A pitch detector listens to a sound and tells you which note it is. That's the whole job. Behind that one sentence sits a little physics, a little history, and one unit — the cent — that confuses almost everyone the first time they see it.
Sound is vibration, and pitch is how fast the vibration repeats. A string wobbling back and forth 440 times per second produces the note we call A. The unit for "times per second" is the hertz, so that note is 440 Hz. Double it to 880 Hz and you get another A, one octave up. Every octave is a doubling. That's why the note names repeat.
A tuner could just show you "437.2 Hz" and leave you to do the math. Nobody wants that. So a pitch detector does two things: it measures the frequency, then compares it against the grid of named notes and reports the nearest one plus the distance to it.
That distance is measured in cents. One semitone — the gap from A to A♯ — is 100 cents. So "A4, −30 ¢" means: closest to A, but almost a third of the way down toward A♭. Trained ears start hearing a note as off somewhere around 5 to 10 cents. Inside ±5 ¢, call it in tune and move on with your life.
Nothing in physics says A must be 440 Hz. It's a standard people agreed on — and only in 1939, at that. Before then, "A" wandered all over Europe. Some historical pitches survive as living traditions:
415 Hz — common for baroque ensembles playing period instruments. Almost exactly a semitone below modern pitch.
440 Hz — the modern standard. Your piano tuner, your guitar tuner, and this site all default to it.
442–443 Hz — many European orchestras tune slightly high. Brighter, allegedly. String players argue about this the way programmers argue about tabs.
This is why decent pitch detectors have an A4 calibration setting. Change the reference and the entire note grid shifts with it. If a recording sounds maddeningly "between notes," it was probably tuned to a different reference — or the tape ran at the wrong speed.
Tuning is the obvious one, and for guitar, ukulele, violin or anything else with strings, it's the fastest route. But a few other jobs come up constantly:
Singing practice. Hold a note, watch the needle. You find out immediately whether you drift flat at the end of a phrase, and by how much. Numbers beat vibes for this — you can't argue with −20 ¢.
Naming a mystery note. A sample, a synth patch, a fragment of melody stuck in your head. Hum it at the mic and get a name for it.
Checking intonation. Fret the twelfth fret and compare against the open string. If the octave comes out sharp, your guitar's intonation needs a setup — a tuner is how you catch it before your ears do.
Transcribing a recording. Feed in a solo voice or instrument file and read the pitch track instead of looping two seconds of audio forty times.
Most real-time detectors, including this one, use autocorrelation: slide the waveform against a delayed copy of itself and find the delay where they line up best. That delay is the period; frequency is one divided by it. It's an old technique because it works. The useful detail for you is the confidence score that comes with it — when correlation is weak, an honest tool reports nothing rather than a guess.
A pitch detector hears one note at a time. Play it a chord and the correlation gets murky — you'll get the strongest component or nothing. Full mixes are worse. Detecting the vocal line inside a produced song is source separation, a genuinely different (and much heavier) problem that no simple tuner solves. If a tool claims to do that instantly in your browser for free, be suspicious of every reading it gives you.
For one voice or one instrument at a time, though? Solved problem. Free, instant, and it never has to leave your browser.