スタッドファインダー 音響式間柱検出器

StudFinder is an acoustic wall stud detector that uses your iPhone's microphone to estimate what's behind your walls.

When you knock on a wall, the sound tells a story about what's inside. A hollow section resonates at higher frequencies and decays quickly. A stud absorbs vibration, producing a mid-frequency dull thud. Concrete and solid walls produce low-frequency sounds that linger. This app captures those differences in real-time and turns them into numbers.

HOW THE ACOUSTIC ENGINE WORKS
The built-in microphone records each knock and runs four acoustic measurements instantly:

1. Peak Frequency — An FFT (Fast Fourier Transform) breaks down the knock sound into its frequency components. The app finds where the most energy is concentrated. Hollow areas typically show peaks above 800 Hz. Studs sit between 400-800 Hz. Concrete falls below 400 Hz.

2. Decay Rate — Measures how fast the knock sound fades, in dB per second. The app compares the RMS (root mean square) energy of the first half vs. the second half of each knock. Hollow spaces decay fast because sound bounces around inside. Solid materials sustain vibration longer, so decay is slower.

3. Spectral Centroid — Calculates the "center of gravity" of the entire frequency spectrum. It's a weighted average of all frequency bins by their energy. Think of it as the "brightness" of the sound. Hollow walls sound bright (high centroid). Dense walls sound dark (low centroid).

4. Sound Level — Converts the overall RMS of the knock to decibels. Even with the same knocking force, different wall structures produce different volume levels.

UNDER THE HOOD: DSP DETAILS
All signal processing runs on Apple's Accelerate framework (vDSP) for native performance. PCM audio sampled at 44,100 Hz goes through a Hanning window function, then a 4096-point FFT transforms it to the frequency domain. This gives a frequency resolution of about 10.8 Hz. Magnitude spectrum is computed from the complex FFT output for peak detection and centroid calculation.

CLASSIFICATION ALGORITHM
Each of the three parameters (peak frequency, decay rate, spectral centroid) casts a weighted vote for one of three categories: Hollow, Stud, or Concrete. The category with the highest total score wins. A confidence percentage shows how strongly the evidence points to that result.

HOW TO USE
- Tap the scan button to activate the microphone
- Hold your iPhone close to the wall and knock with your knuckle
- Results appear instantly on screen
- Knock along the wall every 30 cm (12 inches) to map out stud locations

TIPS FOR BEST RESULTS
- Use in a quiet room — background noise reduces accuracy
- Knock with consistent force each time
- Keep the microphone close to the wall surface
- Compare multiple spots on the same wall — relative comparison is the most reliable method

This app is a reference tool. Before any construction or wiring work, verify with a dedicated stud sensor or a professional.