UPII-Wk6 - University Physics II • Announcements •...

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Unformatted text preview: April 28, 2008 University Physics II • Announcements • Sound Waves CLICKERS TO CHAN. 13 April 28, 2008 Announcements • Thanks for doing RQ5 – There was an error on Previous RQ, I will remove the last question and give everybody the 1 point it was supposed to be • PS5 is posted, due Wednesday by 5pm • PreLab due Monday (TODAY) by 12:30pm • Midterm II one week from today • 0.0000008 – Isn’t it a pain in the ass NOT to use scientific notation? • Much better… 7 8 10- × April 28, 2008 Question #1 A wave is sent along a long spring by moving the left end rapidly to the right and keeping it there. The figure shows the wave pulse at QR —part RS of the long spring is as yet undisturbed. Which of the graphs 1–5 correctly shows the relation between displacement s and position x ? (Displacements to the right are positive.) 1. 1 2. 2 3. 3 4. 4 5. 5 April 28, 2008 Sound Waves • Matter can be represented by a string of oscillators – Disturbance will be propagated • Longitudinal waves – Displacement parallel to the direction of wave propagation – Sound waves are a typical example Compression Expansion April 28, 2008 Traveling Sound Waves • String of harmonic oscillators C E C E C E C E x ∆ p = ∆ p max sin( kx- ϖ t ) p s ( x,t ) = s max cos( kx- ϖ t ) s x Pressure amplitude ∆ p max = ρ v ϖ s max Displacement April 28, 2008 Traveling Sound Waves – Example • The pain threshold for humans occurs at a pressure amplitude of 28 Pa. Calculate the displacement amplitude of eardrums for sound waves of 1 kHz frequency. – The hearing threshold is 6 orders of magnitude smaller. Calculate the amplitude. ∆ p max = ρ v ϖ s max 5 max max 3 28 Pa 1.1 10 m = 11 m 1.2 kg/m 343 m/s 2 1000 Hz p s v μ ρ ϖ π- ∆ = = = ⋅ ⋅ ⋅ ⋅ 5 11 max 6 1.1 10 m = 1.1 10 m = 0.011 nm 10 s-- ⋅ = ⋅ April 28, 2008 Question #2 You stand a certain distance away from a speaker and you hear a certain intensity of sound. If you double your distance from the speaker, what happens to the sound intensity at your new position? 1. Drops to ½ its original value 2. Drops to ¼ its original value 3. Drops to 1/8 its original value 4. Drops to 1/16 its original value 5. Does not change at all April 28, 2008 Sound Intensity • Sound Intensity is the average rate per unit area at which energy is transferred by the wave through on onto the surface • Consider a point source – Intensity is defined • Falls off with r 2 • Related to the displacement amplitude: r P I A = 2 2 max 1 2 I v s ρ ϖ = 2 1 I r ∝ 2 W m 2 4 P I r π = April 28, 2008 Sound Intensity/2 • First, we have to define the power – We consider an element of air oscillating in SHM – Kinetic energy – Average power v A dx dm Adx ρ = 2 1 2 s dK dm u = ( 29 max ( , ) cos s s x t u s kx t t t ϖ ∂ ∂ = =- ∂ ∂ ( 29 2 2 2 max 1 sin 2 dK Adx s kx t ρ ϖ ϖ =- 2 2 max 1 2 2 dE dK P Av s dt dt ρ ϖ = = = dx v dt = ( 29 2 2 2 1 1 1 sin sin 2 2 2 kx t ax dx π ϖ π- = = ∫ April 28, 2008...
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This note was uploaded on 05/29/2008 for the course PHYS 101 taught by Professor Herber during the Spring '08 term at Denver.

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UPII-Wk6 - University Physics II • Announcements •...

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