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# Physics for Kids

## Sound Wave Characteristics

A sound wave is a special kind of wave that can be detected by the human ear. Sound waves have special characteristics that make them unique.

Mechanical Waves

One important characteristic of sound waves is that they are mechanical waves. This means that they travel through a medium. Sound waves can travel through all sorts of mediums. Normally, we hear sound waves that have traveled through air, but sound can also travel through water, wood, the Earth, and many other substances. Sound cannot travel through a vacuum like outer space, however.

The source of sound waves is something vibrating. This vibration causes a disturbance in the molecules around the source. The energy of the wave is transferred from molecule to molecule within the medium.

Longitudinal Waves

Another characteristic of sound waves is that they are longitudinal waves. This means that the disturbance of the wave travels in the same direction as the wave. As the molecules vibrate and transfer energy to each other they cause a wave that moves in the direction of the vibration.

The longitudinal characteristic of sound waves can be seen the picture below. Here you can see how the molecules move in a left to right motion causing the wave and the disturbance to move in the same direction. In some areas of the wave the molecules get bunched together. This is called compression. In other areas the molecules become spread out. This is called rarefaction.

What is the wavelength of a sound wave?

We studied how the wavelength of a transverse wave is measured from crest to crest or trough to trough. This is fairly easy to see when looking at a graph. However, sound waves are different as they are longitudinal. To determine the wavelength of a sound wave you measure from compression to compression or rarefaction to rarefaction.

Pressure Waves

Sound waves can also be thought of as pressure waves. This is because the compressions and rarefactions that move through sound waves have different pressures. The compressions are areas of high pressure while the rarefactions are areas of low pressure.

What is the amplitude of a sound wave?

Sometimes you will see a graph of a sound wave that looks like a sine wave (see below). This is different from the graph of a transverse wave. The peaks and valleys of this wave graph the changes in pressure that occur in the wave. From this graph we can determine the amplitude of the sound wave. The amplitude is the peak of the compression or rarefaction on the graph.

Intensity of a Sound Wave

Sound waves are sometimes measured using a quantity called intensity. The intensity of a sound wave (I) is equal to the sound power (P) over the area (A):

I = P/A

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