Announcements 10/8/12
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Prayer
Exam 1 – one problem left to grade
Pearls
Before
Swine
From warmup
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Extra time on?
a. (nothing in particular)
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Other comments?
a. 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.....?......?
Sound Waves
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What type of wave? What is waving?
Demo: Sound in a vacuum
Demo: tuning fork
Demo: Singing rod
Sinusoidal? Not necessarily.
a. Demo: musical disk
vsound  343 ms
T
293K
Speed of sound
Speed of sound…
a. in gases: ~300-1200 m/s
b. in liquids: ~1000-1900 m/s
c. in solids: ~2000-6000 m/s
 v = sqrt(B/r)
compare to v = sqrt(T/m)
 Speed of sound in air
a. 343 m/s for air at 20C
b. Dependence on temperature (eqn in book and
also given on exam)
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vsound  343 ms
T
293K
Intensity
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Intensity: power/area
a. “Spherical waves”
b. 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?
From warmup
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If the "sound level" of a noise changes by 20
dB, by how much did the intensity change?
a. 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
β = 10 log( I / Io )
I0 = ?
Solve for I?
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
Logarithm Review
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Log10(x) is the inverse of 10y
→ if x = 10y then y = log10(x)
a. I.e. “10 to the what equals 22?”
answer: 1.3424
calculator: log10(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)?
Other Power and Intensity Scales
Power or Intensity
 sound
a. dB
β = 10 log(I/I0)
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microwaves/rf
a. dBm β = 10 log(P/P0)
I0 = 10-12 W/m2
P0 = 1 mW
electronics/electrical circuits
a. dB
β = 10 log(P2/P1) (ratio only)
Clicker question:
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A +3 dB increase is just about a factor of 2
in intensity. How many dB represents a
factor of 4 increase in intensity?
a. +4
b. +6
c. +8
d. +9
e. +10
Clicker question:
dBm:
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β = 10 log(P/P0)
P0 = 1 mW
How much power is -30 dBm?
a. 0.001 mW
b. 0.003 mW
c. 0.030 mW
d. 0.100 mW
e. You can’t have negative dBm because you
can’t take the log of a negative number
Clicker question:
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If 30 dBm is 1 mW, how much power is 33
dBm?
a. 1003 mW
b. 1006 mW
c. 1100 mW
d. 2000 mW
e. 3000 mW
Question:
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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)
Doppler Effect
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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
a. http://stokes.byu.edu/bells.wav (0:32)
The Pie Factory
vbelt
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Spacing between pies = ?
a.  = v/f
vs source speed
f = vnew/old f = vold/new
vo observer speed
v speed of sound (vbelt)
If observer moves toward source (pie maker), she
wavelength but the pies are
would measure the same ___________
speed
coming at her at a faster ________
wavelength
If source moves toward observer, the __________
speed doesn’t change
shrinks, but the pie _______
new=(vbelt-vs)/fs
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Doppler, cont.
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Combine both effects:
v  vo
f f
v  vs
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What does  mean?
Stokes Flash video
a. http://stokes.byu.edu/teaching_resources/doppler_script
_flash.html (1:50)
Distant light source
a. Traveling toward you
b. Traveling away from you
 See HW 19.4 for equation
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Astronomy
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Edwin Hubble, 1929: Distance to galaxies is proportional
to their speed
a. Distance measured through Cepheid variable star
observations, “standard candle”
b. How did he measure speed?
– Doppler shift of spectral lines!
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That’s now a standard technique for today’s
astronomers when they want to measure distance to far
away objects… just measure Doppler shift.
Hubble’s Law and the Big Bang
a. (Yes, it’s OK for LDS to believe in the Big Bang…)
From warmup
Fig 17.10
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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
a. 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.
Sonic Boom
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http://stokes.byu.edu/teaching_resources/boom_script_flash
.html (2:47)
http://stokes.byu.edu/teaching_resources/boomray_script_fl
ash.html (2:53)
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?
Sonic Boom
Sonic boom
manifested by
condensation
of water in air
θ
sinq = vsound/vsource = 1/“Mach number”
Sonic Boom
Sonic boom of
bullet in flight
(holographic
interferometry)
q
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How fast is the bullet traveling?
a. Mach # = 1/sinq