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Science 7–10 · Year 9

Measuring the speed of sound: echoes from a wall and two phones with acoustic stopwatches

Physical sciences — Waves and motion: waves, sound and light (NSW Stage 5 focus area)

Practical, model not builtLow risk

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The idea

Sound travels at a finite, measurable speed in air, about a third of a kilometre each second, which can be found from the time a clap takes to cover a known distance.

What you need

  • two wooden boards or a clapper board to make a sharp clap, 1 set
  • tape measure 50 m, 1
  • stopwatch, 1
  • two smartphones with phyphox (Acoustic Stopwatch), 1 pair
  • a large flat wall with a clear standing point at least 50 m from it
  • thermometer for air temperature, 1

How to do it

  1. Echo method: stand 50.0 m from the wall (measure it). One learner claps at a steady rate, adjusting it until each clap coincides with the echo of the previous one; another times 20 intervals between claps with the stopwatch. Speed = (2 x 50.0 m) x 20 / time. Three trials.
  2. Two-phone method: place the phones 10.00 m apart on the ground, each running the acoustic stopwatch. Clap once near phone A, then once near phone B. Each phone records the interval between the two claps; the difference between the two intervals equals 2 x 10.00 m divided by the speed of sound. Five trials.
  3. Record the air temperature.
  4. Calculate the speed from each method with an uncertainty estimate from the spread of trials, and compare with the tabulated value for the day's temperature.

What you should see

At 20 degrees C the tabulated speed is 343 m/s, rising about 0.6 m/s per degree. Echo method: the clap interval at 50.0 m is 0.292 s, so 20 intervals take about 5.8 s; clapping in time with the echo is the main source of error. Two-phone method: the interval difference at 10.00 m is 58.3 ms, and the phones time each clap from its sound, so the stopwatch reaction-time error of the echo method is avoided. The learner knows it worked when both methods agree within their uncertainty.

What changes

What you change
distance between phones or to the wall (m)
What you measure
time interval (s) and the speed calculated from it (m/s)
What you keep the same
  • same clap source
  • air temperature recorded
  • phones at the same height

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Sound arrives instantly.
  • Sound travels faster on a cold day.
  • Louder sounds travel faster.

Safety card

Low riskLearners carry it out

Hazards

  • outdoor area near traffic or sports

Controls

  • choose a fenced oval or courtyard
  • sun protection outdoors

Note

No hazardous chemicals or naked flames are used. Complete the school's risk assessment for the activity before the lesson; the NSW Department of Education Science safety and compliance page points to CSIS 1.7 (Risk assessment – a pre-requisite for risk control) for how to carry it out.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  2. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-5/faafd3c3df
  3. spark.iop.org/measuring-speed-sound-using-echoes
  4. spark.iop.org/factors-affecting-speed-sound
  5. phyphox.org/experiment/speed-of-sound
  6. phyphox.org/experiment/acoustic-stopwatch

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