Noise is defined as unwanted sound and can be intermittent or continuous, steady or impulsive. Human response to noise is extremely diverse and varies according to the type of noise source, the sensitivity and expectations of the receptor, the time of day, and the distance between the noise source and the receptor.

The decibel (dB) is the accepted unit of measurement for noise. Because human hearing is not equally sensitive to all sound frequencies, various frequency weighting schemes have been developed to approximate the way people hear sound. The A-weighted decibel scale (dBA) is normally used to approximate human hearing response to sound. Example sound noise levels are as follows:

Relative to human receptors, noise levels under 45 dBA are considered quiet, 46 to 65 dBA are considered moderately loud, 66 to 75 dBA are considered loud, 66 to 110 dBA are considered very loud and 111 dB and above are considered uncomfortable.

LAeq refers to the “equivalent” average sound level. During daytime, few people are highly annoyed at LAeq levels below 55 dBA, and few are moderately annoyed at LAeq levels below 50 dBA (World Health Organization 1999); however, in quiet rural settings, noise levels well below 50 dBA could be considered annoying (Leitner undated).

To avoid annoyance and interference with normal human activity, sound levels during the evening and night are recommended to be 5 to 10 dB lower than during the day. Indoor guideline values for bedrooms are 30 dBA LAeq for continuous noise and 45 dBA as the maximum for single sound events. Lower noise levels may be disturbing depending on the nature of the noise source. At nighttime, outside sound levels about 3 feet from the exterior of living spaces are recommended to not exceed 45 dBA LAeq, so that people may sleep with bedroom windows open. This value was obtained by assuming that the noise reduction from outside to inside with the window open is 15 dBA (World Health Organization 1999).