Spark image

Reflection of sound

If you clap your hands in a street surrounded by high buildings you will hear and echo
Shouting or clapping in a canyon also gives a echo
Medieval music played in churches was usually slow to stop the notes being confused with echoes from the walls
An echo sounder is used to detect the seabed and underwater objects like shoals of fish and wrecks
Ultra sonic scans are used to check on unborn babies

These effects can all be explained because sound can be reflected just like light although of course sound is a longitudinal wave.

It obeys the same laws of reflection but the amount of sound and light reflected from different materials is different. The walls of rooms used to test sounds are often covered with soft material or even projections like egg boxes to reduce the sound reflection.

It is important to have enough sound reflection but not to much in concert halls. Too little and the building is described as "dead" or "dry", too much and the echoes from the walls interfere with the original music. The more people in a room the "deader" it will be as regards the sound because the clothed human body does not reflect sound very well.

You can investigate the laws of reflection of sound by the following simple experiment.
Mount a piece of hardboard vertically and lay two cardboard tubes on the bench in front of it. 10cm diameter tubes that have been used for carpets are ideal. Put a clockwork watch or clockwork kitchen timer in one tube and put your ear against the open end of the other tube. When the angles between the two tubes and the hardboard are the same you should hear a loud ticking proving that the angle of incidence = angle of reflection for sound.


© Keith Gibbs 2020