Does a classroom noise meter measure real decibels?

It measures decibels of the microphone signal, not of the air. A browser hands a page samples between -1 and +1 and nothing about the microphone's sensitivity, the gain your system applies or where the laptop sits, so sound pressure in dB SPL cannot be worked out from it. What the meter shows instead is exact about the signal: the level in dB full scale, and the big number, which is that level minus the quiet baseline you took, or minus a default of -60 dB full scale until you take one. Doubling the signal adds about 6.02 dB, and the table under the meter shows it with test tones computed by the same code. The same room can read differently on another device, so compare readings on one device in one spot.

How do I set it up for my class?

Press Start listening and allow the microphone when your browser asks. When the room is quiet, press Set quiet baseline: the meter listens for three seconds and takes the energy average as its zero. Then set Amber from and Red from, in dB above that baseline; they start at +10 and +18 and can go anywhere from -20 to +60 dB. Pick a smoothing time from 0.5 to 4 seconds, and press Projector view, or F, to fill the screen. The bands, the smoothing and the baseline are remembered in this browser for next time, and the Start over button at the top of the page forgets them.

Is the classroom recorded?

No. The microphone opens only when you press Start listening. About 30 times a second the page reads the latest samples into one buffer, reduces them to a single power figure and overwrites them with the next frame, all in your browser. There is no recorder in the page and nothing is sent anywhere. Stop stops every microphone track and closes the audio engine, so the browser's microphone indicator goes off. The session trend lives only on the page and is gone when you leave it.

What do green, amber and red mean?

Only what you set. Green is below your amber threshold, amber runs from it up to your red threshold, and red starts there, all measured in dB above your quiet baseline. They are visual targets for the room, not a hearing-safety limit, and the meter makes no judgement about any student. If you type a threshold that is not a number, sits outside -20 to +60 dB, or puts amber at or above red, the meter says which and keeps the last bands that made sense until you fix it.

What if my browser blocks the microphone?

The message under the controls says what happened and how to allow it, usually through the microphone or lock icon in the address bar, and on a Mac through Privacy and Security in System Settings. Meanwhile Preview without a microphone runs a made-up level up and down through your bands, so you can try the projector view and tune the thresholds. The preview is labelled as not the room and is never added to the session.

What does the session trend show?

While the meter is listening it adds up the time spent in each band and draws one bar a minute, each bar the energy average of that minute, coloured by the band it falls in. The last 90 bars are kept and the totals keep counting past that. A gap of more than a second between readings, such as while the tab is hidden, is not counted. Stop keeps the session, Start listening carries on from it, Clear starts afresh, and Copy puts the session or the trend on your clipboard as text.

How do I show it on a projector?

Press Projector view or the F key when you are not typing in a box. The meter asks the browser for full screen and shows the level, the band name, the bar and the trend at a size readable across a room, tinted with the current band's colour. Where full screen is refused, in an embedded frame for example, the meter fills the browser window instead. Escape leaves either one, and Stop stays on screen the whole time.

Classroom Noise Meter

Put the room's noise on the projector as a colour band. Take a quiet baseline while the room is settled, set where amber and red begin in decibels above it, and the meter shows how far the room has moved from that quiet moment, smoothed over a time you choose from half a second to four seconds; the longer it is, the steadier the colour. A browser cannot measure sound pressure, so rather than print a decibel figure it cannot back, this meter says plainly that its numbers are relative to your microphone and your baseline, and it keeps a session trend: one bar a minute, with the time spent in each band. The microphone opens only when you press Start listening, nothing is recorded or uploaded, and Stop releases it at once.

Not listening

Press Start listening and your browser asks for the microphone first. Nothing is recorded or uploaded.

level: not listening
bands: green below +10 dB, amber +10 to +18 dB, red from +18 dB; you set these below
baseline: -60.0 dB full scale, the default until you set one
now: no reading
floor: -100.0 dB full scale, shown for digital silence
session: nothing measured yet
trend: one bar a minute appears here while listening

Level: not listening
Band: none yet
Baseline, default: -60.0 dB full scale
Smoothing

Amber from: dB
Red from: dB
Range: -20 to +60 dB from the baseline

The microphone is closed. Nothing is listened to until you press Start listening.

Sessiontime in each band while listening
nothing measured yet
Trendone bar a minute, energy average, last 90
no minutes measured yet
MicrophoneStop, or access blocked
closed: it opens only when you press Start listening

Accuracy. Relative levels from your device's microphone, not sound pressure in dB SPL: the number depends on the microphone, any automatic gain the browser applies and where the device sits, so it only compares this room with the quiet baseline you set. It is not a hearing-safety measure and not an assessment of any student. Sound is measured in your browser as it arrives, never recorded or uploaded, and Stop releases the microphone at once.

What the numbers mean

The big number is decibels of microphone signal above the quiet baseline you take with Set quiet baseline, so 0 means as quiet as that moment. Every doubling of the signal adds about 6 dB. The bands are yours: set amber and red where they make sense for your room, as dB above the baseline. The meter asks the browser to switch off automatic gain, noise suppression and echo cancellation, and a browser that ignores the request still changes the level. These rows are computed by the same code the meter runs.

Signal	Level	What it shows
Digital silence, every sample 0	-100.0 dB full scale	The floor, never minus infinity
Test tone at 0.1 amplitude	-23.01 dB full scale	Take a baseline here and this reads 0.0 dB
Test tone at 0.2 amplitude	-16.99 dB full scale	+6.02 dB over the 0.1 tone: amplitude doubled
Test tone at full scale	-3.01 dB full scale	A full-scale sine sits 3.01 dB under full scale: its average power is half its peak

Common questions

Does a classroom noise meter measure real decibels?: It measures decibels of the microphone signal, not of the air. A browser hands a page samples between -1 and +1 and nothing about the microphone's sensitivity, the gain your system applies or where the laptop sits, so sound pressure in dB SPL cannot be worked out from it. What the meter shows instead is exact about the signal: the level in dB full scale, and the big number, which is that level minus the quiet baseline you took, or minus a default of -60 dB full scale until you take one. Doubling the signal adds about 6.02 dB, and the table under the meter shows it with test tones computed by the same code. The same room can read differently on another device, so compare readings on one device in one spot.
How do I set it up for my class?: Press Start listening and allow the microphone when your browser asks. When the room is quiet, press Set quiet baseline: the meter listens for three seconds and takes the energy average as its zero. Then set Amber from and Red from, in dB above that baseline; they start at +10 and +18 and can go anywhere from -20 to +60 dB. Pick a smoothing time from 0.5 to 4 seconds, and press Projector view, or F, to fill the screen. The bands, the smoothing and the baseline are remembered in this browser for next time, and the Start over button at the top of the page forgets them.
Is the classroom recorded?: No. The microphone opens only when you press Start listening. About 30 times a second the page reads the latest samples into one buffer, reduces them to a single power figure and overwrites them with the next frame, all in your browser. There is no recorder in the page and nothing is sent anywhere. Stop stops every microphone track and closes the audio engine, so the browser's microphone indicator goes off. The session trend lives only on the page and is gone when you leave it.
What do green, amber and red mean?: Only what you set. Green is below your amber threshold, amber runs from it up to your red threshold, and red starts there, all measured in dB above your quiet baseline. They are visual targets for the room, not a hearing-safety limit, and the meter makes no judgement about any student. If you type a threshold that is not a number, sits outside -20 to +60 dB, or puts amber at or above red, the meter says which and keeps the last bands that made sense until you fix it.
What if my browser blocks the microphone?: The message under the controls says what happened and how to allow it, usually through the microphone or lock icon in the address bar, and on a Mac through Privacy and Security in System Settings. Meanwhile Preview without a microphone runs a made-up level up and down through your bands, so you can try the projector view and tune the thresholds. The preview is labelled as not the room and is never added to the session.
What does the session trend show?: While the meter is listening it adds up the time spent in each band and draws one bar a minute, each bar the energy average of that minute, coloured by the band it falls in. The last 90 bars are kept and the totals keep counting past that. A gap of more than a second between readings, such as while the tab is hidden, is not counted. Stop keeps the session, Start listening carries on from it, Clear starts afresh, and Copy puts the session or the trend on your clipboard as text.
How do I show it on a projector?: Press Projector view or the F key when you are not typing in a box. The meter asks the browser for full screen and shows the level, the band name, the bar and the trend at a size readable across a room, tinted with the current band's colour. Where full screen is refused, in an embedded frame for example, the meter fills the browser window instead. Escape leaves either one, and Stop stays on screen the whole time.

Relative levels from your device's microphone, not sound pressure in dB SPL: the number depends on the microphone, any automatic gain the browser applies and where the device sits, so it only compares this room with the quiet baseline you set. It is not a hearing-safety measure and not an assessment of any student. Sound is measured in your browser as it arrives, never recorded or uploaded, and Stop releases the microphone at once.