Lesson focus: A sound-speed investigation is only useful when distance, delayThe time gap between a sound being made and it being heard or detected. and uncertaintyHow far a measurement could be from the true value, such as "25 cm, give or take 1 cm". are measured on a suitable scale.
Calculate sound speedHow fast something moves: the distance travelled each second, measured in metres per second (m/s). from measured distance and time, then evaluate a method using repeats and uncertainty.
Build on earlier learning
Echoes and ultrasoundEcho calculations used a known speed to find distance. This investigation rearranges the same relationship to measure speed.
Practical
Measuring the speed of sound
- Stand at least 100 m from a wall and clap.
- Clap in time with the echo and time 20 claps.
- Speed = 2 × distance ÷ time between claps.
Record: The distance, timings and calculated speed, compared with about 340 m/s.
You will be successful when you can…
- Use speed = distance ÷ time with correct units.
- Explain why electronic timing improves the method.
- Calculate a mean from repeated delays.
- Distinguish random spread from systematic bias.
Words that carry the science
- speed
- Distance travelled per unit time, measured here in metres per second.
- delay
- The time difference between a sound reaching two detectors.
- mean
- The sum of repeated values divided by the number of values.
- uncertainty
- A quantified or described range within which a measured value is reasonably expected to lie.
- random variation
- Unpredictable differences that make repeated readings spread.
- systematic error
- A consistent bias that shifts measurements in the same direction.
Design a timing method that resolves a rapid event
01Choose a measurable delay
Use two microphones separated by a measured distance. A sharp sound reaches microphone 1 and then microphone 2; a data logger measures the small delayThe time gap between a sound being made and it being heard or detected..
Calculate speed = distance ÷ delay. Measure the separation along the sound path and use metres and seconds to obtain m/s.
02Human reaction time is too large for short paths
Sound covers 34 m in about 0.10 s in air. Hand-operated stopwatches have reaction delays comparable with short travel times, creating large uncertaintyHow far a measurement could be from the true value, such as "25 cm, give or take 1 cm"..
Electronic timing and a larger separation produce a delay that can be resolved more precisely. The sound should stay at a safe level; a sharp signal does not need to be dangerously loud.
03Repeat, inspect and report
Repeat readings reveal spread and possible anomalies. Calculate a meanAn average found by adding up all the values and dividing by how many values there are. only after checking whether every result came from the intended event and retaining the raw data.
Compare the result with an accepted value only under stated conditions. Temperature affects sound speedHow fast something moves: the distance travelled each second, measured in metres per second (m/s). in air, so disagreement does not automatically prove carelessness.
Measuring the speed of sound
Two microphones are connected to a fast timer. A clap starts the timer at the first microphone and stops it at the second. Speed = distance ÷ time. Repeat for several distances and plot a graph.
Controls
Readings are illustrative, with a small random timing error. The speed of sound in air is about 340 m/s.
| Distance / m | Time / ms | Speed / m/s |
|---|
Calculate a measured sound speed
Microphones are 5.00 m apart. The mean delay is 0.0147 s. Calculate sound speed.
- Write speed = distance ÷ time.
- Substitute 5.00 m and 0.0147 s.
- Calculate 5.00 ÷ 0.0147 = 340.1.
- Round sensibly to match the measurement precision.
How to make the reasoning visible: The measured speed is approximately 340 m/s.
Process repeated microphone data
The microphones were 6.00 m apart. The delays are illustrative.
| Trial | Delay / s | Calculated speed / m/s | Comment |
|---|---|---|---|
| 1 | 0.0176 | 341 | Consistent |
| 2 | 0.0175 | 343 | Consistent |
| 3 | 0.0177 | 339 | Consistent |
| 4 | 0.0240 | 250 | Possible false trigger |
Calculate the mean delay of the three consistent trials and the resulting speed.
Explain what should be done with trial 4.
Check your understanding
Answer from memory, then check against the suggested answer.
What equation calculates speed?
Why take repeat measurements?
What is a systematic error?
A sound travels 8.0 m between microphones in 0.0235 s. Calculate its speed.
Explain why doubling microphone separation can reduce percentage timing uncertainty if the timer resolution stays fixed.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Students time a sound over 20 m using hand-held stopwatches. Their values for the speed of sound are very spread out.
Explain why their results are so spread out.
(2)Suggest three improvements to the method.
(3)Total for this question: 5 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., name each limitation, state its effect on the result and pair it with a specific improvement.