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06 Feb 5 Sound, Interference, Pitch, Beats After Class

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Thursday Feb. 5 lecture slides. Nerf doppler ball demo; lecture hall acoustics (dead spots, beats, wah-wah); noise cancellation.

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06 Feb 5 Sound, Interference, Pitch, Beats After Class

  1. 1. Today: Sound Waves, Interference, Beats, Pitch <ul><li>Essential that you practice the quizzes! </li></ul><ul><li>Next reading, quiz, homework assigned later in week (after exam) </li></ul>Exam #1 is Thursday!!! In this room, BRING A PENCIL!!! Big eared Townsend Bat, Nevada Bureau of Land Management
  2. 2. Quiz # 2 Results
  3. 4. Last week’s puzzler: Wind and Doppler Effect <ul><li>Another perspective, may cause “aha!”: </li></ul><ul><ul><li>What if both the whistle and the person were moving in the same direction? Is that any different than a wind blowing? </li></ul></ul>The compression / rarefaction waves are what you hear… Remember it’s the frequency that determines the pitch
  4. 5. Last week’s puzzler: Wind and Doppler Effect <ul><li>Another perspective, may cause “aha!”: </li></ul><ul><ul><li>What if both the whistle and the person were moving in the same direction? Is that any different than a wind blowing?.... CLICKER Q! </li></ul></ul>
  5. 6. 1. Yes 2. No Does the wind affect the pitch of a factory whistle you hear on a windy day?
  6. 7. Author’s very good analogy, why wind does not affect pitch of sound <ul><li>If the first guy is only putting down items once per second… </li></ul><ul><li>How could a faster belt increase the number of items per second the second guy can pick up? </li></ul>
  7. 8. Let’s explore Doppler effect for sound with demos and applets http://www.falstad.com/mathphysics.html ripple Nerf Doppler Demo Very good animation http://physics-animations.com/Physics/English/waves.htm Relative movement between source and receiver DOES change the frequency and wavelength Does NOT change the wave speed Pay attention to things like frequency, wavelength, and wave speed during applets
  8. 9. Is the frequency of a sound wave affected by bouncing off moving surface? This effect is used by some bats in echolocation! Can determine whether insect is approaching Can detect rapidly beating wings of insects Big eared Townsend Bat, Nevada Bureau of Land Management
  9. 10. Is the frequency of a sound wave affected by bouncing off moving surface? This effect is used in ultrasonography! Doppler shift in sound frequency reveals blood velocity User:Ekko on wikipedia (above) also: http://www.youtube.com/watch?v=L88hNMak4Go
  10. 11. Sound in air: Compression / Rarefaction Waves <ul><li>Most of the sound you hear comes from traveling waves through the air. </li></ul><ul><li>Tricky: Compressions and Rarefactions travel in the same direction </li></ul><ul><li>Applet for visualizing traveling sound wave http://positron.ps.uci.edu/~dkirkby/music/html/demos/PlaneWave/Highlighted.html </li></ul>(a) compression (b) rarefaction (c) wave propegation of a tuning fork as seen from above From http://www.pjwstk.edu.pl/~kmarasek/3-sound.ppt What is going on when I strike this tuning fork?
  11. 12. <ul><ul><li>A) They are carried by the wave to your ear. </li></ul></ul><ul><ul><li>B) They are dislocated only a little by the wave. </li></ul></ul>When a sound is emitted at the front of the lecture hall, what happens to the air molecules in the sound wave?
  12. 13. <ul><ul><li>A) They are carried by the wave to your ear. </li></ul></ul><ul><ul><li>B) They are dislocated only a little by the wave. </li></ul></ul>When a sound is emitted at the front of the lecture hall, what happens to the air molecules in the sound wave? Waves transmit energy not matter! If air molecules are dislocated only a little, how are we able to hear the sound? Pressure on our ear drums!
  13. 14. Speed of sound in air about 770 miles per hour <ul><li>Speed does not depend on frequency or amplitude (usually) pressure or density </li></ul><ul><li>Speed does depend on temperature, humidity (mass of molecules)… wind ! </li></ul><ul><li>Dry air, 68 degrees F: Speed about 770 mph or 340 m/s (Remember this!) </li></ul><ul><ul><li>(Speed of light in vacuum, ~670 million mph or 3 * 10 8 m/s…1 million times faster) </li></ul></ul>
  14. 15. Clicker Question—Speed of sound <ul><li>Above is an amination of the time it takes light to reach the moon from earth (~ 3 seconds). How long would it take the sound from an incredible explosion to reach the moon? </li></ul><ul><li>Same </li></ul><ul><li>1,000 times longer </li></ul><ul><li>1,000,000 times longer </li></ul><ul><li>Sound would never survive the trip </li></ul>
  15. 16. Clicker Question—Speed of sound <ul><li>Above is an amination of the time it takes light to reach the moon from earth (~ 3 seconds). How long would it take the sound from an incredible explosion to reach the moon? </li></ul><ul><li>Same </li></ul><ul><li>1,000 times longer </li></ul><ul><li>1,000,000 times longer </li></ul><ul><li>Sound would never survive the trip </li></ul>Sound waves cannot travel through a vacuum!
  16. 17. So, you can’t have waves in space, right? http://www.youtube.com/watch?v=3drcpE-0Inc
  17. 18. Clicker question In air, what is the wavelength of a 440 Hz sound wave? <ul><li>0.75 millimeters </li></ul><ul><li>0.75 meters </li></ul><ul><li>7.5 meters </li></ul><ul><li>75 meters </li></ul>http://www.phys.unsw.edu.au/~jw/notes.html 440 Hz
  18. 19. Clicker question In air, what is the wavelength of a 440 Hz sound wave? <ul><li>0.75 millimeters </li></ul><ul><li>0.75 meters </li></ul><ul><li>7.5 meters </li></ul><ul><li>75 meters </li></ul><ul><li>First: try order of magnitude </li></ul>http://www.phys.unsw.edu.au/~jw/notes.html 440 Hz ¾ meter = 340 m/s / 440 Hz Wavelength = Wave speed frequency
  19. 20. There can be constructive or destructive interference Constructive Interference “ In Phase” Destructive Interference “ 180 degrees out of phase” Remember the wave table demo with Ashley on Tuesday? This interference happens with sound waves as well.
  20. 21. Noise “cancellation” headphones use wave interference to reduce noise <ul><li>Electronics in headphone detect incoming sound, and create new sound wave 180 degrees out of phase </li></ul>
  21. 22. Sound reflects very well…and interferes with itself <ul><li>Remember the waves on the rope reflecting off wall? </li></ul><ul><li>Waves reflect when the form of matter changes (interfaces) </li></ul>Much work goes into “good acoustics” for orchestra halls…preventing “dead spots” Let’s try this out! w/ speakers and sound level meters
  22. 23. Sound waves of two different frequencies also interfere… “beats” <ul><li>The “beat frequency” is the difference in the two source frequencies: f 2 – f 1 </li></ul><ul><li>Demo w/ big speakers </li></ul>http://www.ngsir.netfirms.com/englishhtm/Beats.htm
  23. 24. Clicker Question—Beats <ul><li>Consider two different beat situations: (A) 440 Hz and 442 Hz sound wave (B) 225 Hz and a 220 Hz sound wave </li></ul><ul><li>Which one will be perceived as beating more quickly? </li></ul>
  24. 25. Clicker Question—Beats <ul><li>Consider two different beat situations: (A) 440 Hz and 442 Hz sound wave (B) 225 Hz and a 220 Hz sound wave </li></ul><ul><li>(A) will beat twice per second, and (B) will beat 5 times per second. (But (A) will sound as higher pitch) </li></ul>
  25. 26. Frequency of wave influences pitch perception http://www.phys.unsw.edu.au/~jw/notes.html 4186 Hz 440 Hz 261.6 Hz 27.5 Hz “ Psychoacoustics” Shepard’s tones http://www.netalive.org/tinkering/shepard-effect/ Lissajous curves & beats http://www.youtube.com/watch?v=DXpntnHxNZQ
  26. 27. Parting question: Is pitch perception physics? Biology? Exam #1 is Thursday!!! In this room, BRING A PENCIL!!!

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