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Auditory Content
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Sound
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  • Defined:
    • A series of vibrations moving as waves through air or other gases, liquids, or solids
  • Our Primary Concern:
    • Air waves (i.e.,successive compressions and rarefactions of air )
Sound Waves
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  • Longitudinal:
    • Whereas light waves are transverse (i.e., the wave is perpendicular to the direction of travel), sound waves are longitudinal (i.e., the wave us in the direction of travel)
  • Traveling:
    • The air molecules disturb neighboring molecules, transferring their energy to them
Sound Waves (cont.)
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A Periodic Pressure Wave

soundwave_molecules
Sound Waves (cont.)
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A Periodic Pressure Wave

soundwave
Hearing
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  • The Ear:
    • ear
  • The Process:
    • Sound waves are collected by the auricle
    • Pressure changes cause the eardrum to vibrate
    • Vibrations are amplified by the ossicles
    • Vibrations are transferred to endolymph fluid in the cochlea
    • The basilir membrane vibrates at a particular location
    • The stereocilia stimulate sensory cells
    • The resulting electrical impulses are transmitted by the auditory nerve
Perception of Soundwaves (An Approximation)
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  • Volume:
    • Difference in amplitude (measured in decibels)
    • Audible Sounds: 0dB
    • Painful Sounds: 120dB
  • Pitch:
    • Differences in frequency (mesaured in Hz)
    • Acoustic Signals: 15Hz to 18kHz
  • Timbre:
    • Pronounced tam-bur
    • Includes the waveform's distribution of energy at different frequencis (i.e., spectra) and its envelopes/transients (i.e., the attack, sustain and decay)
Perception of Sound Waves (cont.)
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sound-envelope
Perception of Sound Waves (cont.)
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  • Localization Defined:
    • Our ability to locate the origin of a sound
  • Localization Mechanisms:
    • The interaural time difference (i.e., the difference in the time it takes for a sound to reach both ears). Most people can detect a difference of about 20 microseconds (millionths of a second).
    • The interaural density difference (i.e., the difference in amplitude caused by our head interfering with the sound wave).
    • Frequency filtering performed by the outer ear.
Perception of Sound Waves (cont.)
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  • Complex Wave Forms:
    • Created when two or more soundwaves travel through the same medium at the same time
  • Our Perception:
    • We can (often) perceive the "original" waves (i.e., we perform Fourier analysis)
  • Biological Basis:
    • Different locations of the basilir membrane are "tuned" to different frequencies (except when the tones are close)
Auditory Output Devices
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  • Conventional Loudspeakers:
    • One or more drivers, each of which consiste of a diaphragm (often a semi-flexible cloth or paper with a ... attached) mounted in a basket
    • The electrical signal turns an electromagnet on and off, causing the diaphragm to vibrate, creating pressure waves
  • Electrostatic Speakers:
    • Use a very thin electrically conductive diaphragm mounted in between two charged panels
  • Planar Magnetic Speakers:
    • Use a thin magnetically conductive diaphragm mounted between two magnetic panels
Rendering
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  • Using a Soundbank:
    • The synthesizer has a database of sounds
  • Using Continuous Waveforms:
    • The synthesizer uses pure waveforms to create complex waveforms
There's Always More to Learn
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