Category CSUEB Harmonic Motion 1200 Waves Sound Physics

Category: CSUEB Harmonic Motion, 1200 Waves, Sound Physics I. Harmonic Motion & Resonance Updated 2014 Apr 09 Dr. Bill Pezzaglia

2 Outline A. Periodic Motion B. Harmonic Systems C. Resonance D. Energy in Oscillations (to be done)

3 A. Periodic Motion 1. Equilibrium 2. Periodic Motion 3. Frequency

4 1. Equilibrium: a system which is not changing with time (no net force). There are 3 types: a) Neutral equilibrium: boring case, if you move the object to another position it will just sit there.

b. Unstable Equilibrium • if you displace system slightly it changes drastically, • e. g. a ball perched on top of a steep hill 5

c. Stable Equilibrium • if you displace the system, there is a “restoring force” which opposes the change • Strength of restoring force increases with displacement 6

7 2. Periodic Motion a) Oscillations: • A system displaced from stable equilibrium will oscillate about the equilibrium point • The motion is “periodic” (repeats in time) • The time for one cycle is called the “period”

8 3. Frequency a). Definition: • Frequency is the rate of vibration • Units: Hertz=“cycles per second” • Relation to Period:

b. Frequency is “Pitch” • 1600 Scraper across grooved board produces notes (relates frequency of vibration to pitch of sound) • Mathematical Discourses Concerning Two New Sciences (1638) most lucid of the frequency equivalence Galileo Galilei (1564 -1642) • Made sound waves visible by striking a wine glass floating in water and seeing the vibrations it made on the water’s surface. • First person to accurately determine frequency of musical pitch was probably Joseph Sauveur (1653 -1716) 9

c. Toothed Wheels & Sirens 10 • 1819 Cagnaird de la Tour’s siren used to precisely measure frequency of sound (disk with holes spun, air blown across holes) • 1830 Savart uses card against moving toothed wheel to equate frequency and vibration • Measures the lowest pitch people can hear is about 16 to 20 Hertz

11 B. Harmonic Systems 1. Pendulum 2. Mass on Spring 3. Helmholtz Resonator

a. Pendulums • Galileo: (1581) showed the period of oscillation depends only upon gravity “g” and length “L” of the string: • Period is INDEPENDENT of: – – Mass on end of string Size (“amplitude”) of oscillation Acceleration of gravity on earth: g=9. 8 meters/second 2. Gravity on moon is 1/6 as strong, so pendulum will go slower! 12

b. Springs i. Hooke’s Law: if you squash a spring by distance x, it will give a restoring force F proportional to x: ii. Spring Constant “k” tells the stiffness of the spring. iii. Period of Oscillation for mass m on spring: 13

c. Helmholtz Resonator In 1863, Hermann von Helmholtz (1821 -1894) wrote a famous book “On the Sensations of Tone as a Physiological Basis for the Theory of Music” summarizing his studies of sound. The book describes the form of resonator that he developed for picking out particular frequencies from a complex sound, which we now call a “Helmholtz Resonator” Behaves like mass (air in neck) on spring (gas in cavity). c = speed of sound, D=diameter of neck, L=length of neck, V=volume of gas cavity 14

15 C. Resonance 1. Driven Oscillator 2. Resonance Curve 3. Bandwidth and Quality Section 11. 3 in Hall

Unwanted Resonance (1850) Angers Bridge: a suspension bridge over the Maine River in Angers, France. Its famous for having collapsed on April 15, 1850, when 478 French soldiers marched across it in lockstep. Since the soldiers were marching together, they caused the bridge to vibrate and twist from side to side, dislodging an anchoring cable from its concrete mooring. . 226 soldiers died in the river below the bridge. 16

Tacoma Narrows Bridge Collapse (1940) 17 “Just as I drove past the towers, the bridge began to sway violently from side to side. Before I realized it, the tilt became so violent that I lost control of the car. . . I jammed on the brakes and got out, only to be thrown onto my face against the curb. . . Around me I could hear concrete cracking. . . The car itself began to slide from side to side of the roadway. On hands and knees most of the time, I crawled 500 yards [450 m] or more to the towers. . . My breath was coming in gasps; my knees were raw and bleeding, my hands bruised and swollen from gripping the concrete curb. . . Toward the last, I risked rising to my feet and running a few yards at a time. . . Safely back at the toll plaza, I saw the bridge in its final collapse and saw my car plunge into the Narrows. ” -eyewitness account Video on Collapse of Bridge: http: //www. youtube. com/watch? v=ASd 0 t 3 n 8 Bnc

Millennium Bridge (London) You’d think that engineers don’t make mistakes like this anymore? Yet again in 2000…. The Millennium pedestrian Bridge opened June 10, 2000. Almost immediately it was discovered to resonate when people walked over it, causing it to be close 2 days later. Video on Millennium Bridge Problems: http: //www. youtube. com/watch? v=g. QK 21572 o. SU&feature=related 18

19 1. Driven Oscillator • Externally force a system to oscillate at “driven” frequency “f”. When you approach the system’s natural frequency, the response will be BIG. • Example: Singing in the shower, if you hit right note, the whole room sings. • Demo: resonances of Chladni plate

20 2. Resonant Curve • Consider a system with resonance at 10, 000 Hertz. • As you move away (above or below) resonance, the amplitude drops off.

21 3. Bandwidth • The bandwidth is the range of frequencies for which the amplitude response is above 70%. • Or, the range for which the power is above 50% (power is proportional to square of amplitude) • Bandwidth “ f” • Resonance f 0

22 3 b. “Q” factor • Quality of Resonance: A big Q has a very sharp response, you have to be very close to resonant frequency for the system to respond (musical instruments) • Small Q: very poor resonance, for example if you make a tube out of paper (seal bottom) and blow on it you will not get as good as a sound as blowing on a glass tube/bottle. The “damping” is too big.

23 Notes • Breaking wineglass with sound resonance • http: //www. youtube. com/watch? v=BE 827 gwnnk 4 • X

24 D. Energy in Oscillations 1. Maximum Speed & Amplitude 2. Kinetic & Potential Energy 3. Damping (energy loss) Energy in H. O. Section 2. 5 in Hall, Damped section 9. 7

1. Velocity The maximum speed of the mass on a spring is related to the maximum displacement (amplitude “A”) by the frequency “ f ” : 25

2. Kinetic and Potential Energy of Spring a) Kinetic Energy (maximum at middle): b) Potential Energy (of displaced spring) where k is the spring constant. c) Total Energy is constant. This gives a relation between maximum velocity and amplitude: 26

3. Damped Oscillations • Including friction the oscillations die out with time. • Frictional force is velocity dependent (for wind instruments its due to viscosity of air) • Critical Damping: If friction is big enough (small Q), system does not oscillate! Section 9. 7 in Hall. 27

4. Impedance of a Spring • Impedance “Z” relates the (maximum) force of a spring “F” to the (maximum) oscillation speed of the mass “v”. • Hooke’s Law relates force, spring constant “k” to maximum stretch, or amplitude “A” • Substitute for amplitude (previous slide) • Substitute for frequency : • Yields impedance: 28

29 Notes 1. 2. 3. 4. Demo: Spring, Pendulum Comb or other similar device Perhaps the “Helmholtz Resonator” should be included here? Need to finish part “D”. 5. 6. Include massive spring Include pendula corrections