Model loudspeaker

Class practical

Students will be surprised how simple it is to make a loudspeaker, and are likely to remember their need for permanent magnets.

Apparatus and materials

For each student group

Iron yokes, 2

Copper wire, thin, insulated, SWG 36, 2.5 m

Scissors

Paper, stiff

Retort stand, boss, and clamp

Signal generator, with high current output

Procedure

a Make a simple loudspeaker with paper and adhesive tape:

• Cut a circle of fairly stiff paper. Cut a 45º wedge out of the circle. Bring the cut edges together to make a shallow cone. Tape those edges together so that the cone will keep its shape.
• Cut a strip of the same paper about 4 cm by 20 cm. Roll the strip up to make a tube about 3 cm diameter, and tape it so that it keeps its shape.
• Place the tube on the point of the cone and fix it there with several strips of tape.
• Wind two dozen or more turns of thin insulated wire (SWG 36) round the tube.

b Prepare the magnets as follows:

• Use two C-cores side by side so that they form a W. (You may tape the central pair of legs together if you like.) Make the paper tube wide enough to fit over that pair of legs.
• Place slab magnets on the inside faces of the two outer legs. Make sure the magnets have thesame poles pointing inward.

c Hang the loudspeaker using thread from a horizontal rod. Position the magnets with their poles outside the lower end of the coil.

d Connect the coil to a low-voltage AC supply. Can you make the coil broadcast a buzz? Try a signal generator instead of the power supply. Vary the frequency; can you hear different notes?

Teaching notes

1 Students could be asked to explain how their loudspeaker works. They could also look at commercial loudspeakers, and identify how the force on a current in a magnetic field produces the motion of the loudspeaker cone.

2 Most loudspeakers have a permanent magnet with a special shape, a sort of 'all round' horse-shoe magnet.

3 The radio drives a rapidly changing current through the coil. The current follows the vibrations of speech and the electromagnetic force follows the current changes, pushing the paper cone. Finally the air in front of the loudspeaker is set into vibration following the cone's motion, and sound waves are transmitted to the listener.
This experiment was safety-checked in April 2006