Psychoacoustics

• + - Perception
  • Perception of spatial sound is very rich
  • Sensations
    • Position
    • Elevation
    • Distance
    • Width/Height
    • Volume
    • Depth
    • Direction of motion
    • Room characteristics
    • Aesthetic experiences
• + - Localization
  • Duplex theory of localization
    • Lord Rayleigh
    • Head shadow + Time difference
    • Binaural cues
      • + - Interaural Level Differences (ILDs)
        • A function of frequency
          • Sound waves are effectively diffracted when their wavelength is longer than the diameter of the head
          • At 500 Hz, sound wavelength is 69 cm (almost 4x the human head)
          • 
        • Above around 800 Hz
        • The auditory system is sensitive to ILD differences of up to 0.5 dB
        • + - Head shadow for higher frequencies that diffract less

          • Minimum ILD at the front and back

            Maximum ILD to the sides

            Since diffraction is a function of frequency, ILDs are frequency dependent for the same direction

            Caveat: Of course it is impossible for the auditory system to know whether level differences come from head shadow or from Pinna or torso filtering

      • + - Interaural Time Differences (ITDs)

        • + - At 500 Hz, a human listener is sensitive to differences of 1-2 degrees in the frontal horizontal plane

          • a 1 degree difference corresponds to a difference of 13 µs ITD

            This has been verified with experiments on subjects using headphones and providing signals with only ITD (and no ILD)

          Jeffress neural model attempts to explain this

          • ~ https://auditoryneuroscience.com/topics/jeffress-model-animation
          • Cross-correlation

          Below around 1500 Hz

          • 
          • Half a period of the wave must be longer than the delay between the ears
          • But modulated tones can be localized even at 4000Hz!
            • Provided the modulation is less than 1000Hz

          + - The ITDs depend on the azimuth direction of the source

          • Minimum ITD is at the front and the back
          • Maximum (and opposite) ITDs occur directly to the side (90 degrees az.)
  • + - Monaural cues
    • + - But the duplex model is not enough...
      • Sounds on the median plane
        • How can we tell front from back?
        • How can we judge elevation?
      • Also headphones produce a strange sensation
        • The sound is "within" the listener's head
        • This is called lateralization instead of localization
        • So providing only ILD and ITD cues is not all there is...
      • Cone of confusion
    • one ear HRTF
      • Head Related Transfer Functions
  • HRTF differences
    • Level differences are a frequency dependent phenomenon
  • + - Head movement

    • Even minor and unconscious head movements can help disambiguate localization

      If the HRTFs change strongly when moving the head to the sides, the source is on the horizontal plane

      If they change little then the source is above or below

      How they change might also provide additional cues

  • + - Precedence effect

    • The psychoacoustic suppression of sound reflections/copies that arrive within certain time and amplitude thresholds with respect to the other sound

      + - Occurs between 2 and 50ms

      • Less than 2ms still causes suppression, but gradually pushes localization towards the later source as time delay decreases

        This is called summing localization, and is actually what determines localization in stereo loudspeaker amplitude panning

      The perceived direction of the sound is determined by the earlier/louder tone

      Also known as the Haas effect

      + - Time intensity trading

      • It is possible to trade time and intensity
      • for a delay of 100us sound is perceived towards the earlier sound
      • However if the later sound is played louder, the image can be centered again

      ~ en.wikipedia.org > Wiki > Precedence effect

  • Continuous process of evaluation
    • Available cues
    • Best hypothesis
    • New information, later time
      • Adjust hypothesis
• + - Distance
  • High frequency attenuation
  • High reverb to dry ratio
  • Reduced Loudness
  • Movement
• + - Size
  • + - Source width
    • Width is an inherent attribute of sound sources
    • A sound source is distinctly different when played back over a single loudspeaker

      • The timbral and temporal characteristics of the sound remain (mostly), but the spatial properties of the original sound field are lost

  • + - ASW
    • + - Concert hall acoustics
      • Spatial Impression (SI)
        • + - Spaciousness
          • + - Apparent Source Width (ASW)
            • “width of a sound image fused temporally and spatially with the direct sound image”
            • Attribute of the source
          • + - Listener Envelopment (LEV)
            • “the degree of fullness of sound images around the listener”
            • Attributes of the enclosing space
          • + - Related to the perception of quality in concert halls
            • Greater ASW and LEV are considered desirable features of a concert hall
      • but...
        • Others have proposed alternate classifications
          • + - Griesinger
            • + - Continuous Spatial Impression (CSI)

              • The sense of spaciousness or envel- opment “perceived as connected to or associated with the direct sound”

            • + - Early Spatial Impression (ESI)

              • Produced by sounds that are not continuous, and depends on the energy of reflections arriving within 50ms. ESI is perceived in small rooms, and it occupies the same direction as the original source and is not fully enveloping

            • + - Background Spatial Impression (BSI)

              • BSI is the equivalent of ESI for large rooms and occurs when there is significant energy after 50ms. BSI is perceived as separate from the source sound

        • + - Sazdov
          • Engulfment

            • the sensation of “being covered over as opposed to simply being surrounded as defined by the term envelopment”

      • Measurements
        • + - Hard problem
          • Perception of source width depends on:
          • Frequency content
          • Level
          • Material itself
        • Lateral Fraction/Efficiency (LF)
        • + - Interaural Cross-correlation Coefficient
          • Many different ways to measure
          • Usually calculated from an Impulse Response on Halls
          • Splitting by bands
          • Using only initial or later part of the IR
          • So what started out as a psychoacoustically informed measurement became a purely empirical measure
    • + - Loudspeaker reproduction
      • Concepts borrowed from the Concert Hall Acoustics studies
      • + - Precedence effect
        • Psychoacoustical suppression of sound reflections, by a louder or earlier one
        • Very significant
        • The loudspeakers are "loud" early reflections
  • + - Envelopment

    • Another spatial attribute, generally associated with the effect of a space on the sound than a characteristic of the sound itself

      However both ASW and LEV (Listener envelopment) seem to be somewhat elusive and hard to pin down characteristics

• + - Timbre and space
  • + - Deeply connected
    • Timbral monaural and interaural differences play an important part in the perception of space
      • HRTF based localization is based on direction dependent timbral changes
    • Changes in timbre can affect space and vice-versa
• + - Multimodal
  • Other senses, particularly sight, have a significant influence on the perception of localization
  • Auditory space is often mistakenly considered analogous to visual space
  • But in fact quite different and less accurate in some respects
  • Although hearing covers all around at all times, not only where the head is facing