The NIOSH Sound Level Meter (SLM) app is a free iPhone and iPad app from the National Institute for Occupational Safety and Health. It measures noise with A, C and Z weighting and reports workplace-style metrics such as TWA and noise dose. NIOSH says it is accurate within ±2 dBA after testing in its acoustics lab. There’s no Android version, because NIOSH can’t verify accuracy across the huge range of Android phones.
It’s the best-validated free sound meter you can put on a phone. It’s also more technical than most people need.
What is the NIOSH SLM app? #
NIOSH is the U.S. federal research agency, part of the CDC, that studies workplace safety, including noise and hearing loss. Its researchers published studies in 2014 and 2016 showing that most smartphone sound apps were unreliable, but that a few iOS apps and external calibrated microphones could perform well. The NIOSH SLM app was the result: a sound meter designed around the standards NIOSH and OSHA use.
According to its App Store listing, the app is developed by EA LAB for NIOSH. It’s free, with no ads and no in-app purchases, requires iOS 15 or later, and also runs on iPad, on Macs with Apple silicon and on Apple Vision Pro.
What does the NIOSH SLM app measure? #
| Feature | What it gives you |
|---|---|
| A, C and Z weighting | Readings that match hearing limits (A), capture loud and low-frequency sound (C), or count all frequencies equally (Z) |
| LAeq | The A-weighted average level over the measurement |
| TWA | Time-weighted average, the number exposure limits are written in |
| Max and peak levels | The loudest time-weighted reading and the instantaneous peak |
| Noise dose and projected dose | How much of the NIOSH or OSHA daily allowance you’ve used, and where you’d end up over a full shift |
| Calibration | Adjust the internal microphone or a calibrated external one |
| Reports and sharing | Save and share measurement data |
| Health app integration | Log noise levels to Apple Health |
| Information screens | Background on hazardous noise and hearing protection |
For background on the numbers, see what is Leq, dB vs dBA and NIOSH vs OSHA noise limits.
How accurate is the NIOSH SLM app? #
NIOSH says the app was tested according to standards in a reverberant chamber at its acoustics laboratory and found accurate within ±2 dBA. With a calibrated external microphone, NIOSH says it meets the Type 2 requirements of the IEC 61672-3 sound level meter standard, the class used for general-purpose field measurements.
That fits NIOSH’s research. The 2016 follow-up study found that apps using external calibrated microphones measured within ±1 dB of a reference system. The built-in microphone is the weak link on any phone. See how accurate phone sound meters are.
Two caveats. Tested accuracy applies to the devices and conditions NIOSH tested, and every phone microphone ages and varies a little. And ±2 dBA is a good result for a phone, but it’s not the same as a calibrated Class 1 meter.
Is the NIOSH SLM app available for Android? #
No. NIOSH says the app is only available on iOS devices, because the Android device marketplace is fragmented among many manufacturers, which makes it impractical to verify the app’s accuracy across devices.
If you use Android, you have two options:
- Use a general sound meter app that offers A-weighting, Leq and a calibration offset, and calibrate it once against a reference meter if you can. Sound Meter dB is free on Android and iPhone. It shows A-weighted or flat readings with Fast or Slow response, a proper Leq, min and max, and NIOSH-based safe-exposure time for the level you’re in. It doesn’t calculate noise dose or TWA, and it isn’t validated the way the NIOSH app is.
- Borrow an iPhone or use a real meter if you need NIOSH-grade readings for work.
For more options, see best decibel meter apps.
How do you use the NIOSH SLM app? #
- Install it and allow microphone access.
- Check calibration. If you have a calibrated external microphone or a reference meter, use the calibration settings to match it. Otherwise, the app uses its built-in settings for your device.
- Hold or place the phone at ear height, microphone uncovered, away from your body and hard surfaces.
- Start a measurement and let it run. For a shift-length estimate, the projected dose tells you where you’ll end up if the noise stays the same.
- Read LAeq for the average, max and peak for the loud moments, and dose or TWA for exposure.
- Save or share the report if you need a record.
For general technique that applies to any app, see how to measure noise level with your phone.
Can the NIOSH app replace a dosimeter or sound level meter at work? #
It’s useful for screening and for workers checking their own environment. For formal compliance with OSHA’s noise standard, employers generally use certified sound level meters and personal dosimeters, operated according to procedure. A phone app, even a validated one, is best treated as a first look that tells you where proper monitoring is needed. See noise dosimeter vs sound level meter.
Who should use the NIOSH SLM app? #
A great fit for: iPhone users who want the most defensible free readings, safety professionals doing quick screens, workers checking their own exposure, and anyone curious about dose and TWA.
Less ideal for: Android users, people who want plain-English explanations of each reading, and anyone who needs certified compliance measurements.
Frequently asked questions #
Is the NIOSH sound level meter app free? #
Yes. It’s free on the App Store, with no ads and no in-app purchases.
Does the NIOSH app work with external microphones? #
Yes. NIOSH says the app can calibrate either the internal or an external microphone, and that with a calibrated external microphone it meets Type 2 requirements of IEC 61672-3. External calibrated microphones gave the most accurate results in NIOSH’s research.
Why is there no NIOSH app for Android? #
NIOSH says the Android market is fragmented across many manufacturers, which makes it impractical to verify the app’s accuracy on the range of Android devices. The agency chose to support only iOS, where it could test.
What is the difference between LAeq and TWA in the NIOSH app? #
LAeq is the A-weighted average level over the time you measured. TWA expresses exposure as an equivalent 8-hour average, the form in which workplace limits are written, so the same measurement can produce a lower TWA if it covers less than a full shift.