Chapter 10a: Sound Localization

Question 1

How do you locate a sound?

Answer 1

Cricket example: You can locate very precisely a cricket way before you are able to SEE it.

Similar dilemma to determining how far an object is

Two ears: Critical for determining auditory locations

Question 2

Interaural time difference (ITD):

Answer 2

The difference in time between a sound arriving at one ear versus the other.

Question 3

Azimuth

Answer 3

Used to describe locations on imaginary circle that extends around us, in a horizontal plane (similar to the Horopter of the eyes).

Let’s analyze Interaural time difference (ITD):
Where would a sound source need to be located to produce maximum possible ITD?
What location would lead to minimum possible ITD?
What would happen at intermediate locations?

Question 4

ITD Graphics

Answer 4

STUDIES SHOW we can detect IDT’s of as little as 10 MICRO SECONDS -> direction accurate within one degree!

QUESTION: HOW can HOME THEATERS SYSTEMS (and movies or big stereos) recreate SOUND LOCALIZATION??

Interaural time differences for sound sources, different positions around head form pointed bell curve.

Question 5

Physiology of ITD

Answer 5

Medial superior olives (MSOs): First place where input converges from the two ears.

ITD detectors form connections from inputs coming from two ears during first few months of life.

THE MORE SYNAPSES, the less precise is the temporal coding of the ITD.

Question 6

FROM frequencies higher than 1000Hz, the ….

Answer 6

HEAD itself blocks some of the energy in the acoustic wave.

Question 7

Medial superior olives (MSOs)

Answer 7

First place where input converges from two ears.

Question 8

Auditory Information Pathway

Answer 8

PARTS OF THE PONS:
LSO = lateral superior olive
MSO = medial superior olive
MNTB = medial nucleus of the trapezoid body

In CATS, people found cells that increased their response with increasing ITD

Connections formed within few months of life.

Question 9

Interaural level difference (ILD):

Answer 9

The difference in level (intensity) between a sound arriving at one ear versus the other.

Question 10

Sounds are more intense at the ear closer to sound source

Answer 10

ILD is largest at 90 degrees and –90 degrees, nonexistent for 0 degrees and 180 degrees.

ILD generally correlates with angle of sound source, but correlation is not quite as great as it is with ITDs

Question 11

Physiology of ILDs

Answer 11

Lateral superior olives (LSOs): Neurons that are sensitive to intensity differences between two ears

Excitatory connections to LSO come from ipsilateral (same side) ear

Inhibitory connections to LSO come from contralateral (opposite side) ear.

Question 12

ITD and ILD compared:

Answer 12

Low frequencies are diffracted by the head (like an ocean wave around a pylon), high frequencies are absorbed.

Low Frequencies / Timing Cues Dominate

High Frequencies / Intensity Cues Dominate

Stimuli on headphones, where ITDs pointing to the left are offset by ILDs pointing to the right, so the sound is perceived as coming from the midline

Question 13

Subwoofer placement

Answer 13

Subwoofer placement is less important in a home theater setup due to our inability to accurately localize the low frequencies.

Question 14

Potential problem with using ITDs and ILDs for sound localization

Answer 14

Cone of confusion:
Regions of positions in space where all sounds produce the same time and level (intensity) differences (ITDs & ILDs)

Experiments by Wallach (1940) demonstrated this problem

Question 15

Cone of confusion

Answer 15

Regions of positions in space where all sounds produce the same time and level (intensity) differences (ITDs & ILDs)

Experiments by Wallach (1940) demonstrated this problem

THE MOST CONFUSION CONE:
ABOVE-infront-below-behind!!!

Graphic: ELEVATION on y axis, azimuth is degrees around you. ITD on the z-axis: same color means same ITDs. CONFUSION!

Question 16

How to resolve confusion

Answer 16

As soon as you move, green frog is no longer a possibility (red is) but the only constant possibility between the two is the blue.

So, you move your head and then you KNOW!

Question 17

Shape and form of pinnae helps determine localization of sound

Answer 17

Directional transfer function:
Describes how pinnae, ear canal, head, and torso change intensity of sounds with different frequencies that arrive at each ear from different locations in space (azimuth & elevation)

Sometimes called Head-Related Transfer Function

PINAE funnel certain frequencies better than others.

Question 18

Directional transfer function

Answer 18

Describes how pinnae, ear canal, head, and torso change intensity of sounds with different frequencies that arrive at each ear from different locations in space (azimuth and elevation)

PINAE funnel certain frequencies better than others.

Sometimes called Head-Related Transfer Function

So, intensity shifts due to AZYMUTH (ILDs) and ELEVATION can help determine the HEAD-related TRANSFER FUNCTION for a set of pinnae.

Question 19

IPOD plugs

Answer 19

SOUNDS come from inside your head.

One can simulate ITDs & ILDs but not HRTFs for everyone, so we lose that piece of localization information when we heard music through earplugs instead of LIVE at the concert!.

Question 20

BINAURAL RECORDINGS

Answer 20

Recording through microphones inside your head, near the ear drums

Direction transfer function preserved. Then you feel sound as coming from outside of your HEAD!!

Try this link at home with headphones:
http://www.youtube.com/watch?v=IUDTlvagjJA

Question 21

Apologetics

Answer 21

attempting to close logical loopholes (more commonly used in theology not science fiction).

The most interesting lessons from sci-fi come when youassume, for the sake of argument, that everything in sci-fi is there for a reason—even things that look like mistakes.
There’s a word for this, apologetics, which usually refers to the act of attempting to close logical loopholes in theology.
Take Star Wars, for instance, in the scene when Luke and Han Solo are in the Millennium Falcon blowing up TIE fighters.

Question 22

How do listeners know how far a sound is?

Answer 22

Simplest cue: Relative intensity of sound

Inverse-square law: Sound intensity decreases with 1/d2 with increasing distance d in 3D space.

A sound 1 meter away is 6dB louder than 2 m
A sound 39m away is only 1dB louder then 40m

Spectral composition of sounds: Higher frequencies decrease in energy more than lower frequencies as sound waves travel from source to one ear (low frequencies travel farther)

Question 23

RELATIVE INTENSITY

Answer 23

can lead to illusions:

soft is not always farther away, local environment might be muffling the sound…

Question 24

INVERSE square law

Answer 24

for example: a sound that is 1 meter away is 6dB more intense than one 2 meters away.
But the same 1 meter difference for sounds located 39-40 meters away produces a much smaller (fraction ) of a dB in intensity difference.

So: Intensity is a good cue for depth, but only for objects within our reach (not so valuable). We always underestimate the distance of farther away sounds (think it is closer than it really is).

Question 25

Intensity works BEST when the sound is MOVING (towards YOU for example, or as you move around in the environment).

Answer 25

Spectral composition differences: “CRACK” vs BOOM of (near vs far) thunder.

Question 26

Direct vs. Reverberant Energy

Answer 26

Relative amounts of direct vs. reverberant energy also help evaluate distance.

CLOSE TO YOU: Most sound is DIRECT (with higher intensity)
Far from you: most sound is reverberant

Question 27

Reverberation

Answer 27

Reverberations that occur in a room can severely distort localization cues.

One strategy that listeners unconsciously employ to cope with this is to make their localization judgments instantly based on the earliest arriving waves in the onset of a sound.

This strategy is known as the precedence effect, because the earliest arriving sound wave—the direct sound with accurate localization information—is given precedence over the subsequent reflections and reverberation that convey inaccurate information.

Question 28

Asymmetrical ears for localization of elevation

Answer 28

For example, Barn owls’ asymmetry is such that the center of the left ear flap is slightly above a horizontal line passing through the eyes and directed downward, while the center of the right ear flap is slightly below the line and directed upward.

Sound originating from below the eye level to sound louder in the left ear, while sound originating from above the eye level to sound louder in the right ear.

Question 29

Echo Localization

Answer 29

Ben Underwood
https://www.youtube.com/watch?v=AiBeLoB6CKE

Bat sense
http://www.youtube.com/watch?v=gZxLUNHEmPw

Question 30

Daniel Kish & World Access for the Blind

Answer 30

Goal: Increase public awareness about the strengths and capabilities of blind people

Question 31

Neural Correlates of Natural Human Echolocation in Early and Late Blind Echolocation Experts

Answer 31

Author Lore Thaler PhD, OSU

Question 32

Invisibilia Podcast with extended interviews

Answer 32

http://www.npr.org/programs/invisibilia/378577902/how-to-become-batman?showDate=2015-01-23

Question 33

Echolocation STUDY

Answer 33

Participant EB: Echolocation Expert

Participant LB: Echolocation Novice

BOLD activity projected on participants reconstructed and partially inflated cortical surface.

Marking of cortical surfaces and abbreviations.

Top panel: Contrast between activations for outdoor recordings containing echoes from objects and recordings that did not contain such echoes for EB and LB.
During the experiment EB and LB listened to outdoor scene recordings and judged whether the recording contained echoes reflected from a car, tree or pole or no object echoes at all. Each participant listened to recordings of his own clicks and echoes as well as to recordings of the other person.

Bottom panel: Contrast between activations for outdoor recordings containing echoes from objects and recordings that did not contain such echoes for C1 and C2. The task was the same as for EB and LB and each participant listened to recordings they had trained with as well as to the recordings of the other person, e.g. C1 listened to both EB’s and LB’s recordings (see Figure 1G for behavioral results). It is evident that both EB and LB, but not C1 or C2, show increased BOLD activity in the calcarine sulcus for recordings that contain echoes (highlighted in white). EB mainly shows increased activity in the calcarine sulcus of the right hemisphere, whereas LB shows activity at the apex of the occipital lobes of the right and left hemisphere, as well as in the calcarine sulcus of the left hemisphere. In addition, both EB and LB, but not C1 or C2, show an increase in BOLD activity in along the medial frontal sulcus. This result most likely reflects the involvement of higher order cognitive and executive control processes during echolocation. There is no difference in BOLD activity along the lateral sulcus for any participant, i.e. Auditory Complex (highlighted in magenta). This result was expected because the Echo stimuli and the Control stimuli had been designed in a way that minimized any spectral, temporal or intensity differences. No BOLD activity differences were found when activations for EB’s recordings were contrasted with activations for LB’s recordings. doi:10.1371/journal.pone.0020162.g003

Question 34

Shepard Tone

Answer 34

“Sonic Barber’s Pole” illusion.

The tone sounds as if it is continually ascends (or descends)

Consists of a superposition of sine waves separated by octaves.

Batpod™ sound effect in The Dark Knight