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:

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:

• L_max — The peak (1-second) decibel level during an event.
• Loudness Index — A metric developed for this website indicating how loud most people would perceive the peak of a noise event to be relative to the ambient noise level at the time, assuming a doubling in loudness for every 10 dB increase in noise level. For example, a noise event that peaks at 100 dBA at a time when the background noise level is 50 dBA will have a peak loudness index of 2⁵ = 32, meaning that the peak of the noise event is 32 times louder than the ambient background noise.
• SEL (Sound Exposure Level) — The total noise energy in an event, expressed as the decibel level that would occur if the noise energy for the event was concentrated in a one-second period.
• DNL (Day-Night Average Sound Level) — Average noise exposure based on spreading the total noise energy from one or more noise events over a 24-hour period, with an added penalty for nighttime events.

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 (L_max) 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.