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Music Boxes

Music Boxes. Introductory Question. When you bow a violin string, that string begins to vibrate. What projects most of the sound that you hear: The bow The string The body of the violin. Observations about Music Boxes. They produce music when you wind them up

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Music Boxes

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  1. Music Boxes

  2. Introductory Question • When you bow a violin string, that string begins to vibrate. What projects most of the sound that you hear: • The bow • The string • The body of the violin

  3. Observations about Music Boxes • They produce music when you wind them up • Each sound has a plucked character to it • The sounds are somewhat tinny and shrill • They don’t play very long before winding down • The tempo holds steady then gradually slows

  4. 8 Question about Music Boxes • What are vibration, pitch, sound, and music? • Why does a tine vibrate after being plucked? • Why do different tines have different pitches? • Why is a tine’s pitch independent of its volume? • How does sound from the music box reach us? • How does the music box produce sound? • Why does a music box sound like a music box? • How does a music box maintain its tempo?

  5. Question 1 • What are vibration, pitch, sound, and music?

  6. Vibration and Pitch • A vibration is a rhythmic mechanical motion • It generally occurs around a stable equilibrium • It occurs as that system settles into its equilibrium • It is sustained while the system has excess energy • Pitch refers to the rate of vibration • Low pitches are associated with slow vibration rates • High pitches are associated with fast vibration rates

  7. Sound and Music • Sound is a vibration in the air itself • Air has a stable equilibrium: uniform density • Disturbed from equilibrium, air vibrates as sound • Music is the arrangement sounds • We hear patterns of sounds • We identify those patterns as music (or maybe noise)

  8. Question 2 • Why does a tine vibrate after being plucked?

  9. A Tine’s Vibration • A tine has a stable equilibrium shape: straight • It experiences a restoring force when bent • Upon release, the tine • accelerates toward equilibrium • and coasts through that equilibrium • and accelerates toward that equilibrium again… • The tine oscillates about its equilibrium, • until it runs out of excess energy

  10. Question 3 • Why do different tines have different pitches?

  11. Tine Length and Pitch • A shorter tine • is stiffer • has less mass • accelerates more quickly • reverses directions sooner • takes less time to complete each cycle of oscillation • and has a higher pitch

  12. Question 4 • Why is a tine’s pitch independent of its volume? • As a tine gets quieter, why doesn’t its pitch change? • Do all vibrating systems maintain constant pitch?

  13. Tines and Harmonic Oscillators • As the tine becomes quieter, its motion shrinks • Its tip covers less distance per cycle • Its restoring forces become weaker • It reverses directions in the usual time • It completes each oscillation cycle in usual time • Its restoring force is proportional to displacement • The tine is a harmonic oscillator

  14. Question 5 • How does sound from the music box reach us?

  15. Air’s Vibrations • Air has a stable equilibrium: uniform density • It experiences restoring forces when disturbed • After being disturbed, the air • accelerates toward equilibrium • and coasts through that equilibrium • and accelerates toward that equilibrium again… • The air oscillates about its equilibrium • until it runs out of excess energy • But air behaves as many harmonic oscillators…

  16. Question 6 • How does the music box produce sound?

  17. Projecting Sound • Sound is a density disturbance in the air • Producing a density disturbance isn’t easy • Air flows easily around narrow moving objects, • so a vibrating tine barely affects air’s density • A vibrating surface has more effect on air’s density • The music box uses two steps to project sound • The vibrating tine causes a surface to vibrate • The vibrating surface actually produces the sound

  18. Introductory Question (revisited) • When you bow a violin string, that string begins to vibrate. What projects most of the sound that you hear: • The bow • The string • The body of the violin

  19. Question 7 • Why does a music box sound like a music box?

  20. Overtones • Each tine is an extended object • that is fixed at one end • but has parts that can move opposite one another • The tine has many modes of vibration • In its fundamental mode, the tine moves as a whole • In its overtone modes, its parts move oppositely • The different modes have different pitches • The fundamental mode sets the main pitch • The overtones create timbre – the music box sound

  21. Question 8 • How does a music box maintain its tempo?

  22. Air Drag Controls Speed • The music box needs something to set its tempo • A speed-dependent force opposes the mainspring • Pressure drag force is highly speed-dependent • Music box uses pressure drag to set its tempo!

  23. Summary about Music Boxes • The tines vibrate as harmonic oscillators • Each tines has its own pitch • The music drum plucks the tines and they vibrate • The vibrating tines convey motion to surfaces • The surfaces project sound that you hear • The mainspring powers the music box • A spinning vane controls the tempo

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