Reflections And Their Impact On Sound Quality - Community ENTASYS Application Manual

Reflections and Their Impact on Sound Quality
Introduction
1
A well known study by Michael Barron examining the subjective effects in a listener's perception of early and late
sound reflections has helped shape how this generation understands what types of sound reflections will contribute
positively to the direct sound output of a sound source. For the purposes of this ENTASYS design handbook, the fol-
lowing chart in Figure 46 details the relationships of reflections at various levels relative to the direct sound and the
amount of time in milliseconds that transpires from the initial sound impulse until the reflection travels to the listener.
Regarding the effects of reflections caused by nearby boundaries near an ENTASYS loudspeaker, take particular note of
the resulting acoustic behavior that occurs up to about 40ms. Reflections from such boundaries effect tonal quality of
sound, psycho-acoustic imaging, and the contribution (or degradation) to intelligibility.
Very Early Reflections
When evaluating ENTASYS array locations, consider the effects of very early (up to about 10 ms), higher level reflec-
tions that contaminate the original direct sound impulse mid-flight before it even arrives at the audience. Often this can
be caused by the combination of wide horizontal dispersion from an ENTASYS system and a nearby room boundary
such as a wall. The term "deep comb filtering" describes the effect of these reflections on the direct sound. Also, this
can cause "image shift" wherein the listener begins to perceive the original sound source as actually emanating from a
phantom position between the loudspeaker and the acoustically reflective surface. The audience will be affected much
like the psychoacoustic phenomenon that generates central phantom acoustic images in stereo sound systems. These
are the most destructive reflections to the audio path. They deserve attention in the system design to consider: another
mounting location, a change in horizontal aiming, acoustic absorptive treatment near the array, or some combination of
these measures may improve the quality of the installation.
Supportive Early Reflections
For reflections which arrive at the audience after a greater delay and/or decreased level, the concern for modifying the
system design will be less critical. In fact, as the delay exceeds 10 ms and less than 35 ms, the reflected sound will gen-
erally create a perceived tonal shift, but will also reinforce the original sound and contribute positively to intelligibility.
Comb filtering will still exist, but the listener will begin to average these signals in their hearing perception. To minimize
the tonal coloration in this time domain window, consider the same modifications as indicated above for very early re-
flections.
Reverberant Field Reflections
As reflections arrive later than 35 ms, they become part of what many call the "reverberant field". So long as these
reflections arrive to the listener at lower level, they contribute positively to the listener's dimensional perception of
the environment. In other words, in this region of the chart the listener will begin to perceive the various features of
the venue's "acoustic signature" and appreciate its spaciousness and unique reverberation.
Harmful Reflections
Most importantly, be aware of the very harmful effects of reflections that arrive in the zone in the chart labeled
"disturbance". These sound reflections will be perceived as distinct secondary sound events commonly called "echoes"
which certainly degrade intelligibility. Every effort must be made to avoid this phenomenon for the sake of preserving
clarity of music and speech.
Low-Level Reflections
If there is any saving grace in dealing with reflected sound, note that reflections which arrive at the listener about 20 dB
to 30 dB lower than the original sound become negligible and can largely be disregarded. In the chart, the region
labeled "Threshold" represents these very low level reflections.
Community ENTASYS Application Guide - Page 54