Minimum Audible Movement Angle as a Function of the Azimuth and Elevation of the Source

1
Minimum Audible Movement Angle as a Function of
the Azimuth and Elevation of the Source

• Strybell, Manlignas and Perrott
• 1992

2

• In the past, location of sounds in a cockpit were
  not very informative.
• Technology of headphones now allows for 3-D
  auditory cues.
• In 1986, Doll et. al pointed out that auditory
  directional cues could be used to enhance
  situational awareness

3
Utility of auditory spatial cues depends on 2
factors

• Quality of the 3-D auditory simulation.
• Pilots ability to use the information.
• (sensitivity, discernment, etc.)

4
Study

• Auditory localizationPilots ability to
  determine location of sound.
• Spatial AcuityPilots ability to discriminate
  between the positions of two concurrent sources.

5

• Past research focused on static sounds that
• were real (not simulated) on a horizontal plane.

6

• MAA (Minimum Audible Angle)

7
The focus on stationary stimulus is of limited
use in cockpit situations where acoustic events
are moving.

• Pilots head is moving
• Aircraft is moving
• Auditory stimulus could also be moving

8

• Relatively few studies about dynamic listening
  conditions.
• Most studies looked only at 0º azimuth.
• MAMA (Minimum Audible Movement Angle) is an index
  used to measure dynamic acoustical events.

9

• Like the MAA, the MAMA acuity increases as
  azimuth angle increases.
• MAMA affected by the spectral content of the
  signal and the velocity of the moving sound. (It
  is smallest for broadband stimulus moving at low
  velocities.)

10

• 2 studies showed that the MAMA acuity
• is somewhat poorer than the MAA acuity.

11
Purpose of Study

• To examine auditory spatial conditions more
  representative of those encountered in
  head-coupled display systems.
• To measure MAMAs for sound sources located
  between 80º and -80º azimuth and between 0º and
  87.5º elevation

12
How they did it
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MAMAs were measured at 16 azimuth elevations

• 9 horizontal azimuths at 0º elevation.
• 5 elevations at 0º azimuth.
• 2 azimuth locations (-40º, -80º) at 80º
  elevation.
• The loudspeaker traveled 20 degrees/second
  clockwise or counterclockwise.
• Adaptive psychophysical procedure.

14
Results
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Conclusion

• MAMA ranges from 1-3 degrees for sources located
  in front of the subject.
• For sources outside the semi elliptical area,
  MAMA increases 3º - 10º.
• No reason to believe that asymmetries exist
  across left/right quadrants, but its likely that
  differences would exist below the horizon due to
  the differential shadowing effect of the torso.

16
Cockpit Applications

• Since moving events in the periphery must travel
  further (200-300) than centrally located events,
  displays of moving acoustic stimuli in the
  cockpit must take into account the location of
  the event in order to insure that the pilot can
  actually detect the direction of movement within
  a critical period.

17
Spatial Intercoms for Air Battle Managers Does
visually Cueing Talker Location Improve Speech
Intelligibility?

• Bolia, 2003

18

• Air battle managers sometimes monitor as many as
  8 communication channels simultaneously.
• Spatial intercoms have been suggested as a way of
  improving intelligibility and situation
  awareness.
• Situation Awareness (SA) is the idea that by
  localizing a virtual sound that mimics the
  location of the real world sound, its easier for
  the listener to keep track of where
  communications are coming from.

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What if the reverse were true?

• If a spatialized audio cue can help
• a listener identify a spatial location, can a
  visual cue be used help a listener attend more to
  an audio cue?

20
To investigate this question, Bolia used a form
of the Coordinate Response System (CRM).

• Call sign
• Color/number combination
• Ready baron, go to the blue five now.

21

• For this experiment, there were 2 talkers (same
  sex) located at 20º or 90º of azimuth (0º
  elevation).
• 2 target call signs were assigned.
• Baron in 87 of the trials
• Charlie in 10 of the trials
• Both Baron Charlie were used in 3 of the
  trials
• Charlie was considered higher priority in the
  case of conflict.

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3 Conditions

• Diotic
• Binaural uncued
• Binaural cued

Modified version of the NASA Task Load Index was
used to rate workload after the trials
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Results

• Binaural conditions were significantly better
  than the diotic conditions.
• But cued conditions did not improve performance
  over uncued conditions.
• However, cued conditions did not degrade
  performance, and it rated as lower in task load
  and was perceived by participants as helping.

24
Effects of Spatial Intercoms and Active Noise
Reduction Headsets on Speech Intelligibility in
an AWACS Environment

• Bolia, 2003

25
USAF-AWACSUnited States Air ForceAirborne
Warning and Control System

• Confusing environment
• Noisysometimes more that 85 dBs
• 8 simultaneous communication channels.

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2 technologies proposed to ameliorate the
situation

• Spatial Intercoms(shown to improve speech
  intelligibility)
• Uses spatial cues from interaural differences in
  sound levels (ILDs). Good below 1500Hz.
• Interaural time differences (ITDs). Good above
  3000Hz.
• ANRActive Noise Reduction
• attenuates external sound by measuring it, then
  adding a waveform with the same spectral
  characteristicsbut an opposite phase. Good above
  1000 Hz.

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Both technologies had been studied extensively,
but Bolia wanted to find out how they interact.
The purpose of the study was to determine,
within the context of an AWACS noise environment,
the extent to which spatial intercoms and ANR
contribute to improving speech intelligibility in
a multi-channel listening task, and the degree
to which a facilitative interaction exists.
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Apparatus

• Used speech phrases from CRM (Coordinate Response
  Measure) convolved with Head Related Transfer
  Functions.
• HRTFs are digital representation of spatial cues
  used for sound localization.
• They capture both ITDs and frequency-dependent
  ILDs for various measured locations.

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Experiment

• Participants assigned a call sign
• Responded by clicking on color/number matrix
• Listened to 2 simultaneous phrasesone with a
  call sign, one withoutpresented at 20º and 90º
  of azimuth. Phrases were always in different
  hemifields.

In the non-spatial condition, both phrases were
convolved with the same pair of HRTFs (0º azimuth
and 0º elevation)
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Asymmetry of Sound Field
Louder in participants right ear
Target Left
Target Right
Diotic
ANR on
ANR off
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Results

• Spatial conditions70 correct
• Diotic40 correct
• Left Hemifield89 correct
• Right Hemifield53 correct
• With ANR increased 20

Enhanced intelligibility with ANR may be due to
the increased audibility in the frequency range
in which ITDs operate, i.e. below 1500 Hz
32
Conclusion

• Generally, ANR doesnt improve speech
  intelligibility.
• In certain high noise environments, ANR can
  improve intelligibility.
• Might be applicable to the cockpit environment.