A decibel is the unit for measuring the intensity, or
loudness, of a sound. Today it is possible to study noise scientifically by
means of electrical instruments. Chief among these is the sound meter, or noise
meter, a device that changes sound into electrical signals and then measures
the signals in terms of decibels. One decibel is about the smallest difference
of sound intensity that the normal human ear can detect. Twenty decibels mark
sound one hundred times as strong as a ten decibel sound. Thirty decibels mark
sound a thousand times as strong as a ten-decibel sound, and so on.
A man’s heart beat creates a sound of a little more than 10
decibels. The purring of a cat is about the same as a whisper 25 to 30
decibels. A single typewriter accounts for 40 decibels.
A few other common noises are rated in decibels as follows:
- Barking dog-65;
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- Piano practice-75;
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- Average motor truk-80;
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- Loud shouting-85;
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- Pneumatic riveter-105
All noises above 130 decibels are actually painful in our
ears.
The noise of city streets ranges from 50 to 85 decibels. The
country noise level is about half that of the city. It should be remembered,
however, that a large volume of steady noise in the city actually seems less
than a series of lesser country noises coming at random after intervals of
comparative silence. The cricket and katydid, the locust and bullfrog and the
mournful whippoorwill often strike the attention to a degree out of proportion
to the noise they make. Just so the sudden slamming of a door and the
unexpected ringing of a telephone bell are more of a menace to sleep or
concentration than the continuous roar of traffic, loud as it may be. Homer
says that stentor (source of our word “stentorian”), famous in the Greek army
for his loud voice, could equal the combined voices of 50 men. His shout was probably
on a decibel level of 100.
