Criteria
4.1 INTRODUCTION
An important part of any noise-control program is the establishment of appropriate criteria for the determination of an acceptable solution to the noise problem. Thus, where the total elimination of noise is impossible, appropriate criteria provide a guide for determining how much noise is acceptable. At the same time, criteria provide the means for estimating how much reduction is required. The required reduction in turn provides the means for determining the feasibility of alternative proposals for control, and finally the means for estimating the cost of meeting the relevant criteria.
For industry, noise criteria ensure the following:
• that hearing damage risk to personnel is acceptably small;
• that reduction in work efficiency due to a high noise level is acceptably small;
• that, where necessary, speech is possible; and
• that noise at plant boundaries is sufficiently small for noise levels in the surrounding community to be acceptable.
Noise criteria are important for the design of assembly halls, classrooms, auditoria and all types of facilities in which people congregate and seek to communicate, or simply seek rest and escape from excessive noise. Criteria are also essential for specifying acceptable environmental noise limits. In this chapter, criteria and the basis for their formulation are discussed. It is useful to first define the various noise measures that are used in standards and regulations to define acceptable noise limits.
LAeq,T ' 10 log10 1 Tm
T
0
10LA(t) / 10dt (4.1)
LAeq,8h ' 10 log10 1 8m
T
0
10LA(t)/10dt (4.2)
LAeq,8h ' 10 log10 1
8 t110LA1/10%t210LA2/10%... tm10LAm/10 (4.3)
EA,T ' m
t2
t1
pA2(t) dt (4.4)
4.1.1 Noise Measures
4.1.1.1 A-weighted Equivalent Continuous Noise Level, LAeq
The un-weighted continuous noise level was defined in Chapter 3, Equation (3.22).
The A-weighted Equivalent Continuous Noise Level has a similar definition except that the noise signal is A-weighted before it is averaged. After A-weighting, the pressure squared is averaged and this is often referred to as energy averaging. The A- weighted Equivalent Continuous Noise Level is used as a descriptor of both occupational and environmental noise and for an average over time, T, it may be written in terms of the instantaneous A-weighted sound pressure level, LA (t) as:
For occupational noise, the most common descriptor is LAeq,8h, which implies a normalisation to 8 hours, even though the contributing noises may be experienced for more or less than 8 hours. Thus, for sound experienced over T hours:
If the sound pressure level is measured using a sound level meter at m different locations where an employee may spend some time, then Equation (4.2) becomes:
where LAi are the measured equivalent A-weighted sound pressure levels and tiare the times in hours which an employee spends at the m locations. Note that the sum of t1...tm does not have to equal 8 hours.
4.1.1.2 A-weighted Sound Exposure
Industrial sound exposure may be quantified using the A-weighted Sound Exposure, EA,T, defined as the time integral of the squared, instantaneous A-weighted sound pressure, pA2(t) (Pa2) over a particular time period, T = t2 - t1 (hours). The units are pascal-squared-hours (Pa2h) and the defining equation is:
EA,T ' 4T× 10(LAeq,T&100) / 10 (4.5)
10 dB
t1 Ti me (sec) t2
Sound pressure level (dB)
Ma xi mum sound level
LAE- S haded area under curve
Figure 4.1 Sound exposure of a single event.
LAE ' 10log10 m
t2
t1
pA2(t)
pref2 dt ' 10log10 EA,T× 3600
pref2 (4.6a,b)
Using Equations (4.1) and (4.4), the relationship between the A-weighted Sound Exposure and the A-weighted Equivalent Continuous Noise Level, LAeq,T , can be shown to be:
4.1.1.3 A-weighted Sound Exposure Level, LAE or SEL The A-weighted Sound Exposure Level is defined as:
where the times t1, t2 and dt are in seconds (not hours as for A-weighted sound exposure in Equation (4.4)) and T = t2 - t1.
A “C-weighted” Sound Exposure Level is defined by substituting the C-weighted noise level for the A-weighted level in Equation (4.6). These two exposure level quantities are sometimes used for assessment of general environmental noise, but mainly for the assessment of transient environmental noise, such as traffic noise, aircraft noise and train noise. When the event is a transient, the time interval, t1 - t2 must include the 10 dB down points as shown in Figure 4.1.
LAeq,8h ' 10 log10 EA,8h
3.2 × 10&9 ' 10 log10 1
28,80010LAE,8h/ 10 (4.7a,b)
Ldn ' 10 log10 1 24 m
07:00
22:00
10 × 10LA(t) / 10dt% m
22:00
07:00
10LA(t) / 10dt dB (4.8)
Ldn ' LAE%10 log10(Nday%Neve%10 ×Nnight)&49.4 dB(A) (4.9)
Lden ' 10log10 1 24 m
07:00
22:00
10 × 10LA(t) / 10dt% m
19:00
07:00
10LA(t) / 10dt
% m
22:00
19:00
3 × 10LA(t) / 10dt dB
(4.10) An equivalent Continuous Noise Level for a nominal 8-hour working day may be calculated from EA,8h or LAE using:
4.1.1.4 Day–Night Average Sound Level, Ldn or DNL
The Day–Night Average Sound Level is used sometimes to quantify traffic noise and some standards regarding the intrusion of traffic noise into the community are written in terms of this quantity, Ldn , which is defined as:
For traffic noise, the Day–Night Average Sound Level for a particular vehicle class is related to the Sound Exposure Level by:
where, LAE is the A-weighted Sound Exposure Level for a single vehicle pass-by, Nday , Neve and Nnight are the numbers of vehicles in the particular class that pass by in the daytime (0700 to 1900 hours), evening (1900 to 2200 hours) and nighttime (2200 to 0700 hours), respectively and the normalisation constant, 49.4, is 10 log10 of the number of seconds in a day. To calculate the Ldn for all vehicles, the above equation is used for each class and the results added together logarithmically (see Section 1.11.4).
4.1.1.5 Community Noise Equivalent Level, Lden or CNEL
The Community Noise Equivalent Level is used sometimes to quantify industrial noise and traffic noise in the community and some regulations are written in terms of this quantity, Lden, which is defined as:
nt ' nmax%0.15 j
24 i'1
ni&nmax (noy) (4.12)
LPN ' 40%33.22log10nt (dB) (4.13)
Lden ' LAE%10log10 Nday%3Neve%10Nnight &49.4 (dB) (4.11) For traffic noise, the Community Noise Equivalent Level for a particular vehicle class is related to the Sound Exposure Level by:
where, LAE is the A-weighted Sound Exposure Level for a single vehicle pass-by, the constant, 49.4 = 10log10 (number of seconds in a day) and Nday, Neve and Nnight are the numbers of vehicles in the particular class that pass by in the daytime (0700 to 1900 hours), evening (1900 to 2200 hours) and nighttime (2200 to 0700 hours), respectively. To calculate the Lden for all vehicles, the above equation is used for each class and the results added together logarithmically (see Section 1.11.4).
4.1.1.6 Effective Perceived Noise Level, LPNE
This descriptor is used solely for evaluating aircraft noise. It is derived from the Perceived Noise Level, LPN , which was introduced some time ago by Kryter (1959).
It is a very complex quantity to calculate and is a measure of the annoyance of aircraft noise. It takes into account the effect of pure tones (such as engine whines) and the duration of each event.
The calculation procedure begins with a recording of the sound pressure level vs time curve, which is divided into 0.5 second intervals over the period that the aircraft noise exceeds background noise. Each 0.5 second interval (referred to as the kth interval) is then analysed to give the noise level in that interval in 24 1/3 octave bands from 50 Hz to 10 kHz. The noy value for each 1/3 octave band is calculated using published tables (Edge and Cawthorne, 1976) or curves (Raney and Cawthorne, 1998).
The total noisiness (in noys) corresponding to each time interval is then calculated from the 24 individual 1/3 octave band noy levels using:
where nmax is the maximum 1/3 octave band noy value for the time interval under consideration. The perceived noise level for each time interval is then calculated using:
The next step is to calculate the tone-corrected perceived noise level (LPNT) for each time interval. This correction varies between 0 dB and 6.7 dB and it is added to the LPN value. It applies whenever the level in any one band exceeds the levels in the two adjacent 1/3 octave bands. If two or more frequency bands produce a tone correction, only the largest correction is used. The calculation of the actual tone-correction is complex and is described in detail in the literature (Edge and Cawthorne, 1976). The maximum tone corrected perceived noise level over all time intervals is denoted LPNTmax. Then next step in calculating LPNE is to calculate the duration correction, D,
LPNE ' LPNTmax%D (4.15) D ' 10log10 j
i%2d k'i
10LPNT(k)/ 10 &13&LPNTmax (4.14)
which is usually negative and is given by Raney and Cawthorne (1998), corrected here, as:
where k = i is the time interval for which LPNT first exceeds LPNTmax and d is the length of time in seconds that LPNT exceeds LPNTmax.
Finally the effective perceived noise level is calculated using:
In a recent report (Yoshioka, 2000), it was stated that a good approximate and simple method to estimate LPNE was to measure the maximum A-weighted sound level, LAmax, over the duration of the aircraft noise event (which lasts for approximately 20 seconds in most cases) and add 13 dB to obtain LPNE.
4.1.1.7 Other Descriptors
There are a number of other descriptors used in the various standards, such as “long time average A-weighted sound pressure level” or “long-term time average rating level”, but these are all derived from the quantities mentioned in the preceding paragraphs and defined in the standards that specify them, so they will not be discussed further here.