3-D Sound and Spatial Audio

MUS_TECH 348

Wightman & Kistler (1989)

Headphone simulation of free-field listening

I. Stimulus synthesis

II. Psychophysical validation

I. Stimulus synthesis

• 200-14,000 Hz• Greater than 20 dB S/N (only 20 dB?)• 8 loudspeakers on movable arch creating 144 directions• With & without bite bar

Measure loudspeaker-delivered HRTFs and compare to headphone-delivered HRTFs

Goal is to be able to capture free-field listening acoustics with headphones.

HRTF measurement

system

Variability in HRTF

measurements

Left ear

Assembly replaced 10 times with bite bar

Assembly left in place with no bite bar

Headphone replacement with assembly in place

Right ear

Left ear

Right ear

HRTF intersubject variability

II. Psychophysical validationGoal is to compare localization performance in free-

field and headphone listening

Stimuli:8 250 msec noise bursts200 - 14,000 Hzrandom spectral changes by critical band

Presentation:6 loudspeakers at a time mounted on archheadphones72 positions

Task:absolute judgment of azimuth and elevationno measure of distance or quality

Types of Errors•Angle error (mean of difference angles)

•Judgment centroid (average direction)

•Dispersion of judgments

•Front-back reversals are removed! (and examined separately)

Results•Substantial individual differences

•Less obvious in global measures•Most evident in elevation judgments

•Performance varies with region•Best localization: side (contradicts other studies)•Worst localization: top rear

•Free-field and headphone judgments very similar•More front-back reversals with headphones

Headphone simulation data in parentheses

SDE has most errors.

SDO has fewest errors, especially for elevation.

Elevation Dependency Function

Interaural intensity difference compared to 0-degrees elevation

Subject SDE’s poor elevation judgments could be explained by the lack of a coherent pattern

Begault: Challenges to the Successful Implementation of 3-D Sound

Focus is on deployable systems, especially audio systems

Individual HRTFs can be quite different

Challenges:Eliminate front-back reversals & improve externalizationReduce HRTF data loadResolve conflicts in data specifications

Begault: Challenges to the Successful Implementation of 3-D Sound

Mismatch of Specification and PerformanceSuccess depends on:

HRTFs: some work better than othersdifferent sets create timbral percepts

Input soundsbroadband sounds localize better

SpecificationHave reasonable expectations

What kinds of HRTFs to use for systems? General HRTFs designed for average listenersHRTFs of good localizer

Reality vs Ideal

From Begault and Wenzel, 1993

Begault: Challenges to the Successful Implementation of 3-D Sound

Localization errorFor dummyhead recordings, 30% of locations

suffer reversals4:1 front-back vs back-frontMany sounds not externalized

Low-frequency Response ErrorsMeasurement equipment can’t get it right

Data-reduction for HRTFsReduce the number of coefficientsAlternative Strategies like pole-zero modeling

Martens: Perceptual evaluation of filters controlling source direction: Customized

and generalized HRTFs for binaural synthesis

Focus is on systems supporting directional hearing with special consideration on HRTF design

Position of sound source and position of auditory event do not always coincide, but that is not necessarily an issue of accuracy

Sound localization might better be called space perception

Martens: Perceptual evaluation of filters controlling source direction: Customized and generalized HRTFs for binaural

synthesis

Binaural Synthesis

Good localizer HRTFs not supported by evidence

Given the variety of approaches to binaural synthesis, better to use the term Directional Transfer Functions (DTFs) when they are created analytically

Target Exact AnalyticOne Individualized HRTFs Customized DTFsMany Averaged HRTFs Generalized DTFs

Performance evaluation (in additional azimuth and elevation);ExternalizationRangeCoherenceNaturalness

Martens: Perceptual evaluation of filters controlling source direction: Customized and generalized HRTFs for binaural

synthesis

Binaural Synthesis Evaluation

What features are needed to make binaural synthesis “ear adequate”

Binaural cues can be based on analysis and selected resynthesisPrinciple Components Analysis (PCA)Selective Reconstruction (for example, leaving out phase

information[Pole-zero design]

Elevation judgments needed only three out of four cues:ipsilateral magnitudeinteraural magnitudeipsilateral phaseinteraural phase

Martens: Perceptual evaluation of filters controlling source direction: Customized and generalized HRTFs for binaural

synthesis

Customizing HRTFs

Calibration methods:Anthropometric (anatomy)Acoustic (HRTFs)Psychophysical (perception)

Source RangeIpsilateral gain and contralateral attenuation are

important