What is noise?
No matter what type of noise you wish to consider, whether it’s transportation noise, occupational noise, entertainment noise, or general community noise, it helps to have an understanding of what noise is. Noise is an unwanted sound. What is a “noise” to you may be a sound to someone else; there is always a subjective component to the term.
A sound is produced when something vibrates rapidly enough to generate a disturbance or pressure wave in the surrounding medium. The university of Toronto has a great article delving into the definition of sound.
Most sounds are created by vibrating surfaces and turbulent air. When we measure noise, we are quantifying a sound pressure level. The unit of measurement of sound was named after Alexander Graham Bell. The bel proved unwieldy for practical purposes, so it was divided by ten (deci-) to arrive at the familiar decibel, abbreviated dB.
The decibel is a logarithmic unit. This means that if you want to add together the noise from two sources, the sound levels cannot be added arithmetically, i.e., 5 dB plus 5 dB does not equal 10 dB. Actually, the sum is 8 dB. Noise levels must be added in a logarithmic manner or, as it’s frequently described, “on an energy basis”.
Types of noise sources
There are basic types of noise sources: a line source, from which the sound waves travel in a linear manner (e.g. cars on a freeway); a plane source, where the noise travels within a specific plane (such as noise created inside a cylinder by a piston); and a point source, from which noise radiates in all directions. For every doubling of distance away from a point source, there is a decrease in noise level of 6 dB. This is known as the inverse square law. For line sources, noise levels decrease by 3 dB for every doubling of distance.
The healthy human ear is most sensitive to sounds in the range of 1,000 Hz to 5,000 Hz. But there are sounds that exist outside this range to which we are less sensitive. Noise measurement devices, called sound level meters, have built-in weighting filters. The filters enable the meter to de-emphasize some frequencies and emphasize others.
The filter most frequently used is the A-weighting filter, which emphasizes frequencies to which the human ear is most sensitive. A measurement taken using the A-weighted filter is noted as dB(A). Other filters include a “C” weighting (used for measuring low frequencies) and a “D” weighting (usually used for aircraft noise). The sound level meter can also be used to obtain unweighted readings. This article produced by the University of Montana can be useful in understanding weighting systems.
Representative Noise Levels
To determine whether a community noise can be described as “quiet”, “noisy”, or just plain “loud”, we need some reference levels. Examples of common sounds we hear are: a soft whisper, 30 dB(A), a noise office, 55 dB(A), feeway traffic, 65 dB(A), and a power lawn mower, 90 dB(A). (We once measured an action movie inside a theater at 93 dB(A). Now that was loud!)