### Question

Why do Foghorns always have very low pitches? Do they have high or low amplitude?

Foghorns have very low pitches because sounds with low pitches have a long wavelength. This is important because a long wavelength means that the sound wave can pass around barriers, like rocks, easily. This property of a wave is called diffraction. Diffraction describes the ability of a wave to pass around a barrier. The longer the wave's length the easier it is for the wave to do this.

To get an idea of how this happens imagine that you and ten of your friends hold hands and walk through a park. Never mind what onlookers will say; you are only imagining this! When you come to a tree you must let go of a hand to get around the tree. One of your friends might even get left behind the tree but once around the tree the remaining nine of you could, if you wanted to, easily get a hold of the hands again. Now, imagine again that you are doing this with only three friends. When you get to a tree with only three of you, one of you could be left at the tree and it would be much more difficult for the remaining two of you to reattach. As a result the two of you could go off in different directions!

A wave is much like this. When a wave with a long wavelength strikes a rock, or a tree, or a building, it breaks up to get around it. A shorter wavelength wave might hit the same rock and bounce off it and not go around it at all. If this were a foghorn than the sound would not get past the nearest rock and ships at sea would not have a chance to hear it.

The amplitude of a wave tells you how much energy the wave has. If the foghorn had a low amplitude its sound would not have enough energy to go very far no matter how long its wavelength. So, a foghorn needs to have a very high amplitude so that the long wavelengths can both get around the rocks and far out to sea.
Answered by: Tom Young, M.S., Science Teacher, Whitehouse High School, Texas

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