Beginner · 9 min
Echoes and sound travel time
Use the return time of an echo to measure distance.
Sound takes time to travel
Sound is a mechanical wave that needs a medium such as air, water, or a solid. An echo is reflected sound arriving after the original sound. Sound speed depends on the medium and its conditions; use the value stated in a calculation. For a constant speed, total path length equals speed multiplied by travel time.
Worked example
If sound travels at 340 m/s for 0.1 s, it covers 34 m of total path.
Count the outward and return paths
When sound travels to a stationary reflector and returns along the same route, the total path is twice the one-way distance. Therefore distance to the reflector is d = v × t ÷ 2, where t is the full echo time. Dividing by two matters. If a question gives only the one-way travel time, do not divide it by two again.
Worked example
An echo returns after 0.2 s with sound speed 340 m/s. Total path = 340 × 0.2 = 68 m Distance to the reflector = 68 ÷ 2 = 34 m
Draw an echo route
- Draw the path from a sound source to a wall and back. Label both parts of the journey.
- Using sound speed 300 m/s and an echo time of 0.6 s, calculate the wall's distance and check it by reconstructing the total travel time.