A couple of snippets from
"Acoustics
Of
Small Rooms" by Kleiner & Tichy:
Spaciousness and diffusivity
Localization of externalized single sound field components was shown to be fairly straightforward but dependent on many factors. Localization of sound field components that have identical sound levels at the ears will depend on further factors such as phase difference.
When sounds are correlated, such as a monophonic signal that is presented binaurally, the auditory event occurs inside the head, inside head localization (IHL). If the sounds at the ears are fully uncorrelated, such as two separate noise signals that are presented binaurally, there will be two auditory events, one at each ear.
An interesting effect can be heard when presenting a monophonic wide bandwidth noise signal in stereo (over loudspeakers or headphones) if the stereo signals are out of phase. The noise frequency components below 2 kHz are then perceived as spatially diffuse—having spaciousness— whereas those for higher frequencies are perceived as located between the loudspeakers (or for headphones, IHL occurs). The time difference in the low-frequency components provides phase cues that are ambiguous thus providing apparent sound field diffuseness, whereas the high-frequency sounds are analyzed by their envelopes and those will be identical at the two ears causing a located auditory event.
Similarly, when a wideband noise signal is provided over headphones to a listener and one of the headphones is fed with the signal delayed by a millisecond or more, the sound is perceived as diffuse.
What constitutes a diffuse sound field is thus different in the physical and psychoacoustic domains. In the latter, a diffuse sound field is that that provides non-locatedness of sounds or, alternatively phrased, that provides a sound that is located over all spatial angles (or rather upper hemisphere in a concert hall that has sound-absorptive seating).
In physics on the other hand, a diffuse sound field is defined as a sound field where all angles of sound incidence have equal probability, where the sound from each spatial angle is out of phase, and where the energy density is the same everywhere.
Obviously, the two ideas of what constitutes diffuseness are different in the two sciences. A physically diffuse sound field will also be psychologically diffuse but not necessarily the reverse. From the viewpoint of listening, it is of course the psychoacoustic properties that are of importance, not the sound field properties.
Auditory source width and image precision
As we listen to sounds, the apparent width of the auditory event, often called the auditory source width (ASW), will depend on many issues. To those listening to stereo or multichannel recordings of sound, it is quite clear that the width of the array of phantom sources treated by the recording or playback is determined by not only the layout of the loudspeaker setup in the listening room and the directional properties of the loudspeakers but also on the listening room itself. The more reflections arriving from the sides of the listening room, the wider will the ASW be. However, the ASW will be frequency dependent above 0.5 kHz and a 2 kHz sound arriving at ±45° relative the frontal direction will produce maximum ASW [38,39]. This is to be expected since the masking by direct sound is the smallest for this angle of incidence of early arriving reflections [16]. The ASW also depends on the low-frequency content of the signal, more low-frequency energy increases ASW [38,40,41]. Psychoacoustic testing shows that the spatial aspects of the early reflections are primarily determined by the reflection spectrum above 2 kHz [33].
Reliable data for sound reproduction in small rooms are difficult to find. A single omnidirectional loudspeaker judiciously placed close to the corner of a room may well create as large an auditory image as a conventional stereo loudspeaker setup placed out in the room as discussed in Chapters 9 and 11.
Using digital signal processing, the ASW can be made to extend far outside the bounds set by the stereo baseline. Sound field cancelation techniques
Symmetry
Early reflected sound will confuse hearing and make the stereo stage and its phantom sources appear incorrectly located or even blurred. As explained in Chapter 8 the listener’s placement of the phantom sources is dependent particularly on the transient nature of the sound that comes from the loudspeakers so it will be affected by the early reflected sound from the room surfaces. The early reflected sound will also affect the global auditory source width for an orchestra for example and may make it extend considerably beyond the baseline between the loudspeakers.
In asymmetric rooms where the walls on the left and right of the listener have different acoustic properties, the stereo stage may become biased towards the wall that reflects the most. The curve in Figure 8.23 shows the dependency more clearly for different levels of unbalance as applied to the center phantom source in a stereo loudspeaker system. The intensity will then be higher at that ear and the sound stage distorted. This distortion is usually compensated by changing the balance in amplification between the stereo channels.
At low frequencies in the modal region, symmetry may not be desirable since someone sitting in the middle of the room may be on or close to modal node lines. One way of avoiding such node lines is to make the room asymmetric in the low-frequency region.
This can be achieved by having an asymmetric rigid shell surrounding the inner room which is symmetric for mid- and high frequencies by suitably reflective side walls, ceiling, and floor. The inner room must be open acoustically to the outer shell at low frequencies, for example through ventilation vents, and similar large openings, for example at corners. In this way, one can have the desired listening position sound field symmetry for mid- and high frequencies while at the same time have asymmetric conditions in the modal frequency range. Bass traps to control the damping—and thus the reverberation times—of these modes can be placed between the outer and inner shell. It is important to remember though that noise transmission to the surrounding spaces will then be dependent on the sound isolation of the outer shell that must be physically substantial.
