- Slides: 23
Music Boxes 1 Music Boxes
Music Boxes 2 Introductory Question n When you bow a violin string, that string begins to vibrate. What projects most of the sound that you hear: A. The bow The string The body of the violin B. C.
Music Boxes 3 Observations about Music Boxes They produce music when you wind them up n Each sound has a plucked character to it n The sounds are somewhat tinny and shrill n They don’t play very long before winding down n The tempo holds steady then gradually slows n
Music Boxes 4 8 Question about Music Boxes What are vibration, pitch, sound, and music? n Why does a tine vibrate after being plucked? n Why do different tines have different pitches? n Why is a tine’s pitch independent of its volume? n How does sound from the music box reach us? n How does the music box produce sound? n Why does a music box sound like a music box? n How does a music box maintain its tempo? n
Music Boxes 5 Question 1 n What are vibration, pitch, sound, and music?
Music Boxes 6 Vibration and Pitch n A vibration is a rhythmic mechanical motion It generally occurs around a stable equilibrium n It occurs as that system settles into its equilibrium n It is sustained while the system has excess energy n n Pitch refers to the rate of vibration Low pitches are associated with slow vibration rates n High pitches are associated with fast vibration rates n
Music Boxes 7 Sound and Music n Sound is a vibration in the air itself Air has a stable equilibrium: uniform density n Disturbed from equilibrium, air vibrates as sound n n Music is the arrangement sounds We hear patterns of sounds n We identify those patterns as music (or maybe noise) n
Music Boxes 8 Question 2 n Why does a tine vibrate after being plucked?
Music Boxes 9 A Tine’s Vibration A tine has a stable equilibrium shape: straight n It experiences a restoring force when bent n Upon release, the tine n accelerates toward equilibrium n and coasts through that equilibrium n and accelerates toward that equilibrium again… n n The tine oscillates about its equilibrium, n until it runs out of excess energy
Music Boxes 10 Question 3 n Why do different tines have different pitches?
Music Boxes 11 Tine Length and Pitch n A shorter tine is stiffer n has less mass n accelerates more quickly n reverses directions sooner n takes less time to complete each cycle of oscillation n and has a higher pitch n
Music Boxes 12 Question 4 n Why is a tine’s pitch independent of its volume? As a tine gets quieter, why doesn’t its pitch change? n Do all vibrating systems maintain constant pitch? n
Music Boxes 13 Tines and Harmonic Oscillators n As the tine becomes quieter, its motion shrinks Its tip covers less distance per cycle n Its restoring forces become weaker n It reverses directions in the usual time n It completes each oscillation cycle in usual time n Its restoring force is proportional to displacement n The tine is a harmonic oscillator n
Music Boxes 14 Question 5 n How does sound from the music box reach us?
Music Boxes 15 Air’s Vibrations n n n Air has a stable equilibrium: uniform density It experiences restoring forces when disturbed After being disturbed, the air n n The air oscillates about its equilibrium n n accelerates toward equilibrium and coasts through that equilibrium and accelerates toward that equilibrium again… until it runs out of excess energy But air behaves as many harmonic oscillators…
Music Boxes 16 Question 6 n How does the music box produce sound?
Music Boxes 17 Projecting Sound is a density disturbance in the air n Producing a density disturbance isn’t easy n Air flows easily around narrow moving objects, n so a vibrating tine barely affects air’s density n A vibrating surface has more effect on air’s density n n The music box uses two steps to project sound The vibrating tine causes a surface to vibrate n The vibrating surface actually produces the sound n
Music Boxes 18 Introductory Question(revisited) n When you bow a violin string, that string begins to vibrate. What projects most of the sound that you hear: A. The bow The string The body of the violin B. C.
Music Boxes 19 Question 7 n Why does a music box sound like a music box?
Music Boxes 20 Overtones n Each tine is an extended object that is fixed at one end n but has parts that can move opposite one another n n The tine has many modes of vibration In its fundamental mode, the tine moves as a whole n In its overtone modes, its parts move oppositely n n The different modes have different pitches The fundamental mode sets the main pitch n The overtones create timbre – the music box sound n
Music Boxes 21 Question 8 n How does a music box maintain its tempo?
Music Boxes 22 Air Drag Controls Speed The music box needs something to set its tempo n A speed-dependent force opposes the mainspring n Pressure drag force is highly speed-dependent n Music box uses pressure drag to set its tempo! n
Music Boxes 23 Summary about Music Boxes The tines vibrate as harmonic oscillators n Each tines has its own pitch n The music drum plucks the tines and they vibrate n The vibrating tines convey motion to surfaces n The surfaces project sound that you hear n The mainspring powers the music box n A spinning vane controls the tempo n