Phys 1240: Sound and Musicwww.colorado.edu/physics/phys1240LAST: pitch/frequencySpeed of soundTODAY: pressure and the nature ofsoundNEXT: waves and oscillationsREAD: Hall 2.4, 2.5CAPA Thursday… Question 1 gotchopped, fixed it ~1 hr after classlast Thurs,Grab a corrected copy if you want, orjust read the screen!)[email protected] 1.2.3What determines pitch?Length/size?“Bendability”/rigidity ?Heaviness/density? (maybe) ?The above may influence it, butit’s really all about the frequency of vibration!What about amplitude? This impacts the loudness, not (generally) the pitch.Periodic motion1 wiggle/sec = 1 oscillation/sec = 1 cycle/sec = 1 Hertz (1 Hz)⇒ motion that repeats itself…(vibrations, waves, …)There’s a time ( “period”) (“P”)that it takes to get back to whereyou started.There’s a frequency (“f”) that tellsyou how often you repeat (eachsecond)Period and frequencyPeriod is “seconds for one cycle”.Frequency is “cycles per second”LARGE (long) period (time per cycle) => SMALL (low) frequency.P = 1/fwiggle at 2 Hz (=2 cycles/sec)⇒ each cycle takes 1/(2 Hz) = 1/(2 cycles/sec) = 0.5 seconds / cycleWe say “the period is 0.5 seconds”CT 2.1.2When the amplitude of an oscillatingobject is doubled, the period becomes:a) twice as bigb) 1/2 as bigc) Stays the samed) 1/4 times as bige) Not enough information to decideWhen the frequency is doubled, the period becomes…CT 2.1.4A vibration has a frequency of 100cycles/second, what is the period(or the amount of time for onecycle)?a) 100 secb) 1 secc) 10 msecd) 100 msece) None of theseIs this tone audible? If so, is it high or low?How fast (frequently) can youtap the desk?CT 2.1.1You get the strobe timed so anobject appears stationary.What if we speed up the strobe’sfrequency just a little?a) The object still staying stationaryb) The object alternating back andforth between two extremesc) The object moving forwardsd) The object moving backwardse) Can’t really guessHow fast can your vocal cordsvibrate? What frequencies can your eardetect?How fast does a tuning forkvibrate?CT 2.1.3If I lower the frequency of this tone whathappens to the pitch?a) It decreasesb) It increasesc) It remains the samed) Not enough informationWhat if I make the same sound louder (keeping the same pitch), the frequency…Production, propagation perception• Instrument (string/tube/etc)wiggling (at some frequency)=> air pressure a little high,then a little low, over and over.• Air pressure “wiggles” too!• Frequency of pressurechanges is the SAME as thefrequency of the instrument• You detect varying pressure atyour ear: frequency => pitchBrief detour: Air pressure• Pressure is not force… it’sforce divided by area.• P = Force/AreaLots of pressure from the air inthis room! Called “1 atm” of pressure = 14.7 pounds/in2 (sea level) = 100,000 N/m2 (sea level) (P ~ 12.5 pounds/in2 in Boulder, about .85 atm)“Amplitude of the pressure wave” is theamount of over (or under) pressure• Normal sound ~ 1 N/m2overpressure⇒ pressure at your ear goes from 100,001 N/m^2 down to 99,999 (and back)(If it’s 100 Hz, that cycle happens 100 times every second)time100,000Pressure100,00199,999CT 1.5.1If there is really a force of 105 N (that’s100,000 N, about 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) Other forces (e.g. from the frame)counteract and balance thisc) Glass is stronger than you thinkd) There is an equal but opposite forceof air pushing against the window fromthe other sidee) Gravity counteracts the forceCT 1.5.2I stand on a scale with both feet andmeasure my weight. If I stand onone foot, does the reading:a) Increaseb) Decreasec) Remain the sameI weigh 500 N, what is the force oneach foot?a) 500 N eachb) 250 N eachc) It depends on the area of my footCT 1.5.2What is the area of my feet in m2(Hint: my feet are very roughly 25 cmx 8 cm)?a) 200b) 20c) 2d) .2e) .02CT 1.5.2What is the approximate pressure on eachfoot?a) .125 N/m2b) 125 N/m2c) 1250 N/m2d) 1.25x104 N/m2e) Not enough informationConvert this to atmospheresa) 1.25x10-5 atmb) 1.25x10-3 atmc) .125 atmd) 125 atme) 1.24x105 atmCT 1.5.3I stand on both feet, when I lift onefoot up the pressure on the other foot(the one that remains on theground)…a) Increasesb) Decreasesc) Remains the sameCT 1.5.4Who is more likely to damage ahardwood dance floor?a) A 250 lb cowboy wearing bootswith 6 cm x 6 cm heelsb) A 100 lb waif-like woman wearing1 cm x 1 cm high heelsc) They will do equal damaged) Not enough information to makean informed choiceCT 1.5.5When the air is sucked out of thisbarrel, what is going to happen tothe barrel?a) The barrel will explodeb) The barrel will completelycollapsec) The barrel will collapse halfwayd) Absolutely nothing, it’s areinforced steel barrel!e) I have no
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