An Intriguing Addition to the Ambiophonic Repertoire
by Ralph Glasgal
In the early papers and tutorials on Ambiophonics, the use of rear speakers has always been recommended. They provide a full rear stage when playing back 5.1 surround music or movies but also are clearly of benefit when just playing normal 2.0 files like CDs, LPs, MP3s, etc. When the rear speakers play the same 2.0 signal source as the fronts but are channeled through a RACE device so that the rear speakers like the front pair are a functioning Ambiodipole, a sense of presence, spaciousness, and an enhanced “you are there” feeling is clearly engendered. In the literature this characteristic of concert halls is often called envelopment and so the use of a rear Ambiodipole, when playing 2.0 files like LPs and CDs, is called Envelophonics.
The level and the location of the rear speaker pair is singularly non-critical for this effect to be noticed. The pair can be closer to the listening position than the fronts, the same distance, or further back. This property means that the ideal listening area can be larger and accommodate more people than say the 5.1 arrangement. The rear speaker level can be close to that of the fronts but is normally set somewhat lower in most cases by a few dB The angle to the listening position is also non-critical but should be relatively narrow, like the front pair. Again, this makes the placement of these “surround” speakers a lot easier than is the case for standard 5.1. The RACE settings, while likely different from those used for the front speakers, do not need to be precise to readily produce audible Envelophonics. There is no set of absolutely right parameters for Envelophonics any more than there is a perfect concert hall or an agreement on what envelopment is.
Indeed, what is envelopment? Normally, as defined by Griesinger, Toole, Rumsey, Soulodre, Bradley, Barron, Ando, etc. it is the feeling of being in, say, a concert hall or similar large space. This imprecise, rather ill-defined effect is thought to be produced mostly by early reflections in the 80 millisecond range and mostly from the sides of the hall. In the case of Evelophonics the spatial sense produced is more a feeling of realism, width, and depth added to the frontal stage than just a sense of general spaciousness. Note that the rear speaker sound is not delayed and may even arrive at the ears ahead of the frontal sound. But despite what one might expect from the precedence effect, the front stage image does not shift to the rear. In contrast to those longer delays thought to be required for conventional envelopment, this Envelophonic effect is one produced by fully directional early reflections in the single or low double digit millisecond range. These are near simultaneous with the direct sound and have very directional localization cues in contrast to later room or hall reflections that are lower in level, have relatively longer delays, and, being more diffuse, lack much directional cue precision or clear discreteness.
While research needs to be done to confirm how this works with many more listening panels, the result is not that surprising. In a concert hall and indeed in most hearing situations, there are constantly changing, very short reflections coming from the seats and heads around you or from nearby furniture in home rooms and offices. So for every frontal sound location there is a distinct and unique pattern of very early reflections originating from the rear. The brain expects to be exposed to such a varying mélange of the frontal sound coming from the rear half circle of almost any indoor space with very short delays but from a wide variety of directions that change with the position of the front instrument, noise, or voice. So again, for every frontal discrete sound source there is a shower of very early reflections at fairly high levels coming from a lot of discrete directions that vary with the location of the frontal sound source. This is what we need to reproduce in a home Ambiophonic system to satisfy this psychoacoustic need when listening to large ensemble music recordings or watching video sources that have a wide front stage.
If we take the front left and right media signals and put them to a second set of stereo speakers to the rear of the listeners, would that provide Envelophonics? No. In a 60 degree stereo system, since all the sound comes from just two front locations, the short rear room reflections are static and always have the same directional characteristics. The same would be true for a rear stereo pair, no directional diversity with front image position except for a change in level. Also the rear pair would have the usual stereo defects of combing, reduced ITD and reduced ILD that limit the sense of envelopment. This is also similar to the reason why just normal room reflections cannot produce an Envelophonic sound field for a stereo speaker system. Again, the room reflection pattern is fixed for any sound from either speaker. Since the room reflection localization cues do not change (except a bit in level) as the front phantom image moves, it is not binaurally realistic. This static pattern is not what the brain likes and expects. This is why simple difference signal approaches like the pioneering Haffler Effect arrangement or other matrixing ideas do not work well in the envelopment sense. The same for the rear speakers in 5.1 or 7.1 where all the ambience has the same localization cues.
Now if you RACE cancel this rear speaker pair, then, compared to say 60 degree rear stereo, (or 110 degree 5.1) the ear receives a much fuller range of undistorted interaural level and time differences for every frontal sound source on the recording. The pattern from the rear is substantially changing or tracking the sound moving around on the front stage as in normal hearing situations. Also the rear pinna cues are combined with the front pinna cues and this fact ameliorates a high frequency localization error for sources that are extreme left or right. This pinna effect may explain the increase in the apparent stage width and depth for many symphonic recordings when the rear speaker pair is turned on.
Another advantage of Envelophonics is that now the longer listening room reflections in the 20 to 100 millisecond range are much less troublesome. For one thing they are swamped by the higher level of the rear Ambiodipole. Also note that such moderate early reflections are more or less normal in most spaces such as studios, recital halls and concert halls where the backs or fronts of distant seats and heads provide a similar group of delayed early reflections. Adding two rear speakers for Envelophonics is likely to be lot easier and more effective than treating a room with panels, diffusors, DSP room correction, etc. just to tame these mid delay reflections that are really part of almost all acoustic venues.
For the ultimate domestic concert hall, one needs the longer concert hall reflections that last on the order of 2 seconds or more. For these when playing 2.0 files one can use hall impulse responses and convolve signals for as many surround speakers as one wants. Again such ambience surround speakers swamp the listening room reflections so room treatment or correction is not needed. Alternatively, if one makes surround recordings using a 4.0 microphone like the Panambiophone, then the rear channels of such a recording, played back via a rear Ambiodipole produces the rear half of the hall as if it were direct sound with all its reflection/ambience directionality intact. (normally just horizontal plane reflections) This provides 4.0 Envelophonics where the rear media pair (as in 5.1) is used to drive a RACE enabled rear Ambiodipole. Some 5.1 movies and classical music DVD/BDs have such sound fields already recorded this way. Thus playback, via just four speakers can easily provide full 360 degree surround sound for a small group of listeners or watchers in a home environment even when playing the existing library of 5.1 media.
Finally, for the best in 3D full horizontal plane sound, one can use six speakers, i.e. three Ambiodipoles. One pair delivers the direct sound fully wide front stage. One rear pair provides Evelophonics for this front stage and the third pair provides the rear sound effects, or the rear direct sound stage, or rear hall reverberation for reproducing the full spectrum of sound found on many 4.0/5.1 media. Since height is the least interesting part of a concert hall ambient field to the human brain, because sound from above is mostly mono, it does not pay to obsess about height speakers. Also, since the sense of envelopment varies with an individual, there will be many possible electronic settings and positions for the rear speakers and even a possibility of using multiple pairs of speakers by purists and hobbyists.