Hearing and Listening: questions

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Hearing and Listening questions!

• How do we know where a sound originates?
• How do blind individuals use sound to avoid
  obstacles?
• What makes a flute sound different than a violin?
• Why do you like listening to some voices and hate
  listening to other voices?
• Why might a concert played at UVIC center
  cafeteria not sound too good?
• How do you manage to ignore the hubbub at a noisy
  party while at the same time picking out and
  listening to a familiar voice?

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Auditory processes

• sound localization knowing where a sound
  originates
• sound quality the mix of frequencies and
  harmonics in a complex waveform
• auditory scene analysis separating individual
  perceptual events from the tangled auditory
  stream

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Where the positioning of sounds in space

• horizontal coordinates provide the azimuth
• vertical coordinates provide elevation
• near or far provides a distance measure
• the cone of confusion locations where a sound
  source would produce the same intensity and time
  differences at the ear

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some sound basics

• a pure tone is called a sine wave
• its rate of variationfrequency
• its height amplitude
• in most listening situations we dont hear a
  single wave, we hear complex, overlapping sound
  waves- a tangle of sound

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sine and complex waveforms
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the problem of localization 1) identifying the
cues

• sounds located on the azimuth provide binaural
  cues - differences between the two ears in the
  time of arrival (spatial separation) and the
  intensity of the sound ( the head acts as a
  sound) shadow
• these are called interaural differences
• interaural time differences (ITD)
• interaural level differences (ILD)
• this is the duplex theory of sound localization

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Interaural time differences

• the time of arrival of a sound at the two ears
  varies with the location of the sound source
  relative to the head
• the largest time difference will occur when sound
  source is at 900 azimuth, .6 to .9 ms
• for sounds in this case the sounds between the
  two ears can signal the azimuth position of a
  sound source

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Rationale

• the head approximates 20 cm in diameter,
  (circumference 62.8 cm), so sound must travel
  31.4 cm from one ear to the other, equivalent to
  a time distance of .6 to .9 msec
• for an ITD one cycle must fall within the maximum
  time difference between the ears
• eg. 200 Hz has a wavelength of 170 cm,
  approximately 1/6 of a cycle will occur between
  ears an ITD of .8 msec
• 6000 Hz has a wavelength of 5.6 cm
  approximately 6 cycles will occur between the
  ears an ITD of .9msec

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the ITD is a cue for low frequency waves

• in the low frequency wave, one cycle or less
  occurs between ears and therefore the time
  difference is not confused with the cycle
  difference
• in the high frequency wave, the cycle of the wave
  is confounded with the time cue as more than one
  cycle arrives between the ears
• measures show that a time difference of lt .1 msec
  can be detected by the auditory system

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Interaural level differences

• a cue for frequencies gt 1000 Hz
• Why consider a wave hitting a rock in the ocean
• a large rock disrupts and reroutes the wave
  whereas a small rock does not disrupt the wave,
  rather creates a ripple
• the head is like the rock it is large or small
  in relation to the frequency of the sound wave

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Example

• 1) a 200 Hz sound wave has a wavelength of 170
  cm, the diameter of the head approximates 20 cm.
  What will happen to the sound wave?
• 2) a 6000 Hz sound wave has a wavelength of 5.6
  cm, the diameter of the head is 20 cm. What will
  happen to the sound wave?

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Conclude

• the head breaks up a high frequency sound wave,
  thus decreasing the intensity and creating a
  interaural level difference between the ears.
• this cue is effective for high frequency waves,
  creating a difference of 20 dB for a 10000 Hz
  wave positioned at azimuth 900
• of interest hearing acuity is related to
  animals body size eg. a mouse hears 80,000Hz
  (wavelength 5 mm) so the 2cm head of the mouse
  provides a sound shadow and the intensity cue is
  useful for localization

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Practical application binaural cues

• echolocation exercise

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Elevation cues what are they?

• elevation does not provide interaural
  differences, rather spectral cues
• the pinna acts like an acoustic antenna- its
  resonant cavities amplify some frequencies and
  attenuate other frequencies
• class exercise

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directional transfer functions
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DTFs for varying elevation
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Other cues to distance

• decreasing sound pressure indicates increasing
  distance
• loss of high frequency sounds
• if listener is moving, the rate of change in
  loudness cues location
• difference between indirect (reflected) sound and
  direct sound. at greater distances indirect sound
  increases

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other cues

• vision when sensory information conflicts,
  visual information dominates
• eg. ventriloquist
• pseudophone

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physiological basis of localization

• recording from the auditory cortex and pathways
• interaural time detectors
• directionally sensitive neurons
• panoramic neurons
• receptive fields for space

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Sound quality (timbre)

• a complex sound is composed of a fundamental
  frequency (F0) and harmonic frequencies (F1.Fn)
• examples guitar, saxophone

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piano harmonics
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attack and decay create timbre
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reverberation and architectural acoustics

• what makes a good concert hall or theatre?
• Greek theatre?
• early churches?
• acoustic analysis makes use of reverberation time
  - the time taken for a sound to drop by 60 db.
• good concert halls have a reverberation time of 2
  2.25 seconds

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How?

• redesign of halls to reduce reverberation time by
  changing building materials
• seat cushions six seat cushions 1 body in
  decreasing reverberation time
• oriental carpets
• felt on the walls
• today, the Sabin the sound absorbing quality
  of an open window 1 sq.ft in area

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precedence effect

• when listening to direct and indirect sound,
  direct sound reaches the ear first and its source
  is localized
• eg. Bekesy and malingering

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Auditory scene analysis

• the auditory system is able to separate sounds
  according to source, it is also able to separate
  sounds when they come from a single source
• the auditory scene
• visual analogy
• YWOEU TSAMSETLEL SLMIOKEE
• the process of separation or grouping is called
  auditory scene analysis

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the problem of auditory scene analysis
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Bregman, 1990

• the Gestalt laws apply to audition
• location ? spatial streams
• harmonic structure? multiples of a frequency are
  grouped
• similarity of pitch
• proximity of pitch
• continuation melody is a pitch moving in time
• temporal proximity
• common fate
• onset/offset

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temporal proximity
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law of similarity
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Conclude

• auditory patterns trigger auditory scene analysis
  to determine identity and location of sounds
• laws function as best guesses in making sense
  of the auditory environment guesses based on
  principles which enable valid inferences about
  the nature of sound in the real world
• preference for auditory pattern is like
  preference for visual pattern--gt good figure
• some operate without attention and are
  primitives, eg. pitch others reflect experience
  to familiar sounds, eg. human word recognition