Waves and Sound

1. Waves and Sound Honors Physics Chapter 14

2. Definition of a wave • A disturbance that propagates from 1 place to another. • Characterized by a large transfer of energy without a large transfer of medium

3. Types of waves • Mechanical waves: require a medium (air, water, ropes) to travel • Electromagnetic waves: do not require a medium to travel (light, radio) • Matter waves: produced by electrons and particles

4. Mechanical Waves • Transverse: the displacement of the individual particles is perpendicular to the direction of propagation.

5. Mechanical Waves • Longitudinal: The displacement of the individual particles is parallel to the direction of propagation.

6. Mechanical Waves • Surface waves: the displacement of individual particles is circular (result of both transverse and longitudinal motion)

7. Waves in motion • Longitudinal and Transverse Wave Motion

8. Properties of waves • Amplitude(A): the maximum displacement from equilibrium position, measured in meters. • Wavelength ( λ): the distance between 2 particles that are in phase with each other, measured in meters.

9. Wave diagram

10. Properties of waves 3.Frequency (f): number of complete waves that pass a point in one second, measured in 1/seconds or Hertz (Hz) 4.Period(T): the time it takes for one complete wave to pass a given point, measured in seconds.

11. Properties of waves • Velocity of propagation (v): horizontal speed of a point on a wave as it propagates, measured in m/s.

12. Relationships/equations • T =1/ f or f = 1/T • v = f × λ

13. Phase • In-phase: when waves are synchronized (crest meets crest) • Out-of-phase: waves are not synchronized • Opposite phase (180º out-of-phase): crest meets trough

14. Reflections • Fixed End: • Explanation • Animation • Open End: • Explanation • Animation • Between different mediumsscroll down (What do you notice about the phases? Transmitted? Reflected?)

15. Principle of Superposition • Occurs when two waves travel through the same medium at the same time. • Each wave affects the medium independently. • The displacement of the medium is the algebraic sum of the displacements. • Animation(scroll down)

16. Interference • Constructive: occurs when wave displacements are in the same direction (in-phase) • Destructive: occurs when wave displacement are in different directions (out-of-phase) • animation

17. What is Sound? • Longitudinal • Mechanical • Rarefaction: low air pressure • Compression: high air pressure • module

18. Speed of sound • Depends on… • Temperature • V air = 331m/s +(0.6 m/s/ºC)*T • Density/kind of medium • Gases <liquids<solids • Chart of speeds

19. Speed of sound

20. Pitch • How we perceive variations in frequency • Audible range 20-20,000 Hz (listen) • Most sensitive to 1,000 to 5,000Hz • Loudness can distort our perception of pitch (listen tape) • module

21. Doppler Effect • Variation in the frequency of sound due to the relative motion of the sound source or the listener. • Animation1 • Picture of a sonic boom • Video of sonic boom • Space shuttle breaking sound barrier and mythbuster

22. Doppler Effect Results • As an moving sound source approaches a listener the frequency (pitch) increase. • As a moving sound source passes by a listener the frequency (pitch) decreases. *Same effect if sound source is stationary and listener is moving.

23. Calculating Frequency change • f’ = f (v ± vR / v± vs) • f’ = new frequency • f = original frequency of source sound • v: velocity of sound • vR: velocity of receiver • vs: velocity of source

24. Loudness • How we perceive variations in amplitude and intensity. • Module • In general, sound waves of higher intensity sound louder but we are not equally sensitive to all frequencies.

25. Sound Intensity • The amount of energy that passes through a given area in a given time. • I=P/A • P=power (watts) • A = area (m2) • I = intensity (W / m2) • Directly proportional to the square of the amplitude. • Inversely proportional to the square of the distance from the source.

26. Relative Intensity • Logarithmic scale used to indicate the intensity level of a sound. • Measured in decibels or dB • β= 10 log (I / Io) • Io =1×10-12 W/m2 (intensity of the faintest sound that can be heard) • I = intensity of sound in W/m2

27. How loud is a decibel? • Threshold of hearing( Io)= 0dB (air pressure 2×10-5 Pa) • Threshold of pain = 120 dB (air pressure = 20 Pa) • We perceive a 10 dB increase as twice as loud. • Every 20 decibels air pressure increases 10 times • Table of sound levels

28. Resonance • Causing the vibration of an object by the influence of another vibrating body. • Must match the natural frequency of vibration of the object • Whole-number multiple of the natural frequency work too. • DEMO (wood blocks) • Breaking a glass with resonance

29. Standing waves • Caused by the interference of reflected waves with incident waves from the source. • Nodes: pts of no displacement • Antinodes: pts of maximum displacement • Animation • applet

30. Vibrating Columns of Air • Column will emit a sound when the air inside achieves resonance. • DEMO (cardboard moose call) • Frequency of vibration depend on • Length of column • Type of column • Open end • Closed end

31. Fundamentals and Harmonics • Fundamental Frequency: lowest frequency of vibration • Harmonics: whole number multiples of the fundamental • Note: the fundamental frequency is the 1st harmonic. • Animation

32. Closed (at one end) pipe • The standing wave created has • Node at closed-end • Antinode at open-end A N

33. Calculations for closed pipe • Fundamental frequency • f1= v /4L (L=length of air column) • Harmonics • fn = nf1 (n=1,3,5,…) • λ =4L/n • Note: only ODD harmonics are produced • Animation

34. Open pipe • The standing wave created has • Antinode at both ends A A

35. Calculations for open pipe • Fundamental frequency • f1= v /2L (L=length of air column) • Harmonics • fn = nf1 (n=1,2,3,…) • λ =2L/n • Note: ALL harmonics are produced • animation

36. Frequency • Light • Sound (listen) • BACK