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Sound can be thought of as invisible air waves that spread or ripple across space. Sound waves are often referred to as vibrations. Sound waves enter through the ear canal and vibrate the eardrum, both in the outer ear. Three small bones of the middle ear (the hammer, anvil & stirrup) amplify and transmit the vibrations to the cochlea in the inner ear. The cochlea is filled with fluid and tiny hair-like structures, cilia, that wave like grass. The waving of the cilia translates the sound waves into nerve impulses for the brain. Our brain then interprets these impulses as sound (hearing). Figure 1 illustrates parts of the ear important for understanding hearing.
Sound has two properties that are important in preventing noise induced hearing loss: frequency and intensity. Sound frequency refers to how many vibrations occur in one second and is measured in Hertz units, abbreviated Hz. Intensity is the power or size of the sound pressure. The perceived loudness of a sound is dependent upon both frequency and intensity along with other factors such as how close one is to the sound source and the health of one’s ears.
Sound as air waves is a commonly used analogy in explanations about hearing and noise induced hearing loss. It is also helpful to think of sound waves as “energy” that radiates out in all directions from the sound source. This analogy is useful in understanding what happens when our ears are exposed—or assaulted—with too much sound. Hazardous sound exposure is related to sound intensity (loudness) and length of exposure to the sound. The energy that is associated with sound that is too loud and/or lasts too long will over-stimulate the cilia (hair cells) of the inner ear and leave it in a “flattened” state, interfering with the nerve impulses sent to and interpreted by the brain (hearing).
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