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 for reasons explained in Chapter 6);

• 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 conventiona\ 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 (Fig. 2-7). (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.)

Fig. 2-7

Multidirectional radiators used as surrounds with the minimum output pointed at the principal listening location delivers an increased surround effect through interaction with the room acoustics of typical home listening rooms by reflecting the surround sound component of the sound field from many surfaces in the room.

Same angles as ITU, only with diffuse-field dominant dipolar radiating surrounds

Square Array

This places left and right at ±45° and surrounds at ±135°. The use of the center channel is generally minimized by these producers in their

work. The surround loudspeaker angle of ±110° for the AES/ITU setup was based on research that showed it to be the best trade-off between envelopment (i.e., best at ±90° when only 2 channels are available for surround) and rear quadrant imaging (which is better at ±135° than ± n0°). A square array was thoroughly studied during the quad era as a means of producing sound alf around, and information about the studies appears in Chapter 6. Nevertheless, it is true that increasing the surround angle from ±110° to ±135° improves rear phantom imaging, at the expense of envelopment.

One rationale given for the square array is the construction of four "sound fields," complete with phantom imaging capability, in each of the four quadrants front, back, left, and right. This thought does not consider the fact that human hearing is very different on the sides than in the front and back, due to the fact that our two ears are on the two sides of our heads. For instance, there is a strongly different frequency response in the ear canal for left front and left surround loudspeakers as we face forward, even if the loudspeakers are perfectly matched and the room acoustics are completely symmetrical. For best imaging all around, GuntherTheile has shown that a hexagon of symmetrically spaced loudspeakers, located at ±30°, ±90°, and ±150°, would work well. So the current 5.1-channel system may be seen as somewhat compromised in the ability to produce sound images from all around. After all is said and done, the 5.1-channel system was developed for use in accompanying a picture, where there was a premium placed on frontal sound images and surround sound envelopment, not on producing sound images all round.

Дата добавления: 2015-10-30