1Phys 1240: Sound and MusicLAST: interference, superposition,beatsTODAY: diffraction, doppler, andoutdoor soundNEXT: sound intensity and loudnessREAD: 5.1 and 5.2CAPA 4 due tonight.CAPA 5 will be “back onschedule”, in TWO weeks, theThursday AFTER the break…(It’ll be in the bins in a day or so)CT 4.5.2xTwo waves traveling in the samedirectionImagine two traveling sin waves (samefrequency, in phase, or “in synch”)traveling together (same direction,starting in the same place)What happens when they “superpose”?A) A traveling wave, same as either oneindividuallyB) A traveling wave, twice the amplitudeof either oneC) A traveling wave that gets bigger andsmaller, bigger and smaller, inamplitude (beats)D) A standing waveE) ?? (something else)CT 4.5.2xTwo waves traveling in the samedirectionImagine two traveling sin waves (samefrequency, in phase, or “in synch”)traveling together (same direction,starting in the same place)What happens when they “superpose”?A) A traveling wave, same as either oneindividuallyB) A traveling wave, twice the amplitudeof either oneC) A traveling wave that gets bigger andsmaller, bigger and smaller, inamplitude (beats)D) A standing waveE) ?? (something else)CT 4.5.2xbTwo waves traveling in oppositedirectionsImagine two sin waves (same frequency)traveling towards each other (oppositedirections, passing through each other),What happens when they “superpose”?A) A traveling wave, same as either oneindividuallyB) A traveling wave, twice the amplitude ofeither oneC) A traveling wave that gets bigger andsmaller, bigger and smaller, in amplitude(beats)D) A standing waveE) ?? (something else)2PhysletsPhyslet: periodic motionBeatsStanding waveWave on a stringCT4.5.4xSuppose 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 else (or ???)DiffractionWhen traveling waves reach a “hole”,they continue but also bend.That’s diffraction.Larger λ => MORE “bending”.Similarly, larger opening =>LESS bending.Decent diffraction if λ > hole sizeTraveling wave approaches asmall slit:3Diffraction: it “spreads out”, thesmall hole acts like a little pointsource of waves on the far side.(Lots of “bending”, wave goes inall directions!)Bigger slit (compared towavelength) => less bending. It’smore just a “shadow” here…Remember the wavelengths oftypical sounds:λ * freq = speed of sound,So λ = (344 m/s) / frequencyLow (34 Hz) => 10 metersMedium (1000 Hz) => .3 mHigh (10,000 Hz) => 3 cmA doorway (size ~1 meter) willdiffract low sounds a lot, highsounds much less.CT 4.2.3You can hear a sound in your left earthat came from your right.There are many physical reasons whythis occurs, but which below is best?a) Because your head is a relativelyrough surfaceb) Because of interferencec) Because the sound just keepstraveling through your head to your leftear drumd) Most sound wavelengths are largerthan your head so they diffracte) Most sound wavelengths are smallerthan your head so they diffract4CT 4.2.4Which sounds are more likely tohead out only in the forwarddirection from a large speaker?A) The low pitchesB) The high pitchesC) All pitchesD) None…(Why might we choose to usesmaller speakers for treble?)Remember the wavelengths oftypical sounds:λ * freq = speed of sound,So λ = (344 m/s) / frequencyLow (34 Hz) => 10 metersMedium (1000 Hz) => .3 mHigh (10,000 Hz) => 3 cmA small speaker (~couple cm)will diffract MOST sounds =>sound goes out in all directions.Ct 4.2.5Would light also diffract if youpass it through a slit?A) SureB) Only if the slit is muchSMALLER than the wavelengthof lightC) Only if the slit is muchLARGER than the wavelength oflightD) ???Decent diffraction if λ > hole sizesimsDiffract sims5 time=0, freq=1 Hz time=0, freq=1 HzTime=1 secdistance=344 mTime=1 secdistance=344 m6T=2 secinner circle at 344 m, outer at 688:T=2 secinner circle at 344 m, outer at 688:6 sec: time=0, freq=1 Hzv(source=172 m/s)7Time=just before 1 secdistance=344 mTime=1 secdistance=344 mTime=1 secdistance=344 mT=2 sec86 sec:s4.4.3Given the following picture of asound source and its sound wavesin still air, what do you conclude?a) The source is movingb) The source is stationaryc) There is some obstruction to theright of the sourced) The sound being produced is ofvarying pitch and wavelengthe)???ssimsDiffract simsDoppler shiftFrequency tells you “number ofwiggles per second”.If a source wiggles the air atfrequency f, but gets closer to youeach cycle, each cycle arrives a littleearlier than it would have.You perceive MORE wiggleseach second => higherfrequency.What if it gets farther from you each cycle?94.4.1You are standing stationary and see anambulance approaching with its sirenson. As the ambulance passes you thepitch of the siren seems to…a) Get higherb) Get lowerc) Stays the samed) Need more informationThe pitch…a) Steadily decreases the whole timeb) Decreases really fast at first then slower astime goes onc) Decreases slowly at first then faster as timegoes ond) None of the above really describes ite) I’m still not entirely convinced that the pitchdecreases4.4.4The picture below shows a travelingobject that is emitting a sound. There is aperson at point A and a person at pointB. Which direction is the sound traveling?a) Rightb) Leftc) All directionsd) Need more informationWho hears the higher pitch?a) Ab) Bc) They hear the same pitchd) Need more informationA B4.4.2A child is swinging on a swingtowards a person blowing a whistle.At which position will the pitchsound the highest to the child?d. The pitch is the same at all thepoints.Whistlera.b.c.endstart4.3.1Adding a shell around aperformer at an outdoor show…a) Improves the music prettymuch no matter where theperformer is positionedb) Allows the performer tobetter hear their own musicc) Creates interference,making the music lessenjoyabled) Is mostly for aestheticpurposese) (depends on where theperformer is positioned withinthe shell)10Outdoor music• Sound is generally weaker thanindoors (why?!)• Shell reflects sound, can improve– Volume for audience– Consistency of sound at various spots– Acoustics for the performers(Why?!)Where should the performer stand toget the maximum benefits out of thiscircular shell?d)Somewhere elsee)There is no
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