If an echo travels from a source to a wall and returns to a receiver beside the source, the recorded time covers both legs. Multiply the speed of sound by the echo time, then halve that distance to find the distance to the wall.
The 2027 SEC G3 Physics K323 syllabus covers using sound reflection to measure distances. This guide contrasts a returning echo with a direct pulse received at a second location.
- Mark the source, reflector and receiver.
- Read exactly when the timer starts and stops.
- Calculate the distance travelled by the pulse.
- Halve only when two equal legs make up the measured path.
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Trace the path recorded by the timer
An echo is a reflected sound. For a source and receiver at essentially the same position, the pulse travels to the reflector and back over equal distances in the simple exam setup.
A directly received sound pulse follows a one-way path. If a question specifies a one-way time, speed multiplied by time already gives the corresponding distance.
Convert milliseconds to seconds by dividing by 1,000 before using a speed stated in metres per second.
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Worked example | An echo arrives after 0.36 seconds
A sound pulse travels at 340 m/s. A receiver next to the source detects the echo from a wall 0.36 s after the pulse is emitted. Find the distance from the source to the wall.
| Quantity | Calculation | Result |
|---|---|---|
| Total path | 340 × 0.36 | 122.4 m |
| Distance to wall | 122.4 ÷ 2 | 61.2 m |
The 122.4 m is the distance from source to wall and back. The wall is 61.2 m away. A check is to divide 61.2 by 340 to get 0.18 s for each leg.
Write ‘round trip’ beside the measured time. That label explains the factor of two in your working.
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Practise with a different timer interval
A receiver 51 m from the source detects the pulse directly. At 340 m/s it arrives after 51 ÷ 340 = 0.15 s. The receiver is on the direct route, so there is no return leg in that reading.
If the source instead receives an echo after 150 ms, the elapsed time is also 0.15 s but the reflector is 340 × 0.15 ÷ 2 = 25.5 m away.
The same time can give a different answer because the questions describe different routes. Sketch the arrows before choosing the operation.
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Frequently asked questions
Which speed should I use?
Use the speed supplied for the medium in the question.
What if source and receiver are far apart?
Use the sum of the outgoing and returning path lengths. They may be unequal, so draw the geometry given.
Can I halve the time instead?
Yes. For equal legs, half the round-trip time is the time for one leg.
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