Spectrum Analyzer Online

How the Spectrum Analyzer works

Spectrum analyzer online: how it works

A waveform shows level against time; a spectrum shows level against frequency. It answers a different set of questions — is there rumble below 40 Hz, is the hiss above 10 kHz, where does the vocal actually sit, why does the mix sound muddy — and it does it in real time, so you can watch the spectrum change as you turn a knob, move a microphone or open a window.

How it works. The browser's analyser node takes a block of samples — the FFT size — and runs a fast Fourier transform on it, which turns the block into a set of frequency bins, each reporting how much energy sits in a narrow slice of the spectrum. The number of bins is half the FFT size, and they are spaced evenly from zero to half the sample rate, so a 4096-point FFT at 48 kHz gives 2048 bins about 11.7 Hz apart. The Resolution line in the table shows that spacing for the current settings.

Choosing the FFT size is a trade between frequency and time. A bigger FFT resolves finer detail — 16384 points separates bins less than 3 Hz apart, enough to see the individual harmonics of a bass note — but each block covers a third of a second, so fast changes blur. A small FFT of 1024 points updates almost instantly but its bins are nearly 50 Hz wide, which lumps all the low bass into a couple of bars. 4096 is a good default for music; drop to 2048 for percussion, rise to 8192 or above for tuning and hum hunting.

Logarithmic or linear. Hearing is logarithmic: every octave doubles the frequency, and the octave from 50 to 100 Hz matters as much to the ear as the one from 5 to 10 kHz. The logarithmic axis gives each octave the same width, so the bass region is spread out and readable and the display looks like an equaliser. The linear axis gives each hertz the same width, which crushes everything below 1 kHz into the left edge but makes evenly spaced harmonics appear evenly spaced — the right view for checking a test tone's distortion products or seeing the comb pattern of a delay.

Smoothing averages each bin with its previous values. None shows every frame raw and flickers with the signal; Medium steadies it for general viewing; Heavy takes a second or two to settle and is best for reading the average balance of a whole track. The peak hold marks, drawn in a paler colour above the bars, remember the highest level each bin has reached and decay slowly, so a short burst is visible for a moment after it has gone.

Range below 0 dB sets the floor of the display. The top is always 0 dB, the loudest a digital signal can be; 96 dB below that is the noise floor of a 16-bit file and shows almost everything. Raise the floor to 48 or 60 dB to zoom in on the loud part of the spectrum and make small differences in the top half easier to see.

The table. The Peak line is the single strongest bin, converted to hertz and to the nearest note name at concert pitch, which is often enough to identify a hum (50 or 60 Hz and its harmonics), a whistle or a feedback frequency. The seven band lines average the energy within the conventional ranges — sub, bass, low mid, mid, high mid, presence and air — so you can compare the balance of two recordings numerically rather than by eye. The table refreshes a few times a second so it stays readable.

Microphone or file. With no file loaded the tool listens to the default microphone with echo cancellation, noise suppression and automatic gain turned off, so the spectrum is the raw input rather than a processed voice call. The output is muted to prevent feedback. Drop a file and it plays through your speakers on a loop instead; the settings can be changed while it runs and the display retunes without restarting.

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