248 ANTHEM LABS · 04 · TEMPO FINDER
Point your phone at the music. We’ll tell you where it lands.
Whatever is playing — the clubhouse, the car, the range speaker — hold the phone up and press Listen. In about eight seconds it finds the beat, gives you the tempo, and puts that tempo on the 112 BPM grid every anthem is cut to.
It does not identify songs. It measures them. Nothing is recorded and nothing leaves the phone.
Listen to the room.
Press once. Keep the phone still and out of your pocket. The number settles as it hears more; the bar under it says how sure it is. If the beat is really twice or half as fast, the two buttons beside it swap the octave.
Press Listen. Your browser will ask for the microphone once.
What it measures, and how. The microphone feed is chopped into 2048-sample windows, and each window’s spectrum is compared with the one before it. Where energy jumps, there was an attack. The last eight seconds of those attacks are autocorrelated over 50 to 220 BPM, and the lag that lines up best is the tempo. That is the standard method behind most beat trackers; ours adds nothing clever, which is why it can run on a phone in the browser with no upload. Onset detection by spectral flux (Bello et al., IEEE TSAP 2005); tempo by autocorrelation with a log-normal prior (Ellis, J. New Music Research 2007).
What it cannot do. It cannot name the track. It cannot tell you the tempo of an anthem in the library — the artists have not supplied one, so we do not publish one. And on live room audio it gets the octave wrong often enough that we always show you the half- and double-time alternative rather than pretend. The confidence bar is real: below a third, do not trust the number.
What the number means.
Tour Tempo counted elite swings on 30 fps video and found four canonical timings — 27/9, 24/8, 21/7, 18/6 frames back and down — all near 3:1. A 24/8 swing is 0.80 seconds back and 0.27 down. At 112 BPM one beat is 0.536 seconds, so that swing is exactly one and a half beats back and half a beat down. The grid is a choice made to fit that arithmetic. It is not a laboratory result, and we will not dress it as one.
So when Tempo Finder tells you the room is at 126, it also tells you what a 24/8 swing would occupy at 126 — 1.68 beats back — and which of the four Tour Tempos would sit on one and a half beats there. That is all the number can honestly say.
Novosel & Garrity, Tour Tempo (Doubleday, 2004); Grober & Cholewicki, arXiv physics/0611291 (2006) — a Yale physics preprint, not peer-reviewed. Full notes in the fine print.
Now play one on the grid.
The room told you its tempo. The library is cut to 112. Pick a course and hear what the grid sounds like when it is built in rather than measured after.