree. In terms of loudness, the farther we were from the tree, the quieter it would be. The closer we were to the tree, the louder it would be.

The standard measurement of sound, or loudness can be expressed in several ways. A pascal, or actual measurement of pressure due to the passage of sound waves is often used, although the measurement of loudness levels, or decibel (dB) rating, is a bit less complex and easier to understand. For instance, threshold of hearing at 0 dB would be equal to 2.0E-5 pascals and the threshold of pain at 140 dB would equal 200 pascals. The loudness of a typical conversation in a conference room would range from 2.0E-2 to 3.56E-2 pascals or 60 to 65 dB.

The human ear is a remarkable item in that it can hear a dynamic range from 0 to 140 dB, although severe physical damage would occur if the sound was any louder than 140 decibels. A common audio tape has a range of 50 decibels and a compact disc has a range of a little over 90 decibels.

The second thing we would notice as we stood in the woods, are the different tones or frequencies created by the tree falling. The crack of the branches, the rustle of the leaves, and the thud of the trunk hitting the ground are all sounds or noises which are made-up of many tones. Think of a piano keyboard...every key has a different tone or pitch. The tone or pitch of the key that is played is heard as a frequency. The higher the pitch, the greater the frequency. Frequency is the number of cycles or oscillations that a tone makes in one second. The lowest frequency that we hear is 20 cycles per second or 20 Hertz (Hz). The highest frequency that we would be able to here as a young child is 20,000 cycles per second or 20,000 Hertz (Hz). (As we get older, our ability to hear high frequencies diminishes.)