0/3/203 Sound and ed insrumens Sound so far: Sound is a pressure or densiy flucuaion carried (usually) by air molecules sound wae is longdiudinal Wha is he difference beween a hud and a musical noe? ecure 4: Sound and s Reminders/Updaes: HW 6 due Friday, 0pm. Eam 2, a week oday! Eam reiew help session: Monday?? Creaing Musical Tones To creae a pure susained one (like concer ), he speaker pushes on he air a regular inerals creaing a series of pressure waes. ooking a a soundwae: Microphone deecs changes in pressure. Sound waes raeling ou Hi microphone, I flees, Creaes elecrical signal Higher P Higher P ower P ower P ll insrumens work wih same principle... push on air a regular inerals. SHOW SPEKER IN CTION 3 olume and ampliude olume and ampliude Microphone Microphone Quesion: If I increase he olume, wha will happen o he signal from he microphone? a. The peaks will go up and he alleys will go down. b. The peaks will ge closer ogeher. c. The whole signal will go up. d. Boh a and b. e. Nohing will happen DO EXPERIMENT. 5 The ampliude () of a wae is he measuremen beween zero and a peak (or rough) s you urn up he olume you increase he ampliude of he pressure wae 6
ir pressure 0/3/203 Wha is he amoun of oer / under-pressure is ha we hear? Microphone Ploing a sound wae mpliude mplify olage am. Ear is deecs ery, ery small pressure changes: Normal pressure of air (a sea) = amosphere Minimum pressure change deecible by ear = 2 X 0-0 amospheres minimum change is par in 5 billion, Maimum pressure change deecible by ear = 3 X 0-4 amospheres maimum is par in 3600. More han ma === OUCH!!!!! 7 0 Time (s) Wha if we waned o change he pich of he one produced by he speaker? To ge a higher pich sound, we need o adjus he speaker so ha: a. I ibraes back and forh more rapidly, aking a smaller amoun of ime for each cycle b. I ibraes back and forh a he same rae as before, bu he range of i s back and forh moion is larger. c. I receies more power d. I ibraes back and forh more slowly, aking a longer amoun of ime for each cycle e. I ibraes back and forh a he same rae as before, bu he range of i s back and forh moion is smaller. Conrols pich of sound Frequency (f) of a sound wae The number of imes per second ha he speaker goes hrough one complee pushing moion The number of imes per second ha he pressure in my ear goes hrough risefall cycle. Unis: Hz (s - ) Hz = cycle per second The frequency of Concer is 440 Hz FGB CDEFGB CDEFGB Ocae below Concer middle C Concer Ocae aboe Concer (220 Hz) (256 Hz) (440 Hz) (880 Hz) Range of Human Hearing : 20 Hz o 20,000 Hz Dogs : 40 Hz o 60,000 Hz Mice : 000 Hz o 90,000 Hz Period of a sound wae l Waelengh Higher P l ower P If he speaker ibraes back and forh 200 imes each second, (has a frequency of 200 Hz) how much ime passes beween each ime i produces a maimum in pressure? a. 0.2 seconds b. 200 seconds c. 0.005 seconds d. 0.02 seconds e. 0.05 seconds Quesion: If he speaker oscillaes a 200 Hz (remember ha is compleing one cycle in 0.005 seconds), wha is he waelengh (disance beween he pressure maimums) Recall: he speed of sound = 330 m/s a. 0.6 m b..65 m c. 66,000 m d. 3.3 m 2
Posiion 0/3/203 If he speaker ibraes back and forh wice as fas (so 400 imes per second), hen he period of he sound wae (he ime beween producing each peak in pressure) is a. wice as long b. half as long c. unchanged Wha happens o he waelengh of he sound wae when we double f? The disance beween each peak in pressure is a. wice as far b. half as far c. unchanged Thinking abou waes: Frequency (f) # of oscillaions/sec (Hz = /s) Waelengh (l) Disance of one complee cycle (m) (e.g. disance beween pressure maimums) Period (T) Time for one complee oscillaion (s) Speed () Disance raeled per second (m/s) More on speed of sound hrough air: all frequencies rael a same speed Wha would happen o orchesra music if frequencies raeled a differen speeds? speed of sound in air is a fundamenal propery of he air pressure and densiy Relaionships among hese ariables: - = l f Disance per second = disance per oscillaion # of oscillaions per second - f = /T # oscillaions per second = /ime for one oscillaion - = l /T (correc ypo on preclass noes) Sound and ed insrumens How does a iolin (or oher ed insrumen) produce sound? How do we ge differen noes from a iolin? How a iolin makes sound Srings oscillae up and down a cerain frequency Make wooden body oscillae in and ou, Body pushes air o make sound waes Why is he sound of each insrumen unique? Remember ha a musical noe is a periodic ariaion of he air pressure Therefore o creae musical noes, all musical insrumens hae somehing ha oscillaes back and forh in periodic fashion. Consider he iolin. Each piece of is like a lile mass hooked o spring. o compuer sound waes raeling ou hi microphone, i flees, makes olage low pressure high pressure (aoms close) low pressure high pressure Noe ha we now hae 2 ypes of waes going on:. Oscillaory (wae) moion of he 2. Sound wae (pressure wae in he air) coming ou from iolin Boh waes hae same frequency bu differen waelenghs and speeds Sound and ed insrumens Firs les hink abou springs a lile bi How does a iolin (or oher ed insrumen) produce sound? How do we ge differen noes from a iolin? Why is he sound of each insrumen unique? Sar a mass bouncing on a spring: Relaed Spring Time for one oscillaion (Period) Remember ha a musical noe is a periodic ariaion of he air pressure Therefore o creae musical noes, all musical insrumens hae somehing ha oscillaes back and forh in periodic fashion. Posiie direcion Mass ime Consider he iolin. Each piece of is like a lile mass hooked o spring. Posiion monior If he spring is siffer, hen a. he ime per oscillaion will increase b. he ime per oscillaion will decrease c. he ime per oscillaion unchanged 3
Posiion Posiion 0/3/203 Firs les hink abou springs a lile bi Now les hink abou energy: Sar a mass bouncing on a spring: Time for one oscillaion (Period) Sar a mass bouncing on a spring: Time of one oscillaion (Period) Relaed Spring Relaed Spring Posiie direcion Mass Posiion monior If he mass is heaier, hen a. ime per oscillaion will increase b. ime per oscillaion will decrease c. ime per oscillaion unchanged ime Posiie direcion Mass Posiion monior B C ime which ime is he kineic energy of he mass greaes? Where does energy go a imes and C? How do we ge noes of differen pich from a iolin? Ge he s o ibrae a differen frequencies Mus conrol he ibraions or wae moion of he s Remember, for NY wae: = f l f = / l Frequency (pich) of iolin noe Speed of wae (on ) To change he pich (frequency) of a noe we can a) Change - he speed of waes on he - change hickness or ension of he b) Change l - he waelengh - change he lengh of he Waelengh of moion Changing pich by speed of waes on a FT m a) How do ioliniss une heir s? - djus he ension (F T) - Mass on spring eps: Increasing F T like increasing he siffness of he spring (same mass) Bigger spring force acceleraes and oscillaes more quickly b) Why does he G produce a lower noe han he E? - G is hick large mass per lengh (m ) - E is hin small mass per lengh - Mass on spring eps: Thicker like bigger mass (same spring) More mass acceleraes and oscillaes more slowly Sring hickness on a iolin Waelengh of waes on a If we could only conrol he frequency of a iolin wih hickness and ension, he iolin would hae 4 noes.. Bu, so we can also change he frequency by changing he waelengh Simples or fundamenal oscillaion of a iolin How much of a waelengh does his fundamenal moion demonsrae? a) waelengh b) 2 waelenghs c) ¼ waelengh d) ½ waelengh 4
0/3/203 Waelengh of waes on a If we could only conrol he frequency of a iolin wih hickness and ension, he iolin would hae 4 noes.. So we can also change he frequency by changing he waelengh 2 onger onger waelengh, lower frequency = ½ l l 2 2 Fundamenal waelengh and hence frequency direcly relaed o lengh of Can change fundamenal frequency by shorening he wih fingers Fundamenal frequency of deermines ha pich ha we hear Finger Shorer Shorer waelengh Higher frequency Wha makes each insrumen sound unique? Now les compare a concer played by he uning fork and he iolin on he ocilloscope. Why does he race from he iolin look so differen? a. iolin is no playing a concer bu a single noe of a differen pich b. You are seeing he effec of all he s on he iolin ibraing c. The is ibraing a muliple frequencies d. The produces a single noe () bu he wood is ibraing a muliple frequencies e. None of he aboe Harmonics Sring is ied down a each end. I oscillaes back and forh. The simples way for he o fle is like his: Bu i can also fle in more complicaed ways and we call hese higher harmonics Fundamenal frequency, s harmonic. f = /2 2 nd harmonic: = l 2 f 2 = /l 2 = / = 2f Harmonics Sring is ied down a each end. I oscillaes back and forh. The simples way for he o fle is like his: Fundamenal frequency, s harmonic. f = /2 Bu i can also fle in more complicaed ways and we call hese higher harmonics 3 rd harmonic: = 3/2l l 3 2/3 More quesions on harmonics is clamped a boh ends and hen plucked so ha i ibraes in he mode shown below, beween wo ereme posiions and C. Which harmonic mode is his? a. fundamenal, b. second harmonic, c. hird harmonic, d. 6 h harmonic C B, B and C are snapshos of he a differen imes. f 3 = /l 3 = 3 /2 = 3f I is he miure of harmonics ha each insrumen produces along wih he fundamenal ha gies i is unique sound 5
0/3/203 More quesions on harmonics More quesions on harmonics + B + B C C When he is in posiion B (insananeously fla) he elociy of poins along he is... (ake upwards direcion as posiie) : zero eerywhere. B: posiie eerywhere. C: negaie eerywhere. D: depends on he posiion. When he is in posiion C (one of he 2 ereme posiions) he elociy of poins along he is... : zero eerywhere. B: posiie eerywhere. C: negaie eerywhere. D: depends on he posiion. More iolin quesions When you pluck he, wha is making he sound you hear? a., b. he wood, c. boh abou he same, d. he bridge Wha will happen if we ouch uning fork o he bridge? a. no effec, b. sound will be muffled (quieer), c. sound will be louder, d. sound will change frequency/one 6