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Ambiophonic Loudspeakers - Ambiopoles, Ambiostats, and Surrstats

In an Ambiophonic system there are two different functions the loudspeakers must perform. The first one is to generate the combfiltering and non-phantom-image free front stage and the second one is to reproduce the surrounding concert hall ambience. I call the pair of speakers (Ambiostats) that generate the front stage an Ambiopole and the other speakers that provide early reflections and reverberation tails, Surrstats. While I will describe the ideal loudspeaker for each Ambiophonic purpose, the ultimate choice for audiophiles will, as always in stereo or home theater, be determined by their, budget, space, and what they already own that can be adapted to this purpose.

We wish to apply the rules of good concert-hall design to the choice of home concert-hall loudspeaker characteristics and speaker placement. Let us assume that we have available the high quality software-generated hall ambience signals described in detail in Chapter 8. Let us also assume that our listening room is treated well enough to eliminate the bulk of the counterproductive listening room reflections using absorption panels and hopefully an electronic room correction system as described in Chapter 5. Furthermore, let us also assume that we will be using the software or barrier Ambiopole arrangement for the left and right front channel speakers which are separated by a ten degree angle directly in front of the listener or listeners as described in more detail in Chapter 7.

There is one general characteristic that applies to all the loudspeakers used in a domestic concert hall: all speakers should be as focused as possible, so as to reduce the number and level of stray listening room reflections. This is desirable because no practical room treatment is ever fully absorptive, so the less there is to absorb in the first place the better the result. In a multi-speaker surround system there is also the possibility of significant reflections from the surround speaker cabinets, diaphragms or frames so room treatment is even more desirable in such systems.

The 5.1 Home Theater Conundrum

Since so many of my readers are devotees of video home theater and its ad-hoc arrangement of two surround speakers placed at the rear sides, I think it would be best to first discuss the shortcomings of this arrangement before proceeding to describe something more realistic and scientifically based. The home theater movie people recommend two dipole speakers placed on edge so that the acoustic null such speakers produce is facing the listening position. Dipole speakers are speakers that radiate sound equally from opposite sides. Additionally, these sounds are of opposite polarity and so cancel where they collide in a room. Some dissenters argue that monopole, that is direct, single polarity, radiators, are better. Either type of rear surround speakers may be reasonable some of the time for movie and video sound reproduction. However, where classical music, jazz, etc. is concerned the Dolby/THX crowd has ignored some serious and seemingly insoluble acoustic problems common to both of these 5.1 rear surround speaker types.

The use of dipole speakers assumes that the listening room is quite live, because otherwise, the dipoles would be relatively inaudible. But a live home theater room means that the direct sound from the front speakers will be reflected, in spades, from all these nearby surfaces, especially since there are three of them up front emitting direct sound. To add insult to degradation, their reflections cannot be thoroughly eliminated by room treatment with absorbers or diffusers, if the dipoles are to function properly. In movies, these spurious early reflections only slightly impede our ability to locate dialog and sound effects because of the precedence effect and because the brain has no preconceived notions of the acoustic spaces the rapidly changing scenes are supposed to be set in. In contrast, in classical music reproduction, these early direct sound home theater wall reflections produce cues indicating the hall is small while the recording and the brain say the hall must be large. The brain usually resolves this contradiction by deciding that the music is canned. This is one of the many reasons realism, as opposed to mere localization, in both stereo or multi-channel 5.1 music reproduction, is such an elusive goal.

But if one forgoes dipole surrounds and uses directional rear speakers in conjunction with room treatment, the front stereo or 5.1 three-speaker stage improves, but music reproduction still sounds unrealistic. This effect in 5.1 is due to the fact that all the rear half-hall ambient sound is coming from two discrete speaker locations where as in a real hall the sound comes equal in power (but not in detail) from all directions (diffuse field). Even if the recording is made so perfectly that no direct stage sound is emanating from these rear speakers there is still no concert hall in the world that delivers all its early reflections and reverberant tails from two small side spots. The resultant pinna angle error added to the abnormally low interaural cross correlation factor, signals the brain that something is rotten in the state of Dolmark and again the result for 5.1 classical music reproduction is disappointment for any experienced audiophile or concert goer. Already this point is being conceded by such as Lexicon, Yamaha, and Holman who now talk about 7.1 or 12.2. Even conceding that monopoles with room treatment are better for music than dipoles without, the problem remains that making such multi-channel recordings or trying to extract hall ambience to feed more than two surrounds (without including erroneous proscenium direct sound or frontal early reflections) is easier said than done. The answer is full surround convolution as discussed in Chapter 8.

The Front Speakers

In an Ambiophonic system the front speakers should be placed almost directly in front of the listener with each speaker aimed at the listening area. (See next chapter). For best results the front main speaker pair, the Ambiopoles should be as directional as possible. In, theory the ideal speaker for this purpose would behave like a flashlight, with a sound beam emanating from a single point at ear level and the rest of the room in deep shadow. The more focused an Ambiopole is, the less sound absorption treatment the room requires and the more effective the software or mechanical crosstalk elimination is.

The front speakers used should be capable of reaching concert-hall volume, the normal speaker selection criteria of good frequency response, low distortion, reasonable time coherence and affordable price naturally still apply. Since the use of a Speaker/Room correction DSP can correct most speaker response anomalies, one can choose the front speakers based primarily on their radiation patterns. An Ambiophonic speaker, designed by Soundlab, called an Ambiostat, can be used in pairs to form a virtually perfect Ambiopole. This speaker is a six-foot by three-foot vertical panel that is slightly curved in the horizontal direction. Such a concave panel focuses sound quite nicely centered about eight feet from the speaker. The focusing prevents sound from being sprayed where it is not wanted and makes the job of combing elimination that much easier. If you are not too critical, this focusing can obviate the need for any other crosstalk elimination, but then the size of the best listening area becomes smaller.

Front Early-Reflection Loudspeakers

At least one pair of the early-reflection speakers should be placed about the critical plus or minus 55-degree angle to the listening position. This angle is where the ear is most sensitive to such spatial cues. Of course if many frontal surround speakers are available then they should be spaced in whatever way is most convenient or specified by the hall convolver. The ideal speaker for this purpose is one that radiates to the listener from as large an area as possible. Large electrostatic or ribbon loudspeakers are excellent in this application since they can be aimed and so do not cause unwanted room reflections especially from ceilings and floors. If they are dipoles don't forget to put sound absorbing material behind them.

A useful property of such large-area full-range sound radiators is that they provide significant diffusion without invoking spurious room reflections, as would physical diffusion panels. Ideally, one wants all surround speakers in an Ambiophonic system to cover as wide a horizontal arc as possible. This corresponds to the situation in a real concert hall, where the predominant early reflections arrive from slightly different side directions (but still hopefully centered around 55 degrees) because the originating sound sources are spread out on the stage and have various angles of incidence and, therefore, reflection. In the home environment, the computer reconstructed early reflections are the same for all the right-channel instruments, the same for all the left channel instruments, and the same for all the center instruments. This moderate lack of precise spreading of the apparent early reflections would seem to detract from the concert-hall ideal. But just as the perfect Philharmonic Hall has yet to be built, so our home room may be real but not 100% ideal.

By mounting speakers on their sides or by leaning tall speakers, at say a 45-degree angle, the ambient signals arrive at the listening position with a greater diversity of direction and delay. As discussed below, the reverberant field needs to be as diffuse as possible. Therefore, to the extent that either recorded reverberation or recreated reverb is present at these front-side loudspeakers, there is an additional benefit to being wide and as horizontal as possible, providing both vertical and horizontal dispersion without risking significant room reflection or diffusion of the main front signals.

Soundlab has also produced a speaker called a Surrstat which is essentially an Ambiostat turned on its side. Again being slightly concave toward the listener it delivers ambient sound most efficiently over a wide angle without aggravating room reflection modes. It rear wave should be absorbed, although the convex side, radiating over a wider angle, reduces, on average, the intensity of the resulting reflections impinging on the listening position in most rooms. Eventually, distributed mode loudspeakers, that are essentially flat panels that radiate equally from their entire surface, will be quite useful in this application.

Side and Rear Reverberation Loudspeakers

Side and rear speaker pairs are fed with largely uncorrelated reverberation tails. Since in a concert hall, various reverb tails reach the listener from virtually all directions, the ideal speaker would be a set of thin squares, which could be hung on all the walls. I find, however, as above, that large electrostatics or ribbon speakers do an excellent job, particularly if they can be mounted horizontally. One could also use multiple small, inexpensive box speakers arranged on pedestals around the rear half of the room. Again, in theory, each reverberation sound source should have its own independent reverberation computer but the Japanese have shown that such speaker walls can easily fool the ear-brain system even when some of the reverberation tail speakers are correlated. Incidentally, there is no reason why Ambiophonic surround speakers need to be matched if they can still be reasonably set to the sound level required.

Since the rear reverberant field often has a strong vertical component coming from the auditorium balconies and ceiling, we have found it advantageous, but by no means critical, to use one pair of rear speakers elevated as much as possible. These sometimes provide a richer simulation and a better match to concert-hall design theory but "better real" is not more real than "real" and this suggestion is, perhaps, gilding the lily unless the measured hall response specifically includes elevation data.