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Chapter 21 Musical Instruments

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Chapter 21 Musical Instruments Lecture 22 Chapter 21 Musical Instruments CR/NC Deadline Oct. 19 15-Oct-10 Musical Instruments Now that we understand some of the physics of sound, let’s analyze how musical sound is produced by different types of musical instruments. 15-Oct-10 Physics 1 (Garcia) SJSU String Instruments Standing wave on the vibrating string causes forced oscillation of the sounding board. Frequency for a string depends on: • Length of string • Thickness and composition • Tension in the string Modern piano has many long, Loudness depends on: massive steel • Amplitude of oscillation strings under • Size of sounding board high tension • Flexibility of sounding (hundreds of pounds) on a board large sounding 15-Oct-10 board. Standing Waves; Resonance Frequencies of standing waves on a particular string are called resonant frequencies. The lowest resonant frequency f1 is referred to as the fundamental and the higher resonant frequencies fn th = nf1 as the n harmonics. Standing Waves of Guitar Strings Thin string In order for different strings to have different fundamental frequencies, they must differ in length and/or mass/unit length. A guitar has strings that are all the same length, but the mass/unit length varies. We change the effective length of a string by pressing it against the Thick string fingerboard. Standing Waves - Piano In a piano, the strings vary in both length and density. This gives the sound box of a grand piano its characteristic shape. Once the length and material of the string is decided, individual strings may be tuned to the exact desired frequencies by changing the tension. Musical Pipe Instruments 15-Oct-10 Physics 1 (Garcia) SJSU Standing Waves: Open/Open Pipe If the tube is open at both ends, both ends are antinodes, and the sequence of harmonics is the same as that on a string. All harmonics appear. Demo: Whirly Tube Whirl a corrugated tube to produce a pure tone at the tube’s natural frequency. L Bernoulli principle creates low pressure A at the moving end, drawing air through the tube. 15-Oct-10 Playing Simple Horns Standing waves of different frequencies (different notes) are produced, depending on how musician blows into the horn. Simple bugle is just a long pipe wrapped in a coil so it’s compact. 15-Oct-10 Brass & Woodwind Vibrations Vibrations in a pipe instrument created by: • Vibrating one’s lips (e.g., trumpet) • Blowing past an opening (e.g., flute) • Blowing & vibrating a reed (e.g., clarinet) 15-Oct-10 Harmonic Series Music for natural horns and bugles is limited by harmonic series, the frequencies of the fundamental and overtones. Fundamental 15-Oct-10 Physics 1 (Garcia) SJSU Changing the Length To play notes beyond the harmonic series requires changing the frequency of the fundamental by changing the length of tube. Crooks Finger-holes Disadvantage: Reduced amplitude Disadvantage: Clumsy Changing the Length Trombone uses a continuous slide to vary its length Cornett 15-Oct-10 Physics 1 (Garcia) SJSU Changing the Length Valves used in trumpet, tuba, and French horn Cornett 15-Oct-10 Similar to using a crook but easy to open & close Woodwind Instruments Resonant standing waves also produced in a pipe but the pipe length varied by air holes (finger-holes, keys, or pads). Flute Oboe Cor anglais Saxophone Clarinet Bassoon Meter stick Using air holes reduces amplitude of the sound Percussion Instruments Create oscillations by striking an object, such as: • Stretched drumhead • Metal rod or disk • Wooden object 15-Oct-10 Drum Heads Drum heads are stretched membranes that vibrate at different frequencies depending on the membrane’s oscillation pattern. Note:15-Oct-10 These animations are not accuratePhysics bec 1 (Garcia)ause SJSUcomplex patterns should oscillate faster. KeyKey PointsPoints ofof LectureLecture 2222 • Musical Instruments z Before Wednesday, read Hewitt Chap. 22 (first half). z Homework #16 due by 11:00 PM Friday Oct. 22..
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