1. Resonance

2. Resonance of Sound waves Strike one fork with a rubber hammer, then cover the fork. Describe what you hear? tuning fork Sound box

3. Resonance Every system has a natural frequency. If the frequency produced by a wave is the same as its natural frequency, the system vibrates the most. This is called resonance. eg. Two forks of the same frequency Tuning radios and TVs Resonance can cause huge damage to buildings and electric systems. Eg Bridge damaged by resonance

4. Sound from two forks Strike both forks with the rubber hammer. Describe what you hear? Which tuning fork has the lower frequency? Why? tuning fork Sound box tuning fork with a piece of metal

5. Beats If two waves of slightly different frequencies pass through each other, the combined wave will have an amplitude which varies between large and small. The variation in amplitude is called beats. If the crest/compression of one wave meets the crest/compression of another wave, constructive inference occurs, resulting in largest amplitude (the sum of both). If the crest/compression of one wave meets the trough/rarefaction of another wave, destructive inference occurs, resulting in smallest amplitude (the difference between the two).

6. Frequency of beats The frequency of variation in the amplitude is the difference between two frequencies. Beat frequency = | f 1 – f 2 | For sound waves, beats are heard as a continuation of a wobble of loud and soft sound.

7. Exercises A tuning fork has a frequency of 300.0Hz. It is sounded together with another one to give a beat period of 0.250 s. What is the frequency of another fork? ans: f 1 = 300.0Hz, beat period=0.250s, f 2 =? beat frequency = 1/beat period =1/0.250 = 4.0 Hz f 2 =f1±4 = 300 ± 4 =296, 304 Hz

9. Beats Beats Animation