Surround Loudspeaker Directivity
Depending on the program material and desires of the producer, an alternate to either a pair of discrete direct radiators, or to surround arrays, is to use special radiation pattern surround loudspeakers. A pair of dipole loudspeakers arranged in the AES/ITU configuration, but with the null of the radiation pattern pointed at the listening area, may prove useful. The idea is to enhance envelopment of the surround channel content, as opposed to the experience of imaging in the rear. These work well in the rooms for which they were designed: consumer homes without particular acoustic treatment. In lower-reverberation time environments for their volume, such as small professional rooms, these are clearly not what is intended. Pros and cons of conventional direct radiators compared to multiradiator surrounds is as follows:
Pros for direct radiators for surround:
-rear quadrant imaging is better (side quadrant imaging is poor with both systems);
-localization at the surround loudspeaker is easily possible if required;
-somewhat less dependence on room acoustics of the control room.
Cons for direct radiators for surround:
-too often the location of the loudspeakers is easily perceived as the source of the “surround” sound;
-pans from front to surround first snap part of the spectrum to the surround speaker, then as the pan progresses, produces strongly the sound of two separate events, then snaps to the surround; this occurs due to the different HRTFs for the two angles of the loudspeakers to the head, the different frequency response that appears in the ear canal of listeners even if the loudspeakers are matched.
Pros for multidirectional radiators for surround:
-delivers the envelopment portion of the program content (usually reverberation, spatial ambience) in a way that is more “natural” for such sound, that is, from a multiplicity of angles through reflection, not just two primary locations;
-produces more uniform balance between front channel sound and surround sound throughout a listening area; in a conventional system moving off center changes the left-right surround balance much more quickly than with the dipole approach;
-makes more natural sounding pans from front to surround sound, which seem to “snap” from one to the other less than with the direct-radiator approach.
Cons for multidirectional radiators for surround:
-not as good at rear quadrant imaging from behind you as direct radiators;
-localization at the surround loudspeaker location is difficult (this can also be viewed as a pro, depending on point of view—should you really be able to localize a surround loudspeaker?);
-greater dependence on room acoustics of the control room, which is relied upon to be the source of useful reflections and reverberation.
There has been a great deal of hand-wringing and downright misinformation in the marketplace over the choice between direct radiator and multidirectional radiators for surround. In the end, it has to be said that both types produce both direct sound and reflected sound, so the differences have probably been exaggerated. (Multidirectional radiators produce “direct sound” not so much by a lack of a good null in the direction of the listener as from discrete reflections.)
This is an excerpt from Surround Sound: Up and Running, 2e by Tomlinson Holman
Tomlinson Holman is President of TMH Corporation and one of the prominent figures in audio today. He is widely known for his development of new products and processes in the field of audio and video, including the THX Sound System, Home THX (with his patents licensed to more than 45 companies), and the THX Digital Mastering program developed while he was Corporate Technical Director at Lucasfilm, Ltd.