How to use the spectrum analyser
Live from the microphone
- Under Source, choose Microphone and press Start.
- Allow microphone access when your browser asks. The page must be on https for this to work.
- Make a sound. Bars rise at the frequencies present, and the line below reads, for example, “Strongest: 440 Hz · A4 · −20 dBFS”.
- Change FFT size at any time; the display adapts while running. Press Stop to release the microphone.
From a file
- Choose Audio file and drop a file (MP3, M4A, WAV, OGG, FLAC, WebM, or a video’s sound).
- Press Play & analyse live to watch the spectrum as it plays, or Analyse whole file for an averaged result: Strongest frequency, Nearest note, Resolution, a line chart and a table of 32 bands you can download as
spectrum.csv.
What it does and when to use it
A spectrum shows how much of each frequency a sound contains. Use it to find the pitch of a hum (mains hum sits near 50 or 60 Hz), check whether a microphone or speaker handles bass, spot a sharp cut-off in the highs that can point to a low-bitrate MP3 source, or teach how notes and overtones work.
Make a reference tone with the tone generator, see a recording’s shape over time in the waveform viewer, or check only your mic level with the microphone tester.
How it works
The live view uses your browser’s Web Audio analyser, which runs a Fast Fourier Transform on the latest block of samples (the FFT size). The page groups the result into 64 bars on a logarithmic scale from 20 Hz to 20 kHz, each showing the strongest bin in its range.
Analyse whole file decodes the file in a Web Worker on your device, mixes it to mono, and averages Hann-windowed FFTs of up to 2,000 blocks spread across the file. A full-scale sine reads 0 dBFS. The strongest bin above 20 Hz is refined by interpolation and named with A4 = 440 Hz. Resolution = sample rate ÷ FFT size.
Worked examples
From Analyse whole file on test signals at 44.1 kHz:
- A 440 Hz tone with a quieter 1,320 Hz overtone, FFT 4096: Strongest frequency 439.9 Hz, Nearest note A4, Resolution 10.77 Hz per bin. In the band table the 458 Hz band reads −6.1 dBFS and the 1,346 Hz band −20.9 dBFS, matching the two tones.
- A 60 Hz hum, FFT 16384: Strongest 60 Hz, Nearest note B1 (−48 cents), Resolution 2.69 Hz per bin.
- The same hum, FFT 1024: Strongest 61.3 Hz, Resolution 43.07 Hz per bin. The coarse bins blur the low end, which is why low sounds need a large FFT.
Limits and tips
- Microphones and speakers colour the result, so the display shows what your equipment hears, not an absolute measurement.
- Whole-file analysis needs WebCodecs for formats other than WAV and has no ffmpeg fallback; AIFF or WMA must be converted with audio to WAV first.
Frequently asked questions
Is the microphone sound recorded or uploaded?
Which FFT size should I choose?
Can I use it as a tuner?
Why do I see sound when the room is quiet?
How do we know nothing is uploaded? Test it yourself on the privacy proof page.