Hearing

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Hearing
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Outline (1) limits of hearing (2)
loudness (3) pitch (4) perceptual organization
(5) localization (6) speech perception
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(1) Limits of hearing a) audibility function b)
detection of complex stimuli c) masking the
critical band d) patterns of hearing loss
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range of hearing is 20 - 20,000 Hz
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How good is the ear at detecting sounds?
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How GOOD is the ear at detecting sounds?
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We can present complex stimuli by breaking up
pure tones.
Acoustic power can be concentrated into a single
frequency (i.e., a pure tone). Or distributed
among multiple frequencies presented
simultaneously (i.e., a complex tone). In
addition, we can vary the bandwidth (i.e., the
difference between the highest and lowest
frequencies) in the complex tone.
P O W E R
bandwidth
low
high
Frequency (Hz)
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• How does detectability of a sound depend on
  complexity?
• when frequencies are close together (narrow
  bandwidth)
• detectability depends on the total acoustic power
  in the stimulus
• the acoustic power is summed across frequencies.
• (2) when the frequencies are far apart (wide
  bandwidth)
• detectability declines when the frequency
  components in a complex tone are too far apart
• the auditory system is no longer capable of
  combining their acoustic power.
• The bandwidth at which the integration begins to
  fail is called the critical bandwidth.

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Detection of Complex Stimuli nearby frequencies
are combined
close together
detection
far apart
critical bandwidth
stimulus bandwidth
frequencies falling within a critical bandwidth
are integrated by the auditory system
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Noise Masking (1) broad-band noise contains all
audible frequencies (white noise) (2) band-pass
noise contains a smaller range of frequencies
(3) bandwidth is the difference between
the high and low frequency cutoffs.
low frequency cut-off
high frequency cut-off
Power
Frequency (Hz)
centre frequency
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Varying Bandwidth Difference of high and low
cut-offs
same centre frequency, different bandwidths
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What does noise sound like? depends on
bandwidth (1) a very narrow band noise sounds
like a tone (centre frequency) (2)
white noise is a hissing sound (no pitch)
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Detecting a signal embedded in noise How does
noise affect pure tone thresholds? Consider a
task in which you are asked to detect a pure tone
(the signal) embedded in a background noise.
(1) The noise is band-pass the
centre frequency equals the signal's
frequency. (2) The bandwidth of the
noise varies across experimental
conditions. How does detectability vary with
noise bandwidth?
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Noise Band-pass Varying bandwidths
Signal (pure tone)
P O W E R
Frequency (Hz)
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Detecting a signal embedded in noise How does
noise affect pure tone thresholds?
Threshold
critical bandwidth
no noise
Noise Bandwidth
threshold increases with noise bandwidth up to a
point, then levels off
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• Detecting a signal embedded in noise
• How does noise affect pure tone thresholds?
• (1) Increasing bandwidth does increase detection
  threshold.
• this increase in threshold is referred to as
  masking.
• we say that the signal is masked by the noise.
• (2) Increasing noise bandwidth results in more
  masking, but only up to a point.
• beyond some critical bandwidth, increasing noise
  bandwidth does not result in more masking.
• The amount of masking levels off and thresholds
  are constant.

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• Interpretation of noise masking
• The critical band usually is interpreted as the
  width of an
• frequency-selective auditory channel/filter.
• detect signal by monitoring response of 1
  channel
• noise falling within channel masks signal
• noise falling outside channel has no effect
• (1) This filter responds well to a small range
  of auditory frequencies.
• (2) Analogous to the spatial frequency channels.

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Frequency-selective filter
P O W E R
P O W E R
Frequency (Hz)
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Frequency-selective filters (or channels)
(1) The assumption is that observers have many
auditory channels tuned to different frequencies.
(2) Some channels respond to low frequencies,
some to high frequencies, some to medium
frequencies. (3) In all cases, the auditory
filter responds to a small range around a
preferred, or best frequency. (4) Generally, the
critical band increases with increasing
frequency.
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Hearing Loss Two types (a) conduction loss (b)
sensory/neural loss
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• Conduction hearing loss
• Disorder of outer and/or middle ear
• problem associated with mechanical transmission
  of sound into cochlea
• Causes
• blockage of outer ear
• punctured eardrum
• middle ear infection
• otosclerosis
• Effects
• broad-band sensitivity loss
• conduction loss can be severe (up to 30 dB)
• loss occurs across a wide range of frequencies
• does not affect neural mechanisms
• treated with hearing aids/surgery (stapedectomy)

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• Sensory/Neural Loss
• Damage to the cochlea and auditory nerves
• age-related hearing loss (Presbycusis)
• progressive sensitivity loss at high frequencies
• high frequency cutoff drops from 15,000 Hz (30
  years) to 6,000 Hz (70 years)

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• Effects of Exposure to Noise
• (1) Temporary Thresholds Shifts (TTS)
• short term exposure to noise (nightclub,
  concert)
• increases your thresholds about 30 dB
• recover over a period of several hours/days
• (2) Permanent Thresholds Shifts (PTS)
• long term exposure to noise (work-related)
• short, intense sounds (explosion)
• can be as large as 60 dB

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? (1) limits of hearing (2) loudness (3)
pitch (4) perceptual organization (5)
localization (6) speech perception
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Loudness

• Loudness is a psychological, not physical,
  attribute of sound!
• loudness is not the same as intensity (it is
  your subjective perception of intensity)
• cannot measure loudness with a sound meter
• How can we estimate loudness?
• (1) Loudness Matching
  Method
• (2) Magnitude Estimation Method

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Loudness Matching Method Task Match the loudness
of 2 tones. (1) present a standard tone at a
fixed intensity (2) present a test tone at a
different intensity (3) adjust test's intensity
until it has the same loudness as the standard
(4) repeat procedure for many test frequencies
(5) graph of intensities at all test frequencies
(6) repeat entire procedure for a new standard
intensity
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Equal Loudness Contour
every point on the contour has the same loudness
as the standard
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• Equal Loudness Contours
• When the standard intensity is low, equal
  loudness contours have the same shape as the
  audibility function.
• When the standard intensity is high, equal
  loudness contours are much flatter than the
  audibility function.
• Another way of stating the results
• At low intensities tones have the same loudness
  when they are equally detectable
• At high intensities they match in loudness when
  they have the
• same intensity.

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Magnitude Estimation In a magnitude estimation
task, an observer rates the loudness of a
stimulus on a numerical scale. (We are really
good at this!) The loudness ratings are related
to intensity according by the formula Loudness
k Intensity0.67 (Stevens Power
Law) Loudness grows more slowly than intensity!
Loudness
Intensity (dB)
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Effects of noise mask on loudness Consider a task
in which observers rate the loudness of a pure
tone signal embedded in a band-pass noise centred
on the signal frequency. Noise will mask the
target, so we would expect to find that loudness
ratings are decreased for low intensity signals.
Q What happens at higher intensities, when the
signal is above threshold? A Once above
threshold, loudness increases rapidly -- much
more rapidly than normal.
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Effects of noise mask on loudness
This figure shows loudness judgements for a 1,000
Hz tone presented in quiet (left curve) and in 2
different amounts of noise (middle and right
curves). Note how all the curves converge at high
intensities. This abnormally rapid growth in
loudness with increasing intensity is referred to
as loudness recruitment.
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Effects of hearing impairment on loudness
Some forms of hearing loss (cochlear defects)
affect loudness judgements in much the same way
as a masking noise loudness for weak, but not
intense, tones is reduced.
This phenomenon is another example of loudness
recruitment.
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• Loudness of Complex Tones
• complex tones vs. pure tones
• Complex tones are sometimes louder than pure
  tones of equal energy. Differences depend on the
  bandwidth of the complex stimulus.
• Zwicker, Flottorp, Stevens (1957)
• compared loudness of pure tone (1,000 Hz) vs.
  complex tone (made up of frequencies ranging from
  900 to 1,100 Hz)
• listeners adjusted the intensity of the pure
  tone to match loudness of complex tone
• experimenters varied the bandwidth of the
  complex tone
• all complex tones had equal energy

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Effect of bandwidth on loudness loudness changes
at critical bandwidth
Intensity of matching tone
160 Hz
Stimulus Bandwidth
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? (1) limits of hearing ? (2) loudness (3)
pitch (4) perceptual organization (5)
localization (6) speech perception
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Pitch

• Pitch is also a psychological, not physical,
  attribute of sound!
• pitch is not the same thing as frequency (it is
  your subjective perception of
  frequency!)
• low frequencies (e.g., 500 Hz) will sound low in
  pitch
• high frequencies (e.g., 1500 Hz) will sound high
  in pitch

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• Place Theory of Pitch (of pure tones)
• peak activity is localized on
• basilar membrane
• different frequencies gt different
• locations
• pitch is encoded by the place of
• activation
• Pros
• (1) damaging parts of basilar membrane results in
  frequency- specific deficits
• (2) stimulating parts of basilar membrane evokes
  different pitches
• Cons
• (1) low frequencies have a pitch, but dont
  produce localized activity on basilar membrane
• (2) Cant account for pitch of complex sounds

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• Frequency Theory of Pitch
• pitch depends on rate, not place, of response
• neurons can fire in-synch with signals
• rate of response might represent frequency of
  stimulus
• But.
• cells cant track high frequency sounds
• Maybe there is a dual coding theory of pitch!
• (place theory for high frequencies, response
  rate for low frequencies)

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• Pitch of Complex Sounds
• most sounds are complex
• not all complex sounds have a pitch (white
  noise)
• those that do tend to have a harmonic structure

Pure Tone
Complex Sound (with pitch)
low
high
low
high
Frequency (Hz)
pitch of the complex stimulus pitch of
fundamental frequency
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Fundamental Frequency The fundamental frequency
of a set of frequencies is the highest common
divisor of the set. What is the fundamental
frequency for each of the following sets?
200, 400, 600,
800, 1000 Hz
200, 600, 800, 1000 Hz
100, 400, 600, 800, 1000 Hz
200, 300, 600, 800,
1000 Hz Each of the above sets has one
fundamental frequency, although the fundamental
itself may not actually be present in the set.
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Problem of the Missing Fundamental (Virtual
Pitch) Is the fundamental really missing? When
the fundamental is missing from the stimulus, do
distortions produced by the ear introduce a
fundamental frequency?
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Is the fundamental really missing? effect of
masking on virtual pitch
band-pass mask centred on the fundamental
frequency
Intensity (dB)
100 200 300 400 600 800 1000
Frequency (Hz)
masking does not eliminate the pitch!
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? (1) limits of hearing ? (2) loudness ? (3)
pitch (4) perceptual organization (5)
localization (6) speech perception
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• Perceptual Organization
• How do you group sounds to identify an object?
• Auditory Scene Analysis
• Gestalt grouping principles apply!
• (1) Good continuation spectral harmonics
• harmonics are grouped together
• doe-rae-mi-fa-so-la-ti-doe
• (2) Similarity common spectral content
• similar frequencies are grouped together
• (3) Proximity common time course
• frequencies that occur at the same time are
  grouped together

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?(1) limits of hearing ?(2) loudness ? (3)
pitch ? (4) perceptual organization (5)
localization (6) speech perception
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Cone of Confusion Binaural cues do not provide
unambiguous information about a sound's location.
For example, the same binaural cues are produced
by sounds directly in front of and behind an
observer. A particular combination of IIDs and
ITDs can be produced by sounds in many different
directions.
All such directions lie on the surface of the
so-called cone of confusion.
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• Minimizing Mislocalizations
• Even though binaural cues are ambiguous, we don't
  make very many localization errors.
• How does the auditory system minimize
  mislocalizations?
• (1) Head movements
• Only one sound source location is consistent with
  multiple head positions.
• (2) Pinna
• (3) Visual cues

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• Cocktail Party Effect
• You can focus on one sound in a noisy
  environment.
• This form of "sound segregation" is due in part
  to being able to tell from where the sound is
  coming.
• Binaural unmasking
• masking reduced when sounds come from different
  locations

• Example of how binaural cues reduce masking
• Present signal with noise over headphones to the
  left ear
• you can't hear signal
• (2) now add identical noise (via headphones) to
  the right ear
• even though there is more noise, you now can hear
  the signal!
• binaural unmasking occurs because the noise and
  signal have different perceived locations

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? (1) limits of hearing ? (2) loudness ? (3)
pitch ? (4) perceptual organization ? (5)
localization (6) speech perception
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Speech Perception

• Speech perception different from perception of
  other auditory stimuli.
• Evidence
• (1) Psychophysical experiments demonstrating
  categorical nature of speech perception.
• (2) Hemispheric differences in auditory
  processing
• left hemisphere specialized for speech and
  language
• right hemisphere seems to be more responsive
  for non-speech sounds

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Categorical Perception
look more closely at physical/acoustic properties
of speech signal
sound spectrogram picture of distribution of
acoustic energy
/ba/
/da/
/ga/
(Hz)
(ms)
density or darkness amplitude (loudness)
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Formants bands of energy - due to resonances of
vocal tract
/ba/
/da/
/ga/
f3

f2
f1

rising transition
falling transition
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Manipulation constant series of 13 syllables
that differ only in f2 transition
Task present to subjects for identification -
many presentations
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Identification function rapid change in
identifying function suggests subjects have
learned category

• Conclusions
• /ba/, /da/, /ga/ (and other phonemes) are
  perceived categorically, unlike non-speech sounds
  
• main cue f2 transition

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Speech Perception
Neurons in the auditory cortex are specific for
speech sounds. For example /ba/ /pa/ /ta/ and
/da/.
There is evidence that these neurons compose
speech-selective channels. Adaptation!!!!
Pre-adaptation listener had to identify an
ambiguous sound as
either /da/ or /ta/ (50 chance of choosing
either one). Adaptation listen to /da/ over and
over again. Post-adaptation listen to ambiguous
sound again. Result listeners will identify the
ambiguous sound as /ta/. The /da/ channel was
fatigued.
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The End ? (1) limits of hearing ? (2)
loudness ? (3) pitch ? (4) perceptual
organization ? (5) localization ? (6) speech
perception