Does a classroom noise meter measure real decibels?

The big number is the microphone signal level minus your quiet baseline, not sound pressure in dB SPL. Until you take a baseline, it uses -60 dB full scale as the default. A doubling of signal amplitude adds about 6.02 dB. Compare readings on the same device in the same spot; microphone sensitivity and system gain can change the result.

How do I set it up for my class?

Press Start listening and allow microphone access. When the room is quiet, press Set quiet baseline for a three-second capture. Set Amber from and Red from; the defaults are +10 and +18 dB relative to that baseline, and the allowed range is -20 to +60. Choose a smoothing time from 0.5 to 4 seconds. Longer smoothing makes the display steadier.

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.

How does the traffic-light noise meter work?

Green means below your amber threshold, amber runs from that threshold up to but not including red, and red starts at the red threshold. The colours are visual room targets you choose. Invalid thresholds show a message and leave the last valid bands in use until you fix them.

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.

Can I use the noise meter on a computer or Smartboard?

Open it in a current Chrome, Edge, Firefox or Safari browser on a computer with a microphone. For a Smartboard, show that computer's browser display on the board and use Projector view. Microphone access needs a secure HTTPS page and browser permission; no app installation is needed.

Can I print a classroom noise meter chart?

Set your thresholds, then choose Save printable chart. Open classroom-noise-bands.html and use your browser's Print command to make a paper chart or save it as a PDF. The file lists the three bands and their relative thresholds; it does not measure sound on paper.

Does the meter sound an alarm or include a countdown timer?

No. Changes appear through the colour band and level display. This meter has no audible alarm or countdown timer; the session clock counts measured listening time.

Classroom Noise Meter

Show classroom noise as a traffic-light display with green, amber and red bands you set. Start the microphone, take a quiet baseline and choose your thresholds. Use Projector view for the room, or save a printable chart of your bands.

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

Reading summary, select to copy

Classroom noise meter. Not listening: the microphone is closed. Baseline -60.0 dB full scale, the default: no quiet baseline taken yet. Bands: green below +10 dB, amber +10 to +18 dB, red from +18 dB. Smoothing 1 s. Session nothing measured yet. Relative microphone levels, not dB SPL.

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

Printable noise band chart

Save a one-page reminder of your current green, amber and red thresholds. Open the saved HTML file and use your browser's Print command. It is a static chart, not a live reading.

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. The examples show microphone signal levels, not classroom sound pressure.

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?: The big number is the microphone signal level minus your quiet baseline, not sound pressure in dB SPL. Until you take a baseline, it uses -60 dB full scale as the default. A doubling of signal amplitude adds about 6.02 dB. Compare readings on the same device in the same spot; microphone sensitivity and system gain can change the result.
How do I set it up for my class?: Press Start listening and allow microphone access. When the room is quiet, press Set quiet baseline for a three-second capture. Set Amber from and Red from; the defaults are +10 and +18 dB relative to that baseline, and the allowed range is -20 to +60. Choose a smoothing time from 0.5 to 4 seconds. Longer smoothing makes the display steadier.
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.
How does the traffic-light noise meter work?: Green means below your amber threshold, amber runs from that threshold up to but not including red, and red starts at the red threshold. The colours are visual room targets you choose. Invalid thresholds show a message and leave the last valid bands in use until you fix them.
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.
Can I use the noise meter on a computer or Smartboard?: Open it in a current Chrome, Edge, Firefox or Safari browser on a computer with a microphone. For a Smartboard, show that computer's browser display on the board and use Projector view. Microphone access needs a secure HTTPS page and browser permission; no app installation is needed.
Can I print a classroom noise meter chart?: Set your thresholds, then choose Save printable chart. Open classroom-noise-bands.html and use your browser's Print command to make a paper chart or save it as a PDF. The file lists the three bands and their relative thresholds; it does not measure sound on paper.
Does the meter sound an alarm or include a countdown timer?: No. Changes appear through the colour band and level display. This meter has no audible alarm or countdown timer; the session clock counts measured listening 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.