Combined Noise Level
90.0 dB
Safety Zone
Loud – Protection Recommended
Sounds Like
Gas lawn mower or shop tools
How far away the dB readings were measured
How far you actually are from the source
OSHA: 90 dB limit for 8 hours, 5 dB exchange rate (workplace standard)
NIOSH: 85 dB limit for 8 hours, 3 dB exchange rate (recommended, stricter)
At 90.0 dB, this sounds roughly 8.0x as loud as a normal conversation (60 dB).
Every +10 dB sounds approximately 2x louder to human ears.
Inputs
Result
Two identical 90 dB sources combine to 93 dB (+3 dB), not 180 dB, because decibels are logarithmic. This cuts your safe OSHA exposure from 8 hours to about 5 hours 17 minutes.
Inputs
Result
A chainsaw (105 dB) and lawn mower (90 dB) combine to 105.1 dB at 3 ft. Moving back to 24 ft (3 doublings) reduces the level by about 18 dB to 87.1 dB, giving nearly 5 hours of NIOSH-safe exposure.
Inputs
Result
Four shop tools at varying levels combine to 96.6 dB. The router at 95 dB dominates the total. OSHA allows about 3 hours 12 minutes; NIOSH recommends only 33 minutes without hearing protection.
Decibels are logarithmic, so you cannot simply add them. Two 90 dB sources produce 93 dB, not 180 dB. The formula converts each source to linear intensity, sums the intensities, then converts back: L_total = 10 × log10(Σ 10^(Li/10)).
| Number of Equal Sources | dB Increase | Example (90 dB each) |
|---|---|---|
| 1 source | +0 dB | 90 dB |
| 2 sources | +3 dB | 93 dB |
| 4 sources | +6 dB | 96 dB |
| 10 sources | +10 dB | 100 dB |
OSHA permits 90 dBA for 8 hours using a 5 dB exchange rate. For every 5 dB increase, the allowed exposure time is halved. Workers exposed above 85 dBA must be included in a hearing conservation program.
| Sound Level (dBA) | OSHA Max Duration | NIOSH Max Duration |
|---|---|---|
| 85 dB | No limit | 8 hours |
| 90 dB | 8 hours | 2 hours 31 min |
| 95 dB | 4 hours | 47 min |
| 100 dB | 2 hours | 15 min |
| 105 dB | 1 hour | 4 min 43 sec |
For a point source in open air, sound decreases by 6 dB each time you double the distance. A lawn mower at 90 dB from 3 feet drops to 84 dB at 6 feet, 78 dB at 12 feet, and 72 dB at 24 feet. Walls and barriers reduce noise further.
| Distance (ft) | dB Reduction | 90 dB Source Level |
|---|---|---|
| 3 ft (reference) | 0 dB | 90 dB |
| 6 ft | −6 dB | 84 dB |
| 12 ft | −12 dB | 78 dB |
| 24 ft | −18 dB | 72 dB |
| 48 ft | −24 dB | 66 dB |
OSHA is the legal workplace standard with a 90 dB threshold and 5 dB exchange rate. NIOSH is a recommended guideline with a stricter 85 dB threshold and 3 dB exchange rate. NIOSH limits are roughly 4x shorter exposure times at the same levels.
Below 85 dB no protection is needed. From 85–100 dB, foam earplugs (NRR 22–33) are sufficient. From 100–105 dB, use over-ear muffs. Above 105 dB, OSHA requires double protection: earplugs plus muffs, which reduces noise by roughly 5 dB beyond the higher-rated protector.
Prolonged exposure above 85 dB can cause permanent hearing loss according to NIOSH. A single exposure above 120 dB can cause immediate pain, and sounds above 140 dB (like a gunshot or jet engine at close range) can cause instant permanent damage.
| Sound Level | Common Source | Risk Level |
|---|---|---|
| 30 dB | Whisper | Safe |
| 60 dB | Conversation | Safe |
| 85 dB | Heavy traffic | Caution (8h limit) |
| 100 dB | Power tools | Dangerous (15 min) |
| 120 dB | Siren | Pain threshold |
| 140 dB | Jet engine | Instant damage |
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Last Updated: Mar 9, 2026
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