Speed of Sound Virtual Lab: Echoes and Two Microphones

This speed of sound virtual lab covers the two classic school methods for measuring the speed of sound in air. First, students time an echo from a wall 50 m away, once with a single clap and once by clapping 20 intervals in time with the echo. Then they place two microphones a measured distance apart, read the delay between the two pulses on a dual-trace oscilloscope, and plot Δt against d. The gradient gives v. Realistic random errors show why repeats and graphs beat single readings. Every value below was read from the simulation.

Curriculum links

• Cambridge IGCSE Physics 0625, 3.4 (Sound): a method involving a measurement of distance and time for determining the speed of sound in air.
• AQA GCSE Physics 8463, 4.6.1.2 (Properties of waves): describe a method to measure the speed of sound waves in air.
• AQA A-level Physics 7408, 3.3.1.1 (Progressive waves): v = fλ, here with ultrasound and an oscilloscope as an extension.

Simulic is not affiliated with or endorsed by Cambridge University Press & Assessment or AQA.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"Microphone B is moved from 0.80 m to 1.60 m away from microphone A. What happens to the delay Δt between the two pulses?"

Some students expect no change, because "the speed of sound is fixed".

Method in the simulation

Part A: echo (5 min)

1. Click the Echo tab. Keep Air temperature θ at 20 °C and Distance to wall d at 50 m.
2. Press Clap once and wait for the stopwatch. Then press Clap 20 intervals. Repeat both once more. The table fills itself.

Method	N	T (s)	t = T/N (s)	v = 2d/t (m/s)
One echo	1
Claps in time	20

Part B: two microphones (15 min)

1. Click the Two microphones tab. Keep Air temperature θ at 20 °C and Time base at 1 ms/div.
2. Set Microphone separation d to 0.40 m and press Clap.
3. Leave Cursor 1 at 1.00 div, where pulse A starts. Move Cursor 2 to the start of pulse B (drag it on the screen or use the slider), then press Record.
4. Repeat at 0.80, 1.20, 1.60 and 2.00 m. Read the best-fit line under the table.

d (m)	0.40	0.80	1.20	1.60	2.00
Δt (ms)
v = d/Δt (m/s)

Expected results

• Echo, on a freshly opened page: one echo read 0.37 s (270 m/s), then 20 intervals read 5.77 s (347 m/s). The repeats gave 0.34 s (294 m/s) and 5.65 s (354 m/s). The true round trip is 0.29 s, so a reaction time of 0.1–0.2 s ruins a single echo.
• Two microphones: careful cursor readings gave Δt = 1.14, 2.34, 3.50, 4.68 and 5.84 ms. The single values of v were 351, 342, 343, 342 and 342 m/s (mean 344 m/s).
• Graph: a straight line with gradient 2.935 ms/m and intercept −0.022 ms, so v = 1/gradient = 341 m/s. The model gives 343 m/s at 20 °C.
• At d = 1.00 m on a fresh page, pulse B starts 2.89 ms after pulse A.

Questions for students

1. (Prediction, asked again after the lab) What happens to Δt when d doubles from 0.80 m to 1.60 m?
2. Which variable must stay the same for every reading?
3. At d = 1.00 m, what is Δt?
4. What speed of sound does the gradient of your graph give?
5. Why is timing 20 clap intervals better than timing one echo?

Answers for teachers: (1) It doubles, from 2.34 to 4.68 ms, because Δt = d/v. (2) The air temperature. (3) Accept 2.82–3.00 ms. (4) Accept 333–353 m/s. (5) The reaction-time error at the start and stop is shared between 20 intervals, so it becomes a far smaller fraction of the time measured.

Common misconceptions

• "The echo time is the time to reach the wall." The sound goes there and back, so v = 2d/t, not d/t.
• "Louder sounds travel faster." The speed depends only on the air and its temperature.
• "One careful reading is enough." At 0.40 m, an error of 0.03 ms is 2–3% of Δt, which is why that point gave 351 m/s. Longer distances and a gradient reduce the effect.

Extension

• Temperature: at d = 1.00 m, set Air temperature θ to 0, 20 and 40 °C. Δt was 3.02, 2.92 and 2.82 ms. With Graph on v against θ, the line had a gradient of 0.59 m/s per °C and an intercept of 331 m/s.
• Ultrasound phase: on the Ultrasound phase tab (40 kHz), record the in-phase positions of R2 at 0, 8.5, 17.0 … 86.0 mm. The fit gave λ = 8.59 mm and v = fλ = 343 m/s.

FAQ

Can the class link open with the right settings?

Yes. It opens on the Two microphones tab by default. To be sure, set Method to Two microphones and an oscilloscope in the link's starting values, and pin Air temperature θ at 20 °C and Oscilloscope time base at 1 ms/div.

Will every student get the same numbers?

Nearly. The random errors are seeded: reloading or pressing Reset repeats the same sequence. A different order of claps gives slightly different values, so questions 3 and 4 accept a range.

Why not use a smaller time base?

At 0.5 ms/div readings are more precise, but pulse B leaves the screen beyond about 1.5 m (the simulation warns you). One time base for every distance keeps the method simple.

Related simulations and guides

Air column resonance – measuring the speed of sound Doppler effect – a moving sound source

To measure the reaction time that spoils the single echo, see the reaction time virtual lab. For more topics, see interactive physics lesson ideas.