Sound. When a drum is struck, the drumhead vibrates and the vibrations are transmitted through the air in the form of waves. When they strike the ear, these waves produce the sensation of sound. There is also sound that cannot be heard, however: infrasound, below the range of human hearing, and ultrasound, above the range of human hearing.

Terms used in the study of sound
Acoustics is the science of sound and of its effects on people.
Condensation is a region in a sound wave in which the sound medium is denser than normal.
Decibel (dB) is the unit used to measure the intensity of a sound. A 3,000-hertz tone of 0 dB is the softest sound that a normal human ear can hear.
Frequency of a sound is the number of sound waves that pass a given point each second.
Hertz is the unit used to measure frequency of sound waves. One hertz equals one cycle (vibration, or sound wave) per second.
Intensity of a sound is a measure of the power of its waves.
Loudness refers to how strong a sound seems when we hear it.
Noise is a sound that is unpleasant, annoying, and distracting.
Pitch is the degree of highness or lowness of a sound as we hear it.
Rarefaction is a region in a sound wave in which the density of the sound medium is less than normal.
Resonance frequency is the frequency at which an object would vibrate naturally if disturbed.
Sound medium is a substance in which sound waves travel. Air, for example, is a sound medium.
Sound quality, also called timbre, is a characteristic of musical sounds. Sound quality distinguishes between notes of the same frequency and intensity that are produced by different musical instruments.
Ultrasound is sound with frequencies above the range of human hearing—that is, above 20,000 hertz.
Wavelength is the distance between any point on a wave and the corresponding point on the next wave.

Technically, sound is defined as a mechanical disturbance traveling through an elastic medium—a material that tends to return to its original condition after being deformed. The medium need not be air; metal, wood, stone, glass, water, and many other substances conduct sound, many of them better than air.

There are a great many sources of sound. Familiar kinds include the vibration of a person's vocal cords, vibrating strings (piano, violin), a vibrating column of air (trumpet, flute), and vibrating solids (a door when someone knocks). It is impossible to list them all, because anything that imparts a disturbance to an elastic medium (as, for example, an exploding firecracker to the surrounding air) is a source of sound.

Sound can be described in terms of pitch—from the low rumble of distant thunder to the high-pitched buzzing of a mosquito—and loudness. Pitch and loudness, however, are subjective qualities; they depend in part on the hearer's sense of hearing. Objective, measurable qualities of sound include frequency and intensity, which are related to pitch and loudness. These terms, as well as others used in discussing sound, are best understood through an examination of sound waves and their behavior.

Speed of sound in various mediums
MediumSpeed in feet per secondSpeed in meters per second
Air at 59 degrees F. (15 degrees C) 1,116340
Aluminum 16,0005,000
Brick 11,9803,650
Distilled water at 77 degrees F. (25 degrees C) 4,9081,496
Glass 14,9004,540
Seawater at 77 degrees F. (25 degrees C) 5,0231,531
Steel 17,1005,200
Wood (maple) 13,4804,110