# Announcements 10/8/12

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Announcements 10/8/12 Prayer Exam 1 one problem left to grade Pearls Before Swine
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Title: Announcements 10/8/12 1 Announcements 10/8/12
• Prayer
• Exam 1 one problem left to grade
• Pearls Before Swine 2 From warmup
• Extra time on?
• (nothing in particular)
• I dreamed that we all failed our tests and you were driving a mechanical claw machine and picking us up with the claws and throwing us off the SWKT.....?......?
• 3 Sound Waves
• What type of wave? What is waving?
• Demo Sound in a vacuum
• Demo tuning fork
• Demo Singing rod
• Sinusoidal? Not necessarily.
• Demo musical disk
• 4 Speed of sound
• Speed of sound
• in gases 300-1200 m/s
• in liquids 1000-1900 m/s
• in solids 2000-6000 m/s
• v sqrt(B/r) compare to v sqrt(T/m)
• Speed of sound in air
• 343 m/s for air at 20?C
• Dependence on temperature (eqn in book and also given on exam)
• 5 Intensity
• Intensity power/area
• Spherical waves
• Non-spherical waves?
• Question you measure the sound intensity produced by a spherically-emitting speaker to be 10 W/m2 at a distance of 2 meters. What will be the intensity at 8 meters away?
• Question What is the total sound power (watts) being produced by the speaker?
• 6 From warmup
• If the "sound level" of a noise changes by 20 dB, by how much did the intensity change?
• 20 dB a factor of 100 change in the intensity. I 10(dB/10)
• (only 6 got this right) add 10 to b ?? ?10 to I ß 10 log( I / Io ) I0 ? Solve for I? 7 Decibels Threshold of hearing 0 dB 10-12 W/m2 Whisper 30 dB 10-9 W/m2 Vacuum cleaner 70 dB 10-5 W/m2 Rock Concert 120 dB 1 W/m2 Nearby jet airplane 150 dB 1000 W/m2 8 Logarithm Review
• Log10(x) is the inverse of 10y
• ? if x 10y then y log10(x)
• I.e. 10 to the what equals 22?
• Review of Laws of Logs
• 1. log(ab) log(a) log(b)
• 2. log(an) n log(a)
• log10(100) ? Translation 10 to what equals 100?
• ln(100) ? (ln loge log2.71828)
• Translation e to what number 100? (4.605)
• Ambiguity log(100)could be either log10 or ln
• Question log10(1,000,000) ?
• Question If log(3) 0.477, what is log(300)?
• 9 Other Power and Intensity Scales
• Power or Intensity
• sound
• dB ß 10 log(I/I0) I0 10-12 W/m2
• microwaves/rf
• dBm ß 10 log(P/P0) P0 1 mW
• electronics/electrical circuits
• dB ß 10 log(P2/P1) (ratio only)
• 10 Clicker question
• A 3 dB increase is just about a factor of ?2 in intensity. How many dB represents a factor of ?4 increase in intensity?
• 4
• 6
• 8
• 9
• 10
• 11 Clicker question dBm ß 10 log(P/P0) P0 1 mW
• How much power is -30 dBm?
• 0.001 mW
• 0.003 mW
• 0.030 mW
• 0.100 mW
• You cant have negative dBm because you cant take the log of a negative number
• 12 Clicker question
• If 30 dBm is 1 mW, how much power is 33 dBm?
• 1003 mW
• 1006 mW
• 1100 mW
• 2000 mW
• 3000 mW
• 13 Question
• You have a 1 volt amplitude sine wave. You want to go up 3 dB in power. How many volts do you need?
• (Recall Power amplitude2 true for voltages, sound, and light waves as well as waves on a string)
• 14 Doppler Effect
• Demo Doppler Speaker
• What happens if the source is moving?
• What happens if the observer is moving?
• Key point
• Frequency is _______________when the source and observer approach each other, ______________ when they go away from each other
• Stokes Come, Come, Ye Saints recording
• http//stokes.byu.edu/bells.wav (032)
• 15 The Pie Factory vbelt
• Spacing between pies ?
• ?? v/f
• vs source speed
• vo observer speed
• v speed of sound (vbelt)
• If observer moves toward source (pie maker), she would measure the same ___________ but the pies are coming at her at a faster ________
• If source moves toward observer, the __________ shrinks, but the pie _______ doesnt change
• f vnew/lold f vold/lnew lnew(vbelt-vs)/fs wavelength speed wavelength speed 16 Doppler, cont.
• Combine both effects
• What does ?? mean?
• Stokes Flash video
• http//stokes.byu.edu/teaching_resources/doppler_s cript_flash.html (150)
• 17 Doppler shift of light
• Distant light source
• Traveling toward you
• Traveling away from you
• See HW 19.4 for equation
• 18 Astronomy
• Edwin Hubble, 1929 Distance to galaxies is proportional to their speed
• Distance measured through Cepheid variable star observations, standard candle
• How did he measure speed?
• Doppler shift of spectral lines!
• Thats now a standard technique for todays astronomers when they want to measure distance to far away objects just measure Doppler shift.
• Hubbles Law and the Big Bang
• (Yes, its OK for LDS to believe in the Big Bang)
• 19 From warmup Fig 17.10 Fig 17.11
• Section 17.4 discusses both the Doppler effect and shock waves. How/why are they related? Hint compare Fig. 17.10 to Fig. 17.11
• With both sound waves and shock waves if the source has a velocity then the side towards which the wave is moving, will experience a condensing of the waves versus having the waves being in the exact center of each wave on all sides. The side in the same direction of the velocity will have less distance between waves than the opposite side.
• 20 Sonic Boom
• http//stokes.byu.edu/teaching_resources/boom_scri pt_flash.html (247)
• http//stokes.byu.edu/teaching_resources/boomray_s cript_flash.html (253)
• Happens with all types of waves whenever the sources is traveling faster than the speed of the wave
• so, what is sonic boom of water waves?
• 21 Sonic Boom Sonic boom manifested by condensation of water in air sinq vsound/vsource 1/Mach number 22 Sonic Boom Sonic boom of bullet in flight (holographic interferometry) q
• How fast is the bullet traveling?
• Mach 1/sinq
• Recommended

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