1Phys 1240: Sound and MusicLAST: decibelsTODAY: decibels and hearingNEXT: Midterm(We’ll finish up Ch 6 next Tues. Catchup on those readings if you haven’talready done them)Extra office hour today@4 (Vincent)(help room)Earlier evening help sessiontomorrow 6 PM (here) (Jeff)5.2.4eA violin is playing at 60 dB. Whathappens to the SIL if there are 20violins playing? It goes to…A) 73 dB B) 76 dB C) 80 dBD) 120 dB E) more than 120 dBSIL difference (in decibels) Intensity ratio 1 dB 1.3 2 dB 1.6 3 dB 2.0 4 dB 2.5 5 dB 3.2 6 dB 4.0 7 dB 5.0 8 dB 6.3 9 dB 7.9 5.2.4eA violin is playing at 60 dB. Whathappens to the SIL if there are 20violins playing? It goes to…A) 73 dB B) 76 dB C) 80 dBD) 120 dB E) more than 120 dBSIL difference (in decibels) Intensity ratio 1 dB 1.3 2 dB 1.6 3 dB 2.0 20x intensity means 2*10,From table that’s +3+10 dBChorus effectAdding instruments => add theintensities(AND, nice, richer sound from thesubtle beating…)If sound from different sources isperfectly IN phase (or OUT ofphase) (constructive ordestructive interference) then it’snot so simple…2Imagine sources with AMPLITUDE=1=> INTENSITY ~ (Amp)2 = 12 = 1If DESTRUCTIVE interference:AMP = 1-1 =0=> INTENSITY ~ 0 2 = 0. Silence!If CONSTRUCTIVE interference:AMP = 1+1 =2=> INTENSITY ~ (2)2 = 4. Loud!!Average of 0 and 4 is 2 (!!)If have TWO such sources, EXPECT total intensity = 1+1 = 2.5.3.1There is a source in the centeremitting sound outward. How doesintensity of the sound at r compareto that at R?a) I(at r) > IRb) I(at r) < IRc) I(at r) = IRd) Not enough informationrR5.3.1b(Assuming no friction/losses, etc)How does the total power(Energy/second) passing throughthe whole spherical surface at rcompare to that at R?A) Pr>PRB) Pr<PRC) Pr=PRD) ??rRHow does the power passing through apatch of 1 cm2 at radius r compare tothat at R?1 cm1 cm1 cm1 cmRemember: I = power/area5.3.13The area of a sphere grows like(Radius)^2Intensity = Power / areaSo, intensity of sound decreaseslike 1/(Radius)^2.5.3.2If R=2r, what is IR/Ir?a) 1b) 2c) 4d) .5e) .25DOUBLE distance to a soundsource (outside) =>sound intensity is 0.25 times as big(you could say it decreases 4 times)Table 5.1 says6dB <=> 4x (look it up!)Doubling distance lowers the SILby 6 dB.(Or even MORE, if some soundenergy is lost to heat!…)(Or LESS, if lots of sound reflects…)5.3.3Standing 10 meters from thespeakers at an outdoorconcert, the SIL is 100 dB.Assuming “no losses”, howloud is it at20 meters? …. 80 meters?A) 50 dB ….. 33 dBB) 50 dB ….. 38 dBC) 94 dB ….. 88 dBD) 94 dB ….. 82 dBE) Something totally different4Proximity effectYou can “focus” on sounds.Tend to focus on loudest sound!(Cocktail party effect, you “hearover the noise”)Adjust your car speaker balance, so nearby is not as loud,otherwise it really dominates!MIDDLEOUTER INNERMiddle earDiscussionWhat is the physics and functionof the “hammer, anvil andstirrup” (ossicles) in the middleear?• Why have a middle ear at all?5CT 6.1.1Why have a middle ear?A) To provide an amplifier ofpressure wavesB) To better couple pressurewaves from “air” medium to“liquid” mediumC) To protect the inner ear fromextremely loud soundsD) To maintain the tympanicmembrane (eardrum) inproper equilibriumE) More than one of the
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