Sounds of all kinds fill the air around us. Refrigerators hum, car horns honk, birds sing, and leaves rustle in the wind and crunch under foot. Although we may not be able to see their underlying cause, all sounds—no matter how loud or soft, high-pitched or booming—begin with vibration.

Vibrating objects create waves in the air that surrounds them. These waves travel in all directions away from their source. Their movement resembles the motion of waves traveling away from a splash caused by a stone tossed into a pond. When sound waves enter your ears, they cause first the eardrum, then the bones of the middle ear, and finally the fluid-filled cochlea of the inner ear to vibrate. The vibration stimulates nerve cells in the ear to send a signal to the brain, which the brain interprets as sound.

Sounds vary in recognizable ways. This is because objects of varying shapes and materials vibrate differently. Some objects vibrate slowly, while others vibrate quickly. Some objects create vibrations that last only a few moments, while others continue for minutes or even hours. The sound waves created by vibrating objects vary accordingly. For example, an object that vibrates quickly produces many sound waves over a short period of time, while an object that vibrates more slowly produces fewer waves spread over the same time period.

Our brains make sense of variations in sound waves. A rapid succession of waves (generated by an object vibrating quickly) is interpreted as a high-pitched sound; waves that come less frequently are interpreted as lower in pitch. The brain also interprets large-amplitude sound waves as louder than small-amplitude sound waves. This is because amplitude is related to how much energy the wave is carrying. The greater the amplitude, the louder the sound. Through experience, we become able to distinguish between many different types of sounds, even when we can't see their source. This information tells us a great deal about our environment.

For people who are hearing-impaired, sound provides much less information. However, it is still possible for someone who cannot hear to feel the vibrations that generate sound, and to distinguish among variations in those vibrations. This often requires direct contact with the vibrating object, such as a drum or a piano. However, if sound waves are large enough, they can be felt as they pass through the air.

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