An ear pressed to a desk hears a knock; spoons tapped underwater are still heard — sound travels through solids, liquids, and gases.
Pump the air from a bell jar with a ringing bell inside, and the sound fades to silence — even as the bell keeps visibly ringing.
Pushes air particles forward, then pulls back, alternately bunching and spreading them.
A region of higher-than-average density, where particles are pushed close together.
A region of lower-than-average density, where particles are spread further apart.
Distance between two consecutive crests, in metres.
Number of oscillations per second, in hertz.
Time for one complete oscillation, in seconds.
The maximum change in density in a compression or rarefaction. Bigger amplitude means more energy.
Sound energy passing through a unit area per unit time. It falls as a wave spreads out from its source.
Perceived frequency — a whistle sounds shriller than thunder's low rumble.
Perceived amplitude, measured in decibels — conversation is about 60 dB.
20 Hz to 20,000 Hz. Below is infrasonic, above is ultrasonic.
A reflected sound heard separately, only if it arrives at least 0.1 s later — the surface must be at least 17 m away.
Many reflections arriving less than 0.05 s apart blend into a lingering persistence of sound.
Detecting earthquakes, volcanic eruptions, and severe storms.
Ultrasonography, breaking kidney stones, industrial cleaning and flaw detection.
Bats and dolphins sense echoes to navigate; SONAR finds distance underwater the same way.
Periodic to-and-fro motion that produces sound.
Region of higher / lower than average density.
Particles vibrate parallel to wave propagation.
λ, distance between crests; ν, oscillations per second; T, time per oscillation.
Size of density change; energy per area per time.
Perceived frequency / perceived amplitude.
A separate reflected sound / overlapping, lingering reflections.
Below 20 Hz / above 20 kHz — outside human hearing.