Sound

Sound is a mechanical wave that results from the oscillation of particles in a material medium. It propagates as a longitudinal wave, with regions of compression and rarefaction that move away from the source. Sound cannot travel in a vacuum, because there are no particles to transmit pressure changes; it requires a medium such as air, water, or a solid.

The speed of sound depends on the medium and its conditions. In dry air at 20 degrees Celsius, it is about 343 meters per second. In liquids and solids, speeds are typically higher, for example about 1,500 m/s in water and several thousand meters per second in metals. Temperature, pressure, and composition affect these speeds.

Sound is characterized by frequency, which determines pitch, and amplitude, which relates to loudness. Humans generally perceive frequencies from about 20 Hz to 20,000 Hz, with sensitivity varying across the range. The loudness depends on the pressure amplitude of the wave.

Sound levels are often expressed as sound pressure level in decibels (dB). The reference is 20 micropascals, roughly the quietest sound audible to a healthy ear. Levels are logarithmic, so a tenfold increase in pressure corresponds to about a 20 dB increase.

When encountering boundaries or obstacles, sound can reflect (echo), refract, diffract, or absorb. These interactions influence hearing, room acoustics, and technologies such as sonar and ultrasound imaging.

Sound is produced by vibrating sources, including musical instruments, voice, engines, and loudspeakers. The field of acoustics studies how sound is generated, transmitted, and perceived, with applications in music, communication, medicine, and industry.

Environmental noise and high-intensity sound can affect health, causing hearing loss or stress. Mitigation includes design of spaces, regulations on exposure, and personal hearing protection.