Hearing - Sound
The basis of sound is simple: there is a vibrating source, a medium in which sound travels, and a receiver. For humans the most important sounds are those which carry meaning, for example speech and environmental sounds. Sounds can be described in two ways, by their frequency (or pitch), and by their intensity (or loudness).
Frequency (the number of vibrations or sound waves per second) is measured in Hertz (Hz). A sound that is 4,000 Hz (like the sound the letter "F" makes) has 4,000 waves per second. Healthy young adults can hear frequencies between 20 and 20,000 Hz. However, the frequencies most important for understanding speech are between 200 and 8,000 Hz. As adults age, the ability to hear high frequency sounds decreases. An example of a high frequency sound is a bird chirping, while a drum beating is a low frequency sound.
Intensity (loudness) is the amount of energy of a vibration, and is measured in decibels (dB). A zero decibel sound (like leaves rustling in the wind), can barely be heard by young healthy adults. In contrast, a 120 dB sound (like a jet engine at 7 m [20 ft]) is perceived as very loud and/or painful. Extremes in both loudness and/or pitch may seriously damage the human ear and should be avoided.
The difference between frequency (pitch) and intensity (loudness) can be illustrated using the piano as an analogy. The piano keyboard contains 88 keys which represent different frequencies (or notes). The low frequencies (bass notes) are on the left, the higher frequencies (treble notes) are on the right. Middle C on the keyboard represents approximately 256 Hz. The intensity or loudness of a note depends on how hard you hit the key. A light touch on middle C may produce a 30 dB, 256 Hz note, while a hard strike on middle C may produce a 55 dB, 256 Hz note. The frequency (or note) stays the same, but the intensity or loudness varies as the pressure on the key varies.