I Built a Binaural Beat Generator — Then Proved It With a Live FFT Spectrum Analyzer

The "frequency healing" corner of the internet runs on faith. Apps ship MP3s labeled "40Hz gamma" and ask you to believe it. I'm a life scientist who builds web tools, and I couldn't ship that. So I built SereneSynth, a browser-based binaural beat and noise generator — and then I built a live spectrum analyzer into the page so anyone can audit the output in their own browser. This is the engineering write-up: the Web Audio graph, the FFT gotcha that almost made me publish wrong numbers, and how I cross-verified everything in Audacity. The honesty constraint first A binaural beat is not a tone in the air. Play 200 Hz into the left ear and 240 Hz into the right, and the listener's superior olivary complex computes the 40 Hz difference. A microphone — or a mono spectrum analyzer — will never show a 40 Hz peak. So the only honest thing a generator can prove is its carriers and its spectral slope. That is exactly what we measure. The synthesis graph Two sine oscillators, hard-panned with StereoPannerNode, summed into a master GainNode, tapped by an AnalyserNode before the destination — the analyzer observes exactly what the headphones receive. Settings that matter: fftSize 16384, smoothingTimeConstant 0.8. The FFT gotcha that almost made me ship garbage My first version used fftSize 1024: one fat bump near 220 Hz instead of two peaks. Bin width = 44100 / 1024 ≈ 43 Hz, and my carriers are 40 Hz apart — same bin, merged. At fftSize 16384 the bin width drops to ≈ 2.7 Hz and the carriers resolve as razor-sharp spikes at 200.0 and 240.0 Hz. Lesson: FFT size is the magnifying glass. If a "frequency proof" doesn't state its FFT size, ask. The widget renders a log axis (20–1000 Hz) because a linear axis wastes 90% of the canvas, and peak detection labels the top bins in the 100–500 Hz range live. Bit-exact, downloadable verification The page also renders 10-second stereo WAVs via OfflineAudioContext (16-bit PCM, 44.1 kHz): same graph, offline render, RIFF encode. No lossy compre