Chapter 10a: Sound Localization Flashcards
How do you locate a sound?
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
Interaural time difference (ITD):
The difference in time between a sound arriving at one ear versus the other.
Azimuth
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?
ITD Graphics
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.
Physiology of ITD
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.
FROM frequencies higher than 1000Hz, the ….
HEAD itself blocks some of the energy in the acoustic wave.
Medial superior olives (MSOs)
First place where input converges from two ears.
Auditory Information Pathway
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.
Interaural level difference (ILD):
The difference in level (intensity) between a sound arriving at one ear versus the other.
Sounds are more intense at the ear closer to sound source
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
Physiology of ILDs
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.
ITD and ILD compared:
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
Subwoofer placement
Subwoofer placement is less important in a home theater setup due to our inability to accurately localize the low frequencies.
Potential problem with using ITDs and ILDs for sound localization
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
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
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!
How to resolve confusion
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!
Shape and form of pinnae helps determine localization of sound
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.
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 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.
IPOD plugs
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!.
BINAURAL RECORDINGS
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
Apologetics
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.
How do listeners know how far a sound is?
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)
RELATIVE INTENSITY
can lead to illusions:
soft is not always farther away, local environment might be muffling the sound…
INVERSE square law
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).
Intensity works BEST when the sound is MOVING (towards YOU for example, or as you move around in the environment).
Spectral composition differences: “CRACK” vs BOOM of (near vs far) thunder.
Direct vs. Reverberant Energy
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
Reverberation
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.
Asymmetrical ears for localization of elevation
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.
Echo Localization
Ben Underwood
https://www.youtube.com/watch?v=AiBeLoB6CKE
Bat sense
http://www.youtube.com/watch?v=gZxLUNHEmPw
Daniel Kish & World Access for the Blind
Goal: Increase public awareness about the strengths and capabilities of blind people
Neural Correlates of Natural Human Echolocation in Early and Late Blind Echolocation Experts
Author Lore Thaler PhD, OSU
Invisibilia Podcast with extended interviews
http://www.npr.org/programs/invisibilia/378577902/how-to-become-batman?showDate=2015-01-23
Echolocation STUDY
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
Shepard Tone
“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
