The Decibel Scale
An explanation of the decibel scale and related units for measuring sound and loudness.
The ear has the remarkable ability to handle an enormous range of sound levels. In order to express levels of sound meaningfully in numbers that are more manageable, a logarithmic scale is used, rather than a linear one. This scale is the decibel scale.
What is a decibel? Zero decibels (0 dB) is the quietest sound audible to a healthy human ear. From there, every increase of 3 dB represents a doubling of sound intensity, or acoustic power.
Loudness and Sound Intensity (Power)
The relative loudness that we perceive is a subjective psychological phenomenon, not something that can be objectively measured. Most of us perceive one sound to be twice as loud as another one when they are about 10 dB apart; for instance, a 60-dB air conditioner will sound twice as loud as a 50-dB refrigerator. Yet that 10-dB difference represents a tenfold increase in intensity. A 70-dB dishwasher will sound about four times as loud as the 50-dB refrigerator, but in terms of acoustic intensity, the sound it makes is 100 times as powerful.
Here's another way of looking at it: If the sound from one typewriter registers 60 dB, then ten typewriters clacking away would register 70 dB (not 600 dB!), and they would sound only twice as loud as one typewriter. You would need 100 typewriters to reach a noise level of 80 dB, and together they would sound only four times as loud as a single typewriter.
|Sound Level||Sound Intensity (Power)||Perceived Loudness|
The potential for a sound to damage our hearing is proportional to its intensity, not its loudness. That's why it's misleading to rely on our subjective perception of loudness as an indication of the risk to our hearing.
See this chart for safe noise exposure limits.
A-weighting (dBA) and C-weighting (dBC)
You will often see noise levels given in dBA (A-weighted sound levels) instead of dB. Measurements in dBA, or dB(A) as it is sometimes written, are decibel scale readings that have been adjusted to attempt to take into account the varying sensitivity of the human ear to different frequencies of sound. (The main effect of the adjustment is that low and very high frequencies are given less weight than on the standard decibel scale.) Many regulatory noise limits are specified in terms of dBA, based on the belief that dBA is better correlated with the relative risk of noise-induced hearing loss.
Compared with dB, A-weighted measurements underestimate the perceived loudness, annoyance factor, and stress-inducing capability of noises with low frequency components, especially at moderate and high volumes of noise.*
Another system of adjustment is C-weighting, the dBC scale. dBC is sometimes used for specifying peak or impact noise levels, such as gunfire. Unweighted dB readings are also used for this purpose; there is usually not much difference between the two.
Phons and Sones
The phon is a non-standard noise unit that is designed to reflect perceived loudness, and is based on psychoacoustic experiments in which volunteers were asked to adjust the decibel level of a reference tone of 1 kHz until it was the same loudness as the signal being measured. So for example, if a sound is 70 phons, that means it sounds as loud as a 70-dB, 1-kHz tone. The dBA scale is now widely used instead of phons.
The sone is another non-standard, psychoacoustic unit of loudness. By definition, 1 sone = 40 phons, and from there upward, the sone measurement doubles for every increase of 10 phons:
The sone is a more intuitive measure of loudness, because a doubling in the number of sones represents a doubling in perceived loudness (unlike the logarithmic phon scale). Noise levels of household fans are often measured in sones.
Measuring Sound Levels
sound level meter
is the instrument normally used to measure noise levels on the decibel scale. Several factors affect the noise level reading:
- The distance between the meter and the source of the sound
- The direction the noise source is facing, relative to the meter
- Whether the measurement is taken outdoors (where noise can dissipate) or indoors (where noise can reverberate)
For a reported sound level value to be most useful, it is necessary to specify the conditions under which the reading was taken, especially the distance from the source.
Richard L St Pierre Jr and Daniel J Maguire, "The Impact of A-weighting Sound Pressure Level Measurements during the Evaluation of Noise Exposure" (paper presented at NOISE-CON, Baltimore, Maryland, July 12–14, 2004).
Chart of Sounds in Decibels
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