Sound

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Sound, by definition, is the reception of vibration waves of high enough amplitude to stimulate a hearing organ and with a frequency within hearing range, transmitted through a medium. It is important to note that in order for a vibration to be classified as sound, it must be heard: strictly speaking, if a tree falls in a forest an no one is around; no, it does not make a sound.

Physics of Sound

Longitudinal

Longitudinal waves are the manner in which sound itself propagates through air. They are defined as waves in which the manipulated medium moves in the same direction as the path the wave travels. As any medium vibrates, its minuscule movement displaces the air molecules surrounding it. As one air molecule is pushed outward, two things occur:

• the molecule invades the space of the next molecule in line, thereby pushing it forward,
• the molecule leaves a space behind it; i.e., a vacuum, that it must immediately return to.

In this manner, each molecule pushes the next and returns back to it's original position until the very last molecule is pushed against the observer's ear by the one behind it. This all occurs at approximately 343 metres per second (768 mph, or about one mile in five seconds), depending on altitude, temperature, and other factors. This figure, known as the speed of sound, varies depending on the medium. For instance, in water the speed of sound travels about 4.4 times this speed (~1,500 m/s), and in solids even significantly faster yet.

Transverse

Transverse waves are the manner in which vibrations pass through a string. They are defined as waves in which the manipulating medium moves in a direction perpendicular (transverse) to the path the wave travels. For instance, as a wave moves from one end of a string to the other, the string is actually vibrating up and down, not forward and backward like a longitudinal wave.

Standing waves

This section requires expansion.

Acoustics

Main article to be written

Biological Uses

Organisms may have originally developed sound as one of their major sense in order to perceive potential danger: fire, oceans, wind, earthquakes, and falling trees all make unique and identifiable sounds. Many organisms also have evolved organs for producing sounds unique to their species, as a means of communication amongst each other or as a warning to others.

Human hearing is said to be in the range of 20 Hz to 20,000 Hz with the sensitivity of the human ear averaging at about 20 uPa or 0dB.