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PHYS16 – Lecture 40

PHYS16 – Lecture 40. Ch. 17 Sound. Ch. 17 Sound. Sound Longitudinal Pressure Waves Sound Velocity Sound Reflection and Standing Waves Intensity Beats Doppler. Sound Waves = Longitudinal Wave. Sound is a longitudinal pressure wave Needs medium to propagate

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PHYS16 – Lecture 40

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  1. PHYS16 – Lecture 40 Ch. 17 Sound

  2. Ch. 17 Sound • Sound • Longitudinal Pressure Waves • Sound Velocity • Sound Reflection and Standing Waves • Intensity • Beats • Doppler

  3. Sound Waves = Longitudinal Wave • Sound is a longitudinal pressure wave • Needs medium to propagate • Propagation is parallel to direction of motion

  4. Ruben’s Tube Demo http://en.wikipedia.org/wiki/Rubens'_tube

  5. 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

  6. Discussion: Sound Velocity • If density affects the velocity of sound then why does vsolid>vliquid>vgas? • If temperature of air were to change would that affect the velocity of sound? • For a sound in water, would temperature affect the velocity of sound?

  7. Sound Reflection • Sound reflects when medium 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

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

  9. Standing Waves with Sound

  10. Sound Intensity • Intensity is the power per unit area in a wave http://hyperphysics.phy-astr.gsu.edu/hbase/acoustic/imgaco/isc2.gif

  11. Sound Level • Sound Level (β) - a relative intensity in decibels (dB) • 0 dB = smallest sound that can be heard • 50 dB = background noise • 70 dB = traffic • 110 dB = rock concert

  12. Example Question • If a Rock Concert is 110 dB, what is the intensity level in W/m2? A) 0.1 W/m2 B) 10 W/m2 C) 1011 W /m2 D) 1098 W/m2

  13. Example Question • A bell is rung in a room and can be assumed to be a point source. If you sit three times the distance your friend is sitting from the bell, what is the decibel drop? A) 0.1 dB B) 1 dB C) 10 dB D) 100 dB

  14. Main Points - Sound • Sound– a longitudinal pressure wave that moves through a medium • Velocity – 343 m/s in air • Reflection – occurs when velocity changes • Resonance and Standing Waves – depends on open/closed ends

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