19-sep-15 chapter 21 musical instruments lecture 22 cr/nc deadline oct. 19
TRANSCRIPT
Apr 19, 2023 Physics 1 (Garcia) SJSU
Musical InstrumentsNow that we understand some of the physics of sound, let’s analyze how musical sound is produced by different types of musical instruments.
Apr 19, 2023
String InstrumentsStanding 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; ResonanceFrequencies 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
= nf1 as the nth harmonics.
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 fingerboard.
Standing Waves of Guitar StringsThin string
Thick string
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.
Standing Waves: Open/Open PipeIf 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.
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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.
A
L
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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.
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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)
Apr 19, 2023 Physics 1 (Garcia) SJSU
Harmonic Series
Music for natural horns and bugles is limited by harmonic series, the frequencies of the fundamental and overtones.
Fundamental
Changing the Length
Finger-holes
Disadvantage: Reduced amplitude
Crooks
Disadvantage: Clumsy
To play notes beyond the harmonic series requires changing the frequency of the fundamental by changing the length of tube.
Apr 19, 2023 Physics 1 (Garcia) SJSU
Changing the Length
Trombone uses a continuous slide to vary its length
Cornett
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Changing the Length
Valves used in trumpet, tuba, and French horn
Cornett
Similar to using a crook but easy to open & close
Woodwind InstrumentsResonant standing waves also produced in a pipe but the pipe length varied by air holes (finger-holes, keys, or pads).
Flute
Clarinet
Saxophone
Oboe
Bassoon
Meter stick
Cor anglais
Using air holes reduces amplitude of the sound
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Percussion InstrumentsCreate oscillations by
striking an object, such as:
• Stretched drumhead
• Metal rod or disk
• Wooden object
Apr 19, 2023 Physics 1 (Garcia) SJSU
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 because complex patterns should oscillate faster.