Phys 1240: Sound and Musicwww.colorado.edu/physics/phys1240LAST: pressure and the nature ofsoundTODAY: waves and oscillationsNEXT: more on wavesREAD: Finish Ch 2!CAPA tonight: 1 was chopped, fixed~1 hr after class last Thurs,Grab a corrected copy if you want, orjust read the screen!)CT 1.5.1If there is really a force of 105 N (that’s100,000 N, over 11 tons!) on eachsquare meter of a glass window, why isit that the window does not shatter?a) That is such a small force it does notmatterb) The frame of the window exerts acounterforce which balances the forcesoutc) Glass is stronger than you thinkd) There is an equal but opposite forcepushing against the window from theother sidee) Gravity counteracts the forceCT 1.5.4Who is more likely to damage a hardwooddance floor, while just standing around?a) A 250 lb cowboy wearing boots with 6 cmx 6 cm heelsb) A 100 lb waif-like woman wearing 1 cm x 1cm high heelsc) They will do equal damaged) Not enough information to make aninformed choice(Pressure = Force/ area)Clap: push the air => slight increase inpressure of the air followed by a slightdecrease. This pressure fluctuation travels outin all directions as a wave. As the pressurewave reaches your ear, you hear sound.Just after clapLaterAIR MOLECULESMore densely packed air molecule… Slight increase in pressureSlight decreaseWhat are you hearing?Creating Musical TonesTo create a pure sustained tone (likeconcert A), the speaker pushes on the air atregular intervals, creating a series ofpressure waves.Higher PLower PIn speaker we vibrate cone: All instruments work with same principle...push on air at regular intervals.Look at sound:Microphone detects changes in pressure.Higher PLower PSound waves traveling outHitmicrophone,It flexes,CreateselectricalsignaltimepressureIf the speaker vibrates back and forth at 200 Hzhow much time passes between each time itproduces a maximum in pressure?a. 0.2 seconds b. 200 secondsc. 0.005 seconds d. 0.02 secondse. 0.05 secondsHigher PLower PSound waves traveling outAnswer is c. 0.005 seconds. Period = 1 second/ 200 cycles = 0.005 seconds/cyclewavesA wave is a disturbance of somematerial (the medium)As a wave passes, the motion ofthe material is “back and forth”Which way does the materialwiggle?(Transverse or longitudinal?)Representing waves(lots of ways!)Wave simSound simCT 1.4.3Sound travels at a speed of 344 m/s (770 mph).How fast does the air flow out of my mouth as Ispeak?a) Much slower than 344 m/sb) 344 m/sc) Much faster than 344 m/sCT 1.4.6When sound travels from a source to a listener,what do air molecules never do?a) Travel from the source to the listener at344 m/sb) Move back and forth with the frequency ofthe sound wavec) Bump into other air moleculesd) Move in the direction of wave propagatione) Air molecules do all of the aboveCT 1.3.1The picture shows “displacement as a functionof location along a string”What is the wavelength (“λ”)?e) None of theseabcdWhat is the amplitude?xA key relationship!Speed = distance / time takenFor all traveling waves, if the wavewiggles at frequency f, the periodP = 1/fTo travel ONE wavelength takesONE period.Speed = wavelength / period = wavelength * frequency = λ fV = λ f Wave
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