What is Sound?
Simply put, sound is a pressure wave traveling through the air. A sound source, such as a speaker, vibrates air molecules back and forth to create small changes in air pressure and cause a sound "wave" to propagate to the listener’s ears. The eardrums pick up this vibration and turn it into a signal that the brain interprets as "sound."
The amplitude of the air molecule vibrations determines sound volume. When the molecules vibrate a small amount, there is little change in air pressure, and thus, a low level of sound is produced that is discernable only to those near the sound source (Fig. 1). Similarly, when the vibration level is very high, the change in air pressure is high, and the sound is strong. When the sound is loud enough, we can actually feel the changes in air pressure!
Sound also is made up of various frequencies -- that is, the rate at which the molecules vibrate. Low frequencies (slow vibrations) produce deep bass sounds, such as the rumble of the firebox. High frequencies (fast vibrations) produce bright sounds, such as the dinging of the bell or hissing of steam.
Frequency is measured as Hertz (Hz), or cycles per second. Thus, if a sound causes an air particle to vibrate back and forth 100 times in a second, it is said to have frequency of 100Hz.
Three Ranges of Sound
Human hearing works in the range of 20Hz to 20,000Hz, which is divided into three categories: bass, midrange, and treble.
Some sounds, such as exhaust chuffs, are complex and are made up of many frequencies that span several sound categories.