Perception of Loudness and Space

1
Perception of Loudness and Space
2
The Perception of Loudness

• Loudness is a psychological attribute of a
  stimulus, not a physical attribute.
• It is related to the intensity of the stimulus.
• Loudness also depends on frequency.

3
Audibility Function

• Pure tones Consist of a sinusoidal change in
  pressure over time (e.g., a tuning fork). A
  single sinusoidal wave.
• Complex tones A combination of pure tones.
• Researchers have measured the minimum intensity
  needed to detect pure tones of different
  frequencies.
• I.e., they measured absolute thresholds for
  intensity.

4
Audibility Function (contd)

• These thresholds can be plotted in the form of a
  function.
• This is known as the audibility function.
• We are not equally sensitive to sounds of all
  frequencies.
• We are most sensitive to tones at approximately
  3000 Hz.

5
Pure Tones (contd)

• As we grow older, we experience a loss of hearing
  sensitivity known presbycusis.
• Particularly, we become less sensitive to higher
  frequencies (see Fig17.2).

6
Equal Loudness Contours

• A way of describing the relationship between
  loudness and sound intensity.
• A 1000 Hz tone is presented at a particular
  frequency (60 dB) and the subject notes the
  perceived loudness.
• Subject is then presented with tones at other
  frequencies.

7
Equal Loudness Contours (contd)

• They adjust the intensity of the other tones
  until they match the loudness of the 1000 Hz
  tone.
• These adjustments can be plotted across
  frequencies as equal loudness contours.
• All tones with intensities and frequencies on
  this contour are equal in loudness (Fig. 17.3).

8
Equal Loudness Contours (contd)

• Loudness is measured in phons.
• The loudness level in phons is defined as the dB
  level of the subjectively equally loud 1000 Hz
  tone.
• At low loudness levels, the equal loudness
  contours resemble the audibility function.
• As loudness increases, the contours change shape.

9
Equal Loudness Contours (contd)

• At low loudness levels, you are more sensitive to
  high frequencies compared to low frequencies.
• The opposite is true at slightly higher loudness
  levels.

10
Fatigue and Adaptation

• Auditory Fatigue A fatiguing stimulus is
  presented for a for a period of time and auditory
  thresholds are measured at various times after
  the stimulus has been turned off.
• E.g., Postman and Egan (1949) presented subjects
  with white noise (115 dB) for several minutes.

11
Fatigue and Adaptation (contd)

• White noise contains a broad number of
  frequencies at equal intensity levels.
• Subjects showed immediate hearing loss that
  decreased with time. Losses were most pronounced
  at higher spatial frequencies (Fig 17.7).
• Perhaps presbycusis is a form of cumulative
  auditory fatigue.

12
Fatigue and Adaptation

• Auditory Adaptation Effects are measured while
  the adapting tone is still present. Usually done
  with the using simultaneous dichotic loudness
  balance (SDLB).
• An adapting stimulus is presented in one ear
  while a subject manipulates the intensity of a
  comparison tone presented to the other ear.
• Subject matches the loudness of the two.

13
Adaptation and Fatigue (contd)

• The intensity of the comparison tone serves as a
  measure of the loudness of the adapting tone.
• The perceived loudness of the adapting tone
  decreases rapidly, then reaches an asymptote at 3
  to 7 minutes.
• Adaptation is equal at both low and high
  frequencies.

14
Critical Bands

• Involve complex tones.
• Band Limited a complex stimulus consisting of
  equal intensities of stimulation at all
  frequencies between two limits.
• Bandwidth the range of frequencies between the
  two limits (see Fig 17.8).

15
Critical Bands (contd)

• For narrow bandwidth stimuli at moderate
  intensity, the loudness of the stimulus is the
  same as a pure tone of the same intensity.
• Specifically a pure tone at the center frequency.
• Loudness is constant as bandwidth increases until
  it reaches a critical point where loudness begins
  to increase (see Fig. 17.9).

16
Critical Bands (contd)

• These bands in which loudness is constant are
  known as critical bands.
• This has led to the critical band theory.
• The auditory frequency range is divided into a
  number of critical bands.
• Combinations of tones that fall within a critical
  band are treated differently than those that fall
  outside the band.

17
Critical Bands (contd)

• When two intense tones excite the same critical
  band, they inhibit each other.
• If two tones excite different critical bands,
  there would be no inhibition.
• If the tones are near or below threshold in
  intensity and excite the same critical band, they
  will have their effects summed.

18
Critical Bands (contd)

• Size of critical bands increase as you go from
  low to high frequencies (Fig 17.10).
• Most likely, critical bands overlap.

19
Auditory Masking

• The minimum intensity necessary to hear a pure
  tone is determined with and without a continuous
  masking stimulus (noise).
• The amount of masking is greatest when the
  frequency of the test stimulus is equal to the
  center frequency of the masking stimulus.
• The more and more separate they become, the lower
  the amount of masking.

20
Auditory Space Perception

• Subjects are very good at localizing sounds to
  the left or right.
• Sound localization is relatively poor at about
  1500 to 3000 Hz (see Fig 17.13).

21
Neural Systems

• Owls have cells primary auditory cortex which
  respond to sounds in specific locations.
• They also have midbrain cells that have
  center/surround excitatory/inhibitory receptive
  fields.
• Cats and humans have been found to have bimodal
  cells
• Respond maximally when activated with a visual
  and auditory stimulus from the same location.

22
Neural Systems (contd)

• The auditory cortex and inferior colliculus also
  appear to be important.

23
Binaural Cues for Localization

• Intensity difference See Fig. 17.16.
• Low frequency sounds can bend around the head.
  High frequency sounds can pass through the head.
• The head acts as a filter, and reduces the
  intensity of sound.
• See Fig. 17.13., errors in localizing decrease
  after 3000 Hz because we use intensity
  difference.

24
Binaural Cues (contd)

• Timing Differences Because the ears are in
  different positions, the receive the sound
  stimulus at different times.
• With pure tones, this means the ears receive
  waves that are out of phase.
• We appear to use this cue with frequencies below
  1500 Hz (see Fig. 17.13).
• We also appear to use timing differences with
  complex stimuli.

25
Binaural Cues (contd)

• We appear to be able to use a difference of 9 ms
  to localize sound.
• Head Movements We possess a cone of confusion
  where timing differences and intensity
  differences are the same (Fig 17.17).
• We get around this using head movement.

26
Cues for Localization (contd)

• Monaural Cues We can use head movements.
• The shape of the pinna is also important.
• Monaural cues appear to be particularly important
  with pure tones.
• The perception of movement of a sound is mediated
  through monaural cues.
• Intensity change is an important cue.

27
Cues for Localization (contd)

• We also use the Doppler shift.
• The frequency in front of a stimulus is higher
  than that behind a stimulus.