This is the 2007 version. Click here for the 2017 chapter 04 table of contents.

The Auditory System

While the eye responds to electromagnetic radiation, the ear responds to pressure waves in the air. Sound energy consists of alternating waves of compressed and decompressed air. That is why you can feel bass notes at a loud concert. The pressure waves, and the vibrations they create in objects, are strong enough to feel with the sense of touch.

To what sort of energy does the sense of hearing respond? Of what does sound consist?


A sound wave consists of alternating periods of compression and decompression (rarefaction).

An audio speaker, like those in stereo systems, produces sound by converting electrical signals into pressure waves. A magnet behind the speaker makes a piston move, and this makes the speaker cone vibrate. When the speaker cone moves forward, it compresses the air; when it moves backward, it decompresses the air, creating a partial vacuum. The alternating periods of pressure and partial vacuum result in the sensation of sound.

Sound requires a substance to pass through—a medium to compress. There can be no sound of space ships blowing up in outer space, because sound cannot pass through a vacuum. With rare exceptions such as Stanley Kubrick's 2001: A Space Odyssey, TV series and movies portraying fights between space ships in outer space ignore this simple law of nature, giving viewers the experience they are accustomed to on earth: hearing explosions.

How are nearly all science fiction movies and TV series inaccurate?

Light waves (and other forms of electromagnetic radiation) require no medium to compress, so they travel easily through interstellar space. Sound travels only through air, water, or other materials. The denser the medium, the faster sound travels. Sound goes 1,100 feet per second in the air, 4,900 feet per second in water, 20,000 feet per second in steel. That is why you can detect a train from very far away by putting your ear directly to a rail. Not only is the vibration from a train strong; it travels quickly for a long distance because it is propagated in the rails, a dense medium.


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