How to Measure Decibels: 4 Methods That Actually Work

You can measure decibels using your phone browser, a dedicated app, or a professional sound level meter — depending on how accurate you need the reading to be. For most everyday purposes, a browser-based or app-based measurement is accurate enough. For occupational assessments or legal documentation, you’ll want a calibrated professional instrument.

This guide covers all four methods in order of effort and accuracy, so you can pick the right one for what you’re actually trying to find out.

Method 1 — Browser-Based Online Decibel Meter (Fastest)

The quickest way to measure decibels is to use the free online decibel meter directly in your browser. It accesses your device’s built-in microphone, processes the audio signal locally, and displays a real-time dB reading in seconds.

How to use it:

1. Open the meter on any device with a microphone — phone, tablet, or laptop
2. Click Allow when your browser asks for microphone permission
3. Hold or place your device near the sound source you want to measure
4. Let the meter run for 15–30 seconds and note the average reading

Accuracy: ±3–5 dB without calibration. Improves to ±2–3 dB with a calibrated microphone.

Best for: Checking room noise, testing a workspace, getting a quick reference reading for everyday environments.

Limitation: Built-in microphones are optimised for voice and often apply automatic gain control (AGC), which can skew readings at extreme levels. The microphone calibration guide covers how to reduce this effect.

Method 2 — Smartphone Decibel Meter App

Dedicated apps give you more control than a browser-based meter — many include data logging, peak hold, A-weighting selection, and time-history graphs. They still use your phone’s built-in mic, so accuracy is similar, but the additional features make them more useful for monitoring over time.

How to use it:

1. Download a sound level meter app (NIOSH SLM for iOS is free and well-regarded; Decibel X is popular across both platforms)
2. Open the app and allow microphone access
3. Set the weighting to dBA for most real-world measurements — this matches how noise regulations define safe exposure
4. Position the phone microphone toward the sound source at a steady distance
5. Let it run and record the reading — use the Leq (equivalent continuous level) for sustained noise, or Lmax for peak levels

Accuracy: ±2–4 dB for most modern smartphones, depending on the microphone. Some phones have frequency-dependent quirks that skew readings at specific ranges.

Best for: Monitoring noise in a space over time, logging data for a complaint, checking multiple locations within a room.

Limitation: Results vary between phone models. For consistent comparisons, always use the same device and app. The online decibel meter vs app comparison covers which approach works better in different scenarios.

Method 3 — Professional Sound Level Meter (Most Accurate)

A professional sound level meter (SLM) is a dedicated hardware device with a calibrated measurement microphone. These are classified as Type 1 (laboratory grade, ±0.7 dB) or Type 2 (general purpose, ±1.5 dB).

How to use one:

1. Set the weighting. For environmental and occupational noise, select A-weighting (dBA). For low-frequency noise or peak measurements, use C-weighting (dBC). The difference is explained in the dBA vs dBC guide.
2. Set the time weighting. Use Slow (1-second averaging) for steady background noise. Use Fast (125ms) for fluctuating noise. Use Impulse for short, sharp sounds like impacts or bangs.
3. Hold the meter correctly. Position the microphone at the measurement point — typically at ear height, pointed toward the source, arm extended away from your body. Your body can reflect sound and skew the reading.
4. Take the measurement. For continuous noise, record for at least 30 seconds and note the Leq (average). For intermittent noise, record over a representative period.
5. Log your result. Record the level, date, time, distance from source, and weighting used. This is especially important for occupational or legal purposes.

Accuracy: ±0.7 dB (Type 1) or ±1.5 dB (Type 2).

Best for: Workplace noise compliance, legal noise complaints, acoustic engineering, OSHA/NIOSH assessments.

Where to get one: Type 2 meters from brands like Extech, Reed, or Sper Scientific are available for £50–£200 and are suitable for most non-laboratory applications.

Method 4 — Calibrating Your Phone for Better Accuracy

If you want to use your phone but need more accuracy than a default reading provides, calibration against a known reference improves results significantly. This won’t reach Type 2 professional accuracy, but it can get you within ±2 dB for everyday practical purposes.

The calibration process:

1. Place your phone’s microphone next to a Type 2 sound level meter (borrow one from a colleague, hire one, or use a sound calibrator device)
2. Play a known reference sound — a calibration tone or consistent ambient environment
3. Note the difference between your phone’s reading and the reference meter reading
4. Apply this offset to all future readings from your phone

For a full step-by-step walkthrough, the microphone calibration guide covers the process in detail including common calibration mistakes.

How to Position the Microphone

Microphone placement affects your reading significantly, regardless of which method you use:

Distance matters. Sound level drops approximately 6 dB every time you double the distance from the source in a free field. Moving from 1 metre to 2 metres away reduces your reading by about 6 dB. Always note the distance when recording a measurement.

Avoid reflective surfaces. Walls, floors, and hard furniture reflect sound and can increase your reading by 3–6 dB compared to open-air measurements. Measure away from corners and hard surfaces when possible, or note that the environment is reflective.

Face the source. Most microphones have some directional sensitivity. Point the microphone toward the sound source rather than holding the device at an angle.

Keep your body out of the path. Your body absorbs and reflects sound. Hold the device at arm’s length with the microphone facing the source, or place it on a stand if you’re measuring ambient room noise.

Reading Your Measurement: What the Numbers Mean

Once you have a reading, compare it against these reference ranges to understand what it means:

Reading	Typical Environment	Safety
Below 40 dBa	Very quiet room, rural night	No limits — completely safe
40–55 dBa	Library, quiet office	No limits
55–65 dBa	Normal conversation, rainfall	No limits
65–75 dBa	Busy restaurant, TV	No limits
75–85 dBa	City traffic, vacuum cleaner	Safe below 80 dBa indefinitely
85–91 dBa	Lawnmower, heavy traffic	8 hrs max at 85, 2 hrs at 91
91–100 dBa	Motorcycle, power tools	Less than 2 hours
100–110 dBa	Jackhammer, live music	15 minutes max at 100
Above 110 dBa	Loud concert, industrial equipment	Immediate risk — use protection

For the complete NIOSH and OSHA safe exposure tables with exact time limits at every level, the noise exposure time limits guide covers both standards in full.

If you’re concerned that levels in your environment are in the 85–100 dBa range, the hearing damage decibel chart explains exactly how much damage accumulates at each level over time.

Common Measurement Mistakes

Using the wrong weighting. If your meter or app is set to flat (unweighted) dB instead of dBA, your readings won’t match noise regulations and will differ significantly from what the environment actually sounds like to a person. Always use dBA for occupational or environmental assessment.

Measuring too close to the source. Measuring a lawnmower at 30 cm will give you a reading 12+ dB higher than measuring at 1 metre. Standardise your distance and always record it.

Single-point measurements. Noise levels vary across a room. A single reading at one location doesn’t represent the full picture. For a space, take readings at 4–5 locations and use the average.

Ignoring background noise. If your background noise level is within 10 dB of the source you’re trying to measure, it will inflate your reading. Measure background noise separately and subtract it (using the decibel calculator to handle the log math).

Measuring during atypical periods. If you’re trying to document a noise problem, a single measurement during an unusually quiet period proves nothing. Log measurements over a representative time window.

FAQ

What is the most accurate way to measure decibels?

A calibrated Type 1 professional sound level meter in a controlled environment. For everyday use, a well-calibrated smartphone running a reputable app or browser-based meter is accurate to ±2–4 dB — sufficient for most non-professional applications.

Can I measure decibels with my iPhone?

Yes. Both the browser-based online decibel meter (works in Safari) and dedicated apps like NIOSH SLM work on iPhone. The accuracy depends on the iPhone model’s microphone and whether you’ve applied a calibration offset. Newer iPhone models have better microphone frequency response and typically give more accurate readings.

What unit should I use when measuring?

For most noise assessment purposes — workplace, environmental, home — use dBa (A-weighted). This is the unit used in OSHA and NIOSH regulations and reflects how the human ear actually perceives loudness. Use dBC if you’re measuring low-frequency or bass-heavy noise sources.

How do I measure decibels accurately on Android?

Use a measurement app rather than a browser-based meter for better access to the raw microphone input on Android. NIOSH SLM is available for Android through NIOSH’s official download. Set weighting to dBa, disable automatic gain control (AGC) in the app settings if available, and calibrate against a reference before measuring.

What’s the difference between Leq and Lmax?

Leq (equivalent continuous level) is the time-averaged sound level over your measurement period — the most useful single number for assessing sustained exposure. Lmax is the highest level recorded during the measurement period. Both matter: Leq tells you about average exposure risk, Lmax tells you about peak events.