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.
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
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
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
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!!!
Sound Waves Molecules in the air vibrate about some average position creating the compressions and rarefactions. We call the frequency of sound the pitch.
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
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
Think of arrows pointing in the direction of the wave motion We can trace the path of these arrows Wave Reflection Angles Equal
Wave Reflection 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
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
The combination of reflection and refraction enables imaging Ultrasonic medical imaging Naval SONAR for detecting submarines Bats catch mosquitoes Catch discussion of dolphins on p. 261 Wave Reflection & Refraction
Objects have “natural” frequencies based on their size and structure Guitar strings are an example Timpani heads Air columns Natural Frequencies
Can externally impose a vibration on an object 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
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