1. 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?”

2. Review on Waves

3. Question 1 PhET website

4. Question 2 If a string suddenly increases its density, a wave propagating along that string will: A) Increase its speed B) Decrease its frequency C) Increase its frequency D) Decrease its wavelength

5. Question 3 PhET website

8. 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 m B)2 m C)1 m D)0.5 m E)0.25 m

9. Outline for Sound Sound – Longitudinal Pressure Waves – Sound Velocity and Reflection – Intensity – Interference and Beats – Doppler – Resonance and Standing waves

10. Sound Waves Sound is a longitudinal pressure wave – Needs medium to propagate – Propagation is parallel to direction of motion

11. Sound Velocity 343 m/s in air at STP Changes with: – Tension of material – Density We need 3D relationship, ρ instead of μ and (Y or B) instead of T So why does v solid> v liquid >v gas ?

12. Example Question If temperature of air were to change would that affect the velocity? For a sound in water, would temperature affect velocity?

13. 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

14. Guitar Demonstration

15. Standing Waves with Sound Need two “ends” where sound reflects Reflections constructively and destructively interfere to setup up standing waves

16. 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