Sound is an oscillating disturbance that passes through matter in the form of pressure waves; the essential requirement for the propagation of such waves is the presence of a material medium that is to some extent elastic. Such a medium contains atoms and molecules that can pass on a disturbance—a sound wave—by bumping into their neighbors. Solids, liquids, and gases all propagate sound waves; pure vacuum does not.
The study of pressure waves is termed acoustics, and the term sound applies principally to pressure waves whose frequencies lie in the range 20 Hz to 20,000 Hz—the range of frequencies detectable by the human ear. When pressure waves reach the eardrum, they cause it to vibrate. The ear then converts these vibrations into signals that pass through nerves to the brain, where they are interpreted as sounds.
Acoustic waves of vibration frequencies below 20 Hz are termed infrasonic, and include most earthquake vibrations. Pressure waves at frequencies greater than 20,000 Hz are termed ultrasonic, and include vibrations produced by equipment for cleaning and therapeutic uses.
Production and transmission
When an object vibrates, molecules next to its surface are alternately pushed together and pulled apart—the medium is first compressed, then rarefied. As these molecules move back and forth, they collide with neighboring molecules in the medium. Those molecules then move back and forth with a slight time lag when compared with the molecules at the vibrating object’s surface, and that time lag increases with distance from the vibrating object. As such, waves of compression and rarefaction spread through the medium from the surface of the vibrating object.
Speed of sound
The speed of sound is the rate at which pressure waves travel through a medium—solid, liquid, or gas—and this rate varies according to the medium and the prevailing conditions of temperature and pressure. One factor is the elasticity of the medium: the more elastic the medium, the greater the ability of one atom or molecule to nudge its neighbors into motion and the greater the speed of sound. Another factor is density: higher density indicates more massive particles, whose greater inertia makes their motions more sluggish, so the speed of sound is reduced. Temperature and pressure act indirectly by changing the elasticity and density of media, thereby changing the rate of passage of pressure waves. In general, the speed of sound, v, is related to the modulus of elasticity, E, and density, ρ, by the formula
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In strict scientific terms, Young’s modulus of elasticity, E, is the ratio of stress (force per unit area) to the resulting strain (change in length per unit original length). The force required to produce a given strain in steel (E = 2 × 1011 Nm–2), say, is much greater than the force required to produce the same strain in rubber (E = 107Nm–2), so steel has a much greater elasticity than rubber in this sense.
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THE COCHLEA |
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The cochlea is the part of the inner ear responsible for the ability to hear sounds. It is present in humans and other mammals, as well as in birds and some species of reptiles.
Sound waves reach the cochlea via the fenestra ovalis (Latin for "oval window"), a membrane that separates the middle and inner ears. They stimulate vibrations of the endolymph, the fluid that fills the cochlea, and these vibrations in turn cause vibrations of tiny hairs that protrude from the lining of the cochlea. Hairs near the basal end of the cochlea vibrate in response to high frequencies, stimulating nerve impulses that originate in the sensitive cells that support them; hairs near the apical end of the cochlea respond to low-frequency sounds in a similar way. The brain interprets the impulses from the different types of cochlear cells to generate a perception of sound.
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In general, the speed of sound is greatest in solids, less in liquids, and least in gases because the decrease in elastic modulus from solids to gases overwhelms the decrease in density. The speed of sound in steel is approximately 11,180 mph (5,000 m/s); in water, it is around 3,130 mph (1,400 m/s); and in air at room temperature and at sea level, it is 769 mph (344 m/s).
The effect of temperature on the speed of sound is much less pronounced in solids and liquids than it is in gases: values of elasticity and density change little with temperature in solids and liquids, whereas an increase in the temperature of a gas causes its molecules to move from collision to collision with greater speed, thereby increasing elasticity. The speed of sound in air, for example, increases from 741 mph (331 m/s) at 32°F (0°C) to 864 mph (386 m/s) at 212°F (100°C)—an increase of 17 percent.
