Physics 1240 Tues 3/14/06Today: Natural modes of vibration, percussionNext time: String instrumentsOutline• Exam II and III•Natural modes of vibration • Percussion instruments051015202530350 20406080100Exam II scoresMedian 68.0, Mean 69.0Exam problems come from concept tests and homework – don’t try to memorize the right answer, know why the answer is rightExam III GoalsKnow what might not exhibit natural mode vibrationsKnow when do natural modes produce a distinct pitchBe able to identify the missing fundamental – virtual pitchCalculate frequencies/pitches of pipes and stringsKnow what happens when mass/tension of a string is changedImportant qualities or things you can do to improve room acousticIdentify important characteristics of good reverberationEstimate reverberation time of a roomNatural modes of vibration – that bend and spring back will exhibit natural modes of vibrationNatural mode – the pattern of a particular vibrationNatural mode frequency – the frequency that the particular natural mode vibrates atPartials = Natural mode frequenciesCT19.1 What object would best exhibit natural mode vibations?A) a lump of clayB) a steel garbage canC) an orangeMarimbaXylophoneVibesNatural modes of vibration for a handbellGuitar top plateViolin top and backObjects that have natural mode frequencies that form a harmonic series are a special caseExamples of harmonic series100, 200, 300, 400, ...100, 300, 500, ...300, 500, 700, 900, 1100, 1300, ...CT19.2 What natural mode frequencies (in Hz) would most likely come from the Liberty Bell in Philadelphia?A) 100, 200, 300, 400, ...B) 60, 240, 300, ...C) 31, 43, 121, 405, ...Virtual PitchThe ear will fill in the missing fundamentale.g. 300, 400, 500The perceived pitch is 100 Hz!CT19.3 Suppose a steady tone had the following 3 harmonics, what would be the perceived pitch?600, 800, 1000 HzA) 100 HzB) 200 HzC) 300 HzCT19.3 Suppose a steady tone had the following 3 harmonics, what would be the perceived pitch?600, 800, 1000 HzA) 100 HzB) 200 HzC) 300 Hz200, 400, 600, 800, 1000, 1200,1400 ...200 = “greatest common divisor”CT19.4 Suppose a steady tone had the following 3 harmonics, what would be the perceived pitch?200, 800, 900 HzA) 40 HzB) 50 HzC) 100 HzD) 200
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