Echoes and reflections
Imagine that you are the captain of a ship. It is a large ship carrying many tonnes of cargo and has over a hundred people on board. You have sailed the ship across the ocean and now you can see land on the horizon. You look at the maps and find that you should come in closer to the coast to avoid some undersea ridges of sand. If you got stuck on them it would take a major rescue operation to set you free. However there is another problem. There are large areas of rocks just under the water near the coast. If you struck any of them you may make a hole in the hull and sink the ship. This does not worry you because you can see the cliffs ahead and as long as you stay eight hundred and fifty metres from them you will have a safe passage into port. Everything looks set for an easy sail into the harbour when the weather changes.

As you approach the coastline a thick mist comes down and blocks your view of the cliffs. You can no longer use your eyes to judge distances but you can use your ears. On board the ship is a loud foghorn. It is used to tell other ships that you are nearby. However you can use it another way: to send a sound and listen for an echo. When you set off the foghorn a blast of sound spreads out in all directions. The sound waves that travel out to sea simply warn other ships of your presence but the sound waves which head towards the cliffs may save your ship. The surface of the cliffs is hard and flat. When the sound waves reach the cliffs they are reflected just like waves of light in a mirror. The energy in the reflected sound waves is so strong that the waves make it all the way back to the ship and are powerful enough to be heard. The reflected sound from the cliff is an echo of the fog horn blast.

Although it might be nice to hear the echo and know that land is not far away, you need to use the echo to tell you more about your distance from the cliffs. While you ponder the facts about sound travel you direct the ship's crew to drop anchor to prevent it drifting too near the sands or the rocks.

Sound travels at about 340 metres in a second. If you send out a blast on the foghorn and then hear an echo two seconds later, for example, it means that the sound has travelled 680 metres in going from the ship to the cliff and back again. This means that the distance between the ship and the cliffs must be half that distance - 340 metres.

When you look at your maps again you see that if you can keep 850 metres from the cliff you can make a safe passage into the harbour. You can calculate this distance by finding how long it takes for an echo to sound when the ship is this distance from the cliffs. Remembering that an echo has to travel from its sound source and back again you calculate that the sound needs to travel twice 850 metres. This distance is 1700 metres. As sound travels at 340 metres in a second it will cover the 1700 metres in five seconds. This is the time delay you need between sending out a blast on the foghorn and hearing the echo.

Armed with the knowledge of the time needed between sending a sound and receiving an echo, you instruct the crew to raise the anchor and order the crew in the engine room to make the ship move forward very slowly. You sound the foghorn and wait for the echo. It is eight seconds. This means that you are too far away from the cliffs and in danger of hitting the sandbanks. You steer the ship a little towards the direction from where the echo came then send another blast. This time the gap is only six seconds. You are moving away from the sandbanks and into safer water. After a short while you send out another blast and this time the echo returns in four seconds. This means the ship is too close to the cliffs and is in danger of hitting the rocks. You steer the ship in the opposite direction and sound the foghorn again. This time the echo returns in five seconds. You must stay at this distance from the coast as you approach the harbour.

As you move ahead slowly you keep sounding your foghorn and checking the echo. After what seems to have been an eternity, you see bright lights through the mist. As you get closer you can see the harbour wall and the opening through which you must go. You turn the ship and enter the harbour. When the ship is tied to the quay you can rest. A little knowledge of science and careful observation has saved your ship, cargo and crew.

What happens to a sound wave when it strikes a surface?
It depends on the surface. If the surface is smooth and hard the sound wave is reflected or bounced straight off it. The angle at which the sound wave leaves the surface is the same as the angle at which it met the surface. This means that when a number of sound waves arrive together from the same direction they leave together in the same direction and the quality of the sound hardly changes. If the surface is rough and soft, sound waves reaching it together are reflected in different directions and some of their energy is absorbed by the surface itself. This makes the reflected sound quieter and less clear.

Why do people tend to sing in the bath?
Many bathrooms walls are covered in glazed tiles for easy cleaning. The tiles have hard smooth surfaces, which reflect most of the sound. This reflected sound adds to the sound of your voice and makes it seem more melodious and richer. If you sing in the bathroom then go into a bedroom and sing, you will find that the soft surfaces of the carpet, curtains and bedclothes absorb the sound and your voice sounds less attractive.

Is the sound in a bathroom just reflected once?
No, it is reflected many times. This kind of reflection is called reverberation. A sound wave leaves your mouth, strikes the wall and is reflected towards the ceiling. It strikes the ceiling and is reflected to another wall. It strikes this wall and then hits the ceiling again. At each reflection some of the energy in the sound wave is lost so eventually the sound dies away. It is these reflected waves which accompany the other sound waves you are making that makes your voice seem richer.

Can other rooms reverberate like a bathroom?
Yes, they can. When a room in the home is being decorated the curtains are taken down, the carpet is taken up and the furniture is removed. This leaves behind the hard smooth surfaces of the walls, floor and ceiling which reflect sound and make reverberations. You may also notice that when a classroom is empty you can hear reverberations but when the classroom is full reverberations cannot be heard. This change is due to the soft rough surfaces of the children's clothes preventing the efficient reflection of sound.

How far from a surface do you need to be to hear an echo?
Your ear can only hear two separate sounds if they reach it more than a tenth of a second apart. If the two sounds reach the ear less than a tenth of a second apart you only hear one sound. Now sound travels at about 340 metres in a second so in a tenth of a second it travels about 34 metres. If you want to hear an echo you need to stand at least 17 metres from a hard flat surface. If you do and clap your hands some sound waves will go directly from your hands to your ears, while some will go to the wall and back again and arrive a tenth of a second later. You will hear this reflected sound as an echo.

Can you hear more than one echo?
In some places you can. If there are several cliffs at different distances from you, you may hear reflected sound waves from them all at different times. In a thunderstorm you may hear one loud thunder clap or a roll of thunder. The roll of thunder is made by the reflected sounds from one thunderclap bouncing off the surfaces of the clouds and making a multiple echo.

What is echolocation?
This is locating the position of an object by using echoes. The captain in the story used echoes to locate the position of the cliffs and used this knowledge to steer a safe course into the harbour. Animals such as bats and dolphins use echolocation to find prey and to steer themselves through the air and sea respectively. A bat, for example, locates prey in the following way. It sends out sound waves, which are too highly pitched for us to hear. When the waves strike a moth they are reflected. The bat receives the reflected rays and uses them to work out the distance and the direction of the moth. It sends out more waves and receives more reflections to guide it towards its prey for a meal. If you look at some large moths you will see that they have hairy bodies. These surfaces reflect sound poorly and make it harder for the bat to catch the moth.

Are echoes used to see inside the body?
Yes, they are. The machine that is used for this purpose is an ultrasound scanner. Ultrasound is sound that is so highly pitched that we cannot hear it. However it can be used to pass through the flesh of the body and be reflected from the parts inside. Ultrasound scanners are used to examine unborn babies. The ultrasounds provide a harmless way of building up a picture of the baby on a television screen to check that it is developing normally.