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Decibels
The word "decibel" comes from the Latin prefix "deci-," meaning "10th," and "bel," which is a unit of measurement named after Alexander Graham Bell, who devised it. Decibel literally means "10th of a bel." It is a way of measuring pressure variations in the air caused by the sound waves sent toward the listener; the greater the pressure variations, the higher the number of decibels.
Measuring Decibels
Decibels are measured using sound level meters. These meters have microphones which sample and measure the sound. Electronic circuits then measure and filter the sound, producing a measurement on the meter's digital display. There are several kinds of sound level meters: the most basic meters can only measure the decibels of a sound at a particular moment, while more sophisticated meters are able to measure sounds over longer periods of time in order to determine patterns and trends in sound levels.
The Decibel Scale
The decibel scale measures sound intensity exponentially, in powers of 10, and not in a linear fashion. To understand this better, consider how a measuring tape works: 20 inches is twice as long as 10 inches, and 30 inches is three times as long. The measuring tape works in a linear fashion. However, the decibel scale measures increases in powers of 10, meaning that a sound of 20 decibels (dB) is 10 times louder than a sound of 10dB -- not twice as loud -- and a 30dB sound is 100 times louder, not three times louder.
Safety
The National Institute on Deafness and Other Communication Disorders warns against exposure -- temporary or prolonged -- to sounds that are too loud because they can damage your hearing. Prolonged exposure to sounds between 85 and 100 decibels can cause gradual hearing loss; sources of these sounds include subways, motorcycles and farm tractors. Exposure of one minute or longer to sounds over 100 decibels may cause permanent hearing loss; such intensity is caused by the sounds of jackhammers, snowmobiles and shotgun blasts.