Because sound is a wave, it is reflected by a surface. An echo is when this reflection is sufficiently loud and is quite common in everyday life. It is heard especially in places such as a mountain top or in a canyon because the surface needs to be quite far for our ears to detect the echo. If it is too close, as in within 340m – the distance covered by sound within one second – the echo overlaps with the original sound and cannot be heard. However, if a room is lined by a smooth surface, such as a bathroom or opera house, an echo may be heard. In these cases the delay between the sound and its echo is so close that it merely sounds as if the sound is amplified, thus explaining why singing sounds better when in a bath.
An echo can be very powerful, an example being St Paul’s Cathedral in London, England. This building is built with a giant, hemispherical roof that can amplify a whisper from one corner and replay it audibly at a corner on the opposite side of the room.
It is also very useful as it is used in technologies such as sonar and ultrasound machines to see through objects.
The word “echo” originates from a nymph named Echo from Greek mythology. Echo was instructed by Zeus to distract Hera from noticing him having an affair by chatting to her constantly. Hera caught on to this and cursed Echo to only be able to repeat what others say. After that, Echo could not even say “I love you” to the man she loved, and eventually lived in the mountains where she repeated whatever travellers said in an attempt to speak.
An interesting fact about echoes is the common myth that a duck’s quack does not echo. This is incorrect in that it does in fact echo, but since a quack is soft and fades quickly, the echo is usually too faint to be audible.