3-D Sound and Spatial Audio

MUS_TECH 348

Cathedral / Concert Hall / Theater Sound

Altar / Stage / Screen

Spiritual / EmotionalWorld

Subjective

MusicAmbience

Cultural Context of Spaciousness

Environmental Acoustics, Perception, and Audio

Applications

Distance Perception

We already discussed that Chowning makes use of the fact that in very reverberant environments like concert halls, perceived distance varies in proportion to indirect/direct sound ratio.

The absolute sound intensity and the relative sound intensity (changing as 1/distance) are cues in themselves.

Distance Perception

Most environments have little or no reverberation in the way we discuss it in concert halls.

In these environments, changes in distance can be cued by changes in the timing of low-order reflections.

Consider too,

Distance Perception

Most important is the time interval of the initial gap between the sound and first-order reflections. For example, as a sound moves farther away, the gap becomes smaller.

Distance Perception

We know that distance judgments can be influenced by sound intensity, relative intensity, HRTFs, the indirect-to-direct energy ratio, and the timing of first-order reflections. In addition, distance judgments can be influenced by cognitive factors like knowledge of the sound source or likely distances given the context.

It seems that distance judgment is multi-faceted.

Position Perception

Early reflections also constitute an element of realism for simulated acoustic spaces independent of distance perception.

The kind of interaural decorrelation of that is experienced in concert hall reverberation is an important aspect of audio applications.

Interaural decorrelation can be created in a variety of ways.

Decorrelation Revisited

Multichannel Reverb

Reverb

L

R

Stereo reverb units produce output channels that are uncorrelated in order for the listener to experience some degree of spaciousness.

uncorrelated outputs

Pro Audio Decorrelation

Decorrelation by Reverb

ReverbL

R

Pro Audio DecorrelationStereo reverb is typically used in combination with the source signal.

diffuse field

Decorrelation by Detuning

Pitch ShiftL

R

Pro Audio DecorrelationA related spatial effect somewhere between spatially diffuse reverberation and dual images can be created by detuning one channnel.

Studio Decorrelation by Recording

• Record the same performance twice• Put one on left and one on right

Pro Audio Decorrelation

Recording a performance twice creates similar dual image effect.

The degree of perceived ‘spaciousness’ and the effectiveness of many of these effects are predicted by level of interaural decorrelation. The objective measure of correlation (and decorrelation) is the cross-correlation measure, Ω(∆t), and the cross-correlation index (peak of the cross-correlation measure).

Decorrelation in Audio

where ∆t represents a temporal offset between y1(t) and y2(t).

Examples of correlation measure:

Decorrelation in Audio

Five Effects of Decorrelation on Stereo Imagery

1. the timbral coloration and combing associated with constructive and destructive interference of multiple delayed signals is perceptually eliminated,

2. decorrelated channels of sound produce diffuse sound fields (akin to the late field of reverberant concert halls),

3. decorrelated channels produce externalization in headphone reproduction,

4. the position of the sound field does not undergo image shift with changes in the position of the listener relative to stereo loudspeakers,

5. the precedence effect which causes the collapse of the image into the nearest loudspeaker is defeated enabling one to present the same sound signal from multiple loudspeakers.

Decorrelation in Audio

Decorrelation in Audio

Effect #1 the timbral coloration and combing

associated with constructive and destructive interference

of multiple delayed signals is perceptually eliminated

Decorrelation in Audio

Effect #2 decorrelated channels of sound produce diffuse sound fields (akin to the late field of reverberant concert halls),

Decorrelation in Audio

Effect #3 decorrelated channels produce externalization in headphone reproduction

Decorrelation in Audio

Effect #4 the position of the sound field does not undergo image shift with changes in the position of the listener relative to stereo

loudspeakers

Decorrelation in Audio

Effect #4

Decorrelation in Audio

Effect #5 the precedence effect which causes the collapse of the image into the nearest loudspeaker is defeated enabling one to present the same sound signal from multiple loudspeakers

Decorrelation in Audio

Effect #5

Decorrelation in Audio

Another Distance Effect

A constant phase offset (rather than cross-correlation index) creates an impression of changing image distance.

Like HRTF reproduction, this distance effect depends on how close the listener is to the sweet spot.