What is "Quiet"? -- Varies between and within communities
Well, FINALLY I am ready to talk about what "quiet" means, as I had promised! Then we'll consider the effect of ambient sound when listening for vehicles.
Varies during the day:
"Residual sound level"
In 1970, Wyle Laboratories went to 18 different locations and measured the level of sound during a 24-hour period.
The locations ranged from a farm next to the Grand Canyon to a New York City window overlooking a highway.
Varies between communities:
They noticed that in each site, when all sounds had died down, there still remained what they called "residual noise level" -- that is, the sound level of what we would call "quiet."
As they described it, residual noise is "the continuous noise one hears in the backyard at night when no single source can be identified, and which seems to come from 'all around' " (Wyle Laboratories, 1971, page 7).
To no one's surprise, they found that the residual noise level varies between communities -- "quiet" was louder in New York City than it was on the farm.
What is surprising is that it also varies during the day in each location.
The daytime sound level of "quiet" (residual noise level) for various communities was as follows:
- Quiet suburban residential: 36-40 dB(A)
- Urban/suburban residential: 42-56 dB(A)
- Cities: 62-69 dB(A).
The range of the sound level of "quiet" for these communities is extraordinary, when you consider that the decibel system is logarithmic, so each increase of 10 decibels means doubling the level of sound.
The residual noise level of "quiet" in cities was almost 30 decibels noisier than for suburban residential communities, so their sound level of "quiet" would be perceived to be almost 8 times as loud.
Apparently, the sound level of "quiet" is the same today as it was in 1971.
When Dr. Rob Wall Emerson and I did our research in 2006, we measured the sound level of "quiet" during the middle of the day in various communities (Wall Emerson & Sauerburger, 2008)
and found that the range of residual sound level for
- suburban Silver Spring intersections was 37 to 49.5 db(A);
- urban / suburban Annapolis intersections was 45.5 to 53 db(A); and
- the city of Atlanta was 63.5 to 69 db(A).
The residual noise level at each site varied during the day, being loudest at about 4:00 PM and quietest at about 4:00 AM.
A sample of the range of residual noise level (sound level of "quiet") in a typical residential community was recorded as follows:
Typical Residential Community
Again, the range in the sound level of "quiet" over time at the same location is remarkable.
There was a difference of almost 20 decibels from the quietest time of the day to the noisiest, which means that "quiet" in the middle of the day at that location would be perceived as 4 times as noisy as it was in the middle of the night.
Wall Emerson, R. & Sauerburger, D. (2008). "Detecting approaching vehicles at streets with no traffic control." Journal of Visual Impairment and Blindness, AFB Press, Volume 102, Number 12, pp. 747-760
Wyle Laboratories (1971). Community Noise. U.S. Environmental Protection Agency, Office of Noise Abatement and Control, Washington, DC http://www.nonoise.org/epa/Roll14/roll14doc65.pdf