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Resonance. IDS Physics: Unit 5. Natural Frequency. All oscillators have a natural frequency. If disturbed from equilibrium, this is the frequency at which they will oscillate. example: mass on a spring. Forced Vibrations.

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## Resonance

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**Resonance**IDS Physics: Unit 5**Natural Frequency**• All oscillators have a natural frequency. • If disturbed from equilibrium, this is the frequency at which they will oscillate. • example: mass on a spring**Forced Vibrations**• A tuning fork is not very efficient at transferring it’s energy to the air. • A table can be forced to vibrate at the frequency of the tuning fork and thus better transfer the sound energy to the air.**Forced Vibrations**• Sounding boards are examples of forced vibrations. • All string instruments use a sounding board that is forced to vibrate**An experiment**• Can Mr. B knock the mass off the spring with a piece of paper? • Yes – just add energy at exactly the right times and the oscillations will get bigger and bigger**Resonance**• Resonance occurs when a system is forced to vibrate at a frequency that is the same as the natural frequency of the system.**Resonance**• A swing (pendulum) is an example of resonance • It has a natural frequency • It is forced to vibrate at the same frequency as the natural frequency.**Resonance Examples**• One tuning fork can cause another to vibrate. • Parts of a car rattle at specific speeds. • English Troops cause a footbridge to collapse in 1831**Resonance Examples**• A singer’s voice can break a wine glass • Link: http://www.blazelabs.com/.%5Cpics%5Cglass.mov • Earthquakes cause buildings and structures to collapse**Reflection of Waves**• When a traveling wave encounters a barrier, it can be reflected. • The reflected wave will have the same amplitude and frequency as the original wave.**Standing Waves**• When an incoming and reflected wave meet, interference occurs • At some places, this interference will be destructive • These places where the medium does not move are called nodes**Standing Waves**• At some places, this interference will be constructive • These places where the medium has a maximum amplitude are called anti-nodes**Standing Waves**• A string instrument produces a standing wave when it is plucked or bowed.**Standing Waves**• The length of the string will be equal to ½ of a wavelength**Standing Waves**• The length of the string can be equal to a wavelength**Standing Waves**• The length of the string can be equal to 3/2 of a wavelength**Standing Waves**• For a string that is fixed on both ends, the length of the string must be equal to an integer number of half-wavelengths.**Standing Waves**• What about a column of air? (Like blowing over a coke bottle)**Standing Waves**• For a medium that is fixed on one end, (like a column of air in a bottle) the length of the string must be equal to an odd number of quarter-wavelengths.**Making Music**• Most instruments involve either strings or columns of air. • The speed of sound never changes, so you have to change the wavelength to change the pitch (frequency)

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