Photo: Stuck in Customs (flickr)
Shout “HELLO” between two cliffs and you’ll hear the echoes as the sound waves bounce back and forth. But two things happen as the echoes bounce between the cliffs. First, the echoes get fainter because when the sound hits the cliff, not all of it comes back: some is absorbed by the rock, some bounces off in other directions, and some misses the cliff altogether.
But your voice also gets distorted so that each succeeding echo sounds less like the original sound. Even though your voice contains a wide range of frequencies, what happens to those frequencies depends on the size, position, and material of the objects they come into contact with as they echo between the cliffs.
Every situation is slightly different, but inevitably some frequencies are more likely to be reflected while others simply fade away. So after a few echoes, the sound loses the characteristics of your voice and takes on new characteristics unique to the area you’re standing in.
Real echoes usually fade away before they become that distorted, but you can see how it works with an artificial echo on two recorders. Say “hello” followed by your name into one recorder. Then play it back into the microphone of the other one. Play that recording back into the first recorder and so on back and forth about five to ten times.
Gradually the two words will sound the same, as they lose their original characteristics and conform to the acoustic characteristics of the room and of the two recorders. Incidentally, this is a problem in concert halls, which have to be specially designed to reflect all the different frequencies accurately.