Introduction%20to%20psycho-acoustics:%20Some%20basic%20auditory%20attributes

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Introduction to psycho-acousticsSome basic
auditory attributes
For audio demonstrations, click on any
loudspeaker icons you see ....
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Terminology objective vs. subjective terms

• Objective terms are physical, and describe the
  outer world.
• Intensity or amplitude (dB SPL)
• Fundamental frequency (Hz)
• Spectral shape (dB SPL)
• Subjective terms are psychological, and describe
  the inner world.
• Loudness
• Melodic pitch
• Timbre
• Psycho-acoustics is about the mapping between
  these two domains.

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A caveat about the word pitch

• As a subjective descriptor, pitch is often used
  to describe aspects related to fundamental
  frequency (melodic pitch) as well as timbre
  (quality or colour).
• More confusingly, pitch is also used as a
  physical descriptor.
• Voice fundamental frequency (the vibration rate
  of the vocal folds, is often referred to as voice
  pitch.

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Thresholds and the auditory area an audiogram
(not the clinical type)
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Thresholds and coding of loudness

• Much of the shape of the curve describing
  absolute threshold as a function of frequency can
  be accounted for by sound transmission into the
  inner ear.
• Important aspects of loudness coding appear to
  arise from properties of the basilar membrane and
  neural encoding of level.
• Logarithmic scales (like the decibel scale) are
  much more appropriate for describing our
  sensations of loudness than a linear scale.
• For example, we can detect a change in intensity
  of about 1 dB across a wide range of absolute
  levels.
• Loudness appears to be related to the total
  amount of firing in the auditory nerve.

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Pitch perception for complex periodic signals

• Sinusoids (at least those below 4-5 kHz where our
  perception of pitch is strong) do not constrain
  theories of pitch perception very much, as there
  is information both in place (tonotopic mapping)
  and time (phase locking) codes.
• Focus here on complex periodic waveforms with
  rich harmonic spectra (for example, vowels).
  These always have a strong and clear pitch.

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Hypothesis 1

• We ascertain the pitch of complex tones by
  listening only to the fundamental component in
  the complex (Helmholtz).
• Contradicted by ...
• The case of the missing fundamental.
• We can use the fundamental if it is there, but we
  dont need it.

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The pitch of complexes with a missing fundamental
?frequency
time?
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Masking spectral and virtual (the pitch we
extract from complexes with a missing fundamental)
?frequency
speech
speech
time?
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What happens to a train of pulses (a complex
periodic waveform with a rich harmonic spectrum)
when it is analysed by the cochlea?
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A similar outcome for a more ecologically valid
sound, a vowel.
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Pitch perception for complex periodic signals

• Appears to rely on a spectro-temporal analysis.
• At frequencies corresponding to the first few
  harmonics, individual harmonics will be
  separately resolved by the cochlea. Therefore,
  information can be coded both in terms of place
  (changes in amount of activity across the
  auditory nerve fibre array) and time (synchrony
  of nerve firing to individual harmonics).
• At higher frequencies, individual harmonics
  cannot be resolved. The fundamental frequency is
  revealed as temporal modulations of the waveform
  amplitude envelope, which will result in
  synchronised neural activity.

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Timbre

• A kind of wastebasket category.
• Can include .
• amplitude envelope variations (attack and decay)
• gradual changes both in spectral envelope and
  fundamental frequency
• the range between tonal and noise-like character
• the effects of a prefix, an onset of a sound
  quite dissimilar to the ensuing lasting vibration

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A special (but very significant) caseTimbre for
steady-state complex periodic signals

• That aspect which distinguishes two different
  sounds of the same perceived loudness, pitch and
  duration.
• Also referred to as colour (in vowels) or
  quality.
• Variations in vowel colour are caused by
  variations in what aspect of their spectra?

Their spectral shape or spectral envelope.
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A change in spectral envelope
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How are changes in spectral envelope (and hence
timbre) likely to be coded in the auditory nerve?

• By the place code.
• The excitation pattern of a vowel (on the basilar
  membrane and across the auditory nerve fibre
  array) will reflect, at least to some extent, its
  spectrum.

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Temporal aspects of sounds are very important for
timbre distinctions in general ...
piano played ... forwards
backwards
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Full demonstration of the effects of tone
envelope on timbre
You will hear a recording of a Bach chorale
played on a piano (click on the loudspeaker at
right) ... Now the same chorale will be played
backwards .... Now the tape of the last
recording is played backward so the chorale is
heard forward again, but with an interesting
difference ...
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This is where the lecture finished, but there are
a few further slides (and an audio demonstration)
you should have no trouble with. (Well, maybe
the audio demonstration is tough to follow, but
its interesting to listen to.)
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Psychoacoustic reflections of auditory frequency
selectivity

• ( the fact that the peripheral auditory system
  does a kind of frequency analysis on all incoming
  signals )

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A masked audiogramFor a fixed narrow-band
masker, determine the change in threshold for
sinusoidal probes at a wide variety of
frequencies.
Excitation pattern (spectrum) or tuning curve
(frequency response)?
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Psychophysical tuning curves Determine the
minimum level of a narrow-band masker at a wide
variety of frequencies that will just mask a
fixed sinusoidal probe.
Excitation pattern (spectrum) or tuning curve
(frequency response)?
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Other auditory abilities also reflect frequency
selectivity

• The ability to hear out individual harmonics in a
  periodic complex sound.

speech
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Really, the End!