Acoustic tools · subterranean & structural void

Sound where radio fails

Two handset tools for collapse and cave work. The FSK modem turns a phone into a slow, echo-tolerant sound modem — callsign, status and position in about 20 seconds, through a slab or a limestone squeeze. The knock locator turns four phones laid on the rubble into a seismic array that triangulates a survivor's taps. Planning aids: the breaching team and the structures specialist decide where to cut.

Incident

Acoustic FSK modem

Send my position by sound

Listen & relay

Band

Each valid frame (CRC-8) lists here; Relay to the map places that callsign's position on the incident map with source acoustic; a MAYDAY / TRAPPED / INJURED frame also raises an escalation for the IC. Paste a frame by hand if you read it off another device:

Knock locator (TDOA)

Sensors & arrivals

Medium Max depth m Timing error ms

Local metres from the array origin (x east, y north, z up = ground surface elevation at each phone). Arrival = the knock's onset on that phone's clock after chirp sync, in ms.

Phone	x	y	z	arrival ms
S1
S2
S3
S4
S5 (opt.)

Array origin lat, lng

Clock sync by chirp

One phone plays a 2–4 kHz chirp; every phone notes the time it heard it. With the array geometry known, each phone's clock offset = heard − leader's time − distance ÷ 343 m/s. Enter the heard times (ms) to compute offsets, which are subtracted from the knock arrivals automatically.

Heard at S1–S5 (ms)

Log to the ICS-214

FSK: Bell-202-style 1200/2200 Hz with a 1700 Hz preamble (air), 450/750 Hz with 600 Hz (rock); 45–55 ms symbols, start/stop framing, CRC-8 per frame; echo- and noise-tolerant Goertzel demodulation. TDOA: grid-seeded Gauss-Newton multilateration on range differences, multi-root aware (a flat four-phone array has mirror solutions — a fifth phone lower in a void resolves depth), GDOP geometry grade. Not engineering advice.