Announcements 10/8/12

Announcements 10/8/12 Prayer Exam 1 – one problem left to grade

PearlsBeforeSwine

From warmup Extra time on?

a. (nothing in particular)

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 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

Speed of sound Speed of sound…

a. in gases: ~300-1200 m/sb. in liquids: ~1000-1900 m/sc. in solids: ~2000-6000 m/s

v = sqrt(B/) compare to v = sqrt(T/)

Speed of sound in aira. 343 m/s for air at 20Cb. Dependence on temperature (eqn in book

and also given on exam)

ms343

293KsoundTv

ms343

293KsoundTv

Intensity 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 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)

add 10 to 10 to I

β = 10 log( I / Io )I0 = ?

Solve for I?

(only ~6 got this right)

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 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 ScalesPower or Intensity sound

a. dB β = 10 log(I/I0) I0 = 10-12 W/m2

microwaves/rfa. dBm β = 10 log(P/P0) P0 = 1 mW

electronics/electrical circuitsa. dB β = 10 log(P2/P1) (ratio only)

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?

a. +4b. +6c. +8d. +9e. +10

Clicker question:

How much power is -30 dBm?a. 0.001 mWb. 0.003 mWc. 0.030 mWd. 0.100 mWe. You can’t have negative dBm because

you can’t take the log of a negative number

dBm: β = 10 log(P/P0) P0 = 1 mW

Clicker question: If 30 dBm is 1 mW, how much power is

33 dBm?a. 1003 mWb. 1006 mWc. 1100 mWd. 2000 mWe. 3000 mW

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)

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 recordinga. http://stokes.byu.edu/bells.wav (0:32)

The Pie Factoryvbelt

Spacing between pies = ? a. = 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 _______ doesn’t change

wavelengthspeed

f = vnew/old

wavelengthspeed

f = vold/new

new

=(v b

elt-v

s)/f s

Doppler, cont. Combine both effects:

What does mean?

Stokes Flash videoa. http://stokes.byu.edu/teaching_resources/

doppler_script_flash.html (1:50)

v vv v

o

s

f f

Distant light sourcea. Traveling toward youb. Traveling away from

you See HW 19.4 for

equation

Astronomy Edwin Hubble, 1929: Distance to galaxies is

proportional to their speeda. Distance measured through Cepheid variable

star observations, “standard candle”b. How did he measure speed?

– Doppler shift of spectral lines! 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

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.

Fig 17.10 Fig 17.11

Sonic Boom http://stokes.byu.edu/teaching_resources/

boom_script_flash.html (2:47) http://stokes.byu.edu/teaching_resources/

boomray_script_flash.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?

θ

Sonic boom manifested by condensation of water in air

Sonic Boom

sin = vsound/vsource = 1/“Mach number”

Sonic boom of bullet in flight(holographic interferometry)

Sonic Boom

How fast is the bullet traveling?a. Mach # = 1/sin