| The SLC80 Rating of a hearing protector, ear plugs or headset is a simple number and class rating which is derived from a test procedure as outlined in the Australian/ New Zealand Standard, AS/NZS 1270: 2002. What it does is provide a consumer with a simple number guide to the level of noise attenuation that can be expected from a particular hearing protection device (HPD). however, because we humans have heads which come in all shapes and sizes some with or without eye glasses, beards or long hair, the level of protection achieved for each person could well be different. The SOUND LEVEL CONVERSION (SLC80) rating is defined as the difference between the C weighted sound level of the environment in which the HPD is worn and the A weighted sound level reaching the wearer's ears. The SLC value includes a mean minus one standard deviation correction to ensure that the stated degree of noise reduction is obtained on 80% of occasions. For this reason it is called SLC80, the 80 indicating
the percentage protection rate.
The test procedure can be separated into two very different areas;
1) Mechanical Testing
Hearing protectors and headsets like all things new seem pretty good when worn for the first time however,all things wear, headbands stretch etc. To compensate for this wear factor and obtain a real world test result the HPD under test is pre-aged. This is done by subjecting the HPD to the effects of heat, cold, being dropped and having the headband flexed many times to simulate normal use. The mass or weight is recorded and expressed in grams on the test certificate as is the clamping force in newtons. This gives the user further information on which to base their decision as to the potential overall comfort the HPD may provide. No one wants to work an 8 hour shift and feel as if they have a lead weight on their head or their head is in a vice.
2) Audiometric Testing
This is a subjective test, a minimum of sixteen (twenty for ear plugs) human test subjects are selected at random. Each subject is then given a hearing test to establish they fall into the category of "Normal Hearing", as outlined in the standard.
The attenuation of the HPD is determined by measuring each subject's hearing threshold with and without the HPD fitted. The difference between these two thresholds is the so called real ear attenuation of the HPD. This is done by presenting each test subject with pulsed 1/3 octave pink noise of a known value at the following frequencies 125, 250, 500, 1000, 2000, 4000 and 8000Hz and recording their hearing threshold. From this data the mean real ear attenuation and standard deviation at each frequency is calculated.
By subtracting the standard deviation from the mean we end up with the mean minus standard deviation, this when subtracted from the band level gives us the attenuation
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SLC80 Calculation |
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| Band level |
|
71 |
81 |
89 |
93 |
95 |
93 |
86 |
|
Subject |
125Hz |
250Hz |
500Hz |
1000Hz |
2000Hz |
4000Hz |
8000Hz |
|
1 |
15 |
20 |
26 |
42 |
34 |
32 |
26 |
|
2 |
15 |
21 |
34 |
36 |
39 |
42 |
43 |
|
3 |
18 |
25 |
33 |
32 |
36 |
43 |
40 |
|
4 |
17 |
21 |
35 |
37 |
37 |
50 |
45 |
|
5 |
11 |
19 |
34 |
34 |
35 |
39 |
43 |
|
6 |
19 |
21 |
33 |
40 |
29 |
30 |
38 |
|
7 |
9 |
14 |
21 |
30 |
30 |
38 |
38 |
|
8 |
22 |
17 |
30 |
32 |
38 |
43 |
50 |
|
9 |
11 |
23 |
27 |
31 |
39 |
38 |
39 |
|
10 |
18 |
20 |
28 |
33 |
34 |
38 |
39 |
|
11 |
16 |
17 |
27 |
35 |
38 |
41 |
38 |
|
12 |
17 |
19 |
35 |
34 |
38 |
43 |
43 |
|
13 |
20 |
19 |
31 |
36 |
30 |
40 |
38 |
|
14 |
13 |
18 |
30 |
37 |
39 |
38 |
45 |
|
15 |
17 |
22 |
27 |
33 |
37 |
40 |
36 |
|
16 |
13 |
20 |
25 |
34 |
31 |
39 |
29 |
| MEAN |
|
15.7 |
19.8 |
29.8 |
34.8 |
35.3 |
39.6 |
39.4 |
| STANDARD |
|
3.6 |
2.6 |
4.1 |
3.2 |
3.5 |
4.6 |
5.9 |
| M-S DEV |
|
12.1 |
17.2 |
25.6 |
31.6 |
31.7 |
35.1 |
33.5 |
| ATTENUATION |
58.9 |
63.8 |
63.4 |
61.4 |
63.3 |
57.9 |
52.5 |
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| SLC80 Calculation |
30.14 |
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HMP-1 |
SLC80 |
30 |
Using the test data for our HMP-1 headset the following formula is then applied:
The SLC80 value is 100 minus the logarithmic summation of all the attenuated levels.
SLC80 = 100 - 10Log10(105.89 + 10 6.38 + 106.34 + 106.14 + 106.33 + 105.79 + 105.25)
= 100 - 69.86
= 30 dB rounded
HPD are also given a class rating as outlined below once the SLC80 rating is known.
| Class |
SLC80, dB |
| 1 |
10 to 13 |
| 2 |
14 to 17 |
| 3 |
18 to 21 |
| 4 |
22 to 25 |
| 5 |
26 & Higher |
It must be remembered that the SLC80 rating is not an exam mark where above a certain number you pass and below you fail, it is a means by which in conjunction with the information contained in the Australian /New Zealand standard AS/NZS 1269.3:2005 Occupational noise management - Hearing protector program you can start to address the problems of hearing loss due to noise exposure for your given environment.
Should you wish to obtain a deeper understanding of the SLC80 rating or how to apply these ratings to your workplace copies of the standards can be obtained from the following:
The Australian National Acoustic Laboratories (NAL) is a great source of information on hearing and it's protection, click on the logo to visit their web site.
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