Sound is a very tiny oscillation of pressure. Imagine moving a metal cymbal up and down. When the surface moves upward, the air above is slightly compressed, which means the pressure is raised a little. When the surface moves downward, the air is drawn out, slightly lowering the pressure. Tapping the cymbal with a drumstick creates a much more rapid up-and-down oscillation of the metal surface. The result is a traveling oscillation of air pressure—a sound wave. Anything that vibrates in contact with matter makes sound waves.
How are sound waves different from other waves?
Sound waves are rapid oscillations compared to waves in springs or in water. The period of oscillations in sounds that humans can hear is less than 0.05 s. This corresponds to a frequency of 20 Hz, which is the low-frequency limit to an average human ear. The high-frequency limit is about 20,000 Hz for a young person, but this declines to around 12,000 Hz by middle age.
Can we feel the pressure of sound waves?
Sound waves have a wide range of amplitudes. Typically the variation in pressure is around one part in 10,000 or 0.0001 atmospheres. This kind of pressure variation is far below the detection threshold of our sense of touch. Nonetheless, sound is such a rich environmental factor that virtually all higher animals have evolved a sense of hearing that is well adapted to detecting sound. The human ear is extremely sensitive and can easily detect pressure oscillations of less than one part in a million.
How fast do sound waves travel?
Sound waves travel faster than familiar objects in your everyday environment. For example, if someone across a 5 m room talks to you, the sound wave reaches your ear in 0.015 s. In air at room temperature (21°C) and one atmosphere of pressure, the speed of sound is 343 m/s (767 mph). The speed of sound is not constant but varies with temperature and pressure. The highest speed people normally attain, about 500 mph on a passenger jet, never exceeds the speed of sound.
Supersonic flight
Traveling faster than the speed of sound was first achieved by a human in 1947 when test pilot Chuck Yeager flew the X-1 rocket plane to a maximum speed of 361 m/s (807 mph). Supersonic flight is dangerous because enormous, turbulent pressure waves can suddenly build up at the nose and extremities of an aircraft that can cause loss of control. In fact, ordinary passenger jets must fly slower at high altitudes because the lower temperature of the air at high altitudes reduces the speed of sound.
