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.