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Fast spectrogram analyzer

08/2023

Works on mobile browsers.

Settings

dbRange controls the brightness of the image with 10*(fft.re^2 + fft.im^2)^(0.5/dbRange). This basic formula allows to see more details than the traditional log10-based scaling.

sampleRate controls how AudioContext decodes audio. It should match the sample rate of the audio file. The max computed frequency is sampleRate/2. Lowering the sample rate allows to zoom the spectrogram vertically:

The downside is that the audio file has to be decoded again.

frameWidth controls the number of audio samples per frame. It doesn't have to be a power of 2, but it must be smaller than frameSize, for each frame is zero-padded to frameSize to compute FFT. Usually, frameWidth correponds to 20-200 ms worth of samples, which is 48000/1000*20..48000/1000*200 samples at 48 kHz. Lowering frameWidth increases time resolution at the expense of frequency resolution:

frameSize is the FFT size, must be a power of 2. Increasing it allows to compute more frequencies at smaller step, and if the number of shown frequencies stays the same, the spectrogram image zooms vertically:

Increasing frameSize and numFreqs simultaneously will add finer details to the image, while preserving its overall shape.

numFrames controls how many equally-spaced frames shall be captured and it's also the image width.

numFreqs controls how many frequencies shall be shown and it's also the image height. It's capped by frameSize/2 because that's how many frequencies FFT computes. The reason numFreqs can be made lower is to see the low frequencies at high resolution. In practice, what really matters is the ratio numFreqs/frameSize, so numFreqs stays the same, while frameSize is changed.

showPhase drops the fft.im component to show the phase. This turns DFT into DCT.

How it works

  • Mic capture: getUserMedia + AudioWorkletNode, lossless, up to 48 kHz, although you may try higher sample rates.
  • Audio decoding: AudioContext, up to 384 kHz.
  • Spectrogram: standard FFT optimized for real-only signals, basic JS, no WebWorkers.

Ultrasound

Dodotronic UM250K captures up to 125 kHz audio frequencies.

getUserMedia doesn't support high sample rates, but you can use ffmpeg to record a 250 kHz wav file:

arecord -L
ffmpeg -f alsa -channels 1 -sample_rate 250000 -i hw:CARD=r4,DEV=0 -t 5 mic.wav