# Young's fringes with sound waves

##### Demonstration

This kinesthetic experience reinforces understanding of the interference pattern that results from two coherent sources producing circular waves. Compare this with a similar ripple tank experiment.

#### Apparatus and materials

Signal generator, low frequency

Loudspeakers, 2

#### Heath & Safety and Technical notes

This experiment is best done in the open air, with any reflecting buildings behind the loudspeakers. If the demonstration has to be done indoors, it helps to keep the sound as quiet as possible so that the reflected sound is not too obtrusive.

Set up the loudspeakers about 50 cm apart, if possible at shoulder height. Connect them to the output of the signal generator and set the frequency at about 2000 Hz. If you are doing this is indoors, you may need to adjust the frequency to achieve optimal results for the room size and surface reflections. A carpetted room with curtains round the walls (e.g. a stage) would be ideal.

If the central position turns out to be a minimum, reverse the connections to one of the speakers. This will make the central position a maximum.

A video showing the use of an signal generator is freely available at the National STEM Centre eLibrary.

#### Procedure

a Turn on the signal generator. Get the students to walk across the area in front of the loudspeakers and listen for loud and quiet places. It may help them if they block one ear with a finger.

b Get students to stand where the sound is loudest. They should discover they are standing on hyperbolae with the two loudspeakers as foci.

c Now have students move to positions where the sound is a minimum. Cover one loudspeaker with something absorbent (for example, a cushion). Students should notice that the sound immediately gets louder.

#### Teaching notes

1 If students have already seen this effect in a ripple tank, encourage them to visualize where they are ‘in the tank’. This will strongly reinforce understanding of the interference pattern. The hyperbolic nodal lines can be mapped out.

2 In experiment c, it is better not to switch off one of the loudspeakers, as the power to the other speaker might change.

3 It is also possible to move a directional microphone connected to a CRO along a line parallel to the plane of the speakers and so see the maxima and minima on the screen.

4 A variation on the experiment which works well in a classroom: Mount two small speakers facing the same way at opposite ends of a stout piece of timber, about 2 m long. Connect the speakers in phase to the signal generator set at between 200 and 250 Hz. Grip the handle, point the speakers at the audience, and slowly rotate the supporting beam so that the interference pattern is swept across the audience.

You can follow this up using tuning forks. Hold vertically close to the ear and slowly rotate – you will clearly hear the increase and decrease in the sound level, as each prong acts as a source.

5 Sound is a longitudinal wave and this experiment reinforces the fact that longitudinal waves interfere. Interference diagrams normally imply transverse waves.

This template with two sets of semi-circular lines can be used with an OHT to simulate the interference of coherent waves from two point sources.

This experiment was safety-checked in January 2007