Sound Production and Perception

Sound is produced when an object vibrates creating pressure waves. In the following diagram, the red line represents the object vibrating in the air while the black dots represent the molecules of air. As the object moves toward the right it pushes molecules of air with it, compressing them together causing a region of higher pressure. The area of higher pressure (more molecules) causes molecules to its right to move right. As the object moves left it leaves open space, a lower pressure area, which allows the previously moved molecules to move left. This causes an area of fewer molecules (a rarefication) which causes molecules to its right to move left.

If you look carefully at the animation, you will notice that each dot (molecule) moves back and forth, right to left while the area of compression appears to move to the right. The movement of compression followed by rarefications of the molecules of air is the pressure wave.

Sound pressure waves can travel though any compressible medium (solid, liquid, or gas.) The speed of propagation depends on the type of the medium. The speed of sound in dry air at 68° F, is approximately 1127 feet per second. Although the diagram shows linear movement, pressure waves will propagate in each unblocked direction much like a wave in water.

When the pressure wave reaches the ear, the series of compressions and rarefications causes to the ear drum to vibrate. The vibrations are coupled through the middle ear (anvil, hammer, and stirrup) to the fluid of the inner ear in a snail shaped structure called the cochlea. The waves cause movement of tiny cells within the cochlea called hair cells. Each hair cell vibrates in response to the presence of certain frequencies in the pressure wave and send signals to the brain which can then be interpreted as sound.

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