Turn a spare phone, tablet or laptop into a sound-activated monitor. It listens through the microphone, shows the sound level live and alerts you with a chime, flash, vibration or notification when noise stays above your threshold. Every event is logged with its time, length and peak level. Audio is analyzed on the device and never recorded or sent anywhere.
Last updated:
Idle
Place this device near your baby, press Start monitoring and allow the microphone. Sound never leaves this device.
Level –Threshold –35 dBRoom average –
Monitoring for–
Sound events0
Last event–
Event log
Time
Lasted
Peak
Alert
Delete
No sound events yet. Each time the level stays above the threshold, it is logged here with its length and peak level.
A browser sound alert is a convenience, not a safety device. It cannot detect breathing or movement, may stop if the device sleeps, the tab closes or the battery runs out, and should never replace direct supervision or a dedicated baby monitor. Follow safe-sleep guidance from your pediatrician.
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How to use the Baby Monitor (Sound Alert)
Put the device near your baby, a safe distance from the crib, plugged into a charger, and open this page.
Press Start monitoring and allow the microphone. The first time, it calibrates to the room for 5 seconds.
Check the threshold: talk or make a soft sound near the device and watch the level line cross the orange threshold. Move the slider if needed.
Choose your alerts: chime, flash, vibration or a browser notification, plus how long a sound must last and how long to wait between alerts.
Review the event log in the morning to see when and for how long your baby was awake, and download it as a CSV if you like.
Choosing a threshold
The level meter uses dBFS, where 0 dB is the loudest sound the microphone can record. Typical readings depend heavily on the device and its distance from the crib, so start with Calibrate to room, which measures 5 seconds of quiet and sets the threshold about 15 dB above it. Then fine-tune:
Problem
What to change
Alerts for small rustles or a cough
Raise the threshold (move the slider right) or set "Sound must last" to 3–5 s
Misses quiet fussing
Lower the threshold a few dB or move the device closer
Fan, heater or traffic keeps triggering it
Keep the voice-range filter on and recalibrate with the noise running
White noise machine in the room
Calibrate with it on; the threshold is then set above its steady level
Too many repeat alerts during one cry
Increase the cool-down to 2 or 5 minutes; events are still logged
How detection works
The microphone signal is optionally band-filtered to roughly 250–4000 Hz, the range that carries most of a baby's cry and voice.
Every 100 ms the tool calculates the RMS level of the last 2,048 samples and converts it to decibels: dB = 20 × log10(RMS).
When the level stays above the threshold for the minimum time (short dips under 0.4 s are ignored), an event starts and an alert fires unless the cool-down is active.
The event ends after 3 seconds below the threshold and is saved with its duration and peak level.
Setup tips
Keep the device and its cable well out of the baby's reach, never in the crib.
Plug it in: keeping the screen awake and the microphone on uses battery quickly.
Turn off auto-lock or low-power mode if your phone stops the microphone when the screen turns off.
Run a test before bedtime: press Test alert to check the volume, then make a sound to check detection.
Frequently asked questions
Does this baby monitor send audio to another phone?
No. It works entirely on one device: it listens, detects and alerts on that same device, and nothing is streamed or uploaded. Use it on a device in a room where you will hear or see the alert, for example a laptop in the doorway between rooms, or use the event log to see when your baby woke.
How does it detect crying?
It measures the loudness of the microphone signal ten times per second. When the level stays above your threshold for the chosen time (2 seconds by default), it counts as a sound event and triggers an alert. The optional voice-range filter focuses on the frequencies where a baby's cry is strongest. It reacts to any loud sound in that range, not only crying; it is not an AI cry recognizer.
What do the dB numbers mean?
They are dBFS: decibels relative to the loudest signal the microphone can capture, so 0 dB is the maximum and quieter sounds are more negative. They are not calibrated sound pressure levels like a professional meter, and they differ between devices, which is why the calibrate button sets the threshold relative to your own room.
Why didn't it alert?
Check that the tab is open and in front, the screen is on, the microphone is allowed and the device volume is up. Phones often pause the microphone in background tabs or when the screen locks. Also check the threshold, the minimum length and the cool-down: events during a cool-down are logged but don't alert again.
Will the alert chime set off the monitor again?
No. Detection pauses for about 3.5 seconds after the chime plays so the device doesn't hear its own alert as a new sound.
Is it safe to rely on this instead of a real baby monitor?
No. A browser tab can stop if the battery runs out, the device sleeps or the tab closes, and sound detection cannot tell you about breathing or movement. Treat it as a handy extra, follow safe-sleep guidance and use a dedicated monitor if you need one.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.