CHAPTER 17 VIBRATIONS AND SOUND Interference of sound To demonstrate sound interference
1. Set up two speakers as in Fig. 17.1. Since both speakers are connected to the same audio signal generator, they will generate a coherent dual sound source.
2. Each speaker will emit in-phase identical sound waves that will constructively and destructively interfere as they propagate through the room.
3. If a person walks from position 1 to position 2 and back again, they will experience interference of sound. This interference will appear as ‘louder’ (antinodal) and ‘quieter’ (nodal) zones where the amplitude has been added together from the two speakers or has been cancelled out (Fig. 17.2). This is due to the combination of compressions and/or rarefactions. This demonstrates the interference and hence the wave nature of sound.
audio signal generator
speaker
12 FIG. 17.1
compression rarefaction
FIG. 17.2
Interference can also be demonstrated by rotating a ringing tuning fork near your ear (Fig. 17.3). As the fork prongs move apart, the air beyond is compressed (Fig. 17.3b). As the prongs come together, air is expanded and a rarefaction is created (Fig. 17.3c). As you turn the fork, you experience nodal and antinodal areas.
compression a c rarefaction NOTE
Take care not to touch your ear with the fork as it can hurt!
STS compression b d compression rarefaction compression rarefaction
Sound interference can also be used as a form of riot control. Where the antinodal areas of waves from two speakers in phase meet, there is constructive interference, which creates a very loud sound. Moving the speakers allows this sound to be directed at any chosen point in a crowd.
This is essentially a louder and more precise version of the demonstration seen in Fig. 17.2.
FIG. 17.3 Turning a ringing tuning fork can demonstrate interference
