Every device you own has a speaker and a microphone. I decided to use them for something useful. The problem worth solving Natural disasters knock out cell towers. WiFi dies at conferences. Underground sensors need to offload data where nothing reaches. Bluetooth pairing is painful and range-limited. LoRa is great but requires hardware you don't have. Sound doesn't care about any of that. Every phone, every laptop, every embedded board with a transducer — they can all talk. And sound propagates through walls. And you don't need a license. So I built SignalHop — a complete acoustic mesh networking stack that turns any device into a peer-to-peer mesh node using nothing but the speakers and microphones they already have. The acoustic modem The core is an FSK (Frequency Shift Keying) modem running at ultrasonic frequencies: 0 → 18,000 Hz (low tone) 1 → 20,000 Hz (high tone) Both are in the ultrasonic range — inaudible to humans, perfectly detectable by any microphone with a 48kHz sample rate.…