1. PHYS16 – Lecture 40
Ch. 17 Sound

2. Ch. 17 Sound Sound Beats Doppler 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

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

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