CS 414 – Multimedia Systems Design Lecture 2 –Auditory Perception and Digital AudioAdministrativeSlide 3Auditory PerceptionChanges in Air PressureAuditory SystemPhysical DimensionsSound Perception and PsychoacousticsPsychological DimensionsPsychological Dimensions (cont.)Sound IntensityDecibel ScaleDecibels of Everyday SoundsInterpretation of Decibel ScaleLoudness from Multiple SourcesExercisesLoudness and PitchFletcher-Munson ContoursMaskingFrequency MaskingSlide 21Slide 22Temporal MaskingSlide 24SummaryCS 414 - Spring 2011CS 414 – Multimedia Systems Design Lecture 2 –Auditory Perception and Digital AudioKlara NahrstedtSpring 2011CS 414 - Spring 2011Administrative Form Groups for MPs Deadline: Latest January 24 to email TA [email protected] 414 - Spring 2011Auditory PerceptionSound – physical phenomenon caused by vibration of material These vibrations trigger pressure wave fluctuations in the airWave formsCS 414 - Spring 2011Changes in Air PressureCS 414 - Spring 2011Auditory SystemEars, parts of brain, and neural pathwaysChanges in pressure move hair-like fibers within the inner earMovements result in electrical impulses sent to the brainPhysical DimensionsAmplitudeheight of a cyclerelates to loudnessWavelength (w)distance between peaksFrequency ( )cycles per secondrelates to pitch w = velocityMost sounds mix many frequencies & amplitudesSound is repetitive changesin air pressure over timeSound Perception and PsychoacousticsPsychoacousticsStudy correlation between physics of acoustical stimuli and hearing sensationsExperimental data and models are useful for audio codecModeling human hearing mechanismsAllows to reduce the data rate while keeping distortion from being audibleCS 414 - Spring 2011Psychological DimensionsLoudnesshigher amplitude results in louder soundsmeasured in decibels (db), 0 db represents hearing thresholdPitchhigher frequencies perceived as higher pitchHumans hear sounds in 20 Hz to 20,000 Hz rangeCS 414 - Spring 2011Psychological Dimensions (cont.)Timbre (tam-bre)complex patterns added to the lowest, or fundamental, frequency of a sound, referred to as spectraspectra enable us to distinguish musical instrumentsMultiples of fundamental frequency give musicMultiples of unrelated frequencies give noiseSound IntensityIntensity (I) of a wave is the rate at which sound energy flows through a unit area (A) perpendicular to the direction of travelP measured in watts (W), A measured in m2Threshold of hearing is at 10-12 W/m2Threshold of pain is at 1 W/m2APtEAI 1CS 414 - Spring 2011Decibel ScaleDescribes intensity relative to threshold of hearing based on multiples of 100log10IIdB CS 414 - Spring 2011I0 is reference level = 10-12 W/m2Decibels of Everyday SoundsSound DecibelsRustling leaves 10Whisper 30Ambient office noise 45Conversation 60Auto traffic 80Concert 120Jet motor 140 Spacecraft launch 180Interpretation of Decibel Scale 0 dB = threshold of hearing (TOH)10 dB = 10 times more intense than TOH20 dB = 100 times more intense than TOH30 dB = 1000 times more intense than TOHAn increase in 10 dB means that the intensity of the sound increases by a factor of 10If a sound is 10x times more intense than another, then it has a sound level that is 10*x more decibels than the less intense soundCS 414 - Spring 2011Loudness from Multiple SourcesUse energy combination equation where L1, L2, …, Ln are in dB)10...1010log(1010102101 LNLLL CS 414 - Spring 2011ExercisesShow that the threshold of hearing is at 0 dBShow that the threshold of pain is at 120 dBSuppose an electric fan produces an intensity of 40 dB. How many times more intense is the sound of a conversation if it produces an intensity of 60 dB?One guitar produces 45 dB while another produces 50 dB. What is the dB reading when both are played?If you double the physical intensity of a sound, how many more decibels is the resulting sound?CS 414 - Spring 2011Loudness and PitchMore sensitive to loudness at mid frequencies than at other frequenciesintermediate frequencies at [500hz, 5000hz]Human hearing frequencies at [20hz,20000hz]Perceived loudness of a sound changes based on frequency of that soundbasilar membrane reacts more to intermediate frequencies than other frequenciesCS 414 - Spring 2011Fletcher-Munson ContoursEach contour represents an equal perceived soundCS 414 - Spring 2011Perception sensitivity (loudness) is not linear across all frequencies and intensitiesMaskingPerception of one sound interferes with anotherFrequency maskingTemporal maskingCS 414 - Spring 2011Frequency MaskingLouder, lower frequency sounds tend to mask weaker, higher frequency soundsFrom http://www.cs.sfu.ca/CourseCentral/365/CS 414 - Spring 2011Frequency MaskingLouder, lower frequency sounds tend to mask weaker, higher frequency soundsCS 414 - Spring 2011Frequency MaskingLouder, lower frequency sounds tend to mask weaker, higher frequency soundsCS 414 - Spring 2011Temporal MaskingWhen exposed to a loud sound, the human ear contracts slightly to protect delicate structuresCauses louder sounds to overpower weaker sounds just before and just after itCS 414 - Spring 2011Temporal MaskingCS 414 - Spring 2011Summary Auditory Perception is very important for understanding digital audio representation Psychoacoustic is used in MP3 audio compression CS 414 - Spring
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