Measure sound speed using two microphones
Use a shared timing system to compare the same pulse at two positions and distinguish microphone separation from source distance.
Section P3.4 · Sound
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Use distance divided by elapsed time.
- Recognise a sound pulse on a time trace.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does the difference between two arrival times measure in the aligned model?
What you will learn
- Describe an electronic distance-and-time air sound method.
- Use microphone separation and matching arrival-time difference.
Measure the incremental journey
Place two microphones at measured separation along a sound pulse's propagation path and connect them to a shared time-recording system. Identify the same pulse feature at each microphone. The arrival-time difference corresponds to the extra path between them, not the full distance from the source.
For microphones 1.70 m apart along the path and an arrival difference of 5.0 ms, v = 1.70/0.0050 = 340 m/s. Convert milliseconds to seconds. Choose the matching first direct pulse rather than comparing unrelated peaks or a later echo.
Control alignment and timing
A collinear source and microphone arrangement makes the extra path equal to microphone separation. If the source lies off that line, the difference of source-to-microphone distances may be smaller than the separation. State the geometry rather than using a straight ruler distance automatically.
Shared timing avoids unsynchronised clocks, but microphone and electronic delays can still matter. Repeat at several separations, inspect traces and check consistent results. Reversing channels or analysing distance versus time-difference data can help investigate fixed offsets; averaging alone need not eliminate them.
Sources for this lesson
Pause and explain
What is 5.0 ms in seconds?
Worked example
Two aligned microphones are 1.70 m apart and receive the same pulse 5.0 ms apart. Calculate sound speed.
Show the worked solution
- Convert 5.0 ms to 0.0050 s.
- The extra path is 1.70 m.
- v = 1.70/0.0050 = 340 m/s.
Answer 340 m/s.
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.
Describe the two-microphone method for air sound speed.
Give me a hint
Use measured separation, a shared clock and the same pulse feature.
Compare my reasoning
- Align source and microphones and measure their separation.
- Record matching pulse arrivals with a shared timing system.
- Calculate separation/arrival difference and repeat the measurement.
An aligned measured extra path divided by matching arrival-time difference.
Look for these in your work
- Include distance and timing.
- Identify the same pulse.
For 2.04 m separation and 6.0 ms delay, calculate speed.
Give me a hint
Convert the interval before division.
Compare my reasoning
- 6.0 ms = 0.0060 s.
- v = 2.04/0.0060.
- v = 340 m/s.
340 m/s.
Look for these in your work
- Convert milliseconds.
- Use microphone separation in the aligned model.
Two microphones 2.0 m apart are equally far from an off-line source. Why can separation divided by arrival difference fail?
Give me a hint
Compare the two actual source paths.
Compare my reasoning
- The equal source distances give equal direct arrival times.
- The time difference is ideally zero despite 2.0 m separation.
- The separation is not the extra sound path in this geometry.
The off-line geometry gives zero path difference; use the actual path geometry rather than separation.
Look for these in your work
- Explain the path difference.
- Do not infer infinite physical sound speed.
Common mistakes
- Use matching pulse features.
- Use the extra path, not source-to-first-microphone distance.
- Convert milliseconds and specify alignment.
What can you explain now?
If the source is off the microphone line, is separation automatically the extra travelled distance?
Compare with the explanation
No.
The relevant path difference depends on the source–receiver geometry.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Describe the electronic method and test whether distance, timing, pulse identity and alignment match.
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.