Tina Royney, of Greymouth Junior High School, asks :-

What causes echos?

John Campbell, a physicist at the University of Canterbury, responded.

An echo is a reflected sound.

But what is sound? When you hit a saucepan, for example, the atoms in the metal saucepan vibrate. This vibration causes air molecules next to the pan to vibrate and they in turn cause their neighbouring molecules of air to vibrate. Hence the sound travels to us through the air.

Because it depends on the type of molecules present and on the forces between them, sound travels at different speeds in different materials, for example 330 metres every second in air, 1400 metres every second in water and 5100 metres every second in aluminium.

Whenever a sound wave meets a surface beyond which the sound would travel at a different speed then the sound wave is partially reflected at the surface. If we shout towards a distant cliff then we will hear a reflected shout a short time later. We can use this as a sound based RADAR to determine how far the cliff is from us. In air, sound travels one kilometre in three seconds. Hence count the number of seconds between your shout and the returning echo and allow 500 metres for every three seconds because the sound has to travel to the cliff and back from the cliff. With the use of ultrahigh-frequency sound, this echo technique is used to study the health of unborn babies, to locate shoals of fish and to search for tiny cracks in metal castings.