Section 1 of 5How an Echo Sounder Works
An echo sounder uses ultrasound pulses sent downwards through water.
Pulse
- Pulse A transducer emits a short pulse of ultrasound downwards into the water.
Reflection
- Reflection The pulse is reflected from the seabed or another boundary and returns to a detector.
Timing
- Timing The instrument measures the time between sending the pulse and receiving the echo.
Depth
- Depth The depth is calculated from the speed of sound in water and half of the measured time.
Section 2 of 5Calculating the Distance
Use the same steps to convert the measured time into a distance.
1
Record the total time t between emitting the pulse and receiving the echo.
2
Multiply the speed of sound in the medium by the time to find the total distance travelled.
3
Divide by two, since the sound travels to the boundary and back again.
4
Give the answer in metres, with the speed in metres per second and the time in seconds.
Section 3 of 5Speed of Sound and Uses
The speed of sound depends on the material, so the medium must be known before a distance is calculated.
Speed of Sound and Uses
Air
- Air Sound travels at about 340 m/s in air, which is much slower than in water.
Water
- Water Sound travels at about 1500 m/s in sea water, so echoes return quickly from the seabed.
Depth finding
- Depth finding Ships use echo sounders to measure the depth of water beneath them.
Fish finding
- Fish finding Fishing boats use echoes to locate shoals of fish below the surface.
| Medium | Approximate speed of sound |
|---|---|
| Air | About 340 m/s |
| Sea water | About 1500 m/s |
Section 4 of 5Worked Example
Question
- An echo sounder sends a pulse into the sea.
- The echo returns after 0.80 s.
- The speed of sound in sea water is 1500 m/s.
- Calculate the depth of the sea.
- Total distance travelled = speed × time = 1500 × 0.80 = 1200 m.
- The sound travels down and back up, so depth = total distance ÷ 2.
- Depth = 1200 ÷ 2 = 600 m.
Answer The depth of the sea is 600 m.
Section 5 of 5Exam Checklist & Tips
6.1.12 – Key Points for the Exam
- Ultrasound Sound with a frequency above 20 000 Hz.
- Echo A reflected sound wave detected after the original sound.
- Echo sounding Measuring distance using the time taken for an ultrasound pulse to reflect back.
- Transducer A device that converts one form of energy into another, such as electrical energy into sound.
- Speed of sound The speed at which sound waves travel through a material.
Common Mistakes
- Forgetting to divide the total distance by two, so the depth is overestimated.
- Using the speed of sound in air for measurements taken in water.
- Using milliseconds as if they were seconds without converting the time first.