Piano Tuner: Stretch & Unison

Piano Tuner is a piano tuning app for owners who want to see which keys have drifted between technician visits. It listens through your phone's microphone, measures the string you play and compares it with a stretched target built from your own piano, not a plain chromatic scale.

WHY A PIANO NEEDS A STRETCHED TARGET
Piano strings are stiff, so their overtones sit slightly above whole-number multiples of the fundamental. Tuners widen the octaves to match those overtones. A general chromatic tuner ignores this, so a well-tuned piano can look sharp in the treble and flat in the bass. Piano Tuner measures the inharmonicity (B) of the keys you save and builds a Railsback-style stretch curve from it. The app asks you to save A3, A4 and A5 first; until enough keys are measured, the targets stay at equal temperament.

TUNE TAB
• Choose any key from 1 to 88 with the arrows, or turn on Auto key and let the app recognize the note you play.
• Tap Listen and play the key firmly: a live cents gauge shows how far it is from its target, together with measured Hz, target Hz and the B value.
• A partial ladder shows partials 1–8 and how many cents each one sits above k × fundamental.
• In the lowest bass, where a phone microphone hears the fundamental poorly, the pitch is worked out from the upper partials.
• When the reading holds steady, tap Save key. In manual mode the app then moves on to the next key.

UNISON MODE
Most keys have two or three strings. Mute all but two, play the key, and the app counts how often the sound swells and fades, in beats per second. Turn the second string until the beating slows and the display reports no clear beating.

KEYS TAB
An 88-key strip colors every saved key by its distance from target: within ±3 cents, 3–10 cents, more than 10 cents, or not measured yet. Above it you see how many keys are measured, how many sit within ±3 cents and the average cents off. A list of the most out-of-tune keys takes you straight back to the Tune tab, and you can clear all saved keys when you start a new round.

CURVE TAB
View the stretch curve for the whole keyboard in cents versus equal temperament, with the keys you measured marked on it. Set the A4 pitch reference anywhere from 415 to 466 Hz. Keep up to 12 named pianos, each with its own A4 and saved keys, and rename, switch or delete them.

GUIDE TAB
Short cards explain how to measure, why to mute down to one string, why targets are stretched, how unison mode works, the limits of a phone microphone and what to know before you turn a tuning pin.

GOOD FOR
• Checking an upright or grand before a lesson, recital or recording
• Finding the few keys that need a touch-up between professional tunings
• Evening out unisons that have started to sound sour
• Students and hobbyists learning how stretch and inharmonicity work

QUESTIONS AND LIMITS
Does it replace a piano technician? No. It is a checking and touch-up aid. For a piano that is far out of tune, or before changing string tension a lot, call a technician.
How reliable are the readings? They depend on your microphone, room noise and the instrument. Hold the phone near the soundboard, mute the other strings and confirm by ear.
Is my audio recorded? Microphone sound is analyzed in memory to find the pitch; it is not recorded or saved.
Do I need an account? No account is needed.
Tuning pins need a proper tuning lever. Use small, slow movements, because a string pulled far sharp can break.