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Sound Waves

Sound Waves. What is sound?. Sound is really tiny fluctuations of air pressure units of pressure: N/m 2 or psi (lbs/square-inch) Carried through air at 345 m/s (770 m.p.h ) as compressions and rarefactions in air pressure. What IS Sound?. wavelength. compressed gas. rarefied gas.

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Sound Waves

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  1. Sound Waves

  2. What is sound? • Sound is really tiny fluctuations of air pressure • units of pressure: N/m2 or psi (lbs/square-inch) • Carried through air at 345 m/s (770 m.p.h) as compressions and rarefactions in air pressure

  3. What IS Sound? wavelength compressed gas rarefied gas

  4. Sound Waves • Remember, sound waves are longitudinal waves • Particles of air vibrate in the same direction the wave travels

  5. Why is Sound Longitudinal? • Waves in air can’t really be transverse, because the atoms/molecules are not bound to each other • can’t pull a (momentarily) neighboring molecule sideways • only if a “rubber band” connected the molecules would this work • fancy way of saying this: gases can’t support shear loads • Air molecules can really only bump into one another • Imagine people in a crowded train station with hands in pockets • pushing into crowd would send a wave of compression into the crowd in the direction of push (longitudinal) • jerking people back and forth (sideways, over several meters) would not propagate into the crowd • but if everyone held hands (bonds), this transverse motion would propagate into crowd

  6. Speed of Sound • 344 m/s in air at 20°C • Depends on: • Temperature of medium • travels faster at higher temps • Type of medium • travels better through liquids and solids • can’t travel through a vacuum

  7. Speed of Sound • Sound speed in air is related to the frantic motions of molecules as they jostle and collide • since air has a lot of empty space, the communication that a wave is coming through has to be carried by the motion of particles • for air, this motion is about 500 m/s, but only about 350 m/s directed in any particular direction • Solids have faster sound speeds because atoms are hooked up by “springs” (bonds) • don’t have to rely on atoms to traverse gap • spring compression can (and does) travel faster than actual atom motion

  8. Example Sound Speeds

  9. Speed of Sound in Air

  10. Human Hearing sound wave vibrates ear drum amplified by bones converted to nerve impulses in cochlea

  11. Sound hitting your eardrum • Pressure variations displace membrane (eardrum, microphone) which can be used to measure sound • my speaking voice is moving your eardrum by a mere 1.510-4 mm = 150 nm = 1/4 wavelength of visible light! • threshold of hearing detects 510-8 mm motion, one-half the diameter of a single atom!!! • pain threshold corresponds to 0.05 mm displacement • Ear ignores changes slower than 20 Hz • so though pressure changes even as you climb stairs, it is too slow to perceive as sound • Eardrum can’t be wiggled faster than about 20 kHz • just like trying to wiggle resonant system too fast produces no significant motion

  12. Human Hearing • Pitch • highness or lowness of a sound • depends on frequency of sound wave • human range: 20 - 20,000 Hz ultrasonic waves subsonic waves

  13. Human Hearing • Intensity • volume of sound • depends on energy (amplitude) of sound wave • measured in decibels (dB)

  14. Sound frequency • Frequency is equivalent to pitch Humans can hear 20- 20 000 Hz Demo: How high can you hear? Elephants & Whales- Infrasonic Bats & Dolphin- Ultrasonic http://www.whaleacoustics.com/audiobaleenwhales.html http://www.seaworld.org/animal-info/info-books/bottlenose/communication.htm http://www.cbmwc.org/education/echo.asp

  15. Human Hearing • Humans hear sounds in a limited frequency range 20 Hz-20,000 Hz • Any sound below the human range of hearing is known as an infrasound • Any sound above the human range of hearing is known as an ultrasound

  16. Human Hearing DECIBEL SCALE 120 110 100 80 70 40 18 10 0

  17. Interaction of Sound Waves

  18. Reflection of sound When sound encounters an obstacle the sound waves bounce off and reflect This causes an ECHO

  19. Refraction of Sound • Sound waves can bend if there are different temperatures in the medium • The wave always bends towards warmer temperatures

  20. Refraction of sound

  21. Diffraction of Sound • Since sound diffracts, we can hear sound around corners or through openings • This is why we use megaphones that are shaped like cones

  22. Doppler Effect • Doppler Effect • change in wave frequency caused by a moving wave source • moving toward you - pitch sounds higher • moving away from you - pitch sounds lower

  23. Doppler Effect- Pitch increases as an object approaches and decreases as it moves away. Click herefor simulation Click here for simulation

  24. lower frequency higher frequency Doppler Effect Stationary (non-moving) source Moving source Supersonic source waves combine to produce a shock wave called a sonic boom same frequency in all directions

  25. More about the sonic boom • Shock wave generated by planes and bullets

  26. Sound Barrier • Mach 1- the speed of sound- 331 m/s or 741 MPH) • Breaking the Sound Barrier SR-71 Blackbird Mach 3.31 Chuck Yeager 1947 X-1

  27. F-18 Breaks the Sound Barrier at 741 MPH (approx.) Click here to see the video!

  28. Medical Imaging SONAR “Sound Navigation Ranging” Seeing with Sound • Ultrasonic waves - above 20,000 Hz

  29. Sound Music • Music vs. Noise • Resonance • Harmonics • Interference • Acoustics

  30. Music vs. Noise • Music • specific pitches and sound quality • regular pattern • Noise • no definite pitch; no set pattern

  31. Resonance • Forced Vibration • when one vibrating object forces another object to vibrate at the same frequency • results in a louder sound because a greater surface area is vibrating • used in guitars, pianos, etc.

  32. Resonance • Resonance • A phenomenon that occurs when two objects naturally vibrate at the same frequency • special case of forced vibration • object is induced to vibrate at its natural frequency

  33. Resonance “Galloping Gertie” The Tacoma Narrows Bridge Disaster Wind through a narrow waterway caused the bridge to vibrate until it reached its natural frequency.

  34. Tacoma Narrow Bridge 1943 VideoTACOMA.mpeg

  35. Harmonics • Fundamental • the lowest natural frequency of an object • Overtones • multiples of the fundamental frequency

  36. Constructive - louder Destructive - softer Interference • Interference • the ability of 2 or more waves to combine to form a new wave

  37. Interference • Beats • variations in sound intensity produced by 2 slightly different frequencies • both constructive and destructive interference occur

  38. Sound Wave Interference and Beats signal A in phase: add signal B A + B beat (interference) out of phase: cancel

  39. Beats and Average Frequency http://library.thinkquest.org/19537/java/Beats.html fa-fb = fbeat Frequency A – frequency B = Beat frequency Ex1: A couple is taking a walk. If Jim takes 264 steps per minute and Sue takes 262 steps per minute, they will be in step twice during a one minute walk and these two steps will be louder than others. The listener will hear a total of 263 steps. Ex2: If tuning fork A vibrates 264 times each second and fork B vibrates 262 times each second, they will be in step twice each second and the listener will hear a beat of 2 Hz and an overall tone of 263 hertz. Musicians use beats to tune instruments. Dolphins use to detect motion.

  40. Another example of Beats

  41. Beats

  42. AM vs FM Radio

  43. Anechoic chamber - designed to eliminate reverberation. E. Acoustics • Acoustics • the study of sound • Reverberation • echo effect produced by the reflection of sound

  44. UCSD: Physics 8; 2006 All Shapes of Waveforms • Different Instruments have different waveforms • a: glockenspiel • b: soft piano • c: loud piano • d: trumpet • Our ears are sensitive to the detailed shape of waveforms! • More waveforms: • e: french horn • f: clarinet • g: violin http://www.st-and.demon.co.uk/AudioMisc/asymmetry/asym.html

  45. How does our ear know? • Our ears pick out frequency components of a waveform • A DC (constant) signal has no wiggles, thus is at zero frequency • A sinusoidal wave has a single frequency associated with it • The faster the wiggles, the higher the frequency • The height of the spike indicates how strong (amplitude) that frequency component is

  46. Speakers: Inverse Eardrums • Speakers vibrate and push on the air • pushing out creates compression • pulling back creates rarefaction • Speaker must execute complex motion according to desired waveform • Speaker is driven via “solenoid” idea: • electrical signal (AC) is sent into coil that surrounds a permanent magnet attached to speaker cone • depending on direction of current, the induced magnetic field either lines up with magnet or is opposite • results in pushing or pulling (attracting/repelling) magnet in coil, and thus pushing/pulling on center of cone

  47. Speaker Geometry

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