Interactions of Sound Waves:


    1.    Give examples of forced vibrations.

    2.    Describe natural frequency.

    3.    Describe resonance.




Wave Interactions: Forced Vibration.

Acoustics is the science of sound.  The study of acoustics is not limited to building instruments or a better home theater but can be applied to structural engineering as well.  Has anybody ever asked you to be a "sounding board" for an idea that they have?  What they are really trying to see is if you are elastic to their ideas or not or whether you will give them positive feedback to their input.  This is the concept of forced vibration.  A forced vibration occurs when one object sets the molecules in another moving.  The result is an increase in the amount of sound.  The more elastic the material, the more forced vibration is possible.  A sounding board is one that amplifies sound by adding to the amount of vibrations.  Consider the resonance chamber below.

tuning fork




( Class/sound/u11l4b1.gif)


The tuning fork sets the molecules in the molecules in the box moving which in turn increases the amount of sound waves in the air.  The added constructive interference you interpret as a louder sound.

When a forced vibration, in one substance causes a second substance to vibrate, the sound produced by the first is at the second substance's natural frequency.   The natural frequency of an object is based on the elasticity of a material and is one at which the minimum amount of energy is required to set its molecules into motion.

tuning fork


    ( Class/sound/u11l4b2.gif)




Whether you are talking about forced vibration, natural frequency, or resonance (when one wave sets another object in motion), it is important to know that these three usually act in concert or together.  The guitar string is strummed which causes a forced vibration in the wood.  The forced vibration is at the natural frequency of the wood so that the maximum sound is produced.  The sound waves in the chamber of the guitar causes more of the wood molecules vibrating which is resonance.  Remember these facts.


A forced vibration is caused by another vibrating object


Everything has a natural frequency  


The frequency at which things vibrate at the lowest energy is called the natural frequency


The second vibration caused by the wave energy is called resonance