Section 1 of 7Wave speed equations
| Equation | Symbols | Units |
|---|---|---|
| v = d ÷ t | v = speed, d = distance, t = time | m/s, m, s |
| v = f × λ | f = frequency, λ = wavelength | m/s, Hz, m |
| T = 1 ÷ f | T = period | s, Hz |
Section 2 of 7Speed of sound: two people
1
Two people stand about 100 m apart; measure the distance with a trundle wheel.
2
One person bangs two wooden blocks together above their head.
3
The other starts a stopwatch on seeing the blocks meet and stops it on hearing the bang.
4
Repeat several times and calculate the mean time.
5
Calculate speed = distance ÷ mean time, which should be about 340 m/s in air.
Section 3 of 7Speed of sound: using echoes
1
Stand about 50 m from a large wall, with the distance measured using a trundle wheel.
2
Clap two blocks and listen for the echo, then clap again in time with each echo.
3
A helper times 20 claps from the first to the last.
4
Each clap-to-echo trip covers 2 × 50 m, so the total distance is 20 × 2 × 50 m.
5
Repeat, average the time, and calculate speed = total distance ÷ time. Timing many claps reduces the percentage error from reaction time.
Section 4 of 7Other methods
Oscilloscope
- Oscilloscope Place two microphones about 5 m apart and connect them to an oscilloscope.
- A clap at the first microphone starts the trace; the time delay between the two signals gives the time,
- and speed = distance between microphones ÷ time difference.
Ripples on water
- Ripples on water Over a calm water surface, mark out a measured distance, make a ripple, and time how long the first ripple takes to travel it.
- Repeat about ten times and average the time, then use v = d ÷ t.
Ripple tank
- Ripple tank Measure frequency from the vibrator or by counting waves per second, and wavelength from the pattern, then use v = f × λ.
Section 5 of 7Improving accuracy
1Long distances
- Long distances A longer distance gives a larger time, so human reaction time is a smaller fraction of the reading.
2Repeats
- Repeats Averaging reduces random error and highlights anomalies.
3Electronic timing
- Electronic timing Microphones with a data logger remove human reaction time.
4Conditions
- Conditions Wind affects the speed of sound outdoors, so repeat in opposite directions and take the mean.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Wave speed | The distance a wave travels each second. |
| Echo | A reflected sound heard after the original sound. |
| Oscilloscope | A device that displays electrical signals, such as microphone outputs, against time. |
| Trundle wheel | A wheel used for measuring long distances on the ground. |
| Random error | An unpredictable variation in measurements. |
Section 7 of 7Exam technique
1
Using the distance to the wall as the total distance in the echo method: the sound travels there and back.
2
Forgetting that timing 20 claps reduces error compared with timing one.
3
Mixing up frequency and wavelength in v = f × λ or using cm instead of m without converting.
4
Assuming the speed of sound depends on loudness: it depends on the medium and temperature.