Measuring the speed of sound – echo timing, two microphones on an oscilloscope, ultrasound in phase
PhysicsOscillations & WavesAges 16–17
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Sign in to playA virtual practical that measures the speed of sound in air in three ways: timing an echo from a wall with a stopwatch (one echo, or clapping in time with the echo for many intervals), reading the delay between two microphones on a dual-trace oscilloscope (v = d/Δt), and moving an ultrasound receiver until the two signals are in phase again to find the wavelength (v = λf). Includes a results table, mean and standard deviation, straight-line graphs with a best-fit line, and the effect of air temperature.
Lesson: Speed of sound in air: echo method, two microphones and an oscilloscope, and v = fλ; effect of temperature
What it shows
Sound is a longitudinal wave, and in air at 20 °C it travels at about 343 m/s; its speed rises by about 0.6 m/s for each degree Celsius. This virtual practical measures it three ways. In the echo method, a stopwatch times sound going to a wall and back, so v = 2d/t. With two microphones, an oscilloscope shows the delay Δt as the sound passes from one microphone to the other, so v = d/Δt. With an ultrasound transmitter, moving one receiver by a whole wavelength brings the two signals back in phase, so v = fλ.
How to use
Choose a tab. Echo: set Distance to wall d and press Clap once or Clap N intervals; the table fills itself. Two microphones: set Microphone separation d, press Clap, move Cursor 1 and Cursor 2 to the start of each pulse, then press Record. Ultrasound phase: move R2 position x until the traces line up and press Record in-phase position each time. Change Air temperature θ and use Graph to compare.
Parameters you can change
- Method Echo and stopwatch, Two microphones and an oscilloscope, Two ultrasound receivers in phase
- Air temperature θ -10–40 °C
- Distance to the wall d (echo) 20–200 m
- Number of clap intervals N 5–40
- Microphone separation d 0.2–2 m
- Oscilloscope time base 0.5 ms/div, 1 ms/div, 2 ms/div, 5 ms/div
- Ultrasound transmitter frequency f 38–42 kHz
- Starting position x of R2 from R1 0–120 mm
Questions to explore
- Why does clapping 20 intervals in time with the echo give a better result than timing one echo?
- Why is the graph of Δt against d a straight line, and how do you get v from its gradient?
- How does the wavelength of the ultrasound change when the air gets warmer, if f stays the same?
