Echoes and using sound to measure distance
Physics · Key Stage 3 · The School
What an echo actually is
Sound waves reflect off hard surfaces, the same way light reflects off a mirror. An ECHO is that reflected sound, arriving back at your ear after the original sound has already passed. The bigger the gap in time between the original sound and its echo, the further away the reflecting surface must be.
Turning time into distance
Sound travels at about 340 m/s in air. If an echo takes 2 seconds to return, the sound has travelled there AND back in that time, covering a total distance of 340 × 2 = 680 m. So the reflecting surface is HALF that distance away: 340 m. Forgetting to halve the answer is the single most common mistake in this topic.
Sound waves you cannot hear
ULTRASOUND is sound at a frequency higher than 20,000 Hz — above the range of human hearing, but it reflects and echoes exactly the same way audible sound does. SONAR uses ultrasound pulses to map the seabed or find objects underwater, timing the echo to calculate depth or distance. Medical ultrasound scanners use the same reflecting-echo principle to build up an image of inside the body, entirely without cutting it open.
A ship sends an ultrasound pulse straight down and detects the echo 0.4 seconds later. Sound in water travels at about 1,500 m/s — faster than in air, because water's particles are closer together. Total distance travelled = speed × time = 1,500 × 0.4 = 600 m. That 600 m is the distance THERE AND BACK, so the actual depth of the seabed is half of that: 300 m. Exactly the same method that finds a wall in a room finds the bottom of the ocean — only the numbers change.