The velocity of sound in water was measured by Daniel Colladon in 1826, using a rather intriguing method. He took two boats out on Lake Geneva and anchored them 13,000 metres apart (about 8 miles). One carried a large bell suspended below the boat in the lake. A lever was used to hit the bell under the water. As the lever moved, it set off a charge of gunpowder from the boat, producing a flash of light. an observer on the second boat timed the interval between seeing the flash and hearing the bell. The sound was detected using a long ear trumpet with the wide end submerged in the water. Science was much more fun then. Students should be able to suggest examples of sound travelling through liquids, such as whale "song" and hearing voices underwater at the swimming pool.
Sound experiments are ideal for project work and for independent research.
Student investigations might include the following:
* Explore how the "singing" note made by touching the edge of a glass with a moistened finger can be altered.
* Use a violin bow to make sound patterns on a thick metal or glass plate.
Cover the plate with a fine dust of chalk and then draw the bow along the edge.
* Design a speaking tube that can carry speech clearly for a fixed distance, say 10 metres.
Here are some activities that may help to get students started.
Opera singers know a bit about sound and how to make it travel. Many opera stars have unpronounceable names and come from distant lands. But not one of them comes from the Moon. This is a conundrum for your students to think through. Space musicians might whip off their helmet on the lunar surface and start to sing. The lack of an atmosphere means that there is nothing to carry the sound. Silent singing. And the singer will expire rapidly for lack of air. You can simulate the problem without any loss of life.
* Tune a small portable radio to a music station and switch up to maximum volume.
* Put the radio inside a bell jar attached to an air pump. The sound can still be heard.
* Pump out the air and there is silence. Some students may be able to detect faint music by pressing an ear to the table near the radio. Sound can travel through solids rather well.
* Let the air back in and you can hear the music again. Ask about the implications for communication between Moon dwellers. Is lip-reading an option? Or charades?
The Australian railway puzzle
Fractious neighbours demonstrate the effect of sound travelling through solids by banging on the party wall to communicate their displeasure. This is particularly popular at night. The railway puzzle is a thought experiment.
Imagine two travellers lost in the outback. They stumble across a railway line. They are naturally anxious to stop the next train and be rescued. Of course, they have no timetable. The next train may arrive in 10 minutes or 10 days. How can they gain the maximum warning of the approach of a train? One traveller says that they should sit by the track and listen for the sound of an approaching train. The other says no, the thing to do is put your ear on the track and listen. Who is right?
The speed of sound in a steel rail is about 15 times faster than in air (5,000 metres per second against 330ms). The person whose ear is on the rail will hear the train first, in good time to avoid being decapitated.
Ask students to think about other examples of sound travelling through solids. These could include sending signals by tapping on pipes, deep bass notes from a nightclub being transmitted through the building or even a road drill.
A spoon stethoscope
You know you have chosen the wrong party when someone says they are going to play the spoons. Seize the initiative and demonstrate the transmission of sound from a spoon. If the room does not empty immediately, this is what your audience will see. For health reasons, this is an experiment for your students to try at home, with their own string.
* Tie the handle of a very large metal spoon to the centre of a piece of string, about one metre long. Hold the free ends of the string just inside your ears, with the spoon hanging freely.
* Rock gently forwards to swing the spoon until it hits a chair or table.
Listen to the sound. The effect is like standing next to a bell as it chimes.
Tuning forks offer lots of opportunities for experiments on sound. The vibrations of the two prongs are usually too rapid to see, but there are ways to make them visible.
* Strike the prongs of a tuning fork on a rubber stopper to set it moving.
Lightly touch the surface of water in a shallow dish. The vibrations of the fork cause showers of water droplets to fly off the surface. If you use a deep dish to contain the spray, you can demonstrate the effect on an overhead projector.
* Stick two identical pieces of aluminium foil to the ends of a large tuning fork. There must be one on each side to maintain the balance of the fork.
Clamp the fork vertically and shine a narrow beam of light on to one of the foil reflectors. Strike the tuning fork and catch the moving reflection with a mirror or card screen. You can produce a ripple-like effect which dies away as the tuning fork loses its energy.
* Use a burning candle to blacken the surface of a white tile. Fix a pin to one prong of a tuning fork. Strike the fork and gently draw the pin over the blackened surface. It will draw out a pattern as it moves.
Energy by Jack Challoner (Dorling Kindersley Eyewitness Guides pound;9.99) Physics in Everyday Life edited by C Sutton (Guild Publishing)Eurekaaargh! by Adam Hart-Davis (Past Times) Ray Oliver teaches science at St Albans Girls' School in Hertfordshire