Noise and Hearing Protection
Decibels and how they add, exposure time limits, noise reduction ratings and how much protection you really get.
Decibels and how they add
Sound level is measured in decibels (dB), usually A-weighted (dBA) to match the ear’s sensitivity. The scale is logarithmic: a rise of 10 dB is ten times the sound energy and is heard as about twice as loud; a rise of 3 dB is twice the energy. Levels do not add like ordinary numbers; their energies do.
L = 10 log₁₀(Σ 10^(Li ÷ 10))
Worked example: adding machines
- Two equal sources of 90 dB give 10 log10(2 × 109) = 93.0 dB: only 3 dB more, not 180.
- Adding a third source of 85 dB gives 93.6 dB: a quieter source adds little.
- A 100 dB machine next to a 90 dB one gives 100.4 dB: the loud one dominates, and quietening the 90 dB one achieves almost nothing. Fix the loudest source first.
- Ten sources of 80 dB each together make 90 dB.
- Distance: in the open, the level falls 6 dB for each doubling of distance. A 100 dB source at 1 m gives 94 dB at 2 m, 88 dB at 4 m and 82 dB at 8 m. In a small hard-walled room the sound reflects and falls much less (nearer 3 dB per doubling), so moving away from a machine in a shop helps less than the numbers suggest.
How long is it safe?
Hearing damage depends on level and time. The NIOSH recommendation allows 8 hours at 85 dBA and halves the time for every 3 dB increase (the exchange rate). The older OSHA permissible limit uses 90 dBA for 8 hours and halves the time for every 5 dB. The NIOSH rule is the more protective and is based on the energy of the sound.
T = 8 h ÷ 2^((L − 85) ÷ 3)
Worked example: permitted times
- At 85 dBA the time is 8.0 h; at 88 dBA 4.0 h; at 91 dBA 2.0 h; at 94 dBA 1.0 h; at 100 dBA 15 min; at 106 dBA 4 min.
- By the OSHA 90/5 rule the same levels allow 4.0 h at 95 dBA and 2.0 h at 100 dBA: 100 dBA is 8 times as long as the NIOSH time. OSHA’s limit is higher than the level at which many people lose hearing; use the more protective one.
- Rule of thumb: if you must raise your voice to talk to someone an arm’s length away, the level is about 85 dBA or more. Normal conversation is about 60 dBA, a raised voice 75 and a shout 90 dBA.
A day of mixed noise: the dose
Real days contain several levels. The dose adds up the fraction of the permitted time used at each level: D = Σ (time spent ÷ time permitted). A dose of 100 % is the full allowance.
Worked example: an afternoon in the shop
| Level | Time spent | Time permitted | Dose |
|---|---|---|---|
| 100 dB | 15 min | 15 min | 100 % |
| 90 dB | 1.0 h | 2.5 h | 40 % |
| 82 dB | 2.0 h | 16.0 h | 13 % |
| Total | 152 % |
- The total is 152 %: 1.52 times the daily allowance, even though only 15 min was spent at the loudest level.
- The equivalent steady level is 85 + 3 log2(1.52) = 86.8 dBA over 8 hours (a time-weighted average): above the 85 dBA criterion.
- On the OSHA 90/5 criterion (levels below 80 dBA are ignored) the dose is only 33 %: the same day is “within the limit” by that rule and over the limit by NIOSH. Hearing protection for the saw and the sander brings the dose well under 100 % (see below).
- The loudest short task dominates: the 15 minutes at the saw is 100 % of the allowance by itself.
How loud are workshop tools?
Typical levels at the operator’s ear (they vary with the machine, the material and the room). The permitted time is by the NIOSH 85/3 and the OSHA 90/5 criteria.
| Tool or task | Level (typical) | NIOSH time | OSHA time |
|---|---|---|---|
| Hand sanding, hand saw | 70 dBA | 256.0 h | 128.0 h |
| Cordless drill | 85 dBA | 8.0 h | 16.0 h |
| Vacuum or dust extractor | 85 dBA | 8.0 h | 16.0 h |
| Compressor | 85 dBA | 8.0 h | 16.0 h |
| Orbital sander | 90 dBA | 2.5 h | 8.0 h |
| Router | 100 dBA | 15 min | 2.0 h |
| Table saw cutting | 100 dBA | 15 min | 2.0 h |
| Planer | 103 dBA | 8 min | 1.3 h |
| Angle grinder cutting | 105 dBA | 5 min | 1.0 h |
| Hammering on steel | 110 dBA | 1 min | 30 min |
- Impact noise (hammering, nail guns, grinding on steel) has sharp peaks that damage hearing at lower averages: wear protection even for short bursts.
- A tool being cut under load is louder than the idle level on its data sheet.
- Measure it: a cheap sound meter or phone app is accurate to about 2 to 3 dB and is good enough to tell 85 from 100.
Hearing protectors: how much do you really get?
Ear plugs and muffs carry a noise reduction rating (NRR) in decibels, measured in a lab with ideal fitting. In real use the protection is lower. A simple estimate for A-weighted levels is (NRR − 7) ÷ 2 dB; a poor fit is much worse.
Worked example: protecting a 100 dBA task
| Protection | Reduction | Level at the ear | Permitted time |
|---|---|---|---|
| None | −0.0 dB | 100.0 dB | 15 min |
| Plugs, NRR 29, poorly fitted (half) | −5.5 dB | 94.5 dB | 53 min |
| Muffs, NRR 25 | −9.0 dB | 91.0 dB | 2.0 h |
| Plugs, NRR 29, well fitted | −11.0 dB | 89.0 dB | 3.2 h |
| Plugs and muffs together | −16.0 dB | 84.0 dB | 10.1 h |
- Plugs with NRR 29 well fitted reduce the level by (29 − 7) ÷ 2 = 11.0 dB to 89.0 dB (3.2 h allowed). Muffs with NRR 25 give 9.0 dB.
- A poor fit, such as plugs not fully inserted, may give only half the reduction (5.5 dB): 94.5 dB and 53 min.
- Double protection (plugs under muffs) adds about 5 dB to the better one: 16.0 dB in total, 84.0 dB at the ear (10.1 h allowed). Use it above 105 dBA.
- Protection only works while it is worn. Taking muffs off for a quarter of the time in a 100 dBA shop cuts the real attenuation of a 25 dB protector to about 6 dB (over the whole period the energy that gets in is dominated by the unprotected time).
- Roll the foam plug thin, pull the ear up and back, insert it deeply and hold it until it expands; the end should not stick out far.
- Muffs seal on skin: glasses arms, hair and a hat break the seal; check for a good fit with a cushion all round the ear.
- Do not over-protect when you need to hear alarms, a person shouting or the sound of a machine: use level-dependent muffs or electronic ones where that matters.
- Replace foam plugs when dirty, muff cushions when hard or cracked.
Reducing noise at the source and on the way
Worked example: what a 10 dB reduction means
- Cutting the sound level by 10 dB removes 90 % of the sound energy and sounds about half as loud (the ear hears a 10 dB change as a factor of two).
- A machine with an enclosure that gives 20 dB of attenuation brings a 100 dB source to 80 dB: the permitted time goes from 15 min to 25.4 h (over a full working day), more than any ear defender gives.
- Many small fixes add up: sharp blades, rubber feet, a damped table, a quieter extractor placed outside or in a box, a muffler on the exhaust.
- Buy quiet: check the declared sound power of new tools; a 3 dB lower tool is half the energy.
- Maintain: a blunt blade, a loose guard or a worn bearing is louder.
- Isolate: rubber mounts under machines keep the floor from becoming a sounding board.
- Absorb: soft surfaces on walls and ceiling cut the build-up of reflections in a small room.
- Separate: put the compressor and extractor in another room or a ventilated box (see Planning the Workshop).
- Limit the time at the loudest tasks and take breaks in a quiet place.
How hearing is damaged
- Hair cells in the inner ear are destroyed by loud sound and do not grow back. The loss starts at around 4 000 Hz (the “noise notch”) and spreads to speech.
- It is painless and gradual. By the time speech sounds muffled, much of the damage is done.
- Tinnitus (ringing) after noise is a warning that damage has happened.
- Temporary threshold shift: muffled hearing for hours after loud noise means the ear has been overloaded; repeated exposure makes it permanent.
- Impulse noise (a gunshot, a hammer on steel) can cause damage at once.
- Test your hearing every year or two if you work in a noisy shop; an audiogram shows a loss early.
Worked example: a decade of a loud hobby
A hobbyist who spends two hours a week at 100 dBA (104 hours a year) uses 8 times the daily allowance every week, with the damage adding up. Over 10 years that is 1 040 hours at a level that gives a daily allowance of 15 min. Ten dollars of ear plugs would have cut the level to about 89 dBA and the dose by a factor of 13.
Rules of thumb
- If you have to raise your voice, wear protection.
- Put protection on before you start the machine, not after the noise has begun.
- Louder means shorter: every 3 dB halves the time.
- Fix the loudest source first; quieting a lesser one achieves little.
- Protection works only when worn and fitted.
- Keep headphones for music below 85 dBA; music through earbuds is noise exposure too.
Troubleshooting
| Problem | Likely cause | Try |
|---|---|---|
| Ears ring after work | Noise too high or protection not used | Use plugs or muffs; double up above 105 dBA; reduce the source |
| Plugs keep falling out | Wrong size, not rolled and inserted deeply | Re-fit; try a different type or size |
| Muffs press on glasses | Frame arms break the seal | Thin-arm glasses or use plugs |
| Cannot hear warnings | Over-protection | Use level-dependent muffs; set up visual signals |
| The shop is too loud overall | Reflections and several machines | Absorbent panels; separate the compressor and extractor; quieter tools |
| Hearing seems muffled for hours after work | Temporary threshold shift | Stop; wear protection next time; get a hearing test |
Related
- Workshop Safety: the overview of this guide.
- Personal Protective Equipment: the other protective equipment.
- Machine and Hand Tool Safety: machine safety and quieter, guarded machines.
- Planning the Workshop (where loud machines go) and Power Tools and Shop Machines.
FAQ
When do I need ear protection?
Whenever you have to raise your voice to be heard at arm’s length (about 85 dBA), and for any short loud task such as a grinder, a router or hammering on steel.
Do I need both plugs and muffs?
Only above about 105 dBA or when one alone does not bring the level low enough; together they add about 5 dB to the better of the two.
Why is 3 dB such a big deal?
Every 3 dB doubles the sound energy, so the safe time halves: 85 dBA for 8 hours, 88 for 4, 91 for 2, 94 for 1 hour.
How do I measure the noise in my shop?
A sound-level meter or a good phone app, set to A-weighting and slow response, at the position of your ear; it is accurate enough to tell whether you are near 85 or 100.
References
- NIOSH, Criteria for a Recommended Standard: Occupational Noise Exposure (1998): 85 dBA, 3 dB exchange rate.
- OSHA 29 CFR 1910.95 (occupational noise exposure: 90 dBA, 5 dB exchange rate) and the EU Directive 2003/10/EC (80 and 85 dBA action levels).
- ISO 9612 (determination of occupational noise exposure) and ANSI S3.19 / ISO 4869-2 (hearing protector attenuation).
Last reviewed: October 2026. Tool levels and protector ratings are typical: measure your own shop and read the label on your protectors.