Prof. Greg Francis - Purduegfrancis/Classes/PSY310/L31.pdfProf. Greg Francis 2 PSY 310: Sensory and...
Transcript of Prof. Greg Francis - Purduegfrancis/Classes/PSY310/L31.pdfProf. Greg Francis 2 PSY 310: Sensory and...
Prof. Greg Francis
1PSY 310: Sensory and Perceptual Processes
Purdue University
Sound Localization
PSY 310
Greg Francis
Lecture 31
Physics and psychology.Purdue University
Audition
We now have some idea of how sound propertiesare “recorded” by the auditory system
So, we know what kind of information the nervoussystem has to work with
We now return to the sound stimulus and look atwhat kinds of information in the stimulus providesinformation about the environment
Sound localization
Where does a sound come from in the environment?
Demonstration
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Localization In many respects, vision is better suited for
localization than audition
Light sources in different places in the environmentproject to different places on the retina
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Visual localization Of course, it’s not trivial to figure out the distance
But at least we can get the relative direction of lights with
information that is on the retina
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Auditory localization
It seems worse for the auditory system
The basilar membrane has no representation of positionof a sound
Only frequency and amplitude (size of wave)
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Coordinates The location of a source of sound can be described by
three coordinates
Azimuth (left-right, relative to some reference, often the front ofthe face)
Elevation (up-down, relative to some reference, often the frontof the face)
Distance (how far away)
Vision easily deals with
the first two
Prof. Greg Francis
2PSY 310: Sensory and Perceptual Processes
Purdue University
Judging the azimuth Just as we use two eyes to estimate distance for
vision
We can use two ears to estimate azimuth foraudition
Two main cues for azimuth location
Interaural time difference: sound hits one ear before theother
Interaural level difference: the sound hitting one ear isstronger than the sound hitting the other ear
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Time difference Sounds from straight ahead reach both ears at the same time
Sounds to the side reach the closer ear a bit sooner than thefarther ear
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Time difference The time differences are small, but they are enough for the auditory
system to judge the left-right position of the sound source
The speed of sound is around 340 m/s (it depends on air pressure,temperature, humidity,…)
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Time difference Neurons in the
brain’s auditorysystem respond toneural responseswith the right kind ofdelay
Superior olive
Think of Reichardtdetectors for motionperception
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Level difference A sound off to one side reaches the close ear unimpeded
To reach the opposite ear, the sound has to travel around thehead
This is not a problem for a relatively low frequency sound
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Level difference A sound off to one side reaches the close ear unimpeded
To reach the opposite ear, the sound has to travel around thehead
It can be a problem for a high frequency sound
Acoustic shadows (physics)
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3PSY 310: Sensory and Perceptual Processes
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Level difference High frequency sounds are reduced in amplitude on the far
side of the head
How much depends on the frequency and on the location ofthe sound source
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Level difference Recordings of sounds at different ears shows some evidence
of this effect
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Judging elevation The interaural
cues do not helpto judgeelevation
Instead theauditory systemtakesadvantages ofreflections andinteractionsfrom the head,shoulders, andouter ear
Consider theear
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Judging elevation The folds and
crevices of thepinnae willproducereflections
Differentfrequencies ofsounds willreflect differently
It’s physicsagain, itdepends on thesize of the folds
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DTF There are similar reflections and interference from the shape of the
head and the shoulders
Farther away body parts don’t usually make much difference
We can characterize the effect as a directional transfer function(DFT)
The impact of reflections on reducing different frequencies of sound
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DTF The DFT depends on the vertical location of the source of sound
Sometimes also called the HRTF (head-related transfer function)
The three curves havedifferent shapes
This means that theyaffect frequenciesdifferently
So, the frequenciesof a sound can provideinformation aboutit’s vertical location
Exactly how it worksis too complicatedto get in to
Prof. Greg Francis
4PSY 310: Sensory and Perceptual Processes
Purdue University
Distance Similar to distance perception with vision, we depend on
various cues to estimate distance of sound sources
Interaural level difference
Overall sound level
Frequency
Movement Parallax
Reflection
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Interaural level difference
The intensity of sound falls off with an inverse squarelaw
This means the ILD between the ears will be large ifthe sound is close and small if the sound is fartheraway
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Interaural level difference
If a sound is within arm’s length, the ILD can be usedto give a pretty good estimate of the distance
Beyond that distance, the ILD can only say that thesource is farther away than arm’s length
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Overall sound level
Many sounds have characteristic source intensities
E.g., speech tends to be between 40-70 dB
If you know the normal intensity of a sound, thenyou can judge the distance by comparing what youhear to what is normal
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Frequency
As sound travels through air, some frequenciestransmit better than others
Air tends to absorb the energy in high frequencysound more than low frequency sounds
Sounds that are far away are more dull or muffledthan near sounds
Kind of like the atmospheric perspective effectswith light
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Movement parallax If you move, then the projection of light from nearby objects
moves more quickly on the retina than for far objects
Similar behavior if objects are moving
Prof. Greg Francis
5PSY 310: Sensory and Perceptual Processes
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Movement parallax Likewise, if you can use sound cues to identify the azimuth
positions of objects
Then those positions will change differently according to theirdistance from you
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Reflection Reflections lead to differences in time
when the sound(s) reach your ears
These time differences tend to degradethe sound
This can be a cue to distance
But reflections tend to cause moreproblems than help
How do you know the source’s locationis from the direct sound wave insteadof the reflection?
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Reflections The auditory system distinguishes the direct sound wave from
reflections by noting when they arrive
Reflections should always take longer to arrive than the directsound wave
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Precedence For determining the location of a sound source, the auditory system
gives precedence to the apparent source of the first sound thatarrives
Set up two speakers and play a sound
If simultaneous sounds, the location is between the speakers
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Precedence For determining the location of a sound source, the auditory system
gives precedence to the apparent source of the first sound thatarrives
Set up two speakers and play a sound
If one speakers lags the other just a little bit, the sound location shiftstoward the leading speaker
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Precedence For determining the location of a sound source, the auditory system
gives precedence to the apparent source of the first sound thatarrives
Set up two speakers and play a sound
With a longer lag, the sounds appears to come only from the leadingspeaker
Prof. Greg Francis
6PSY 310: Sensory and Perceptual Processes
Purdue University
Precedence For determining the location of a sound source, the auditory system
gives precedence to the apparent source of the first sound thatarrives
Set up two speakers and play a sound
With a still longer lag, one hears two sounds
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Conclusions Sound localization involves many different cues in
the sound stimulus
Azimuth, elevation, distance
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Next time
Sound quality
Acoustics
Listening
Auditory grouping