A ship uses sound waves to detect the depth of the ocean floor beneath it. A pulse of sound is sent straight downwards from the ship. The echo is detected 3.0 seconds later. Explain how the properties of sound waves make this depth-finding technique possible, and describe how the crew would use the time measurement to determine the depth of the ocean floor.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sound travels at approximately 1500 m/s in sea water. The technique of using reflected sound pulses to find the depth of water is known as echo sounding.
Model answer (4 marks)
Sound waves are longitudinal waves that can travel through a medium such as sea water, so the pulse can propagate downwards.
The pulse reaches the ocean floor, where it is reflected back; the reflected wave is the echo that the ship detects.
The sound therefore travels the distance to the floor and back again, so the total distance is twice the depth.
Using the speed of sound in water (1500 m s⁻¹) the crew calculate the depth from the time interval:
depth = (speed × time)/2 = 1500 m s⁻¹ × 3.0 s ÷ 2 = 2250 m.
The pulse reaches the ocean floor, where it is reflected back; the reflected wave is the echo that the ship detects.
The sound therefore travels the distance to the floor and back again, so the total distance is twice the depth.
Using the speed of sound in water (1500 m s⁻¹) the crew calculate the depth from the time interval:
depth = (speed × time)/2 = 1500 m s⁻¹ × 3.0 s ÷ 2 = 2250 m.
Examiner tips
- Mention that sound needs a medium to travel.
- State that the echo is the reflected wave.
- Explain the round‑trip distance is twice the depth.
- Show the calculation: depth = speed × half the time.
Common mistakes
- Forgetting that sound requires a medium.
- Using the full time instead of half the time to calculate depth.
- Using the wrong speed of sound for water.
Mark scheme (4 marks)
- Sound waves are longitudinal waves that can travel through a medium (sea water) / sound requires a medium to travel through
- The sound pulse is reflected at the ocean floor / the echo is the reflected sound wave returning to the ship
- The total distance travelled by the sound is twice the depth (the sound travels to the floor and back)
- The depth is calculated using speed = distance / time (or depth = speed × half the recorded time), giving depth = 1500 × 1.5 = 2250 m
Key terms in this question
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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