Sensory Systems:

How Can You Localize Sound?Ponder this:Imagine digging two trenches in the sand beside a lake so that water can flow into them. Now imagine hanging a piece of cloth in the water in each trench. Your job is to determine the number and location and type of every fish, duck, person, boat, etc. simply by examining the motion of the cloth. Thats what your auditory system does!- Al Bregman1All you have is a pair of instruments (basilar membranes) that measure air pressure fluctuations over timeLocalization

2There are several clues you could use:Localization3LocalizationLeft EarRight EarCompressionWaves4There are several clues you could use:arrival time - sound arrives first at ear closest to sourceLocalization5LocalizationLeft EarRight EarCompressionWaves6There are several clues you could use:arrival timephase lag (waves are out of sync) - wave at ear farthest from sound source lags wave at ear nearest to sourceLocalization7LocalizationLeft EarRight EarCompressionWaves8There are several clues you could use:arrival timephase lag (waves are out of sync) - wave at ear farthest from sound source lags wave at ear nearest to sourceHead shadowLocalization9LocalizationArrival TimePhase Lag

Head ShadowInteraural Timing Differences (ITD)Interaural Intensity Difference (IID)What are some problems or limitations?Localization11Low frequency sounds arent attenuated by head shadow because sound bends around the head with little loss of amplitudeLocalizationLeft EarRight EarCompressionWavesSound is the sameSPL at both ears12Low frequency sounds arent attenuated by head shadow

Your brain preferentially uses ITD cues for low-frequency soundsLocalization13High frequency sounds have ambiguous phase lag because more than one wavelength fits between the earsLocalizationLeft EarRight EarLeft EarRight EarTwo locations, same phase information!14High frequency sounds have ambiguous phase lag

Your brain preferentially uses IID cues for high-frequency sounds

Localization15These cues only provide azimuth (left/right) angle, not altitude (up/down) and not distanceLocalizationLeft EarRight EarAzimuth16LocalizationAdditional cues:

17LocalizationAdditional cues:

Head Related Transfer Function: Pinnae modify the frequency components differently depending on sound location

18LocalizationAdditional cues:Room Echoes:For each sound, there are 6 copies (in a simple rectanguluar room!). Different arrival times of these copies provide cues to location of sound relative to the acoustic space

19LocalizationWhat would be the worst case scenario for localizing a sound?20Pitch and Music21PitchPitch is the subjective perception of frequencytime ->

Air PressurePeriod - amount of time for one cycleFrequency - number of cycles per second (1/Period)22PitchPure Tones - are sounds with only one frequencyf = 400 hzf = 800 hz

23Tone HeightTone Height is our impression of how high or low a sound is

but theres something more to our impression of how something sounds than just its tone height24ChromaTone Chroma is the subjective impression of what a tone sounds likeNotes that have the same Chroma sound similar

400 hz

500 Hz

800 Hz25ChromaTones that have the same Chroma are octaves apart

26Chromachroma is best represented as a helixchroma repeats every octavetones with the same chroma are above or below each other on a helix

27ChromaTones that are octaves apart have the same chroma

one octave is a doubling in frequency28Chromafrequency is determined (in part) by location of stimulation on the basilar membrane

29Chromafrequency is determined (in part) by location of stimulation on the basilar membrane

but that relationship is not linear (its logarithmic)30Chromadoublings of frequency map to equal spacing on the basilar membrane

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