Noise Rating Systems

An example of the wording in the measurement techniques section of a noise ordinance might read: "The measurement must be made for a period of at least 10 minutes and any noise source that exceeds those levels set forth in Table X at least 10% of the time (L10) shall be deemed in violation of the law." An L10 or L10 is known as an exceedance level whereby a given dB level is exceeded 10% of the measurement period.

Other terms found in noise ordinances and their definitions follow.

Ldn OR Ldn (level day night)—A noise exposure level computed on a twenty-four-hour daily basis whereby 10 dB is added to all readings between the hours 10 P.M. (2200

hours) and 7 A.M. (0700 hours). This adjustment penalizes noise exposure levels that occur during what is considered normal sleeping hours for a community.

Leq OR Leq (level equivalent continuous equal energy)— That constant noise level that, over a given period of time, expends the same amount of energy as the fluctuating level over the same time period. It is expressed as follows:

Leq = 10 log X 1QLi/10ti

where n is the total number of samples taken, L is the noise level in dBA of the ith sample, t is the fraction of the total sample time.

Ldn and Leq are used in the U.S. Environmental Protection Agency (EPA) published noise criteria levels that it deemed necessary to health and welfare of U.S. citizens (Table 6.5.2). An Lnd of 45 provides a fair margin of safety. These terms appear in the Federal Housing Authority (FHA) noise standards for construction (Table 6.5.3).

Lnp OR Lnp (level noise pollution)—The sum of the A-weighted SPLs (dBA characteristics of each piece of equipment) added together using the Leq equation plus 2.56 times the standard deviation of the SPL to account for annoyance due to fluctuation. Figure 6.5.1 shows the use of the Lnp in the provisional criteria relating the noise pollution level (NPL) to community noise level acceptability at construction site boundaries. TNI (or traffic noise index)—Derived from the equation

TNI = L(90) + 4d - 30 dB; and d = L (10) - L (90) 6.5(4)

Some older ordinances call for octave-band analysis. These ordinances were derived from research which showed that some frequencies have a higher annoyance factor and therefore should have a lower allowable dB level. These laws are not as common since they demand a rather arduous measurement procedure and require a specialized instrument.

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