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. That’s what your auditory system does!
- Al Bregman
Localization
• All you have is a pair of instruments (basilar
membranes) that measure air pressure
fluctuations over time
Localization
• There are several clues you could use:
Localization
Left Ear
Right Ear
Compression
Waves
Localization
•
There are several clues you could use:
1
arrival time - sound arrives first at ear
closest to source
Localization
Left Ear
Right Ear
Compression
Waves
Localization
•
1.
2.
There are several clues you could use:
arrival time
phase lag (waves are out of sync) - wave at
ear farthest from sound source lags wave at
ear nearest to source
Localization
Left Ear
Right Ear
Compression
Waves
Localization
•
1.
2.
3.
There are several clues you could use:
arrival time
phase lag (waves are out of sync) - wave at
ear farthest from sound source lags wave at
ear nearest to source
Head shadow
Localization
• Arrival Time
• Phase Lag
• Head Shadow
Interaural Timing Differences (ITD)
Interaural Intensity Difference (IID)
Localization
•
What are some problems or limitations?
Localization
•
Low frequency sounds aren’t attenuated by
head shadow because sound bends around the
head with little loss of amplitude
Sound is the same
SPL at both ears
Left Ear
Right Ear
Compression
Waves
Localization
•
Low frequency sounds aren’t attenuated
by head shadow
•
Your brain preferentially uses ITD cues
for low-frequency sounds
Localization
•
Left Ear
High frequency sounds have ambiguous phase
lag because more than one wavelength “fits”
between the ears
Left Ear
Right Ear
Right Ear
Two locations, same phase information!
Localization
•
High frequency sounds have ambiguous
phase lag
•
Your brain preferentially uses IID cues
for high-frequency sounds
Localization
•
These cues only provide azimuth
(left/right) angle, not altitude (up/down)
and not distance
Left Ear
Right Ear
Azimuth
Localization
Additional cues:
Localization
Additional cues:
Head Related Transfer Function:
Pinnae modify the frequency
components differently depending on
sound location
Localization
Additional 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
Localization
• What would be the “worst case” scenario for
localizing a sound?
Pitch and Music
Pitch
• Pitch is the subjective perception of frequency
Period - amount of time for one cycle
Frequency - number of
cycles per second
(1/Period)
Air Pressure
time ->
Pitch
• Pure Tones - are sounds with only one
frequency
f = 400 hz
f = 800 hz
Tone Height
• Tone Height is our impression of how high or
low a sound is
• but there’s something more to our impression
of how something sounds than just its tone
height…
Chroma
• Tone Chroma is the subjective impression of
what a tone sounds like
• Notes that have the same Chroma sound
similar
500 Hz
400 hz
800 Hz
Chroma
• Tones that have the same Chroma are
octaves apart
Chroma
• chroma is best represented as
a helix
• chroma repeats every octave
• tones with the same chroma
are above or below each other
on a helix
Chroma
• Tones that are octaves apart have the same
chroma
• one octave is a doubling in frequency
Chroma
• frequency is determined (in part) by location
of stimulation on the basilar membrane
Chroma
• frequency is determined (in part) by location
of stimulation on the basilar membrane
• but that relationship is not linear (it’s
logarithmic)
Chroma
• doublings of
frequency map
to equal spacing
on the basilar
membrane