Exam 2 corrections – due today Today’s lecture Announcements Exam 2 corrections – due today Today’s lecture Comments of some previous HW problems Summary of the physics of waves The physics of sound 11/7/2018 PHY 113 -- Lecture 18

Pressure due to water: P1=0 P2=rg(2d) P3=rg(3d) 11/7/2018 PHY 113 -- Lecture 18

Solutions: y(x,t) = f (x ± vt) Examples: pulse wave periodic wave The wave equation: Solutions: y(x,t) = f (x ± vt) Examples: pulse wave periodic wave velocity function of any shape 11/7/2018 PHY 113 -- Lecture 18

11/7/2018 PHY 113 -- Lecture 18

Longitudinal waves propagating in a fluid or solid Sound waves Longitudinal waves propagating in a fluid or solid 11/7/2018 PHY 113 -- Lecture 18

11/7/2018 PHY 113 -- Lecture 18

Sound intensity: (energy/(unit time × unit area)) Periodic sound wave In terms of pressure: Sound intensity: (energy/(unit time × unit area)) Decibel scale: 11/7/2018 PHY 113 -- Lecture 18

Source b (dB) Lawn mower 100 Normal conversation 70 Mosquito buzzing Some representative values Source b (dB) Lawn mower 100 Normal conversation 70 Mosquito buzzing 40 Threshold of hearing 11/7/2018 PHY 113 -- Lecture 18

Peer instruction question Suppose that you are trying to sleep but your roommate has the stereo on at a sound level of b=100. In trying to diplomatically and accurately achieve an appropriate sound reduction, which of the following is better to request: Please reduce the stereo intensity by half. Please reduce the stereo decibel level by half. Please reduce the pressure amplitude of the sound wave produced by the stereo by half. 11/7/2018 PHY 113 -- Lecture 18

String instruments (Guitar, violin, etc.) The sound of music String instruments (Guitar, violin, etc.) (no sound yet.....) 11/7/2018 PHY 113 -- Lecture 18

coupling to air 11/7/2018 PHY 113 -- Lecture 18

Peer instruction question Suppose you pluck the “A” guitar string whose fundamental frequency is f=440 cycles/s. The string is 0.5 m long so the wavelength of the standing wave on the string is l=1m. Assuming the speed of sound is 343 m/s, what is the wavelength of the sound wave which is produced? (A) 1m (B) 0.78 m (C) 1.28 m 11/7/2018 PHY 113 -- Lecture 18

“Wind” instruments (standing waves in air) 11/7/2018 PHY 113 -- Lecture 18

11/7/2018 PHY 113 -- Lecture 18

The “Doppler” effect v=sound velocity observer moving, source stationary observer source vS=0 d 11/7/2018 PHY 113 -- Lecture 18

The “Doppler” effect v=sound velocity observer stationary, source moving d toward away 11/7/2018 PHY 113 -- Lecture 18

Example: toward away Velocity of sound: v = 343 m/s 11/7/2018 PHY 113 -- Lecture 18

Peer instruction question Is Doppler radar described by the equations given above for sound Doppler? yes (B) no Is “ultra sound” subject to the sound form of the Doppler effect? (A) yes (B) no 11/7/2018 PHY 113 -- Lecture 18

Relative velocity of source toward observer away Relative velocity of source toward observer 11/7/2018 PHY 113 -- Lecture 18