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