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Recipe

Turn an echo time into a distance

Use when: an ultrasonic module reports a round-trip echo time

Goal: the distance to the target in centimetres, corrected for air temperature

Taught in Distance and proximity

Quick recipe

  1. Reject a missing echo
  2. Find the speed of sound for today's air
  3. Halve the round trip
  1. 1

    If the measured time is 0, report "no echo" and stop.

    Example

    pulseIn returned 0 after its 30,000 µs timeout.

    Why: 0 means nothing came back in time, not that the target is touching the sensor.

  2. 2

    c \approx 331.3 + 0.606\,T m/s, with T in °C.

    Example

    At 25 °C: c = 331.3 + 15.15 = 346.5 m/s.

    Why: sound is faster in warm air, and a fixed 20 °C value is off by about 0.17% per degree.

  3. 3

    d = t \times c / 2, with t in seconds and d in metres; multiply by 100 for centimetres.

    Example

    t = 1166 µs: d = 0.001166 \times 346.5 / 2 = 0.2020 m, about 20.2 cm.

    Why: the echo time covers the distance twice, out and back.

Common mistakes

  • Forgetting to halve: every distance comes out twice as far.
  • Mixing units: µs times m/s gives micrometres until you scale it.
  • Treating 0 as a real distance.