Noise is measured in decibels (dB). Noise is measured in decibels (dB). As noise increases, so does the decibel level.

Two primary comparisons are made using decibel readings. First, for every increase of 3 decibels, the sound intensity doubles. For example, a decibel increase from 60 dB to 63 dB represents a doubling of sound energy. Similarly, an increase of 9 decibels, from 60 dB to 69 dB, represents an eight-fold increase of noise. This energy enables sound to penetrate physical structures and rattle windows and vibrate dishes, etc.

The second comparison subjectively characterizes how an individual "perceives" the loudness of a sound event. That is, for every increase of 10 decibels, the sound is perceived as doubling. An increase of 20 dB, amounts to a four-fold increase in perceived noise. In both cases, the noise events being measured are actual noise events - as opposed to average noise levels. The Navy now acknowledges that the number and magnitude of individual noise events are important when assessing the noise impact on communities.

To learn more, see Measuring Noise - Real Differerence in Sound Intensities in this Section.

Day-night Average Sound levels (DNL) are used to describe the various noise zone contours (see map for further detail) aaround Oceana and are primarily a tool for urban planners to ensure that non-compatible development is not permitted within high noise areas - and are specifically relevant to residential development and schools.

DNL contours are established by averaging the magnitude and duration of single noise event decibel levels known as Sound Equivalent Levels (SEL) - intersperced with periods of relative quiet - over a 24 hour period over the course of one year. These noise zone descriptors include late night decibel penalties for the purpose of accounting for the additional annoyance and intrusiveness of single event noises during periods of very quiet ambient conditions.

For practical purposes, an increase of 10 dB (e.g. between 65 dB and 75 dB) represents a doubling in the average noise level. Heretofore, the Navy has relied upon DNL as the sole descriptor for assessing noise impact on communities and has ignored the impact of individual noise events.
To learn more, see Difference Between Day Night Average Levels (DNL)and Sound Exposure Level (SEL).