Wallrap

Wallrap is an acoustic stud finder. It teaches your iPhone the carpenter's knock test — rap the wall, listen for the pitch shift, know whether you're tapping over a hollow drywall cavity or a solid stud underneath.

THE TECHNIQUE
Walls over a stud sound brighter and shorter. Walls over a hollow cavity sound lower and longer — like a small drum. A trained ear can hear it instantly. Wallrap measures it for you.

HOW IT WORKS
Wallrap listens through your iPhone's microphone, detects each knock by its amplitude envelope, takes a 50 ms FFT snapshot of the sound, and computes two features that separate hollow from solid: the spectral centroid (where the energy is) and the low-band-energy ratio (50–500 Hz vs everything up to 4 kHz). The classifier maps each knock onto a hollow → solid axis and reports HOLLOW, EDGE, or SOLID with a soft confidence ring.

CALIBRATION
Different walls (drywall, plaster, paneling, tile) ring differently. Calibrate once per wall: tap a place you know is hollow drywall, then tap a place you know is a stud or door frame. Wallrap learns your reference and uses it for every later knock. Recalibration takes about 15 seconds and is stored locally on your device.

TAP-AND-TRACE MODE
Hold your phone vertically and slide it slowly across the wall, knocking every couple of inches. Wallrap records the sequence and draws a horizontal strip showing the inferred stud locations — amber bands for solid, slate bands for hollow, purple bands for the edges where it's not sure.

WHAT WALLRAP DOES NOT DO
Wallrap is an estimate, not a precision instrument. It will not find every stud in your wall with sub-centimeter accuracy. It can be fooled by noisy environments, inconsistent knocking, very thick walls, plaster-on-lath, and metal studs. Confirm with a drill pilot hole or a hardware stud finder before driving large fasteners.

PRIVATE BY DESIGN
- No accounts, no ads, no analytics, no tracking, no subscription
- Audio is processed live on device and never recorded or uploaded
- Your calibration profile is stored on this device in UserDefaults
- Microphone permission is requested only when you tap Listen, Trace, or Calibrate; you can revoke it at any time in Settings → Privacy

ACCESSIBILITY
- VoiceOver labels on every interactive control
- Dynamic Type-friendly typography (system SF Pro rounded)
- Variable-intensity Core Haptics on every classified knock (stronger and crisper for solid, softer for hollow)
- Dark mode primary; cool slate base with warm amber accent

REQUIREMENTS
- iOS 17 or later
- iPhone or iPad with a working microphone
- $0.99 one-time purchase, no in-app purchases

Wallrap is a single-purpose utility for the DIYer who'd rather knock once than guess. Calibrate, listen, knock, build.