Subjective quality:

Spaciousness or spatial impression is a term that was introduced in the 1970s to refer to a listener’s feeling of being enveloped in the music. Much research on this concept has occurred in the past three decades, and now two aspects of spaciousness have been identified: Auditory Source Width (ASW) and Listener Envelopment (LE). ASW describes how large and wide the sound source appears to the listener. Listener envelopment, meanwhile, addresses how the listener feels surrounded by the music, rather than listening to it as if through a window.

Objective measure:

Early studies on spaciousness determined that lateral reflections, or those coming from the side, play a large role in its perception. The distinction between ASW and LE is dependent on when those lateral reflections arrive. Early lateral reflections (within 80 ms of the direct sound) seem to affect ASW, while late lateral reflections (after 80 ms of the direct sound) affect LE.

Various objective measures have been suggested for spaciousness, each of which suggests a different mechanism for its perception. Among the first was Lateral Energy Fraction (LF), which is a ratio of sound energy arriving laterally over sound energy arriving from all directions. Interaural Cross-Correlation Coefficient (IACC) was soon introduced; it measures the cross-correlation between the signals that arrive at the two ears of a listener. The more dissimilar the signals, the more spaciousness is perceived. Each of these may be calculated for early reflections or for late reflections, and thus indicate ASW or LE respectively.

More recently, a measure known as Late Lateral Level (GLL) has been introduced for listener envelopment. This measure quantifies the strength of laterally arriving sound energy, similar to the strength factor G for loudness. Research is still ongoing about which objective measure gauges spatial impression the best.

Listen to different values of LF.

Optimum values:

Higher values of LF and lower values of IACC are said to correlate with greater feelings of spaciousness. LF(E4), the early lateral fraction over four mid-frequency octave bands, is around 0.2 for popular halls. The quantity (1-IACC(E3)), where IACC(E3) is the early IACC over three mid-frequency octave bands, ranges from 0.6 to 0.7 for popular halls.

Click for IACC/LF’s of famous concert halls.

How to design:

To increase lateral reflections, add irregularities to the hall: use reflective surfaces, ornate decorations, or sculptures, particularly on the sides of the audience.

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