Mechanical Waves and Sound

1. Mechanical Waves and Sound Chapter 17 Physical Science

2. Mechanical Waves 17-1

3. 17-1 Learning Targets • Compare and contrast the three types of mechanical waves

4. Mechanical Waves • A disturbance in matter that carries energy from one place to another • Require a medium • Medium- material through which a wave travels • Solid, liquid, or gas • All waves except electromagnetic waves

5. Mechanical Waves • Created when a source of energy causes a vibration to travel though a medium • 3 types: Longitudinal, transvers, and surface

6. Transverse Waves • Wave moves horizontally from left to right • Medium moves at right angles compared to the direction of the wave

7. Crest- highest point of transverse wave • Trough-lowest point of transverse wave

8. Transverse wave

9. Longitudinal Wave • Vibration of the medium is parallel to the direction the wave travels • Like a spring • Made of alternating compressions and rarefactions • Sound Waves • P waves produced by earthquakes

10. Longitudinal Wave

11. Compression-particles in medium spaced closed together • Rarefaction- particles in medium are spread out

12. Surface Waves • A wave that travels along a surface separating two media • Objects move in circular pattern due to combination of movement ( from transverse and longitudinal like wave)

14. https://www.acs.psu.edu/drussell/demos/waves/wavemotion.html

15. Properties of Waves 17-2

16. 17-2 Learning Targets • Determine how frequency, wavelength, and speed are related. • Determine how amplitude and energy are related.

17. Periodic motion • - motion that repeat at regular time intervals • Pendulums • Period- time required for one cycle • complete motion that returns to its starting point

18. Frequency • Number of full wavelengths that pass a point in a given amount of time • Measures how rapidly vibrations occur in the medium or source of wave • Symbol is f • SI unit for frequency is Hertz (Hz) – vibrations per second

20. Wavelength • Distance between two neighboring crests or troughs in transverse wave • Distance between two neighboring compressions or rarefactions in a longitudinal wave • λ (lambda)

21. Wavelength is indirectly related to frequency • As Frequency increases, wavelength decreases

22. Wave Speed • Wave velocity (v)= frequency (f) x wavelength(λ) • (m/s) (waves/ s) (m/wave) • v= fλ • Period • T= 1/f

23. Wave Speed Problem • The average wavelength in a series of ocean waves is 15.0 m. A wave crest arrives at the shore on average every 10s, so the frequency is 0.100 Hz. What is the average speed of the waves? • v= fλ • V= 0.1 Hz x 15.0 m • V= 1.50 m/s

24. The speed of sound in air is about 340 m/s. What is the wavelength of a sound wave with a frequency of 220 Hz? • v= fλ • rearrange to find λ = v/ f • λ= 340 m/s 220 Hz • λ= 1.7 m

25. What is the period of a 5.2 Hz wave? • T= 1/f • T= 1/ 5.2 Hz • T= 0.19 s

26. Amplitude • The maximum displacement of the medium from its rest position • Transverse wave = vertical distance between the line of origin and crest or trough • Longitudinal wave= difference in pressure between maximum compression and resting state

28. Amplitude and energy is direct relationship • The more energy a wave has, the greater the amplitude

29. https://www.youtube.com/watch?v=CVsdXKO9xlk • http://onlinetonegenerator.com/hearingtest.html

30. Behavior of Waves 17-3

31. 17-3 Learning Targets • Describe reflection • Describe refraction • Describe factors that affect diffraction • Describe two types of interference • Describe what will make standing waves

32. Reflection • Occurs when a wave bounces off a surface that is cannot pass through • Does not change speed or frequency of wave • Wave may be flipped upside down • Transverse wave – if hits fixed boundary the reflected wave is upside down compared to original wave

35. Refraction • Bending of light waves as they pass from one medium to another • Due to changing speed of wave (one side of wave moves more slowly than other side) • Objects may appear to be bent if light waves

36. Diffraction • Bending of a wave as it moves around an obstacle or through a narrow opening • Longer wavelengths are better at bending • Think of radio waves

37. Diffraction

38. Wave Interference • Mechanical waves are not matter, but displacement of matter. • Two wave can occupy the same space at the same time • Known as principle of superposition • Superposition principle is the method of summing the displacement of waves • When two or more waves travel through a medium, the resultant wave is the sum of the displacements of the individual waves at each point

39. Constructive interference • 2 or more waves combine to make a resulting wave bigger than the original waves • New wave has greater amplitude • Crests align with crests

40. Destructive interference • 2 or more waves combine so the resulting wave is smaller than the largest original wave • Complete destructive interference is when two pulses completely cancel each other • Crests align with troughs

42. Standing wave • A wave that appears to stay in one place • Combination of two waves moving in opposite directions • Each having the same amplitude and frequency • Interference of reflected wave on self • Plucked strings

43. Standing wave forms only if half wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating cord Or if frequency is doubled or triples

44. Standing Waves • Node- point on standing wave that has no displacement • Antinode- point where crest or trough occurs midway between two nodes

46. https://www.youtube.com/watch?v=_S7-PDF6Vzc

47. Sound and Hearing 17-4

48. What affects the speed of sound? • 1- Medium • Density • sounds moves best through more dense materials • Material • sounds travel fastest through solids and slowest through gases • 2-Temperature • increase in temperature= increase in sound speed

50. Intensity • Rate at which a wave’s energy fowls through a given area