Show HN: What if two devices could chat using only sound?

No Wi-Fi. No Bluetooth. No SIM card. No server. Just two ordinary devices, their speakers and microphones, and the air between them.

That was the question behind my holiday project: could a phone and a computer exchange secure text messages using sound alone?

The result is SonicChat, an experimental open-source messenger with a custom acoustic modem written in Rust and a cross-platform Flutter interface.

I tested it physically between an Honor Android phone and a MacBook. They discovered each other acoustically, completed a verified cryptographic pairing, created an encrypted group and exchanged messages in both directions—all through audible sound. Underneath the chat interface, SonicChat includes: Noise XX pairing with a six-digit verification code Ed25519 sender authentication XChaCha20-Poly1305 encryption Error correction, fragmentation and replay protection Carrier sensing with ACKs and bounded retries Encrypted local history and group epoch keys The project was built with extensive help from OpenAI Codex as an engineering partner across Rust, DSP, Flutter, testing, documentation and live hardware debugging. The most interesting lessons came from the physical tests: simulations can look perfect while real microphones, speakers, reflections and missed ACKs quickly bring you back to reality. SonicChat is still an alpha. It is currently audible, slow and half-duplex. Compatibility depends on the hardware and room, and it is not ready for emergency use. But the idea opens some interesting possibilities.

Could nearby devices exchange short messages when cellular networks are overloaded at a concert or stadium? Could adaptive frequencies and near-ultrasonic modes make communication less intrusive? Could this become a useful offline coordination channel—or simply a good platform for learning about DSP, protocols and applied cryptography?

I would love feedback from people working with DSP, mobile audio, Rust or applied cryptography.