Decibel Shield
Decibel meter › Spectrum analyzer
Sound spectrum analyzer
A live view of which frequencies the sound around you is made of, from 20 Hz to 20 kHz, with the loudest frequency named as you go. A decibel meter gives one number for the whole sound; this shows what that number is made of, which is how you identify a hum, a whine or a rumble.
Press Start and allow the microphone to see the live frequency spectrum
- Peak frequency
- --
- Level
- -- dB
- Range
- 20 Hz – 20 kHz
The spectrum is computed in your browser and thrown away frame by frame. Nothing is recorded and nothing is uploaded. The level is an uncalibrated estimate, roughly within 10 dB; the frequency readout is as accurate as the analysis window, about 12 Hz.
Last updated:
Reading the bands
| Band | Name | What you find there |
|---|---|---|
| 20–60 Hz | Sub-bass | Traffic rumble, machinery, HVAC, the thump through a neighbour’s wall. Felt as much as heard. |
| 60–250 Hz | Bass | The body of music and most engine noise. Room resonances live here. |
| 250 Hz – 2 kHz | Midrange | Speech. Almost all the information in a voice sits in this band. |
| 2–6 kHz | Presence | Where the ear is most sensitive, so alarms and sirens are built here to be hard to ignore. |
| 6–20 kHz | Treble | Sibilance, cymbals, whine from electronics. The first band to fade with age. |
Finding the source of a noise
The frequency of a noise narrows down what can be producing it, which is the practical reason to look at a spectrum at all. A steady peak at 50 or 60 Hz with smaller peaks at multiples of it is mains-related hum. A broad hill in the tens to low hundreds of hertz that rises and falls is usually rotating machinery: a fan, a pump, a compressor. A narrow peak high in the treble that appears when a device is powered is typically a switching supply or a display. Nothing above the floor but a general rumble that comes and goes is traffic.
For a neighbour or workplace complaint, the frequency matters as much as the level, because low-frequency noise travels through structure and is discounted by the A-weighting most limits are written in. That is the gap our noise complaint guide covers, and the dBA against dBC entry explains why a bass-heavy noise can be a genuine problem and still pass an A-weighted test.
What this can and cannot tell you
- Reliable: the frequency of a strong, steady peak, to within the resolution of the analysis window, about 12 Hz.
- Indicative: the overall shape, and which band dominates.
- Not reliable: the exact height of one bar against another. Phone and laptop microphones roll off in the bass, have their own response curve, and often apply processing the browser cannot switch off.
- Not a measurement: the level shown is the same uncalibrated estimate as the meter, within roughly 10 dB, and is not valid for legal or workplace purposes.
The rest of the toolkit
To measure how loud something is rather than what it is made of, the online decibel meter gives the level with A-weighting, a running average and a saved report. To play a known frequency rather than analyse an unknown one, use the tone generator, and to check the microphone itself first, the mic test confirms your input is working before you trust anything on this page.
What does a spectrum analyser show?
It splits the sound reaching your microphone into frequency bands and draws how much energy is in each one, updated many times a second. A sound level meter gives you one number for the whole sound; the spectrum tells you what that number is made of, which is how you tell a low rumble from a high whine when both read the same level.
How do I find the frequency of a hum or a whine?
Start the analyser near the source in an otherwise quiet room and read the peak frequency. A steady tonal noise shows up as a tall narrow bar that stays put, and the readout names it. Mains hum is usually 50 or 60 Hz with harmonics above it, fans and pumps sit in the tens to low hundreds of hertz, and switching electronics often whine between 8 and 16 kHz.
Is this accurate enough to identify a noise problem?
For identifying which band a noise lives in, yes. For a formal measurement, no. Phone and laptop microphones have their own frequency response, usually roll off below 100 Hz, and often apply processing the browser cannot disable, so the shape of the spectrum is indicative rather than calibrated. The frequency of a strong peak is reliable; the relative heights of the bars are not.
Why does the spectrum show something when the room is quiet?
Every microphone has a noise floor of its own, and most rooms have a background of ventilation and distant traffic that you have stopped noticing. What you are seeing is real sound plus the microphone’s own electrical noise. If the bars sit low and flat with no peaks, that is the floor, not a signal.
Does it record what my microphone hears?
No. Each frame of audio is analysed and discarded immediately; nothing is stored and nothing is uploaded. There is no account and no recording feature on this page at all.
What is peak hold for?
It freezes the highest level each band has reached, so a short event leaves a trace you can read after it has passed. That is the way to catch something intermittent, such as a door slam or a compressor kicking in, when you cannot watch the screen at the exact moment it happens.