FAQ's
The following are some of the most commonly asked questions we get on environmental or community noise. See the FAQ General for more discussion on sound measurement. The FAQ link on our Links-Educational Sites page will take you to a broader FAQ file on acoustics.
ENVIRONMENTAL
How does sound spread outdoors?
FAQs
How does sound spread outdoors?
In the simplest idealized model, the sound level outdoors decreases 6
decibels for each doubling of distance from the source. The farther one
is from a sound source, the farther away one must go to notice a significant
decrease in sound level. In the real world, the sound level often decreases
more rapidly with distance. This is due to barriers blocking the line
of sight, absorption by the air, winds, and temperature variations in
the atmosphere. The atmospheric effects become most important at distances
beyond about 1000 feet from the source. Dry air absorbs sound and reduces
its level rapidly over distance. A humid environment allows sound to travel
further with less reduction in level. Downwind and under many night-time
conditions (cooler air near the surface), sound waves will bend downward.
Thus, the noise-reduction benefits of barriers can be negated by these
atmospheric effects beyond a few hundred feet. Levels will be lower upwind
and under most daytime conditions. Sound waves curve upward in the face
of a wind or towards cooler air higher in the atmosphere. For the same
sound output at the source, distant levels can be as much as 20 decibels
less than predicted by the distance effects alone. Back to
Top
There
is an annoying siren-like noise in our neighborhood, but it does not even
change the sound level when it starts and stops. Why is it so annoying?
A warning siren is designed to concentrate all its sound at a single frequency
or pitch, or a few such frequencies. Our ability to hear it and be warned
is then determined only by the relative level of the siren sound and other
sounds of similar frequency. We can hear the siren even if it is not the
loudest sound around. Unfortunately, some mechanical systems produce sounds
very much like sirens. Typical examples are certain kinds of fans, positive
displacement blowers, and compressors. A new type of air conditioning
compressor with a strong siren-like sound has become popular for large
commercial systems in recent years. Sounds like this are intended to get
attention and can be very hard to ignore even if they meet normal noise
limits. Back to Top
