What is a Decibel?

Mary Elizabeth
Mary Elizabeth

A decibel is one unit on the decibel scale, which is a logarithmic scale. The name means one-tenth of a bel, a bel being an eponymous unit named for Alexander Graham Bell and used to compare power in electrical communication, voltage, or intensity of sound. The abbreviation of bel is B and decibel, dB.

The decibel level of a lawn mower is roughly double that of rainfall and half that of a shotgun blast.
The decibel level of a lawn mower is roughly double that of rainfall and half that of a shotgun blast.

10 dB = 1 B

A quiet whisper has the same number of decibels as leaves rustling in the breeze.
A quiet whisper has the same number of decibels as leaves rustling in the breeze.

The primary use of the decibel scale today is to test audibility, and the results are called the sound-pressure level (SPL), which is similar to loudness. The Occupational Safety and Health Association (OSHA) has set guidelines to help people determine the maximum loudness they should be exposed to. The SPL level that OSHA recommends is 85 decibels, above which special ear protection is required.

An airplane taking off is about 140 decibels.
An airplane taking off is about 140 decibels.

Eighty-five decibels is the threshold for the possibility of noise-related hearing loss, and this guideline is intended to prevent such hearing loss. This figure suggests that many people who do not currently use ear protection should consider it. The following chart reveals that a great deal of the sound we’re exposed to is above that 85-decibel threshold. Because conditions may vary and distances are not specified, these figures are approximate.

The average whisper is around 20 decibels.
The average whisper is around 20 decibels.
Decibels Sound Source
0 low threshold of hearing
10 leaves rustling in the breeze; quiet whisper
20 average whisper
20-50 quiet conversation
40-45 conversation between acts at a theatre; hotel lobby conversation
50 rainfall
50-65 loud conversation
65-70 moderate traffic; hair dryer
65-90 train
75-80 factory (medium)
90 heavy traffic; power lawn mower
90-100 thunder
100 boom box with volume turned high; chain saw
110 shouting; symphony concert
115 rock concert
120 ambulance siren
130 threshold of pain
140 airplane takeoff
140-190 space rocket takeoff
170 shotgun
Ear plugs can help prevent trauma from loud noises that over time can cause hearing loss.
Ear plugs can help prevent trauma from loud noises that over time can cause hearing loss.

Sound pressure level meters, sound level meters, or sound meters are tools used to measure the SPL, the intensity of sound, at a given moment. Such meters use frequency-weighting scales that account for the difference in human sensitivity to different frequencies. The A scale, a decibel scale abbreviated as dBA or dB(A), and the C scale, a decibel scale abbreviated as dBC or dB(C), are the most frequently used. The meters also can be set to slow or fast response. The testing that OSHA specifies when sound has been found to exceed the level it recommends is A scale testing set to slow response.

Dosimeters are like sound level meters, but they monitor, store, and integrate readings over time and can give an average reading for a specified duration. Dosimeters are used for personal noise monitoring, while sound level meters are used for area noise monitoring. Because the dosimeter is used for longer periods and to track the sound exposure one person experiences, it is made to be worn and has a clip-on microphone to pick up sound.

A rock concert is often around 120 decibels, so fans should consider wearing hearing protection.
A rock concert is often around 120 decibels, so fans should consider wearing hearing protection.
Mary Elizabeth
Mary Elizabeth

Mary Elizabeth is passionate about reading, writing, and research, and has a penchant for correcting misinformation on the Internet. In addition to contributing articles to wiseGEEK about art, literature, and music, Mary Elizabeth is a teacher, composer, and author. She has a B.A. from the University of Chicago’s writing program and an M.A. from the University of Vermont, and she has written books, study guides, and teacher materials on language and literature, as well as music composition content for Sibelius Software.

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Discussion Comments

anon46579

what is the decibel level of a car horn?

anon19810

what is the decibel of a jackhammer?

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