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Digital media I: Audio

Digital media I: Audio. Glenn Bresnahan Robert Putnam glenn@bu.edu putnam@bu.edu. Outline. Part I (Glenn) What is sound? How do we hear? Part II (Robert) Qualities of sound Sound reproduction analog v. digital Sound in VR. Waves revisited. Waves – (non)artistic rendering.

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Digital media I: Audio

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  1. Digital media I: Audio Glenn Bresnahan Robert Putnam glenn@bu.eduputnam@bu.edu

  2. Outline • Part I (Glenn) • What is sound? • How do we hear? • Part II (Robert) • Qualities of sound • Sound reproduction • analog v. digital • Sound in VR BPC: Art and Computation – Fall 2006

  3. Waves revisited BPC: Art and Computation – Fall 2006

  4. Waves – (non)artistic rendering BPC: Art and Computation – Fall 2006

  5. Wave properties • How might we describe waves? BPC: Art and Computation – Fall 2006

  6. Wave properties • How might we describe waves? • Height • Time between waves • Speed of the wave BPC: Art and Computation – Fall 2006

  7. Shoals and tides BPC: Art and Computation – Fall 2006

  8. Tide tables BPC: Art and Computation – Fall 2006

  9. Cause of tides • Gravity from moon and sun 1 day New moon 27.3 days (29.5 days) Full moon 365 days BPC: Art and Computation – Fall 2006

  10. Phases of the moon Moon phases: New moon Waxing crescent First quarter Waxing gibbous Full moon Waning gibbous Last quarter New moon BPC: Art and Computation – Fall 2006

  11. Moon phases BPC: Art and Computation – Fall 2006

  12. Sunrise and sunset BPC: Art and Computation – Fall 2006

  13. Sunrise and sunset solstice solstice BPC: Art and Computation – Fall 2006

  14. Waves – sine waves • Sine wave is the fundamental wave BPC: Art and Computation – Fall 2006

  15. Waves – properties Wavelength (distance) Amplitude BPC: Art and Computation – Fall 2006

  16. Waves in motion – properties Period (time for one cycle) 2 Frequency cycles per time interval 1 Time BPC: Art and Computation – Fall 2006

  17. What is sound? • Examples BPC: Art and Computation – Fall 2006

  18. What is sound – vibration • Striking an object will cause it to vibrate • The vibration is a sine wave • Objects have a natural vibration frequency • Resonance frequency • Frequency depends on type of material, thickness, length/size, tension • May have multiple vibrating frequencies • The pitch depends on the frequency • Loudness (amplitude) depends on size of the object BPC: Art and Computation – Fall 2006

  19. What is sound – vibrations moves air Wave Energy (pluck) Air pressure level vibration string BPC: Art and Computation – Fall 2006

  20. Properties of sound • Pitch is perception of frequency • Frequency is measured in cycles per second (cps) • Hertz (Hz) = cycles per second • The A above middle C is 440 Hz. • Humans hear appox. 20-20,000 Hz • Sound travels at approx. 1100 feet/second • Speed depends on pressure and temperature • Approx. 750 miles/hour • Approx. 1 mile every 4.8 seconds • Perceived loudness depends on pressure level • Sound pressure is measured in (micro)pascals (20uPa) • Loudness is usually expressed in decibels (dB) BPC: Art and Computation – Fall 2006

  21. Real Waves BPC: Art and Computation – Fall 2006

  22. Properties of sound • Real sounds are far more complex than simple sine waves • Objects produce vibrations at multiple frequencies • Sound waves interact with other objects • Waves bounce (reflect) off surface • Reverberation/echo • Wave are absorbed by materials • Sound waves interact with each other BPC: Art and Computation – Fall 2006

  23. Combinations of waves BPC: Art and Computation – Fall 2006

  24. Properties of sound – real sounds BPC: Art and Computation – Fall 2006

  25. Electrification of sound • Microphones • Convert pressure levels into electrical signals (voltages) • Guitar pickups • Converts string vibration to voltages • The pickup contains a magnet and a coil • The vibrating metal strings alter the magnetic field and induce a voltage in the coil • Loud speakers convert an electrical signal back into air pressure BPC: Art and Computation – Fall 2006

  26. How do we hear? • Sound waves move through the air from the sound source to the ear BPC: Art and Computation – Fall 2006

  27. Anatomy of the ear BPC: Art and Computation – Fall 2006

  28. Anatomy of the ear - outer • Divided into three principal sections • Outer ear • Middle ear • Inner ear • Outer ear • External ear, aka pinna • Ear canal • Outer ear funnels the ear have to the eardrum BPC: Art and Computation – Fall 2006

  29. Anatomy of the ear - middle • Middle ear • Eardrum • Set of 3 ear bones • the 3 bones are rigid • Act as a mechanical amplifier • The 3rd bone, stapes, induces a vibration into the inner ear, i.e. the cochlea BPC: Art and Computation – Fall 2006

  30. Anatomy of the ear - inner • Inner ear / cochlea • Where the real work is done • Cochlea is a spiral tube and filled with fluid • Stapes causes a wave to pass through the fluid BPC: Art and Computation – Fall 2006

  31. Anatomy of the ear - inner • Cochlea is a spiral tube lined with hair cells on a membrane (~15K HCs) • Hairs vary in length and thickness along the tube • Hairs resonate at different frequencies • High freq on near end, low at far end BPC: Art and Computation – Fall 2006

  32. Anatomy of the ear - inner BPC: Art and Computation – Fall 2006

  33. Anatomy of the ear - inner • Hair cells are connected to the auditory nerve cells • The vibrations excite the nerve cells and cause them to fire (electrical signal) • A series of nerve cells pass the signal to brain BPC: Art and Computation – Fall 2006

  34. Binaural hearing - why two ears? • Two ears, so we can identify locations of sounds • Time difference • Intensity difference • Sound color difference (caused by movement of sound around head and shoulders) BPC: Art and Computation – Fall 2006

  35. Sound localization – pinna back • Sound waves interact with the asymmetric Pinna • The effect on the sound varies with the direction • Up/down, back/front waves result in different sounds entering ear canal front BPC: Art and Computation – Fall 2006

  36. Digital media I: part II • Other qualities of sound: pitch, timbre, “noise”, envelope • Sound reproduction: analog v. digital • Sound in VR BPC: Art and Computation – Fall 2006

  37. What is pitch? • Our perception of the highness or lowness of a tone. • Closely related to frequency • When frequency doubles, pitch rises by an “octave” • Examples • But, what happens when there’s more than one frequency in a sound? BPC: Art and Computation – Fall 2006

  38. Review: modes of vibration of a string Fundamental [e.g., 110 Hz] 2nd harmonic [e.g., 220 Hz] 3rd harmonic [e.g., 330 Hz] Examples BPC: Art and Computation – Fall 2006

  39. Timbre • Sound color, or “timbre” is a quality of sound that derives from the particular combination of frequencies (a.k.a., “harmonics” or “partials”) in a tone. • Two sounds can contain the same harmonics but sound very different because their individual harmonics are of different amplitudes. • Examples BPC: Art and Computation – Fall 2006

  40. Timbre, continued • Easy to demonstrate timbre with human voice • Hum. • Slowly open mouth. • Hear how the sound color changes from “dark” to “bright” • Example BPC: Art and Computation – Fall 2006

  41. Timbre, continued. • Timbre changes as a wind instrument is played louder or softer. • Example BPC: Art and Computation – Fall 2006

  42. Unpitched sounds • Can use human voice to demonstrate another distinction: pitched versus unpitched sounds • Make “s” sound • No identifiable “pitch” • Related to concept of “noise” • Examples BPC: Art and Computation – Fall 2006

  43. Examples • Pitched sounds • Birdsong • Flutes • Stringed instruments • Etc. • Examples of unpitched sounds • Certain percussion instruments (cymbals, ratchets, etc.) • Wind, rain, footsteps in snow • Listen now. What do you hear? Frequencies, amplitudes. Pitched, unpitched. External versus internal sources. BPC: Art and Computation – Fall 2006

  44. Time variation of sounds • Most naturally occurring sounds are not static; i.e., they vary over time • Amplitude • Pitch • Timbre • Examples BPC: Art and Computation – Fall 2006

  45. Sound recording technologies • Analog • Digital BPC: Art and Computation – Fall 2006

  46. Analog recording • Analog: “device or system that represents changing values as continuously variable physical quantities.” • Example: clock with hour, minute and second hands • Question: what values are changing when we hear sound? BPC: Art and Computation – Fall 2006

  47. Analog recording technologies • Phonautograph BPC: Art and Computation – Fall 2006

  48. Analog recording technologies • Mechanical: Gramophone, LP record, etc. BPC: Art and Computation – Fall 2006

  49. Analog recording technologies • Magnetic: Wire, tape recorder. BPC: Art and Computation – Fall 2006

  50. Analog recording technologies • Optical: movie soundtrack. BPC: Art and Computation – Fall 2006

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