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Decibel Sound Meter

How accurate is an iPhone decibel meter, and how do you calibrate it?

Accurate enough to compare places and spot when something is clearly loud; not accurate enough for legal, workplace or hearing-test use. You can get much closer on your own phone by matching a reference meter once, using the Calibration offset in the app's Exposure tab.

4 min read · Updated 2026-09-17

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Are decibel meter apps accurate?

Short answer: not in the way a lab instrument is. A phone microphone is built for voices, not for measuring. It varies from one model to the next, hears very low and very high frequencies less well, and squashes sudden loud bangs. Two phones sitting side by side in the same room can disagree by a few decibels, and fireworks read lower than they really are.

What a phone does well is relative change. If the same spot reads louder today than yesterday with the same phone, that difference is real even when the absolute number is off. We don't publish an accuracy figure for the app, because it depends on your phone and on whether you calibrated it.

The app says so on the meter itself: A-weighted estimate — a phone is not a certified sound level meter.

Why do different apps show different numbers?

  • Weighting: this app uses A-weighting (dBA), which follows human hearing. An app showing flat or C-weighted values reads higher with bass-heavy noise.
  • Response: this app uses Fast (125 ms). A Slow response smooths the number more.
  • Built-in offset: because phone microphones read low, apps add a correction. This app adds +10 dB on iPhone and +8 dB on iPad out of the box.
  • Averaging: some apps show a simple mean. This app's AVG is an energy average (LAeq), which sits closer to the loud moments.

What can you calibrate in the app?

There are three controls and they add up: auto offset plus manual adjustment plus the Exposure calibration offset. The total applies to every reading, on the Meter tab and in Exposure.

The three calibration controls
ControlWhereRangeUse it when
Auto CalibrateMeter > CalibrateAutomatic, within -30 to +30 dBYou have a genuinely quiet room and no reference meter
Manual AdjustmentMeter > Calibrate-20 to +20 dB, whole dBYou want a quick correction in whole decibels
Calibration offsetExposure > Calibration-15 to +15 dB, 0.1 dB stepsYou can stand next to a calibrated meter

How to calibrate against a reference meter

This is the method that actually ties your phone to a real instrument. Borrow or rent a calibrated sound level meter set to A-weighting and Fast response.

  1. Find a steady noise, such as a fan, a running tap or pink noise from a speaker, and put the phone right next to the meter's microphone.
  2. In the Exposure tab, tap the Calibration row (Match a reference meter).
  3. Under Live check, read the Live reading (if it shows --, tap Start preview) and compare it with the meter.
  4. Move the Calibration offset slider, or tap minus and plus for 0.1 dB steps, until both numbers match. Reset to 0.0 undoes it.
  5. Tap Done. The offset is remembered between launches and saved with every exposure session.
Worked example: matching a reference meter
StepValue
Reference meter shows73.0 dBA
Phone Live reading shows71.4 dB
Difference (73.0 - 71.4)+1.6 dB
Set Calibration offset to+1.6 dB
Phone now reads73.0 dB

How to auto calibrate in a quiet room

Auto Calibrate takes 30 readings over about 3 seconds and sets the offset so that their median becomes 32 dB. It assumes the room is really quiet, around 30 dB, and it needs the live reading to be running.

  1. Go to a genuinely quiet room: windows shut, no fridge, fan or air conditioning running nearby.
  2. On the Meter tab, check that the live reading is running, then tap Calibrate.
  3. Tap Auto Calibrate and stay silent until it finishes.
  4. Check the new Auto Calibration Offset. Reset to Default brings back the built-in offset and sets Manual Adjustment to 0; the Calibration offset in Exposure has its own Reset to 0.0.
Worked example: auto calibration (manual adjustment and calibration offset at 0)
StepValue
Median reading before45.0 dB
Auto offset before+10.0 dB
Raw microphone level (45.0 - 10.0)35.0 dB
New auto offset (32 - 35.0)-3.0 dB
Reading after32.0 dB

Questions and answers

What is the most accurate decibel meter app?
Any app is limited by the phone's microphone, so the phone and calibration matter more than the app. Look for one that tells you its weighting and response and lets you enter a calibration offset, then match it against a real meter.
Does calibration make my phone a professional meter?
No. It removes the steady offset of your microphone at the level you calibrated with. It does not fix the frequency response, clipping on loud bangs or differences in how you hold the phone.
Do I need to calibrate at all?
Not for comparisons: which room is quieter, whether the level went up or down. Calibrate when the absolute number matters to you, for example before estimating your noise dose.
Why does my phone read differently from my friend's?
Microphones differ between models and even between units, and each phone may have a different calibration. Compare phones against the same reference meter, not against each other.

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