PHYS16 – Lecture 40 Ch. 17 Sound

Ch. 17 Sound Sound Beats Doppler 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 Propagation is parallel to direction of motion

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

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

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?

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

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

Standing Waves with Sound

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

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

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

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

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