Sound:
Imagine that you are immersed in an ocean of thin, compressible, somewhat sticky fluid. In fact, you are. The fluid is air, and vibrations in it are acoustic waves. When one part of that fluid is set into vibrating waves, the quivering vibrations are communicated throughout the fluid. When the vibrations are in the range of about 20 times a second to about 20,000 times a second they can be perceived by people as sounds.
Information in sound
Wherever you are, there is sound, even if it is only the sound of your own heartbeat or your breath. If analyzed correctly, sound can reveal information about what produced it. In legal cases, it is sometimes possible to ascertain when and where a recording was made by analyzing sounds in the environment; for example, when a recording is made with a television playing in the background or when a vehicle such as a bus passes an intersection on a known schedule.
Speech sounds
When you speak, air passes from your lungs past the vocal folds, which are bands of cartilage in your larynx or “voice box.” In men, the larynx is called the “Adam’s apple” because it is generally larger than it is in women, which accounts for the fact that men generally have lower pitched voices than women do. By moving the speech articulators, such as the tongue, lips, velum and jaw, you can form speech sounds. Vowel sounds are formed when the air stream sets the vocal folds into vibration and then flows smoothly through the upper vocal tract. For some sounds (p, t and k) a sound is made when a closure between articulators is released suddenly and a sharp burst of air results. They are called “plosive” sounds or “stops”. For others (s, sh, f, and voiceless th as in “thin”) a turbulent stream of air produces a hissing sound when it passes through a narrow opening. They are called “fricative” sounds. Various combinations of articulations are also possible, as with voiced stops (b, d and g) and voiced fricatives (z, v, and voiced th as in “then”). Speaker differences: Differences among people in the physical characteristics of their vocal folds and other articulators partly account for the distinctive characteristics of individual voices. Additionally, people use their vocal apparatus in slightly differing ways, resulting in dialectal differences. Because of these differences, people can be identified by their voices.