Combined Level
91.2 dB
Sources
2
OSHA Max
6.78 hrs
Combined Sound Level
91.2
decibels (dB SPL)
OSHA Max
6.78 hrs
NIOSH Max
1.91 hrs
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Inputs
Result
Sum = 10^(85/10) + 10^(90/10) = 316,227,766 + 1,000,000,000 = 1,316,227,766. Total = 10*log10(1,316,227,766) = 91.2 dB. OSHA allows 8/2^((91.2-90)/5) = 6.06 hours at this level.
Inputs
Result
PA adjusted: 115 - 20*log10(2/1) = 108.98 dB. Drums adjusted: 110 - 20*log10(5/1) = 96.02 dB. Combined: 10*log10(10^(108.98/10) + 10^(96.02/10)) = 109.1 dB. Hearing protection essential.
Decibels add logarithmically, not linearly. The formula is: Total dB = 10 * log10(10^(dB1/10) + 10^(dB2/10) + ...). Two identical 90 dB sources combine to 93 dB, not 180 dB. This is because decibels measure ratios of sound intensity on a logarithmic scale.
| Sources at 90 dB | Combined Level | Increase | Perceived Change |
|---|---|---|---|
| 1 source | 90.0 dB | +0 dB | Baseline |
| 2 sources | 93.0 dB | +3 dB | Slightly louder |
| 4 sources | 96.0 dB | +6 dB | Noticeably louder |
| 8 sources | 99.0 dB | +9 dB | Much louder |
| 10 sources | 100.0 dB | +10 dB | About twice as loud |
Sound follows the inverse square law: every doubling of distance reduces the level by approximately 6 dB in free field. At 1 meter a lawn mower measures 90 dB; at 2 meters it drops to 84 dB; at 4 meters to 78 dB. Indoors, reflections slow this drop to about 3-4 dB per doubling.
| Distance (m) | Level from 100 dB source | Reduction |
|---|---|---|
| 1 m | 100 dB | 0 dB |
| 2 m | 94 dB | -6 dB |
| 5 m | 86 dB | -14 dB |
| 10 m | 80 dB | -20 dB |
| 50 m | 66 dB | -34 dB |
OSHA permits 90 dB for 8 hours with a 5 dB exchange rate: each 5 dB increase halves the allowed time. At 95 dB the limit drops to 4 hours, and at 100 dB to just 2 hours. NIOSH uses a stricter 85 dB criterion with a 3 dB exchange rate.
| dB Level | OSHA Max Hours | NIOSH Max Hours | Protection Needed? |
|---|---|---|---|
| 85 dB | 16 hrs | 8 hrs | NIOSH: Yes |
| 90 dB | 8 hrs | 2.5 hrs | Yes |
| 95 dB | 4 hrs | 47 min | Yes |
| 100 dB | 2 hrs | 15 min | Yes |
| 110 dB | 30 min | 1.5 min | Yes |
Noise dose is the ratio of actual exposure time to the maximum allowed time, expressed as a percentage. A 100% dose means you have reached the full permissible exposure. For example, 4 hours at 95 dB = 100% OSHA dose. Exceeding 100% increases hearing damage risk.
Active noise cancellation works by generating a sound wave 180 degrees out of phase. This works well for steady low-frequency noise (engine hum, HVAC) but is ineffective for random or high-frequency sounds. In practice, ANC headphones achieve 20-30 dB reduction for low frequencies and less for higher ones.
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Last Updated: Mar 25, 2026
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