Sound and Noise

Sound is the physical phenomenon of pressure fluctuations in air at frequencies detectable by human ears.

Noise is undesired sound, making it a subjective classification of sound.

Measuring sound

The magnitude of sound pressure waves easily varies by a factor of a million. For this reason, sound pressure is normally expressed on the decibel (dB) scale: each 10dB increase translates to a factor of 10 increase in the magnitude of sound pressure waves. Sound meters measure sound pressure waves relative to a reference level (sound pressure level or SPL) and record the level in decibels.

Weighting factors are used to de-emphasize low and high sound frequencies that aren't as important for human hearing. The A-weighted scale (dBA) is most common—and is the one used in government assessments of noise. However, that scale does downplay low-frequency noise from loud aircraft like fighter jets.

Because sound levels can change rapidly, sound meters capture both fast-response (1/8th second) and slow-response (1 second) readings.

Loudness is how people subjectively experience sound. Experiments have shown that people generally perceive a 10dB increase in sound as a doubling in loudness, though this varies with the nature of the sound.

The table below shows the relationships among decibels, sound pressure level and loudness, expressed relative to 50dB base level, which is typical of an urban environment like Madison's:

Decibels Relative
50 1 1
60 10 2
70 100 4
80 1,000 8
90 10,000 16
100 100,000 32

Click below to see what the noise from a typical military-fighter departure from the Madison airport looks like when plotted in terms of decibels, sound pressure level and loudness

(Recorded on August 5, 2021 at Meter Location 01)

Noise Metrics

Many metrics have been developed for characterizing noise events and overall noise exposure. Some important ones used here and elsewhere are:

Note that metrics such as SEL and DNL, while expressed in decibels, do not represent anything that one would directly experience during a noise event. Instead they are representations of total sound energy exposure over time. For instance, the example above has a peak noise level (Lmax) of 104.5 dBA, an event SEL of 111.7 dBA and two such daytime flights per day would yield a DNL level of 65 dBA.

Further Reading

Appendix A of the recent noise expsoure map study prepared for the Dane County Regional Aiport covers noise metrics in greater detail.