An echo crosses the gap twice
A clap squeezes the air near your hands; that patch squeezes the next; sound travels outward one nudge at a time. The air itself only rocks in place—it does not fly away with the sound.
When the wavefront reaches a wall, the surface vibrates too. Part of the energy enters the wall, and part is handed back to the air as a reflected wave. That reflection is the “second sound” you hear.
The key is that an echo travels there and back. A wall 10 metres away means a 20-metre sound journey.
Too quick a return (a nearby wall) and the echo blends into the original clap, making the room merely sound “full”. Only past a couple dozen metres does the second sound separate.
Clap-and-time ranging
- Face a large wall or cliff and find a spot where the echo is clearly separate.
- Clap rhythmically and adjust until the echo lands exactly halfway between claps—the round-trip time is half your clap interval. Claps 0.4 seconds apart give a 0.2-second echo delay.
- A farther wall usually gives a later echo. Open the equation below if you want to calculate a distance, and remember that sound travels out and back.
Then try: clap facing a wall covered by a thick curtain. What happens to the echo?
Sound speed, materials and limits
More: write the observation as an equation
Open: sound speed depends on temperature
In air, sound speeds up with temperature:
with T in degrees Celsius: about 331 m/s at 0 °C and 349 m/s at 30 °C. A 15-degree error shifts an echo distance by roughly 3%—negligible for play, not for surveying.
Open: hard walls return, soft curtains absorb
The large mismatch between air and a hard wall usually reflects much of the sound; thick soft curtains let fibres grind that energy into heat. At the same distance, a hard wall echoes loudly and a curtain barely whispers back. The model only uses relative “returned fractions”, not invented decibels.
Limits of the model
One pulse and one reflection are tracked—no room reverberation, diffraction, frequency-dependent absorption or directional hearing. The “0.1 s to hear two sounds” figure is an intuitive scale, not a strict human threshold. Sonar and bat navigation push this time-for-distance trade to its limits.