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Vibrations & Waves & Sound ‘”a periodic motion of the particles of an elastic body or medium in alternately opposite directions from the position of equilibrium when that equilibrium has been disturbed” When things vibrate, they move the air and create sound waves Music, talking, sirens, etc.

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Vibrations & Waves & Sound

‘”a periodic motion of the particles of an elastic body or medium in alternately opposite directions from the position of equilibrium when that equilibrium has been disturbed”

When things vibrate, they move the air and create sound waves

Music, talking, sirens, etc.



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Vibrations & Waves

Wavelength - Distance between peaks

Amplitude - Max Height above resting spot

Frequency - Number of Bounces/second

Period - Time of a complete vibration

Period = 1 / Frequency

Frequency = 1 / Period


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Vibrations & Waves

Water Waves - move stick up and down in a pond

Sound Waves - move air back and forth

Electromagnetic Waves - turn electric and magnetic fields on and off

Remember, changing magnetic field produces electric field and vice versa


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Wave Motion

Consider our stick moving up and down in the water

Create circular waves that move out from the disturbance

Water moves up and down, the wave moves outward away from the disturbance

Waves carry energy of the moving stick

A bug in the water will move and and down


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Wave Speed

Wavelength is distance between peaks

Frequency is number of peaks per second going past a point in the water

Frequency depends on wavelength and how fast the wave moves!!!




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Sound Waves

Molecules in the air vibrate about some average position creating the compressions and rarefactions. We call the frequency of sound the pitch.



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Depends on the material of the vibrating medium

Sound can vibrate water, wood (speaker enclosures, pianos), metal, plastic, etc.

Sound speed in dry air is 330 meters/second at 0o C

Faster in warm air, slower in cold

Water 4 times faster, steel 15 times faster

Speed of Sound


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When a sound wave reflects from a surface we generate an echo

Wave reflection from surfaces depends on the characteristics of the surface

Smooth hard surfaces reflect best

Rough soft surfaces reflect poorly

Energy not reflected is absorbed or transmitted through the material

Wave Reflection


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Think of arrows pointing in the direction of the wave motion echo

We can trace the path of these arrows

Wave Reflection

Angles Equal


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Wave Reflection echo

Acoustics of room design is very interesting. Need some reflections to “liven” the room. Too many reflections and the sound gets mushy. Look in a concert hall or auditorium to see the different sound treatments


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If there is a change in the characteristics of a medium, waves are bent

This occurs because different parts of the wave front travel at different speeds

Think of a marching around a curved track

The inside people have to move more slowly than the outside people to keep the lines straight

Wave Refraction


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Wave Refraction waves are bent


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The combination of reflection and refraction enables imaging waves are bent

Ultrasonic medical imaging

Naval SONAR for detecting submarines

Bats catch mosquitoes

Catch discussion of dolphins on p. 261

Wave Reflection & Refraction


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Objects have “natural” frequencies based on their size and structure

Guitar strings are an example

Timpani heads

Air columns

Natural Frequencies


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Can externally impose a vibration on an object and structure

Guitars and violins and pianos

Set the wood into motion at the frequency of the string

This provides a larger surface to interact with the air

Harp vs. Piano

Forced Vibrations


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When the forced vibration matches a natural frequency we get a “resonance” condition

Think about a swing on a playground

You go high when you pump the swing at its natural vibration frequency

Sympathetic vibrations in tuning forks

Famous Tacoma Narrows bridge collapse

Resonance