FoundationalConceptsNeuroscience

—exposure to loud sounds. The standard way of measuring sound
intensity, or loudness, is a unit called the decibel (dB), named in
honor of telecommunications pioneer Alexander Graham Bell (1847-
1922). The decibel scale enjoys many uses in engineering; its defin-
ing feature is that it is a logarithmic scale relative to some specified
reference. When applied to the measurement of sound loudness, 0 dB
represents the approximate threshold for human hearing, and each
increase of 10 dB represents a factor of ten increase in loudness. Thus,
a 20-dB increase in sound intensity corresponds to a 100-fold (102)
increase in sound loudness, and a 50-dB increase in sound intensity
corresponds to a 100,000-fold (10%) increase in sound loudness. For
example, a 75-dB sound is 100,000 times louder than a 25-dB sound.
On this acoustic decibel scale, where O dB represents the approximate
threshold of human hearing, a quiet room measures approximately
25 dB, the loudness of normal human speech is 40-50 dB, the ambient
sound level in a noisy restaurant is 80 dB, ajackhammer breaking up
concrete a meter away might be 100 dB, and the sound level at a loud
concert could be 120 dB or more. Firing a rifle may produce a transient
sound intensity of more than 150 dB.
It is now appreciated that even brief exposure to very loud sounds
(acute acoustic trauma), such as gunshots and explosions, can
result in permanent hearing loss. So also can chronic exposure to
sounds that are moderately loud. Musical events (music in clubs,
rock concerts, and so forth) and work environments that have sound
levels above 85 dB (working outside at airports, certain construction
environments, and so on) are all potential sources of chronic acoustic
trauma. Many places now have health regulations that limit exposure
to loud sounds and require ear protection. Also increasingly appreci-
ated is the possibility that, with the proliferation of highly portable
digital music players, listening to these devices at high volume consti-