Classroom noise meter

How to use it in class

1. Open this page on the computer connected to the board or projector and press Start. The browser asks for the microphone; nothing happens with it before that.
2. During the kind of work you want (quiet reading, partner talk), press Set limit from now. The limit goes 6 dB above the level of that moment. Drag the limit line on the bar, or use the arrow keys on it, to fine-tune.
3. Choose Too loud after: how many seconds (1 to 10) the room may be over the limit before the display turns red. 3 seconds ignores a dropped pencil case or a laugh, but not a room that has drifted up.
4. Set a Quiet goal. The quiet time counts up while the room is under the limit; when it reaches the goal a star appears. Going red puts the quiet time back to zero and adds one to Too loud.
5. Press Large view and Full screen so the number and the face fill the projector. Esc or the button leaves full screen.

What the number means

The page reads the microphone in blocks of 4,096 samples (about 85 ms), applies the A-weighting curve and turns the energy into a level. A-weighting is the curve every ordinary sound level meter uses: it follows how much less sensitive our ears are to low and very high pitches. The page builds it from the formula published in IEC 61672-1, the sound level meter standard (Annex E), so a 1,000 Hz tone counts in full, a 100 Hz hum counts 19.1 dB less and a 10,000 Hz hiss 2.5 dB less.

The level is then smoothed with one of the two standard time weightings from the same standard: Fast (time constant 0.125 s) follows speech syllable by syllable, Slow (1 s) is steadier and better for judging a whole room. Slow is also less jumpy on a projector.

Without calibration the number is dBFS(A): decibels below the loudest sine wave the microphone can pass, which reads 0. So a reading of −40 is 40 dB below the microphone's maximum. Every 6 dB up is roughly double the sound pressure; every 10 dB is what most people hear as twice as loud. Where a room lands depends on the device: a typical phone or laptop microphone of the MEMS kind is specified at about −26 dBFS for 94 dB of sound pressure, which would put a 45 dB(A) quiet room near −75 and 70 dB(A) group work near −50, but the computer's input volume and the distance to the children move all of it, which is why the limit is best set from the room itself.

Calibrate if you have a real sound level meter: with a steady sound, type the meter's reading and the page works out the offset. Example: the meter says 62 dB(A) while the page reads −38 dBFS(A); the offset is 100 dB, so from then on the page shows ≈ 62 dB(A) for that sound and ≈ 70 dB(A) for one 8 dB louder. The offset belongs to this device, this microphone and its settings only.

What it cannot do

• Without calibration it cannot tell you decibels of sound pressure. A phone or laptop microphone's sensitivity is unknown to the browser, and automatic gain changes the numbers, so the default scale is relative (dBFS(A)).
• Browsers apply noise suppression, echo cancellation and automatic gain to microphones unless told not to. The page asks for all three off and tells you after Start if the browser kept any of them on; with automatic gain on, a loud room can read lower than it is.
• It is not a certified sound level meter. Do not use it for noise complaints, workplace noise limits or hearing safety.
• Very low sounds read high: below about 50 Hz the 85 ms block is too short to separate the pitch cleanly, so a deep hum can read up to 2 dB more than a real meter would. Most laptop and phone microphones hardly pick up these pitches anyway.
• The soft chime is heard by the microphone too, so it can keep the display red a moment longer.
• It records nothing and stores only the settings. It runs only while this page is open; if the screen locks or the tab is put in the background, the browser may slow or pause it.

Privacy

The sound never leaves this device and is not kept: each block is reduced to one number and overwritten. Your limit, seconds, goal, display choices and calibration offset are saved in this browser's local storage under polyatic-noise-meter-v1; Clear settings deletes them. The page counts an anonymous page view and two fixed words, noise-start and noise-limit; never a level, a time or anything heard.

Questions

Is there a free noise meter for the classroom that needs no app?

Yes, this page. Open it on the computer that drives the board or projector, press Start and allow the microphone. It needs no account and no install, and it records nothing: the sound is measured in the browser and thrown away about 20 times a second.

What limit should I set for a classroom?

Use the room itself rather than a number from the internet: during normal partner talk press Set limit from now, which puts the limit 6 dB above the current level. 6 dB is roughly the step from two people talking to four. Then pick how many seconds the room may be over it (3 is a good start) so a single dropped book does not set it off.

Why does it show dBFS(A) and not decibels?

A browser cannot know how sensitive your microphone is, so the honest number is the level relative to the loudest sound the microphone can pass (0 dBFS), A-weighted like a sound meter. If you have a real sound level meter, type its reading into Calibrate and the page shows ≈ dB(A) from then on, on this device only.

Does the noise meter record the class?

No. Each 85 millisecond slice of sound is reduced to one number and discarded. Nothing is saved, uploaded or kept. Only your settings (limit, seconds, goal, look, calibration) are stored in this browser, and Clear settings deletes them.