Sound Level Meter

A sound level meter answers one question: how loud is it here? Noise at work, a neighbour’s party, a quiet bedroom, a PA at a concert. This page uses the microphone of your phone or laptop and shows the level in decibels, as numbers and as a trace of the last 18 seconds. It is a good tool for comparing and tracking, and for getting an idea of a situation, though it is not a certified instrument.

How the level is calculated

The browser delivers the microphone signal as samples between -1 and 1. The meter takes the root-mean-square of a short block, which measures the energy, converts it to decibels relative to full scale (dBFS), and adds a calibration offset to shift the scale toward sound pressure. When A-weighting is chosen, the signal is filtered first with the standard weighting curve, which reduces low frequencies and very high ones, as the ear is less sensitive to them. The page updates ten times a second and tracks the lowest, highest and average level (Leq, the energy average) over the most recent ten minutes. The peak figure is the largest single sample in the latest block.

A worked example

Press Sample to see the display filled with fixed demonstration values without using the microphone: 62.4 dB, a peak of 78.1 dB, a lowest of 41.2, a highest of 74.8 and an average of 61.7. For a real measurement, a 110 Hz sine at half scale reads 67.2 dB(A) with the default offset of 94 dB. The arithmetic is: half scale gives an RMS of 0.354, which is -9.0 dBFS; adding 94 dB gives 85.0 dB; the A-weighting curve is -17.85 dB at 110 Hz, giving 67.2. The peak reading is 88.0 dB, from the half-scale sample of -6.0 dBFS plus 94. The same tone shown with flat weighting reads 85.0 dB(Z).

Calibration

The offset is the key setting. Place a reference meter next to the phone, make a steady noise such as a fan or pink noise from the White Noise Generator, type the reference reading into the field and press the button that fits the offset to the current reading. Phones and laptops differ widely, and automatic gain control in the browser or the operating system can also change levels, so a calibration holds only for that device and that setup. The offset is saved in your browser; it is just a number.

Reading the display

The big number is the current level with the peak beside it, and the lowest, highest and average beneath. The trace shows the last 18 seconds, which makes it easy to see a pattern, such as a compressor switching on. The octave-band bars show where the energy is: low bars in a loud room suggest rumble or bass, and high bars hiss. Bands are shown relative to the same scale, so they help to identify the source more than to measure it.

Edge cases

Wind noise and touching the microphone give false peaks, so hold the device steady and still. Very quiet levels fall into the noise floor of the microphone, and the readings below 30 to 40 dB are not meaningful. Loud sounds can clip the microphone before the scale ends, so the peak value can be too low. The meter clamps at 0 dB so the display does not go negative.

Limits

It is not a certified meter and should not be used for legal, workplace or health-and-safety decisions, which need a calibrated Class 1 or 2 instrument. Time weighting (fast, slow) is not offered as a switch. Microphone processing in some phones cannot always be turned off. The browser needs your permission to use the microphone, and the audio stays on your device.

Related tools

• Mic Test checks that the microphone works and shows its level.
• White Noise Generator provides a steady noise for calibration.
• Audio LUFS Meter measures loudness of a file instead of a room.
• Tone Generator makes a steady tone to test the meter.