In GCSE Science students will spend some time looking at both electromagnetic and mechanical waves. This is the third of six quizzes on waves and it looks at one particular type of longitudinal wave - sound.
All waves are one of two different types - transverse or longitudinal. When something vibrates, it creates a sound wave. Sound waves are longitudinal waves and the energy they carry causes vibrations in a medium, so they are mechanical waves. Like all mechanical waves, sound waves can only travel in places where there are particles that are sufficienty close to make the next-door particles vibrate. This limits sound to being heard only in solids, liquids and gases. Science fiction films nearly always show sounds in space. This is technically wrong but let's face it, without the sounds these films would be dull and boring!
The pitch describes how high or low the wave sounds. This depends on how many times it vibrates every second - its frequency. Sounds that are pitched higher than about 20,000 Hz are too high for most human ears to detect, but some animals have much more sensitive hearing than humans and can hear these sound waves. A device that is used by walkers and cyclists to keep aggressive dogs away from themselves uses sound above 20,000 Hz - that irritates the dog and it moves away to a distance where the sound is no longer annoying. Sound below about 20 Hz is too low for most people to hear but is used by whales to communicate with one another. These low pitched sound waves can travel for hundreds of kilometres through the oceans.
The volume of a sound depends on the amplitude of the vibrations. A loud sound has a large amplitude and vice versa. Large amplitude loud sounds carry more energy than small amplitude quiet sounds. When you listen to loud music, for example at a party or disco, or if you are using noisy machinery, the loud sounds carry enough energy to damage your ears. It is generally only a problem to people regularly exposed to these levels of noise as your ears have a defence mechanism. When exposed to loud sounds, muscles tighten the eardrum so that it vibrates less. It takes a while for the muscles to relax after the loud sound which is why your hearing seems a bit 'muffled' for a while.
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