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CU-Boulder PHYS 1240 - Lecture Notes

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1Phys 1240: Sound and MusicLAST: sound propagation (echoes &wave properties)TODAY: interference andsuperposition and diffractionNEXT: diffraction and outdoor sound(and maybe doppler)READ: (4.4? 5.1?) Wrapping up Ch 4.CAPA 4 due next TUES!(The one after that will be due Wed,right before the fall break)Do you want to learn about“Doppler shift”? (Section 4.4)(That’s the sudden drop in pitchas siren goes by you. It’ll takeabout half a lecture…)A) Sure, sounds interestingB) No, let’s move on to otherthings…C) No opinion, Prof. P shoulddecide…4.1.1Which of the following statements isfalse?a) A reflected wave is weaker than theoriginalb) Sound can reflect off of any surfacec) Sound waves travel indefinitelyd) Weather can affect how soundtravelse) A sound wave can be reflectedmultiple timesSuperposition& InterferenceWaves can run on top of each =>they “add up”This is superposition!If the peaks align => get bigger!CONSTRUCTIVE interferenceIf the peaks align with valleys =>cancel each otherDESTRUCTIVE interference2PhysletsPhyslet: periodic motionBeatsStanding waveWave on a string4.5.1Looking at the following two waves:Here they are graphed together:Which of the waveforms shown belowrepresents the sum of the above two?a) Pinkb) Greenc) Blued) Rede) None of theseTime(seconds)Over Pressure (N/m2)Time(seconds)Time(seconds)Time(seconds)Over Pressure (N/m2)Over Pressure (N/m2)Over Pressure (N/m2)4.5.1(from the previous question)This is a form of:a) Constructive interferenceb) Destructive interferencec) A little bit of bothd) Neither4.5.2Same question as before, but just a tadharder…And graphed together…Which is a representation of the sum of thetwo waveforms?a) Pinkb) Greenc) Blued) Rede) ??Over Pressure (N/m2)Over Pressure (N/m2)Time(seconds) Time(seconds)Over Pressure (N/m2)Time(seconds)Over Pressure (N/m2)Time(seconds)34.5.2(from the previous question)This is a form of:a) Constructive interferenceb) Destructive interferencec) A little bit of bothd) NeitherSuperposition wrapupWaves pass through one another!AFTERWARDS, no effect of“interaction”, they keep going.While they pass through eachother, superpose (interfere)(just ADD the two displacements)CT 4.5.2xTwo waves traveling in the samedirectionImagine two sin waves (samefrequency, in “synch”) travelingtogether. (They superpose.)As time goes by, do we haveA) A (big) traveling waveB) A traveling wave, same as eitherone individuallyC) A standing waveD) A traveling wave that gets biggerand smaller, bigger and smaller, inamplitudeE) ??4beatsTwo sounds of slightly differentfrequencies can addAt some times, they’re “in synch” (“inphase”) => add up(constructive, LOUD)At other times, they get out of synch=> cancel(destructive, SOFT)Alternates back and forth, atbeat freq = freq(#1) - freq(#2)4.5.2Which pairs of tones would have a slowvery distinct beat?a) 10 Hz and 20 Hzb) 100 Hz and 120 Hzc) 100 Hz and 102 Hzd) None of thesee) Not enough information4.5.3Suppose there are two tones. The first toneis 440 Hz and the second is 442 Hz. What would happen to the beats, if the second tone was lowered to 438 Hz?a) The beats would be more frequent.b) The beats would be less frequent.c) The beats would be a little softer.d) The beats would be a little louder.e) Nothing.Spatial interferencePicture two speakers: samepitch, in synch, separated fromeach other.If distance (from you to one) isSAME as distance (to other),sound you hear is “in synch”(high amplitude)But if distances DIFFER by half awavelength, they cancel!5CT4.5.4xSuppose the two speakers in theroom are playing the same steadyfrequency, but one is “reverse wired”(so they’re exactly out of phase) I’mstanding on the “midline”. What do Ihear?A) Steady, loud pitchB) Steady pitch, same as either oneof the speakers aloneC) SilenceD) BeatsE) Something


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CU-Boulder PHYS 1240 - Lecture Notes

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