An echo is the reflection of a sound. You hear it a little while after the sound is made.
You can only hear an echo if it comes back more than 0.1 seconds after the original sound.
Sound travels 34 m in 0.1 seconds, so you only hear echoes from surfaces that are at least 17 m away.
Smooth hard surfaces give the best echoes because they break up the sound waves the least.
Acoustics is the study of how sounds are created, transmitted and received.
The acoustics of a space is how sound is heard and how it echoes around that space, whether it is a room or a large concert hall. Electricity, magnetism and radiation
When concert halls are designed, the idea is not to eliminate echoes altogether but to use them effectively.
A hall with too much echo sounds harsh and unclear, as echoing sounds interfere with new sounds.
A hall without echoes sounds muffled and lifeless.
Even in the best concert halls, the music can be heard fading after the orchestra stops playing. This delay is called the reverberation time. Concert halls typically have a reverberation time of two seconds. A cathedral may reverberate for up to eight seconds, giving a more mellow, but less clear, sound.
Sydney Opera House in Australia is famous for its stunning design, but some orchestras have complained about its acoustics.