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Unit 8 Waves and Sound

Unit 8 Waves and Sound. Chapter 13 Sections 7 thru 11 Chapter 14. 13.7 Waves. Properties of Waves: Medium: environment through which a disturbance can travel Waves move through the medium, but the medium doesn’t travel with the wave Sound travels through air or water

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Unit 8 Waves and Sound

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  1. Unit 8 Waves and Sound Chapter 13 Sections 7 thru 11 Chapter 14

  2. 13.7 Waves • Properties of Waves: • Medium: environment through which a disturbance can travel • Waves move through the medium, but the medium doesn’t travel with the wave • Sound travels through air or water • EM waves do NOT require a medium to travel

  3. 13.7 Waves Continued • Types of Waves: • Transverse Wave: vibrations are perpendicular to the wave motion • Longitudinal Wave: vibrations are parallel to the direction of the wave • Sound waves : the air particles vibrate back and forth parallel to the direction of the sound wave • Compression: when particles are close together • Rarefraction: when particles are far apart

  4. 13.8 Frequency, Amplitude, and Wavelength • Wavelength (): the distance between two successive points • Transverse: from crest to crest or trough to trough • Longitudinal: from compression to compression or rarefraction to rarefraction • Amplitude (A): the maximum displacement of the wave from the equilibrium position and is proportional to energy • Transverse wave: from crest or trough to the equilibrium position • Longitudinal wave: amount of stretch from original position • Frequency (f): # of waves passing a given point per period (or time frame)

  5. 13.8 Frequency, Amplitude, and Wavelength Continued f= f is frequency (1/s or Hz) and T is period (s) v==()=f v is wave speed (m/s) and  is wavelength (m) f and  are inversely proportional because v only changes from one medium to another (or if the medium itself changes) vsound=331 m/s and vlight=3.0x108m/s

  6. Practice (pg. 445) • A wave traveling along the x-axis has a frequency of 8.00 Hz. The change along the y-axis is 15.0 cm, and the length from one crest to the following trough is 20.0 cm. Determine the amplitude, wavelength, speed, and period.

  7. Practice (pg. 445) • A wave has a wavelength of 3.00 m. Calculate the frequency of the wave it if it is a sound wave and then if it is a light wave.

  8. 13.9 The Speed of Waves on Strings • Not important right now

  9. 13.10 Interference of Waves • Because waves are not matter but instead are displacement of matter, they CAN occupy the same space at the same time. • Superposition Principle: When 2 or more traveling waves encounter each other, the resultant wave is found by adding together the amplitudes of the individual waves

  10. 13.10 Interference of Waves Continued • Each wave maintains its own characteristics after interference. • Interference: • Constructive: waves are in phase and the resultant comes from wave amplitudes added together. • Destructive: waves are out of phase and the resultant comes from wave amplitudes subtracted • Completely Destructive: when the two waves completely cancel each other out

  11. 13.11 Reflection of Waves • When a wave interacts with a boundary, part or all of the wave is reflected. • Free boundary: the wave reflects in the same direction and same amplitude Fixed boundary: the wave is reflected and inverted • The wave exerts an upward force on the boundary, which exerts an opposite but equal force on the wav

  12. Homework • Complete the “Properties of Waves” Worksheet.

  13. 14.1 Producing a Sound Wave • Sound waves always start with a vibrating source • As it moves toward air molecules, the air is compressed (this high density and high air pressure=compression) • As it moves away, the air molecules spread out in all directions (this low density and pressure=rarefraction) • These compressions and rarefractions spread out in all directions

  14. 14.2 Characteristics of Sound Waves • Types of Sound Waves: • Audible Waves: those that we can hear (between 20-20,000 Hz) • Infrasonic Waves: frequencies below audible (earthquakes) • Ultrasonic Waves: frequencies above audible (dog whistles) • Due to their high f and short , these can be used to produce images of small objects (ultrasounds)

  15. 14.3 The Speed of Sound • Speed of sound and temperature are directly proportional. • At STP (0°C and 1 atm), the speed of sound is always 331 m/s. • At room temperature (25°C), the speed of sound is always 343 m/s. • Sound can travel through solids, liquids, or gases. • Travels faster through solids because particles are closer together

  16. Practice Complete the “Wave Speed” worksheet.

  17. 14.4 Energy and Intensity of Sound Waves • As sound waves travel, energy is transferred from one molecule to the next • Intensity: the rate at which energy flows through an area • Intensity=Power/Area=(W/m2) • Relative intensity is measured in decibels (dB) • Intensity is related to the “loudness” of a sound

  18. 14.4 Energy and Intensity of Sound Waves Continued • Audibility of sound depends on frequency and intensity. • Threshold of hearing: the faintest sounds we can detect (1000 Hz and 1.0x10-12 W/m2) • Prolonged exposure to sound lower will damage the ear. • Threshold of pain: the loudest sounds we can tolerate (1.0 W/m2) • Any more intense sounds will damage the ear

  19. 14.5 Spherical and Plane Waves • Sound waves travel in all directions. • Each circle represents a wave front. • The wave front occurs at the center of a compression. • The distance between wave fronts is equal to a wavelength. • Wave fronts spread in 3D spheres • I=Power/4r2

  20. 14.6 The Doppler Effect • An observed change in frequency when there is relative motion between the source and observer. • Sound waves towards the observer: increase in frequency, decrease in relative wavelength • Sound waves away from the observer: decrease in frequency, increase in relative wavelength

  21. 14.7 Interference of Sound • Speaker use constructive interference to add sound waves to produce a louder sound. • If the sound waves are completely destructive, no sound will be heard. • THIS IS WHY SPEAKER WIRES ARE COLOR-CODED

  22. 14.8 Standing Waves • A wave pattern that results when two waves of the same f, , and A travel in opposite directions and interfere. • Node: complete destructive interference • Antinode: halfway between nodes where the largest displacement occurs • The end of fixed strings will be nodes because the can’t move.

  23. 14.8 Standing Waves Continued • Harmonic Series: series of frequencies that include the fundamental frequency of an instrumental string • Fundamental frequency: the lowest frequency of vibration for a string (first harmonic, f1, 1=2L) • Second harmonic: f2=2f1, 2=L • Third harmonic: f3=3f1, 3=2/3L

  24. 14.9 Forced Vibrations and Resonance • Resonance: a phenomenon that occurs when the frequency of an applied force matches the natural frequency of the system (results in a max. amplitude) • Allows opera singers to shatter class • Collapse of Tacoma Narrows bridge due to wind setting the structure in oscillation

  25. 14.10 Standing Waves in Air Columns • Air columns are used in instruments like trumpets, saxophones, or organs. • Open tube: both ends are antinodes • f1, 1=2L (same as string instruments) • Fundamental frequency can be varied by changing the length of the air column • Closed tube: one end is a node and the other is an antinode • f1, 1=4L • f3, 3=4/3 L

  26. 14.11 Beats • Frequency of a sound wave determines the pitch (low versus high). • Intensity and amplitude determines the loudness of sound. • Beats: the alternation of loudness • If two waves with slightly varying frequencies interfere, they will have periodic variation in amplitude

  27. 14.12 Quality of Sound • Different mixtures of harmonies per instrument produces a spectrum of sound. • Timbre: characteristic sound of any instrument

  28. 14.13 The Ear • Outer Ear: sound waves travel down the canal and hits the eardrum • Middle Ear: eardrum transfers vibrations to the hammer, anvil, and stirrup, which transfers them to the cochlea • Inner Ear: the basilar membrane of the cochlea has various natural frequencies which resonate and send impulses to nerve cells, which send information to the brain where they are interpreted.

  29. Reflection, Refraction, and Diffraction • Reflection: bouncing off a barrier causing a change in direction of a wave • Refraction: passing from one medium to another causing a change in direction of a wave • Diffraction: passing through an opening or moving around a barrier causing a change in direction of a wave • ALL OF THESE EQUATIONS AND DEFINITIONS APPLY TO LIGHT WAVES AS WELL (Electromagnetic Spectrum)

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