Characteristics of Sound Waves

Author: N.H. Crowhurst

Generation of a sound wave

Sound waves combine these two forms of movement so as to cover great distances with only small air movements. Take the sound of a hand clap - when your hands come together, a small amount of air is forced out quite suddenly at the last instant.

The air pushed out by the hands clapping has a momentary movement and some pressure, although both are quite small compared to the movement produced by the fan or the pressure buildup of the supersonic jet. Two things now happen at the same time:

1. Because the pressure of the air close to the hands is momentarily greater than that further away, air moves outward.

2. Because the air close to the hands is moving outward, the air immediately

beyond it also gets compressed.

The two actions combine to keep the pressure and movement wave traveling at a natural speed - the propagation velocity of sound in air. This is not to be confused with the speed at which individual air particles move due to the wave.

How a sound wave travels

Note that, after the wave has passed, the air stops moving. A body of air does not move with the wave, but the energy in the wave is passed on from particle to particle, in much the same way that (in the illustration) the impact of one penny is transmitted along a line of pennies.

How sound travels: the impact passes from one particle to the next

The natural speed, or propagation velocity, of sound is controlled by two properties of the air through which it travels:

1. How much squeezing more of it into a given space will increase its pressure - its compressibility (known technically as elasticity). This feature takes control of the air piled up in front of the supersonic jet.

2. How much force is needed to get a given quantity of it moving, or to stop its motion - its density (or mass per unit volume). This feature is responsible for carrying the draft created by the fan.

The natural speed, or propagation velocity, of sound in air is roughly 1100 feet per second, or 750 miles per hour.