Noise rarely hurts on the day it happens. A worker leaves a loud workshop with ringing ears, sleeps, and feels fine the next morning. But the damage adds up, and hearing that is lost this way does not come back. That is why noise is measured over the whole shift, not just at one moment.
Enter the noise levels and the hours spent at each one below, and the calculator will give you the daily dose and the 8-hour average.
What is noise exposure?
Noise exposure is the total amount of sound energy a person takes in during a working day. It depends on two things: how loud the noise is, and how long the person is in it. A very loud noise for a short time can be as risky as a moderate noise for a long time.
Noise is measured in decibels, written dBA. The “A” means the reading is adjusted to match how the human ear hears. Because the decibel scale is logarithmic, a small rise in the number means a big rise in the sound energy. A job at 91 dBA is not slightly worse than one at 85 dBA. It is several times harder on the ears.
Daily noise dose and the 8-hour TWA
There are two numbers you will see in noise reports:
- Daily noise dose: a percentage. A dose of 100 percent means the person has reached the daily limit for the standard you are using.
- 8-hour TWA: the time-weighted average. It is the single steady noise level that would give the same dose over eight hours.
They are two ways of saying the same thing. A dose of 100 percent is the same as a TWA at the limit of your standard.
The formulas
Allowed time for one noise level: T = 8 / 2^((L – criterion) / exchange rate)
Dose % = 100 x sum of (hours spent / allowed time)
TWA = criterion + (exchange rate / log10(2)) x log10(Dose / 100)
L is the noise level in dBA. The criterion and the exchange rate depend on the standard you choose.
OSHA or NIOSH: which standard?
- NIOSH and ISO: criterion of 85 dBA, exchange rate of 3 dB. Every 3 dB rise cuts the allowed time in half.
- OSHA: criterion of 90 dBA, exchange rate of 5 dB. Every 5 dB rise cuts the allowed time in half.
The NIOSH method is stricter. Many companies and clients follow it, or an even lower limit. Use the one your employer, client or local rule asks for, and always write which one you used next to the result.
Worked example
A worker spends 2 hours near a grinder at 88 dBA, and 4 hours in the general workshop at 85 dBA. The rest of the day is quiet.
Using the NIOSH method
- At 88 dBA, the allowed time is 8 / 2^((88 – 85) / 3) = 4 hours. The worker spent 2 hours, so that is 2 / 4 = 0.5.
- At 85 dBA, the allowed time is 8 hours. The worker spent 4 hours, so that is 4 / 8 = 0.5.
Dose = 100 x (0.5 + 0.5) = 100%. The TWA is exactly 85 dBA, which is the limit.
Using the OSHA method
- At 88 dBA, the allowed time is about 10.6 hours, so 2 hours is about 0.19.
- At 85 dBA, the allowed time is 16 hours, so 4 hours is 0.25.
Dose = about 44%, and the TWA is about 84 dBA.
The same day gives 100 percent under one standard and 44 percent under the other. This is why you must always say which standard you are using.
How to read your result
- Under 50%: low. Keep monitoring when the work changes.
- 50% to 99%: medium. You are close to the limit, so start controls and hearing protection. Under OSHA, a hearing conservation programme starts at an 8-hour average of 85 dBA, which is a 50 percent dose on the OSHA scale.
- 100% or more: high. The limit is exceeded, and you must reduce the exposure.
How to reduce noise exposure
Hearing protection is the last step, not the first. Work through this list in order:
- Buy quiet: choose quieter machines and tools when you replace them
- Fix the source: worn bearings, loose panels and dry parts make a lot of noise, so maintenance helps
- Enclose or screen it: use covers, barriers or sound-absorbing panels
- Move people away: noise falls fast with distance
- Limit the time: rotate jobs so no one spends the whole shift in the loudest area
- Use hearing protection: choose the right type, fit it well and check that people really wear it
- Test hearing: regular hearing checks show early damage while it can still be stopped
Step by step
- Measure the noise at the worker’s ear with a calibrated meter, or use a dosimeter.
- List each noise level and the hours spent at it.
- Choose the standard you must follow.
- Enter the levels and hours in the calculator.
- Read the dose and the TWA.
- Plan controls and check again after any change.
Rough signs that a place is too loud
Meters give the real answer, but your ears can warn you first. If you have to raise your voice to talk to someone one metre away, the noise is probably in the range where a check is needed. Other signs are ringing in the ears after work, muffled hearing in the evening, or workers who start to ask people to repeat themselves.
Common mistakes
- Averaging decibel readings like normal numbers, which gives the wrong answer
- Measuring at the machine instead of at the worker’s ear
- Forgetting short, very loud tasks, which can add a large part of the dose
- Using the OSHA method when the client asks for the NIOSH one
- Giving out ear plugs and calling the job finished
- Using a meter that has not been calibrated
Linking it to your risk work
Noise is one hazard among many. Score it with the risk matrix calculator, and after you add controls, use the residual risk calculator to show how much the risk came down. In hot workplaces, check the heat stress calculator as well, because loud places are often hot too.
Frequently asked questions
What is a safe noise level at work?
It depends on time. Under the NIOSH method, the limit is 85 dBA over eight hours. Under OSHA it is 90 dBA. Louder noise must be limited to shorter times.
What does a 100% noise dose mean?
It means the person has used up the full daily allowance for the chosen standard.
What is TWA?
TWA stands for time-weighted average. It is the steady level that would give the same dose over an eight-hour day.
Can I use this for a day that is longer than eight hours?
Yes. Enter all the hours at each noise level. The dose will show if the longer day is too much.
Do ear plugs remove the problem?
No. They lower what reaches the ear, but only when they are the right type, fitted well and worn all the time.
Learn this properly
Noise, hearing protection and exposure limits are covered in NEBOSH, IOSH and OSHA courses. ISH Institutes teaches these in Peshawar. Chat with us on WhatsApp and we will help you pick the right course.