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PHYS16 – Lecture 35
Sound December 3, 2010
“Since light travels faster than sound, is that why some people appear bright until you hear them speak?”
Review on Waves
Question 1
PhET website
Question 2
• If a string suddenly increases its density, a wave propagating along that string will:
A) Increase its speedB) Decrease its frequencyC) Increase its frequencyD) Decrease its wavelength
Question 3
PhET website
PhET website
PhET website
Question 4
• A string of 0.25 kg/m is stretched to a tension of 4 N between 2 plates that are 0.5 m apart. What is the wavelength of the first harmonic in the string?A) 4 mB) 2 mC) 1 mD) 0.5 mE) 0.25 m
Outline for Sound
• Sound– Longitudinal Pressure Waves– Sound Velocity and Reflection– Intensity– Interference and Beats– Doppler– Resonance and Standing waves
Sound Waves
• Sound is a longitudinal pressure wave– Needs medium to propagate– Propagation is parallel to direction of motion
Sound Velocity
• 343 m/s in air at STP• Changes with:– Tension of material– Density
stringon for wave ,Tv We need 3D relationship, ρ instead of μ and (Y or B) instead of T
So why does vsolid>vliquid>vgas?
Example Question
• If temperature of air were to change would that affect the velocity?
• For a sound in water, would temperature affect velocity?
Sound Reflection
• Sound reflects when velocity changes– Tensional changes between solids and fluids– Density changes within solids and fluids
• Reflections behave the Law of Reflection
• Reflections are similar to waves on string
refin
Guitar Demonstration
Standing Waves with Sound
• Need two “ends” where sound reflects• Reflections constructively and destructively
interfere to setup up standing waves
closedor open both 1,2,3...n ,2
n
L
closed oneopen one 1,2,3...n ,12
4
n
L
Main Points - Waves
• Sound– a longitudinal pressure wave that moves through a medium
• Velocity – 343 m/s in air• Reflection – occurs when velocity changes
• Standing waves – waves that constructively interfere within a cavity and appear to “stand” in one spot
3Din except ,Tv