Noise-induced hearing loss

Exposure to excessive noise can make hearing less sensitive. The loss may be temporary, with hearing recovering after exposure, or permanent. Its extent depends on the exposure, including how loud the noise is and how long a person is exposed to it.

Noise-induced hearing loss commonly affects higher frequencies more strongly, often producing a noticeable loss near $$4\ \text{kHz}$$. At affected frequencies, a sound generally needs a greater sound pressure level to be heard, or to produce a given perceived loudness, than it would for a person with normal hearing. The loss is not necessarily the same at every frequency.

Noise-induced hearing loss and equal-loudness curves

An equal-loudness curve shows the sound pressure level needed at each frequency for sounds to be perceived as equally loud. Frequency is plotted along the horizontal axis and sound pressure level along the vertical axis. Two points on the same curve represent sounds that may have different frequencies and sound pressure levels, but are perceived as equally loud.

To compare hearing, consider curves for the same perceived loudness. If a person's hearing loss affects frequencies near $$4\ \text{kHz}$$, their curve may lie above the normal-hearing curve in that range: a greater sound pressure level is needed there to produce that loudness. Other frequency ranges may be less affected, so hearing loss can change the shape of the curve rather than shifting the whole curve upwards by the same amount.

Age-related hearing loss

Hearing can deteriorate as a person gets older. This is called age-related hearing loss. It commonly affects high-frequency sounds more than low-frequency sounds: a high-frequency sound may need a greater sound pressure level to be heard, or to be perceived at a given loudness.

This can make speech harder to distinguish because some differences between speech sounds depend on their high-frequency components. If those components are harder to hear, words may be harder to tell apart.

Age-related hearing loss on equal-loudness curves

An equal-loudness curve shows the sound pressure level required at each frequency for sounds to be perceived as equally loud. Frequency is plotted on the horizontal axis and sound pressure level on the vertical axis.

For a person with age-related hearing loss, the curve can lie higher than the corresponding curve for normal hearing, especially at high frequencies. At a particular high frequency, a higher point on the curve means that a greater sound pressure level is needed for the sound to be perceived at the same loudness.