Section 1 of 9Speed of sound: two people
1
Stand two observers about 100 m apart and measure the gap with a trundle wheel.
2
One observer bangs two wooden blocks together above their head.
3
The other starts a stopwatch on seeing the blocks strike and stops it on hearing the sound.
4
Repeat several times and average the times.
5
Calculate v = d ÷ t. Light travels almost instantly, so the delay is effectively the sound travel time.
Section 2 of 9Speed of sound: echo method
1Clap near a wall
- Clap near a wall Stand about 50 m from a wall and clap in time with the returning echo.
2Time many claps
- Time many claps Time 20 claps to reduce reaction-time error.
3Find the distance
- Find the distance Each clap needs a round trip, so total distance = 20 × 2 × 50 m.
4Calculate
- Calculate Divide total distance by total time to find the speed.
Section 3 of 9Water ripples
Ripple tank
- Ripple tank A motor-driven dipper creates waves in a shallow tank.
- A lamp above projects the waves as bright and dark lines onto paper below.
Measurement
- Measurement Use a strobe light to freeze the pattern, measure wavelength, then use the strobe or stopwatch to find frequency.
Calculation
- Calculation Wave speed v = f × λ.
Section 4 of 9Worked example: speed of sound
1
Two observers stand 165 m apart and the measured mean time for the sound is 0.50 s.
2
Write the equation: v = d ÷ t.
3
Substitute: v = 165 ÷ 0.50.
4
Calculate: v = 330 m/s, which agrees with the typical value for air.
5
To improve the result, use a larger distance so the human reaction time is a smaller fraction of the total time.
Section 5 of 9Typical wave speeds
| Wave | Medium | Typical speed |
|---|---|---|
| Sound | Air | About 330 m/s |
| Sound | Water | About 1500 m/s |
| Water ripples | Shallow water | Less than 1 m/s |
| Light and other electromagnetic waves | Air or vacuum | 3.0 × 108 m/s |
Section 6 of 9Sources of error
Improving the experiment
Reaction time
- Reaction time Starting and stopping a stopwatch by hand adds an error of about 0.2 s, which matters more for short times.
Repeats
- Repeats Repeating the timing and taking a mean reduces the effect of random errors.
Wind and temperature
- Wind and temperature Wind and air temperature change the speed of sound, so measurements should be made in calm conditions.
Electronic timing
- Electronic timing Microphones connected to a data logger measure the time interval far more precisely than a person can.
Section 7 of 9Key terms
| Term | Meaning |
|---|---|
| Wave speed | The distance a wave travels each second. |
| Echo | Sound reflected from a hard surface. |
| Trundle wheel | A wheel that clicks once per metre, used to measure long distances. |
| Ripple tank | A shallow tank used to study water waves. |
| Reaction time | The delay between an event happening and a person responding to it. |
| Mean | The sum of repeated measurements divided by the number of measurements. |
Section 8 of 9Syllabus checklist
| Quantity | Symbol | Unit |
|---|---|---|
| Speed | v | metres per second, m/s |
| Distance | d | metres, m |
| Time | t | seconds, s |
| Equation | v = d ÷ t | Typical speed of sound in air about 330 m/s |
Section 9 of 9Exam technique
1
Using one-way distance in the echo method; sound goes to the wall and back.
2
Blaming human reaction time but only doing one timing; averaging and timing many claps reduces the error.
3
Starting the stopwatch on hearing the sound instead of on seeing the blocks.