sound localization - purduegfrancis/classes/psy310/l31b.pdf · psy 310: sensory and perceptual...

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

Prof. Greg Francis


Purdue University

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 withinformation that is on the retina

Prof. Greg Francis


Purdue University

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 withthe first two

Prof. Greg Francis


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 the

other

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

Prof. Greg Francis


Purdue University

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

Prof. Greg Francis


Purdue University

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)

Prof. Greg Francis


Purdue University

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

Prof. Greg Francis


Purdue University

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

Prof. Greg Francis


Purdue University

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


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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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

Prof. Greg Francis


Purdue University


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

Prof. Greg Francis


Purdue University

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


Purdue University

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?

Prof. Greg Francis


Purdue University

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

Prof. Greg Francis


Purdue University




If one speakers lags the other just a little bit, the sound location shiftstoward the leading speaker

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With a longer lag, the sounds appears to come only from the leadingspeaker

Prof. Greg Francis


Purdue University




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

Prof. Greg Francis


Purdue University

Next time

Sound quality

Acoustics

Listening

Auditory grouping

sound localization - purduegfrancis/classes/psy310/l31b.pdf · psy 310: sensory and perceptual...

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