Sound is measured as sound level in units of decibels, dB. Environmental sound is typically measured as A-weighted decibels, symbolized dBA. The A-weighting is a frequency-dependent level adjustment that corresponds approximately to the sensitivity of human hearing at various sound frequencies. In particular, it takes into account the ear’s higher sensitivity to mid- and high frequencies.

People normally experience sound levels between about 30 and 90 dBA, depending on their activity. For example, a loud nearby vehicle, radio or power tool may produce 80 to 90 dBA, normal conversation is about 50 to 60 dBA, and a bedroom or quiet office is about 30 to 40 dBA. Table 1 lists typical sound levels associated with common noise sources, activities, or environments.

The decibel scale is a logarithmic rather than a linear scale, i.e., it is not directly proportional to the energy or loudness of the noise. The smallest “just noticeable” increase in noise is about 3 dBA. A 5-dBA increase is clearly perceptible, and a 10-dBA increase causes a doubling of judged loudness. For example, 70 dBA is judged to be twice as loud as 60 dBA and four times as loud as 50 dBA. The following summarizes how increases in perceived loudness correlate with sound level increases:

Sound levels from different sources combine logarithmically, not by adding the levels. For example, two noise sources each producing 50 dBA produce a combined level of 53 dBA. It follows that sound levels increase logarithmically with the number of noise sources or noise events. Doubling the amount of traffic on a street, for example, increases sound levels by about 3 dBA, which is a slight increase in loudness, not a doubling of loudness.

Because sound levels fluctuate over time, several A-weighted sound level descriptors are used to characterize the sound. The following are typical descriptors used in noise studies:

Leq      Equivalent sound level is the most commonly used descriptor for measuring fluctuating sound. The Leq is the level of a constant sound that, over a given time period, contains the same amount of sound energy as the measured fluctuating sound.

Lmax      Maximum Sound Level is the maximum instantaneous sound level measured; usually very short in duration.

Lmin       Minimum Sound Level is the lowest instantaneous sound level and is typically the level of background noise that may be expected if all the fluctuating noise sources were removed.

Ldn      Day-night average sound level is the 24-hour Leq with 10 dBA added to the hourly Leq levels measured during the more noise-sensitive nighttime hours of 10:00 p.m. to 7:00 a.m.

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Noise Descriptors | Noise Studies and Limits | Environmental Noise Guidelines | Measurements and Predictions | Putting It Together