The ABCs of Stereo Magic
by Dick Olsher
The goal of this month's tip is to offer practical advice about how to improve the spatial performance of one's audio system. What has been written about this subject in the popular audio press could be etched onto the head of a pin with plenty of room left over. Perhaps that's why many of the questions I've received over the years dealt with this very topic. It would appear that many audiophiles are frustrated by a lack of stereo magic, even after investing in expensive components recommended by audiophile magazines.
Audio terms such as stereo sound and Dolby surround, once considered technical in nature, have by now become part of our pop culture jargon. Stereo sound, or colloquially just stereo, is now synonymous with an audio system. Ask the average consumer what stereo is all about, and you're likely to hear that it's about two speakers. My hat off to an industry that no longer needs to sell stereo on the basis of its original promise of 3-D sound. The obligatory two-channel purchase is based on the perception that stereo must by definition be better than mono. Ironically, the following scenario plays itself out countless times: a rack system is purchased with matching speakers, which are then positioned two feet apart for minimum stereo effect. Even the Bose Wave radio is being touted as a stereo device!
Audiophiles, on the other hand, have developed their own detailed terminology to deal with the spatial perception of two-channel audio. Soundstaging, imaging, focus, depth perspective, stage width, and image palpability are descriptive of spatial impressions. These terms originate from our everyday experience in localizing sounds, yet they go beyond the concert hall experience. My personal conclusion is that the home hi-fi experience is distinct from that of live music. First of all, you can't beat the illusion of being there when you are actually there. The combination of aural and visual cues, the sensation of being enveloped by sound, these are all unique to concert hall sound. Its quite a trick, even for a high-end system, to transport the listener to a different space or, alternatively, to import a concert hall acoustic into your own living room. Bottom line: it's all an illusion. Close your eyes, dim the lights, have a glass of wine, smoke your favorite herb - do whatever it takes to heighten the illusion. But in truth, there are no musicians arrayed between the speakers. Second, high-end audio has succumbed to a quest for heightened reality, with more detail, and greater focus than that of the real thing. I've yet to hear holographic imaging or pinpoint focus in a concert hall. The effect is partly an artifact of multi-mic, multi-track recording techniques, which generate extreme near-field recordings with artificially etched detail and jumbled perspectives. The quest for detail and focus at all costs has driven a wedge between audiophiles and music lovers. If you still value timbre and dynamic integrity above all else, count yourself as a music lover. If, on the other hand, you delight at the discovery of subtle noises such as lip smacking or foot tapping, previously buried in the mix, then take my word for it: you're an incurable audiophile.
In either case, something we all appreciate is a 3-D soundstage. Unfortunately, getting there is often a non-trivial task, made all the more difficult by the necessity of having to integrate a sound system into the confines of a domestic living room. It is critical at the outset to understand the limitations of two-channel or conventional stereo. Its major fault is that all of the sound, direct and reverb, is presented from a plane between the speakers. Natural reverb, by its very nature, envelops the listener from all directions. The immediate impact of funneling reverb entirely through the right and left speakers is to place the listener's perspective outside that of the venue, looking in through a window. Ironically, it is room reflections, whose role is often misunderstood by the average audiophile, that distribute ambient information more naturally in the listening room. The misguided motto of "the only good reflection is an absorbed one," has driven too many of us toward heavy absorptive treatment of the wall surfaces immediately adjacent to and behind the speakers. The ultimate expression of this approach is the dead-end/live-end listening room. Although this makes some sense in a recording studio monitoring environment, it rarely works in a domestic environment. Try listening to a stereo recording in an anechoic chamber – an entirely echo-free acoustic. You'll be surprised by the sterility of the presentation. In the case of two-channel audio, the room is not the enemy, but rather it must be used intelligently to achieve the most believable illusion of space. Incidentally, over half of the sound intensity at the listening seat in a typical domestic listening environment derives from room reflections.
Because intensity cues are used to generate stereo localization, it is important to locate the listening seat equidistant from the left and right channels. Check to see that a solo instrument that is meant to be center stage, is in fact reproduced precisely between the speakers. Many speaker manufacturers don't bother to match driver sensitivity in a left/right pair. As a result, midrange and/or tweeter level differences between channels of as much as two dB are common. This is audible and always results in image shifts, and sometimes in defocused image outlines. By all means use the balance control to correct for any mismatch in speaker sensitivity. However, if the midrange and tweeter are mismatched in opposite directions between channels, an exact balance will be impossible to achieve. It's a shame that so many expensive speakers are sold without any frequency response documentation that might assure the consumer of basic QA testing.
With many speaker systems, the stereo sweet spot resembles a bulls eye, with optimum results confined to a narrow central spot that accommodates a single listening seat. For your listening enjoyment, plan to listen solo. This is especially true in small rooms, where moving just a couple of feet away from the sweet spot collapses the soundstage to the nearest speaker. I recall listening panel evaluations of loudspeakers conducted at Stereophile Magazine many years ago, where not only was I seated well off center, but my view of the tweeters was blocked by listeners in the rows ahead of me. Needless to say, judgments made under such circumstances leave a lot to be desired. Stereo does not lend itself to a concert-like presentation with a large audience in attendance. Mr. Susumu Sakuma, aka Sakuma san in Japanese audio circles (http://www10.big.or.jp/~dh/), is famous for his concerts of reproduced music using his own electronics system. He insists on using a monophonic system for his public concerts. When I caught up with him at the 1997 VSAC Show he was happily listening to a single Altec A7. When I questioned him specifically about his preference for mono, he responded with a wry smile to the effect that he was too poor to afford a stereo system. The truth is that mono sounds much more realistic than stereo when reproduced in a large hall in front of a large audience.
Room reflections arriving at the listening seat within a time window of 10 mS not only color timbre but can also confuse image outlines. In most cases, the major offender is early sidewall reflections whose path length is less than 10 feet delayed relative to the direct sound. As a rule of thumb, sound is delayed 1 mS for every foot of travel. Early specular reflections off hard surfaces are particularly nasty because they can be almost as intense as the direct sound. There are three basic approaches to dealing with sidewall reflections. The first method is quite inexpensive and often very effective. Simply rotate or toe-in each speaker so that the tweeter axes are either aimed at the listening seat, or intersect in front of the listening seat. This is most effective in controlling sound dispersion in the 2 kHz to 4 kHz octave, by aiming the radiation lobe of the tweeter away from the sidewall toward the listening seat. The optimum degree of toe-in should be arrived at on the basis of experimentation and depends on a couple of factors. First, the tweeter may be too hot sounding directly on-axis. In this case, listening to it at 20 to 30 degrees off axis may just be what the doctor ordered. Second, the sort of speaker balance that sounds best depends on your cultural bias and the tonality of the ancillary equipment. Since you have no one else to please but yourself, don't be afraid to experiment.
In conjunction with speaker toe-in, a light absorptive treatment along critical sidewall spots may further improve stereo performance. This maybe achieved by means of ordinary domestic items such as wall hangings and book cases. Alternatively, the absorptive treatments maybe replaced by diffusive surfaces such as RPG diffusion gratings and Art diffusors.
For example, the RPG Omniffusor shown here provides uniform, omnidirectional, broad bandwidth diffusion in an attractive wall design element that should have a high spouse acceptance factor. The basic advantage of such a device is that it converts harsh specular reflections into a uniform dispersion pattern and does not deaden the listening room. Broad bandwidth diffusion gives the listening room a sweet ambient signature with much of the reverberant sound energy outside the critical 10 mS window.
Having worked hard to minimize early reflections, it would be a shame to undermine these gains by the following sins of commission. Most common of all is the placement of a coffee table right in front of the listening seat. That's a definite no no, as it provides a hard reflective surface capable of early specular reflections. In this day and age of home theater systems, placement of a large direct view monitor or rear TV projection system between the speakers is also common. Expect image focus to suffer as a result. On the occasion when such an audio/video system is used strictly for music playback, the monitor should be covered by a heavy blanket or rug to absorb specular reflections from the surface of the display.
A spacious soundstage is probably the most difficult 3-D attribute to realize in a typical domestic listening room. Ideally, speakers should be allowed several feet of breathing space away from the rear wall. If the speakers are positioned against the short wall of the room, moving them away from the rear wall from a quarter to a third of the length of the room usually gives the best depth perspective. The reason for the specified range is that bass balance is also affected. The closer to the rear wall and corners, the more pronounced the bass range becomes. Hence, a position in the room that is too far removed form the rear wall may give a cavernous spatial impression but less than optimum bass balance. The best room position is both speaker and room size dependent. The major benefit derives from the automatic time delay afforded to sound energy radiated to the rear wall. Ambient energy is fanned out from the rear wall and arrives at the listening seat with sufficient delay to give a much more natural presentation. In addition, early reflections off the rear wall are also minimized.
In the case of dipole radiators such as magnetic planars or electrostatics, it is absolutely imperative to allow an adequate breathing zone. The sound energy directed toward the rear wall is just as intense as that from the front of the panel. Without sufficient delay, major colorations and image confusion are inevitable. If you don't have a large room, or are unable to correctly position such speakers due to domestic constraints, do yourself a favor and don't even go there.
Usually, it is also beneficial to allow breathing space directly behind the listening seat. Having your head located inches from a bare reflective wall is not a good thing for imaging. The least you can do in a situation like that is treat the surface behind you with diffusive or absorptive elements.
Image Size and Height
Since a stereo recording is made with two or more microphones that sample essentially points in the soundfield, such recordings do not encode height or adequate image size information. As a result, most speakers reproduce conventional stereo recordings with image size that is smaller than the real thing. An eight foot piano might occupy a space of two feet between the speakers. The chest of a singer might be collapsed to several inches. In other words, the original size of the instruments is miniaturized and given much tighter placement in the soundstage than that experienced live.
The exception to the rule is the planar speaker. By virtue of its large radiating area it is able to synthesize a more realistic surface loudness density and hence a more realistic impression of image size. A piano outputs significant acoustic energy, but it is spread out over a large sounding board. A piano's characteristic surface loudness density just can't be reproduced by an 8-inch woofer. But a large planar such as the Magnepan MG-20 has no problem in reproducing the wave launch of a piano. This speaker is capable of the most realistic piano reproduction I've ever heard. If you value a believable impression of size, check out a good planar speaker, but be sure that you have the room to accommodate its needs.
I firmly believe that the primary objective of a sound system is to enjoy the music. That's really the philosophical cornerstone of our online review magazine. The concert hall experience has always been rich in spatial detail. Hence, there's no question that a major step toward musical enjoyment in the home is the reproduction of a credible soundstage. The fact that a credible illusion is possible at all has to do with the subjective side of auditory perception. Our brain paints a picture of the external world based on its analysis of a complex stream of information. And in front of our stereo rig, our auditory system attempts to analyze various and often conflicting spatial clues and come up with a definite conclusion in a fraction of a second. With a bit of care in positioning and room treatment it is possible to greatly facilitate this process.