Noise Rating (NR) and Noise Criteria (NC)

Background

A method of rating noise for indoor environments (NR Curves) was developed by the International Standards Organization (ISO) for use in various standards where this application was needed. The most current standard that uses NR curves is ISO 1996-1.

This general approach was also developed in America, although with a slightly different set of curves.

Noise Rating (NR) Curves

When plotted, the full set of NR curves looks as below. Note that the curves are only plotted every 5dB.

NR Curves Example

The basic premise of this system was to measure the sound environment in Octave Bands, then plot each value onto the curves (this would originally have been done manually). The NR value is the nearest curve under which all the plotted values sit. The black line on the chart above shows an example plot. In this case, NR=90

Sound Meter Automatic Calculation

The result can also be derived mathematically, which is now used in many sound level meters. The following formula is used to calculate the NR figure:

Castle sound meters use the A weighted octave band L_eq data for each frequency (L) to obtain n in each case.

NR figure Equation

Constants a and b are different for each frequency and are shown in the table below: –

	Frequency (Hz)
Constant	31.5	63	125	250	500	1000	2000	4000	8000
a	55.400	35.400	22.000	12.000	4.800	0.000	-3.500	-6.100	-8.000
b	0.681	0.790	0.870	0.930	0.930	1.000	1.015	1.025	1.030

The NR is the largest n value from each frequency and is rounded up to the nearest whole number. This can be done in 1dB steps, which gives a higher resolution than using the manual method

As an example, assume we calculated each frequency NR figures and got the following:

	Frequency (Hz)
	31.5	63	125	250	500	1000	2000	4000	8000
NR	64	63	54	63	65	60	59	64	48

The overall NR = 65@500 Hz , noting that the frequency is also recorded.

Noise Criterion (NC) curves

When plotted, the full set of NC curves looks as below. Note these are only shown every 5dB

NC Curves

Deriving the NC value manually is the same as for NR curves, with each octave band value plotted onto the curves and the NC level taken as the nearest curve below which all the plotted values sit.

In modern Castle sound meters, this is calculated using the following method

Using the rounded up whole number A weighted octave band data L_eq, for each frequency and obtain n in each case.

Example Round Up: 74.3 becomes 75

Using the look up table below, each frequency is scanned to determine which NC level should be reported. If a blank exists, then use the next value in the table.

The NC is the largest n value from any frequency. Note the table shows 1dB steps allowing for more resolution in the answer.

NC	63	125	250	500	1000	2000	4000	8000
15	47	37	29	22	17	14	12	11
16	48	38	30	23	18	15	13	12
17					19	16	14	13
18	49	39	31	24	20	17	15	14
19	50	40	32	25	21	18	16	15
20	51	41	33	26	22	19	17	16
21		42	34	27	23	20	18	17
22	52		35	28	24	21	19	18
23	53	43	36	29	25	22	20	19
24		44	37	30	26	23	21	20
25	54	45	38	31	27	24	22	21
26				32	28	25	23	22
27	55	46	39			26	24	23
28	56	47	40	33	29	27	26	25
29				34	30	28	27	26
30	57	48	41	35	31	29	28	27
31		49	42	36	32	30	29	28
32	58	50	43	37	33	31	30	29
33	59	51	44	38	34	32	31	30
34		52	45	39	35	33	32	31
35	60	53	46	40	36	34	33	32
36	61	54	47	41	37	35	34	33
37			48	42	38	36	35	34
38	62	55	49	43	39	37	36	35
39	63	56	50	44	40	38	37	36
40	64	57	51	45	41	39	38	37
41				46	42	40	39	38
42	65	58	52		43	41	40	39
43	66	59	53	47	44	42	41	40
44				48	45	43	42	41
45	67	60	54	49	46	44	43	42
46	68	61	55	50	47	45	44	43
47			56	51	48	46	45	44
48	69	62	57	52	49	47	46	45
49	70	63	58	53	50	48	47	46
50	71	64	59	54	51	49	48	47
51				55	52	50	49	48
52	72	65	60		53	51	50	49
53	73	66	61	56	54	52	51	50
54				57	55	53	52	51
55	74	67	62	58	56	54	53	52
56		68	63	59	57	55	54	53
57	75		64	60	58	56	55	54
58	76	69	65	61	59	57	56	55
59		70	66	62	60	58	57	56
60	77	71	67	63	61	59	58	57
61		72	68	64	62	60	59	58
62	78			65	63	61	60	59
63	79	73	69	66	64	62	61	60
64		74	70	67	65	63	62	61
65	80	75	71	68	66	64	63	62
66		76	72	69	67	65	64	63
67	81				68	66	65	64
68	82	77	73	70	69	68	67	66
69		78	74	71	70	69	68	67
70	83	79	75	72	71	70	69	68

NR Curve Data

The following table show the NR data used to generate the NR curves

NR	31.5	63	125	250	500	1000	2000	4000	8000
120	137.1	130.2	126.4	123.6	116.4	120	118.3	116.9	115.6
115	133.7	126.3	122.1	119	111.8	115	113.2	111.8	110.5
110	130.3	122.3	117.7	114.3	107.1	110	108.2	106.7	105.3
105	126.9	118.4	113.4	109.7	102.5	105	103.1	101.5	100.2
100	123.5	114.4	109	105	97.8	100	98	96.4	95
95	120.1	110.5	104.7	100.4	93.2	95	92.9	91.3	89.9
90	116.7	106.5	100.3	95.7	88.5	90	87.9	86.2	84.7
85	113.3	102.6	96	91.1	83.9	85	82.8	81	79.6
80	109.9	98.6	91.6	86.4	79.2	80	77.7	75.9	74.4
75	106.5	94.7	87.3	81.8	74.6	75	72.6	70.8	69.3
70	103.1	90.7	82.9	77.1	69.9	70	67.6	65.7	64.1
65	99.7	86.8	78.6	72.5	65.3	65	62.5	60.5	59
60	96.3	82.8	74.2	67.8	60.6	60	57.4	55.4	53.8
55	92.9	78.9	69.9	63.2	56	55	52.3	50.3	48.7
50	89.5	74.9	65.5	58.5	51.3	50	47.3	45.2	43.5
45	86	71	61.2	53.9	46.7	45	42.2	40	38.4
40	82.6	67	56.8	49.2	42	40	37.1	34.9	33.2
35	79.2	63.1	52.5	44.6	37.4	35	32	29.8	28.1
30	75.8	59.1	48.1	39.9	32.7	30	27	24.7	22.9
25	72.4	55.2	43.8	35.3	28.1	25	21.9	19.5	17.8
20	69	51.2	39.4	30.6	23.4	20	16.8	14.4	12.6
15	65.6	47.3	35.1	26	18.8	15	11.7	9.3	7.5
10	62.2	43.3	30.7	21.3	14.1	10	6.7	4.2	2.3
5	58.8	39.4	26.4	16.7	9.5	5	1.6	-1	-2.9
0	55.4	35.4	22	12	4.8	0	-3.5	-6.1	-8