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PERCEIVED NOISE REDUCTION
The following table shows various dB levels and the corresponding reduction in actual sound pressure level (SPL) as well as the human perceived volume reduction for reducing noise levels.

dB Actual SPL Reduction Perceived Volume Reduction
3 dB 50.00% 18.77%
6 dB 75.00% 34.02%
9 dB 87.50% 46.41%
12 dB 93.75% 56.47%
15 dB 96.88% 64.64%
18 dB 98.44% 71.28%
21 dB 99.22% 76.67%
24 dB 99.61% 81.05%
27 dB 99.80% 84.61%
30 dB 99.90% 87.50%
33 dB 99.95% 89.85%
36 dB 99.98% 91.75%
39 dB 99.99% 93.30%
42 dB 99.99% 94.56%
45 dB 100.00% 95.58%
48 dB 100.00% 96.41%
51 dB 100.00% 97.08%
54 dB 100.00% 97.63%
57 dB 100.00% 98.08%
60 dB 100.00% 98.44%
63 dB 100.00% 98.73%
66 dB 100.00% 98.97%
69 dB 100.00% 99.16%
72 dB 100.00% 99.32%
75 dB 100.00% 99.45%
78 dB 100.00% 99.55%
81 dB 100.00% 99.64%

Signal to Noise ratio (S/N) is expressed as the dB difference between the sound source (such as someone speaking) and the sound at the listeners ear. So if a person were speaking at 65dB, and a car noise was 60dB, the Signal to Noise ratio would be 5dB. In a classroom the recommended background noise level is 35dBA to result in S/N ratio of 20-25 dBA and +15 dBA at the back of the room, with a .6 second reverb time.

A few other important points to note:
  1. A change of 1 dB is generally not perceptible.
  2. A change of 3dB is just perceptible by most humans.
  3. Speech is somewhat understandable at S/N ratios of 0dB (mostly by adults).
  4. Speech is highly understandable at S/N ratios of >15dB by most children.