sound

The diagram illustrates the motion of a longitudinal wave. Sound, for example, travels through air in longitudinal waves: the waves vibrate back and forth in the direction of travel. In the compressions the particles are pushed together, and in the rarefactions they are pulled apart.

Physiological sensation received by the ear, originating in a vibration causing sound waves. The sound waves are pressure variations in the air and travel in every direction, spreading out as an expanding sphere. Sound energy cannot travel in a vacuum.

All sound waves in air travel with a speed dependent on the temperature; under ordinary conditions, this is about 330 m/1,080 ft per second. The pitch of the sound depends on the number of vibrations imposed on the air per second (frequency), but the speed is unaffected. The loudness of a sound is dependent primarily on the amplitude of the vibration of the air.

Sound travels as a longitudinal wave; that is, its compressions and rarefactions are in the direction of propagation. Like other waves – light waves and water waves – sound can be reflected, diffracted, and refracted. Reflection of a sound wave is heard as an echo. Diffraction explains why sound can be heard round corners. When sound is refracted (see refraction), sound is bent when it passes into a denser or less dense material because sound travels faster in denser materials, such as solids and liquids. The lowest note audible to a human being has a frequency of about 20 hertz (vibrations per second), and the highest one of about 20,000 hertz; the lower limit of this range varies little with the person's age, but the upper range falls steadily from adolescence onwards.