Chapter 21 Musical Instruments

Transcription

1 Lecture 22 Chapter 21 Musical Instruments CR/NC Deadline Oct. 19 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. Physics 1 (Garcia) SJSU

2 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 Loudness depends on: Amplitude of oscillation Size of sounding board Flexibility of sounding board Modern piano has many long, massive steel strings under high tension (hundreds of pounds) on a large sounding board. Standing Waves; Resonance Frequencies of standing waves on a particular string are called resonant frequencies. The lowest resonant frequency f 1 is referred to as the fundamental and the higher resonant frequencies f n = nf 1 as the n th harmonics.

3 Standing Waves of Guitar Strings 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. Thin string 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.

4 Musical Pipe Instruments 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.

5 Demo: Whirly Tube Whirl a corrugated tube to produce a pure tone at the tube s natural frequency. Bernoulli principle creates low pressure at the moving end, drawing air through the tube. L A 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.

6 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) Harmonic Series Music for natural horns and bugles is limited by harmonic series, the frequencies of the fundamental and overtones. Fundamental Physics 1 (Garcia) SJSU

7 Changing the Length To play notes beyond the harmonic series requires changing the frequency of the fundamental by changing the length of tube. Finger-holes Crooks Disadvantage: Reduced amplitude Disadvantage: Clumsy Changing the Length Trombone uses a continuous slide to vary its length Cornett Physics 1 (Garcia) SJSU

8 Changing the Length Valves used in trumpet, tuba, and French horn Cornett 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 Meter stick Bassoon Using air holes reduces amplitude of the sound

9 Percussion Instruments Create oscillations by striking an object, such as: Stretched drumhead Metal rod or disk Wooden object Drum Heads Drum heads are stretched membranes that vibrate at different frequencies depending on the membrane s oscillation pattern. Note: These animations are not accurate Physics because 1 (Garcia) SJSU complex patterns should oscillate faster.

10 Musical Instruments Key Points of Lecture 22 Before Wednesday, read Hewitt Chap. 22 (first half). Homework #16 due by 11:00 PM Friday Oct. 22.