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Creating Sound Waves

by Ron Kurtus (revised 18 March 2012)

Sound consists of compression waves in some material—usually air. Sound waves are created by the vibration of some object or even air itself. They can be created by a back-and-forth motion of an object, by having air pass over a vibrating object, or by moving air causing the vibrations.

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Vibration creates sound waves

Whenever an object in air vibrates, it causes compression waves in the air. These waves move away from the object as sound. There are many forms of the vibration, some not so obvious.

The back and forth movement of a loudspeaker cone, guitar string or drum head result in compression waves of sound. When you speak, your vocal cords also vibrate, creating sound.

Blowing across a bottle top can also create sound. In this case, the air inside the bottle goes in a circular motion, resulting in sound waves being formed. Wind blowing through trees can also create sound this indirect way.

Sound can also be created by vibrating an object in a liquid such as water or in a solid such as iron. A train rolling on a steel railroad track will create a sound wave that travels through the tracks. They will then vibrate, creating sound in air that you can hear, while the train may be a great distance away.

Back-and-forth vibration

A vibrating object will cause these compression waves in air or another medium.

A good example is a tuning fork. The metal rods in the tuning fork move back-and-forth, causing the compression wave in the air. A tuning fork could also be put in water, creating sound waves within the water.

Note that the waves created by the tuning fork on the surface of a pool of water are not sound waves. They are surface waves, which are circular instead of compression waves.

Other examples back-and-forth vibrations that create sound are a guitar string, drum, and loudspeaker.

Passing by vibrating object

If something vibrates as air passes through or by it, the air can be also made to vibrate as sound waves. The best example of that phenomenon is how your vocal chords create sound.

The air causes some of the vibration and that vibration then amplifies the sound. You can see this by putting a thick blade of grass between your thumbs and blowing on it. Another example is wrapping a comb in wax paper, putting it up to your mouth and blowing on it to make the noise, similar to a kazoo.

Air causes its own vibration

When moving air passes by an object, it can start some air to vibrate and make sound. This type of sound can be heard on a windy day or when you blow across the top of a bottle.

One example is an explosion, which results in a sudden surge in air, causing the high-volume sound. Thunder and a balloon popping are other examples of this way of creating sound.

Summary

Sound can be created by a vibration or back-and-forth motion of an object, by having air pass over a vibrating object, or by moving air causing the vibration.


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Resources and references

Ron Kurtus' Credentials

Websites

Loudspeaker Basics - From HyperPhysics

Physics Resources

Books

Top-rated books on Sound Waves


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