Criteria
4.8 AMBIENT NOISE LEVEL SPECIFICATION
4.8.1 Noise Weighting Curves
Although the specification of an A-weighted level is easy and convenient it gives no indication of which frequency components may be the source of non-compliance. For most acoustic design purposes it is more useful to make use of a weighting curve, which defines a spectrum of band levels in terms of a single number. Five currently
NRB ' Lp
B& AB
BB (4.44)
used sets of single-number weighting curves are shown in Figures 4.8-4.12. These figures provide, respectively, noise rating (NR), noise criteria (NC), room criteria (RC), balanced noise criteria (NCB) and room noise criteria (RNC) weighting curves.
4.8.1.1 NR Curves
Noise Rating (NR) curves have been adopted by the International Standards Organisation and are intended for general use, particularly for rating environmental and industrial noise levels. They are also used in many cases by machinery manufacturers to specify machinery noise levels.
The Noise Rating, NR, of any noise characterised in octave band levels may be calculated algebraically. The NR of a noise is equal to the highest octave band noise rating (NRB) which is defined as:
where AB and BB are as listed in Table 4.7. However, the family of curves generated using Equation (4.44) and shown in Figure 4.8 is in more common use than the equation. By convention, the NR value of a noise is expressed as an integer.
Table 4.7 Constants used to calculate NR curve numbers for octave bands between 31.5 and 8000 Hz Octave band centre
frequency (Hz)
AB BB
31.5 55.4 0.681
63 35.5 0.790
125 22.0 0.870
250 12.0 0.930
500 4.8 0.974
1000 0.0 1.000
2000 -3.5 1.015
4000 -6.1 1.025
8000 -8.0 1.030
To determine the NR rating of a noise, measured octave band sound pressure levels are plotted on Figure 4.8 and the rating is determined by the highest weighting curve which just envelopes the data. If the highest level falls between two curves, linear interpolation to the nearest integer value is used. Note that it is also possible to use 1/3 octave band data on 1/3 octave band NR curves, which are obtained by moving the octave band curves down by 10log10(3) = 4.77 dB.
Specification of an NR number means that in no frequency band shall the octave band sound pressure in the specified space exceed the specified curve (tangent
Figure 4.8 Noise rating curves (NR).
method). In practice, a tolerance of ±2 dB is implied. In specifications, an allowance of 2 dB above the curve is usually acceptable in any one octave band, provided that the levels in the two adjacent bands are only 1 dB below the criterion curve.
Ex ample 4.2
Find the NR number for the sound spectrum of Example 3.1.
0 10 20 30 40 50 60 70 80
65
55
45
35
MA F 15
63 125 250 500 1k 2k 4 k 8 k
O ctave band centre frequency (Hz)
Octave band sound pressure level (dB re 20 Pa)µ
(N C )
25
Figure 4.9 Noise criteria curves (NC). “MAF” = minimum audible field”.
Solution
Plot the unweighted sound spectrum on a set of NR curves. The highest curve that envelopes the data is NR = 81 (interpolated between the NR80 and NR85 curves).
4.8.1.2 NC Curves (Figure 4.9)
Noise criteria curves (Figure 4.9) were developed in response to the need for specification of just acceptable noise in occupied spaces with all systems running.
They are still used extensively in the building services industry even though they have
LB ' RC% 5
0.3log10 1000
f (4.45)
been largely superseded by NCB criteria (see Section 4.8.1.4). Noise criteria curves are not defined in the 31.5 Hz octave band and thus do not account for very low frequency rumble noises. They are also regarded as too permissive in the 2000 Hz and higher octave bands and do not correlate well with subjective response to air- conditioning noise. This has resulted in them now being considered generally unsuitable for rating interior noise. The NC rating of a noise is determined in a similar way to the NR rating, except that Figure 4.9 is used instead of Figure 4.8. The NC rating, which is an integer value, corresponds to the curve that just envelopes the spectrum. No part of the spectrum may exceed the NC curve that describes it. Note that linear interpolation is used to generate curves corresponding to integer NC numbers between the 5 dB intervals shown in Figure 4.9. The simplicity of the procedure for determining an NC rating is the main reason these curves are still in use today. The more complex procedures for determining an RC or NCB rating (see below) have prevented these latter (and more appropriate) ratings from being universally accepted.
4.8.1.3 RC Curves
Room criteria (RC) curves have been developed to replace Noise Criteria curves for rating only air conditioning noise in unoccupied spaces. The RC curves include 16 Hz and 31.5 Hz octave band levels (see Figure 4.10), although few sound level meters with external octave band filters include the 16 Hz octave band. Interest in the 31.5 Hz and 16 Hz bands stems from the fact that a level of the order of 70 dB or greater may result in noise-induced vibrations that are just feelable, especially in lightweight structures. Such vibration can also give rise to objectionable rattle and buzz in windows, doors and cabinets, etc.
For spectrum shapes that are ordinarily encountered, the level in the 16 Hz band can be estimated from the difference in levels between the unweighted reading and the 31.5 Hz octave band level. A difference of +4 dB or more is evidence of a level in the 16 Hz band equal to or greater than the level in the 31.5 Hz band.
The RC number is the average of the 500 Hz, 1000 Hz and 2000 Hz octave band sound levels, expressed to the nearest integer. If any octave band level below 500Hz exceeds the RC curve by more than 5 dB, the noise is denoted “rumbly” (e.g. RC 29(R)). If any octave band level above 500Hz exceeds the RC curve by more than 3 dB, the noise is denoted, “hissy”. (e.g. RC 29(H)). If neither of the above occurs, the noise is denoted “neutral” (e.g. RC 29(N)). If the sound pressure levels in any band between and including 16 Hz to 63 Hz lie in the cross hatched regions in Figure 4.10, perceptible vibration can occur in the walls and ceiling and rattles can occur in furniture. In this case, the noise is identified with “RV” (e.g. RC 29(RV)). The level, LB of the octave band of centre frequency f, corresponding to a particular RC curve is given by:
45
35
25 30 40 50 (RC )
16 31 .5 63 125 250 500 1k 2k 4k 8k
10 20 30 40 50 60 70 80 90
Octave band sound pressure level (dB re 20Pa)µ
Oct ave band centre frequency (Hz)
AB
Figure 4.10 Room Criterion (RC) curves. Regions A and B: noise-induced feelable vibration in lightweight structures and induced rattles in light fixtures, doors, windows, etc. Region A, high probability; region B, moderate probability.
RC curves represent the shape of the least objectionable noise spectrum. For this reason they are only defined up to an RC number of 50. By contrast, NR and NC curves are intended for specifying an upper bound of acceptability for background noise, and do not necessarily represent the least objectionable noise spectrum. Thus, in certain cases, such as in open plan offices, where it may be advantageous to introduce background noise to ensure speech privacy, noise with a spectrum shape of an NR or NC curve is unsatisfactory, being both too rumbly and too hissy. However, RC curves are suitable for specifying the introduction of acoustic “perfume”, and noise with a spectrum of that shape has been found to be the least objectionable.
Acceptable RC ratings for background sound in rooms as a result of air conditioning noise are listed in Table 4.8
Table 4.8 Acceptable air conditioning noise levels in various types of occupied space. Note that the spectrum shape of the noise should not deviate from an RC curve by more than 3 dB and should contain no easily distinguishable tonal components. Adapted from ASHRAE (2007)
Room type Acceptable RC(N) Room type Acceptable RC(N)
Residence 25-35 performing arts spaces 25
Hotel meeting rooms 25-35 Music practice rooms 30-35
Hotel suites 25-35 Laboratories 40-50
Other hotel areas 35-45 Churches 25-35
Offices and conf. rooms 25-35 Schools, lecture rooms 25-30
Building corridors 40-45 Libraries 30-40
Hospitals
Private rooms 25-35 Indoor stadiums, gyms 40-50
Wards 30-40 Stadium with speech ampl. 45-55
Operating rooms 25-35 Courtrooms (no mics.) 25-35 Public areas 30-40 Courtrooms (speech ampl.) 30-40
4.8.1.4 NCB Curves
Balanced Noise Criteria (NCB) curves are shown in Figure 4.11. They are used to specify acceptable background noise levels in occupied spaces and include air- conditioning noise and any other ambient noise. They apply to occupied spaces with all systems running and are intended to replace the older NC curves. More detailed information on NCB curves may be found in American National Standard ANSI S12.2-1995 (R1999), “Criteria for Evaluating Room Noise”. The designation number of an NCB curve is equal to the Speech Interference Level (SIL) of a noise with the same octave band levels as the NCB curve. The SIL of a noise is the arithmetic average of the 500 Hz, 1 kHz, 2 kHz and 4 kHz octave band decibel levels, calculated to the nearest integer. To determine whether the background noise is “rumbly”, the octave band sound levels of the measured noise are plotted on a chart containing a set of NCB curves. If any values in the 500 Hz octave band or lower exceed by more than 3 dB the curve corresponding to the NCB rating of the noise, then the noise is labelled
“rumbly”. To determine if the noise is “hissy”, the NCB curve which is the best fit of the octave band sound levels between 125 Hz and 500 Hz is determined. If any of the octave band sound levels between 1000 Hz and 8000 Hz inclusive exceed this NCB curve, then the noise is rated as “hissy”.
4.8.1.5 RNC Curves (Figure 4.12)
The RC and NCB curves have a number of limitations that can lead to undesirable results. The RC curves set criteria that are below the threshold of hearing to protect
0 10 20 30 40 50 60 70 90
80 100
65
55
45
35
15 10
0
63
16 31 .5 125 250 500 1k 2k 4k 8k
O ctave band centre frequency (Hz)
Octave band sound pressure level (dB re 20 Pa)µ
(NB )
25
A
B
Figure 4.11 Balanced Noise Criterion curves (NCB). Regions A and B: noise-induced feelable vibration in lightweight structures, induced audible rattle in light fixtures, doors, windows, etc.; region A, high probability; region B, moderate probability.
against large turbulence fluctuations that generate high levels of low frequency noise for which the level can vary by up to 10 dB in synchronisation with fan surging.
(RN C)
16 31 .5 63 125 250 500 1k 2k 4k 8k
10
0 20 30 40 50 60 70 80 90
Octave band sound pressure level (dB re 20 Pa)µ
Oct ave band centre frequency (Hz) 50
45
35
20 25
10 30
15 40
Figure 4.12 Room Noise Criterion curves (RNC). The lowest curve is the approximate threshold of hearing for octave band noise.
However, the RC curves could unnecessarily penalise a well designed HVAC system such as may be used in a concert hall, requiring 10 dB or more of unnecessary noise attenuation at low frequencies. On the other hand, NCB curves are intended for well designed HVAC systems. They do not sufficiently penalise poorly designed systems that may be energy efficient but are characterised by high levels of turbulence induced low frequency noise.
Schomer (2000) proposed a new set of curves (room noise criteria or RNC - see Figure 4.12) that are intended to address the limitations associated with the RC and NCB curves. The intention is for noise criteria to be above the threshold of hearing for
∆ ' 10log10 1 Nj
N i'1
10(Li&Lm) /δ % Lm & Leq (4.47)
Leq ' 10log10 1 Nj
N i'1
10Li/ 10 (4.46)
well-behaved systems, while at the same time preventing a turbulence-producing, fan- surging HVAC system from being labelled acceptable.
It is unlikely that the RNC curves will receive general acceptance because of the complexity of the rating process. Essentially, the RNC rating of a sound is obtained using the tangency method in much the same way as obtaining an NR number. That is, the RNC rating is the integer value of the highest RNC curve that intersects the plotted spectra. The complexity arises in the determination of what values to plot.
These are the measured octave band values with correction terms added in the 31.5 Hz, 63 Hz and 125 Hz bands. It is the determination of the correction terms that is complex. For broadband noise radiated from an air conditioning duct without the presence of excessive turbulence or surging, the correction terms are zero. In cases where there is excessive turbulence, the correction at 31.5 Hz can be as high as a 4 dB increase and if surging is present it can be as high as 12 dB. The correction at 125 Hz is usually zero, except in the case of surging it may be up to 2 dB. A straight line is drawn between the plotted corrected value at 31 Hz and the corrected value at 125 Hz to obtain the corrected value at 63 Hz. The 16 Hz value is not plotted for cases involving excessive turbulence or surging. Details on the calculation of the correction to be added in the 31.5 Hz and 125 Hz octave bands are provided by Schomer (2000).
Briefly, the correction is calculated by taking a large number of octave band sound pressure level measurements, Li, from 16 Hz to 8 kHz, over some reasonable time interval (for example, 20 seconds), using “fast” time weighting and sampling intervals between 50 and 100 ms. The mean sound pressure level, Lm, is calculated by taking the arithmetic mean of all the measured dB levels in each octave band. The energy averaged, Leq, is also calculated for each octave band using:
The corrections ∆31 and ∆125 to be added to the 31.5 Hz and 125 Hz octave band measurements of Leq, is calculated for each of the two bands using:
The quantity, δ, is set equal to 5 for calculations in the 31.5 Hz band and equal to 8 for calculations in the 125 Hz octave band and N is the total number of measurements taken in each band. Note that for the 31.5 Hz correction term calculation, data for the 16 Hz, 31.5 Hz and 63 Hz bands are all included in each of the three terms in Equation (4.47). Before inclusion in Equation (4.47), the 16 Hz data has 14 dB subtracted from each measurement and the 63 Hz data has 14 dB added to each measurement. The noise is considered well behaved if the correction, ∆, for the 31.5 Hz octave band is less than 0.1 dB. In this case, the actual measured octave band data are plotted on the set of RNC curves from 16 Hz to 8 KHz, with no correction applied to any band.
Note that RNC values are reported, for example, as: RNC41-(63 Hz). In this example the highest RNC curve intersected was the RNC 41 curve and this occurred at 63 Hz.
4.8.2 Comparison of Noise Weighting Curves with dB(A ) S pecifications For the majority of occupied spaces, advisory limits can be placed on maximum permissible background noise levels, but recommended levels will vary slightly depending upon the source of information.
As mentioned earlier, when an attempt was made to use NC rating curves as a guide for deliberate spectral shaping of background noise, the result was unsatisfactory. The internationally accepted NR curves do not provide an improvement in this respect. For example, if the NR weighting curves 15 to 50 are taken as background noise spectra, then A-weighted according to Table 3.1, and their overall equivalent A-weighted levels determined as described in Section 3.2, it is found that in all cases the low-frequency bands dominate the overall level. As A-weighting accords with subjective response, one would intuitively expect trouble with a spectrum that emphasises a frequency range which ordinarily contributes little to an A-weighted level. In this respect the RC weighting curves are much more satisfactory. When A- weighted it is found that neither the high- nor the low-frequency extremes dominate;
rather, the mid-frequency range contributes most to the computed equivalent A- weighted level.
Since the various noise rating schemes are widely used and much published literature has been written in terms of one or another of the several specifications, Table 4.9 has been prepared to allow comparisons between them. In preparing Table 4.8, the dB(A) levels equivalent to an NR, NC, NCB, or RNC level were calculated by considering levels only in the 500 Hz, 1000 Hz and 2000 Hz bands, and assuming a spectrum shape specified by the appropriate NR or NC curve. On the other hand, the entries for the RC data were calculated using levels in all octave bands from 16 Hz to 4000 Hz and assuming the spectrum shape specified by the appropriate RC curve.
It is expected that the reader will understand the difficulties involved in making such comparisons. The table is intended as a guide, to be used with caution.
Table 4.9 Comparison of ambient level criteria
Specification
dB(A) NR NC, NCB and RNC RC Comment
25-30 20 20 20 Very quiet
30-35 25 25 25
35-40 30 30 30 Quiet
40-45 35 35 35
45-50 40 40 40 Moderately noisy
50-55 45 45 45
55-60 50 50 50 Noisy
60-65 55 55 –
65-70 60 60 – Very noisy
Judgment is often necessary in specifying a noise rating for a particular application. Consideration must be given to any unusual aspects, such as people’s attitudes to noise, local customs and need for economy. It has been found that there are differences in tolerance of noise from one climate to another. People in those countries in which windows are customarily open for most of the year seem to be more tolerant of noise, by 5-10 dB(A), than people in countries in which windows are customarily tightly closed for most of the year.