Loudspeaker Topology

Primary Multi-Use Assumptions

It is easy to envision a wide range of uses for such a space. They include, but are not limited to, the following:

Home Theater

Home theater uses a 5.1 loudspeaker array with a screen and video projection system, subject to an array of conventions. It may be possible to replace the subwoofer by using identical full-range speakers and route the LFE channel to at least two of the full-range loudspeakers. At present, it is recommended to place speakers in a circle (or time-corrected polygon) at the following angles:

	Figure 3. Recommended Home Theatre 5-channel array.

It has been my experience that these angles can be altered significantly without trouble, so long as lateral symmetry is maintained. This is particularly true when high-quality loudspeakers are used. Note also that the rear, surround angles are derived from movie theater practice and may not be optimum for either musical or home theater use.

Stereophonic Playback

The system should also permit high-quality two-channel stereophonic playback, which requires quite high quality loudspeakers, plus careful and symmetrical placement.

Multi-Channel Playback of traditional music

The system should permit high-quality multichannel playback of traditional “acoustic” recordings, supporting a variety of approaches to those recordings.

Multi-channel Playback of original Surround Sound Music

The system should permit high quality multichannel playback of dedicated “surround-sound” music composed specifically for a loudspeaker array. This means that no constraints to the playback system should be applied that are based on assumptions about “how” various channels will be used. It reinforces the call for all channels to be identical, high-quality full-range channels.

The 5.1 Channel Array

The 5.1 channel array in current use evolved from film production and movie theater playback requirements and here we DO make assumptions about how the channels will be used. A center channel is needed for on-screen dialogue, left and right channels are used for stereophonic music and off-screen effects, the surround channels are used for reverberance and ambience as needed, plus rear-of-hall effects, while the LFE channel is used for low-frequency extension for dramatic effect as needed. These usages have evolved over fifty years and are well-known and understood by the film production community as well as accepted by audiences.

The application of this cinematic theater tradition to the home theater is less well-developed and implemented, due to manufacturers’ urges to reduce system costs by reducing bandwidth of speakers and related economies. As a result, the performance of the speakers in home theaters has often been degraded on the assumption that the subwoofer can provide the low-frequency bandwidth (this assumption is not part of the movie theater experience, or planned for in film production). Surround speakers are often reduced in performance (and cost) on the assumption that they ONLY carry band-limited low-power reverberant information. The result has been a chaotic and often quite poor implementation of home theater for middle-class consumers, an implementation that is not really adequate for satisfying film viewing, much less musical playback.

Nonetheless, extremely high-quality installations can be obtained with existing technology and often are realized in purpose-built rooms in upper middle class homes. An entire niche industry has evolved to serve this market.

A proposed 6-channel Array

For my own work, I have been using a six-channel array (five full-range channels in approximately the conventional home theater arrangement plus a sixth full-range channel mounted directly overhead).

The height channel is an extremely interesting one, whose virtues aren’t particularly obvious at first, but whose impact is quite substantial. For original multitrack music, its availability as a source channel suggests a variety of musical gestures, meanings and spatial relationships that are simply not available in “horizontal” music.

In acoustic recordings, the overhead channel is usually captured by an upward facing hypercardioid microphone positioned somewhere behind the main front stereo array. The result of this ambient information is a sense of open “airiness,” additional fullness and richness to the sound and a “coupling” of front and rear speaker arrays.

Because of the low-frequency extension of my BeoLab 5 loudspeakers, I simply send the LFE channel to Left and Right, which works entirely satisfactorily. I have been able to use this topology successfully for both stereophonic and surround production and mastering of both music and sound for film/video.

A possible pentagonal topology

Noting the benefits Tomlinson Holman has derived from his 10.2 experimental playback system for more solid “side phantom” images, I have experimented with a pentagonal horizontal array, as shown below:

	Figure 4: Pentagonal loudspeaker array, showing also the approximate locations of phantom images derived from individual pairs.

Such an array can be used successfully for all of the above applications, with some simple but interesting modifications. For surround music, the ability to have robust lateral images (there is a solid source at +/-72° and a L/Ls or R/Rs phantom at ca. 90°) is quite attractive and significantly expands the compositional repertoire.

Meanwhile, really effective stereo (I find it more satisfying than conventional stereo) can be created by sending the mono LR sum to the Center speaker for 3-channel stereo. In the normal +/- 30° stereo configuration, this causes the stereo image to collapse unacceptably around the center channel. However, because Left and Right are both at 72° from Center in the pentagonal array, Left/Center and Right/Center phantoms are generated at +/- 36° and the result is an extremely satisfying stereo image with a solid Center image and phantom Left and Right images (as opposed to the conventional phantom Center image and solid Left and Right images). Meanwhile, the rear stereophonic field is much more stable with robust solid sources and phantom images and reverberance wrapping forward from 180° forward to 90° on each side.

It is my intention to return to this as my primary working topology by the end of 2006.