Write to [email protected] — a real person reads it, usually answering within two business days.
The app version (Settings → the version line at the bottom), your phone model, and what you expected to happen. In Settings there is Send feedback, which attaches the app version, the platform and the in-app log to the message — it tells you exactly what is being sent before it goes.
Frequency is reliable up to roughly 10 kHz on a calibrated device. Depth is an estimate and is always shown with an error bar. The method, including its limits, is published in the Guide.
The measurement uses the camera's rolling shutter, whose exact row rate differs per device. Pointing at any mains lamp calibrates it in seconds.
Some Android phones do not expose manual exposure control, without which the method cannot work. The app says so plainly rather than returning a number it cannot stand behind.
Purchases are handled by the store, so refunds are requested there: on Android through Google Play (Order history → Report a problem), on iOS through Apple (reportaproblem.apple.com). If a refund is refused and you believe the app is at fault, write to us anyway — we would rather refund you than argue.
What the app does with data is written out in full on the privacy page.