Measure sound speed using a synchronised visible event
Describe a distance-and-time method, count the direct path and assess manual reaction error.
Section 3.4 · Sound
Core and Extended; Physics 0625, 2026–2028. Follow the selected spectrum or sound section. Diagrams illustrate stated ideal models and supplied data. Written practice is self-checked.
Before you begin
- Use v = d/t.
- Distinguish direct transmission from an echo.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why can the seen-to-heard delay approximate sound travel time here?
What you will learn
- Describe a method measuring distance and time for air sound speed.
- Explain synchronisation, repetition and reaction-time limitations.
Synchronise the start event
A supervised source makes a visible action and a sound at effectively the same instant. An observer at a measured distance times from seeing the event to hearing its sound. Over an appropriate ground distance, light travel time is negligible compared with sound travel time, so the delay approximates sound's direct journey.
Measure the separation along the stated path, use a clear event and repeat at the same conditions. A larger appropriate separation increases delay relative to human timing error. The source action must be genuinely synchronised; a separate unsynchronised flash introduces an unknown start delay.
Treat timing as evidence
For a measured 102 m separation and mean delay 0.30 s, v = 102/0.30 = 340 m/s. This is a direct one-way measurement, so do not double the distance or halve the answer. Record repeated observations and discuss spread rather than selecting the one nearest an expected value.
A systematic start or stop bias can survive repeated averaging. Background sound, reflected arrivals and reaction differences also matter. A written method describes measurement reasoning; performing it safely and judging real data remain supervised practical work.
Pause and explain
For 102 m and 0.30 s direct travel, what is the calculated speed?
Worked example
A synchronised visible/sound event is 102 m away and its mean delay is 0.30 s. Calculate air sound speed.
Show the worked solution
- Use direct path d = 102 m.
- Use mean sound travel delay t = 0.30 s.
- v = d/t = 340 m/s.
Answer 340 m/s, subject to the method's timing and synchronisation limits.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Outline the visible-event sound speed method.
Give me a hint
Include synchronised source, measured separation, timing and calculation.
Compare my reasoning
- Measure a suitable source–observer distance.
- Time the delay from the synchronised visible action to the sound, repeating observations.
- Use v = distance/mean delay and discuss timing limits.
Measure direct separation and repeated synchronised delay, then calculate d/t.
Look for these in your work
- Include both measurements.
- Explain synchronisation and repetition.
Find the speed from 100 m and a mean delay of 0.30 s.
Give me a hint
Treat the distance as a direct path.
Compare my reasoning
- d = 100 m and t = 0.30 s.
- v = 100/0.30.
- v ≈ 333 m/s.
Approximately 333 m/s.
Look for these in your work
- Use a direct path.
- Report suitable precision.
A source flash always occurs 0.05 s before the sound begins. Can repeated timing alone correct the speed estimate?
Give me a hint
Separate the launch offset from propagation delay.
Compare my reasoning
- The measured delay includes the 0.05 s source offset.
- Repeating reproduces that offset.
- The offset must be established and accounted for, or the source synchronised.
No; averaging alone cannot remove the fixed launch offset.
Look for these in your work
- Name the systematic effect.
- Give a meaningful correction or redesign.
Common mistakes
- The visual and sound event must be synchronised.
- Averaging does not remove every systematic bias.
- The direct path is not an echo round trip.
What can you explain now?
Does averaging repeated times remove a fixed start delay?
Compare with the explanation
No.
A shared systematic offset can remain in every repeated observation.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Describe and evaluate the synchronised visible-event method without confusing direct and reflected paths.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.