StretchwiseWhat we've tested, and what we haven't
A tuning device earns trust by being checked, so here is where Stretchwise stands. In short: the measurement and the stretch calculation have been checked against published data, but Stretchwise has not yet been used to tune a real piano in a real room. We're looking for technicians to help with that.
Tested
Stretch curves compared with TuneLab's published sample tunings
TuneLab ships sample tuning files for real pianos (among them a Steinway D, a Mason & Hamlin A and a Petrof). For seven of them, the curve Stretchwise calculates from the same inharmonicity data is within 0.9–1.7 cents rms of the curve TuneLab calculated, with a largest single-note difference of 5 cents.
Readings from recordings of a real piano
We ran the app's own listening pipeline, the same code the app uses, on 18 published University of Iowa recordings of a Steinway from B0 to C8. Every note produced confirmed readings, and each note's median pitch agreed with an independent analysis within 3.5 cents (6.5 cents for A7, where the recording shows split unisons).
Automated tests
Each build runs a suite of automated tests covering the tuning maths, note matching, noise rejection, pitch raise, temperaments, records and sync logic, plus launches on the simulator and an iPhone.
Not tested yet
- Tuning a physical piano, and comparing the result with an aural tuning or another tuning device.
- Behaviour in occupied or noisy rooms in real use.
- External microphones.
- iCloud sync between two devices.
The screenshots on this site were captured with the app's built-in demo piano, not an acoustic piano.
Help test it on real pianos
If you tune pianos and use an iPhone or iPad (iOS/iPadOS 17 or later), you can try Stretchwise through Apple's TestFlight on pianos you're tuning anyway and send a short report after each one. Test builds are free, there's no obligation to buy or recommend the app, and nothing you send is quoted without your permission.