Transcript 2320Lecture4

Hearing
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Detection
Loudness
Localization
Scene Analysis
Music
Speech
Detection and Loudness
• Sound level is measured in decibels
(dB) - a measure of the amplitude of
air pressure fluctuations
Detection and Loudness
• Sound level is measured in decibels
(dB) - a measure of the amplitude of
air pressure fluctuations
• dB is a log scale - small increases in
dB mean large increases in sound
energy
Detection and Loudness
• Sound level is measured in decibels
(dB) - a measure of the amplitude of
air pressure fluctuations
• dB is a log scale - small increases in
dB mean large increases in sound
energy
• We have a dynamic range that is a
factor of 7.5 million!
Detection and Loudness
• minimum sound level necessary to be
heard is the detection threshold
Detection and Loudness
• detection threshold depends on
frequency of sound:
• very high and very low frequencies
must have more energy (higher dB) to
be heard
• greatest sensitivity (lowest detection
threshold) is between 1000 hz to
5000hz
Detection and Loudness
• Detection can be compromised by a
masking sound
• even masking sounds that are not
simultaneous with the target can
cause masking (forward and
backward masking)
Detection and Loudness
• Loudness is the subjective impression
of sound level (and not identical to it!)
Detection and Loudness
• For example,
tones of
different
frequencies
that are judged
to be equally
loud have
different SPLs
(dB)
Detection and Loudness
• Hearing loss due to exposure to high-intensity
sounds (greater than 100 dB) is frequency-specific
and can last many hours
Detection and Loudness
• Incidence of noise-related hearing loss is increasing dramatically
• iPods and other “earbud” music players are thought to be partly
responsible
• How loud is an iPod?
– maximum volume is approximate but is somewhere between 100 dB
(hearing damage in about 2 hours) to 115 dB (hearing damage in about 15
minutes)
• Consequences: difficulty understanding speech, tinnitus, deafness
• Your perception of loudness adapts so it’s hard to tell how loud your
iPod is - LOCK THE VOLUME ON YOUR iPOD!
Localization
• recall the lake analogy: task is to
localize the positions of the boats on a
lake using the pattern of ripples at two
points on the shore
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
•
1
There are several clues you could
use:
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
•
What are some problems or
limitations?
Localization
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Low frequency sounds aren’t
attenuated by head shadow
Sound is the same
SPL at both ears
Left Ear
Right Ear
Compression
Waves
Localization
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Left Ear
High frequency sounds have
ambiguous phase lag
Left Ear
Right Ear
Right Ear
Two locations, same phase information!
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
Pure Tones are Very Rare in
Nature!
• What are real sounds composed of?
Pure Tones are Very Rare in
Nature!
• What are real sounds composed of?
• Virtually all sounds are composed of
several (or many) frequencies all going
at once
Pure Tones are Very Rare in
Nature!
• What are real sounds composed of?
• Virtually all sounds are composed of
several (or many) frequencies all going
at once
• “Extra” frequencies are called
harmonics