Physics · Sound · Virtual lab
How fast is sound?
Sound is fast, but not instant. In this lab you’ll measure the speed of sound in air using the two methods scientists and students use outdoors, with a real stopwatch that relies on your reaction time.
Learning outcomes
- Describe how a sound from a stationary source changes as you get further away.
- Know how to measure the speed of sound in air.
- Use speed = distance ÷ time.
You will need
- Sound on (headphones are best)
- Your exercise book or a sheet for notes
- A calculator
- Quick reactions!
The key equation
speed (m/s) = distance (m) ÷ time (s)
Light travels at about 300 000 000 m/s, so you see things almost instantly. Sound is nearly a million times slower. That gap is what both methods use.
Part 1 · Warm-up
What happens as you walk away?
An alarm clock is ringing. Move the listener further away and play the sound each time.
How loud it is
Pitch (frequency)
1000 Hz
Watch this number as you move.
Part 2 · Method 1
The one-way method
How it works
- Your partner stands far away with two blocks of wood. The distance is measured with a trundle wheel.
- When they clap the blocks together, you see it almost instantly, so you start the stopwatch.
- When you hear the clap, you stop the stopwatch.
- Repeat 3 times, find the mean time, then calculate the speed.
Your controls
Choose a distance, press Get ready, and watch your partner. They’ll clap at a random moment.
Use the yellow button, or press the space bar, to start and stop the stopwatch.
Results table
Your times go in automatically. You work out the mean and the speed, then press Check.
Part 3 · Method 2
The echo method
How it works
- Stand a measured distance from a large, flat wall.
- Clap the blocks together and start the stopwatch at the same time.
- The sound travels to the wall, reflects, and comes back. Stop the stopwatch when you hear the echo.
- The sound travelled to the wall and back, so the distance it travelled is 2 × the distance to the wall.
Your controls
Press Get ready. You’re the timekeeper standing next to the clapper. Start when you hear (and see) the clap; stop when you hear the echo.
Results table
★ Stretch: the clapping-in-rhythm method
Echo times are short, so reaction time causes big errors. A better trick: clap again every time you hear the echo, so the claps and echoes fall into a steady rhythm. Then time 10 gaps between claps.
- Press Start the rhythm. The claps will count up on screen.
- Start the stopwatch on clap 0; stop it on clap 10.
- Type your time below and calculate the speed.
Time for 10 gaps: – s
Time for one echo (s):
Speed of sound (m/s):
Part 4 · Evaluate
How good were your results?
The speed of sound in air at 20 °C is about 343 m/s. Here’s how close you got:
Part 5 · Checkpoint
Practice questions
Show your working in your book, then type your answer. Give answers to the nearest whole number unless the question says otherwise.
Lab complete
Your lab summary
Conclusion
Write 2–3 sentences: what was your best value for the speed of sound, which method was most accurate, and why?