Conclusions
Still with me? Good. It's payoff time. Here's my explanation of what happens in most control rooms. The loudspeakers will emit sound in every direction despite the best efforts of the designer to limit their directivity. Those which pay no attention to directivity issues will also suffer from lumpy off-axis and power response as mentioned earlier. If the side walls of the control room are damped, they will further low pass filter the room reflections. The net effect is to remove, reduce or distort the room reflections that the ear/brain uses to help localize the sound sources (i.e. the loudspeakers and their related phantom images). If there is diffusion in this transmission path, then the reflected energy is robbed of its phase coherence and becomes useless in both the localization process and for the maintenance of timbre, blurring images and timbre. If the reflected energy comes from behind or the same direction as the direct sound, it will not support the localization process as well as laterally reflected energy does. Without sufficient laterally reflected energy, the sound will generally lack a sense of envelopment and, dare I say, musicality. If the decay time of the room is not short and reasonably similar in each octave band from 63 Hz. through 8 kHz., the playback room will impart its own "color" upon the perceived sound.
Us humans are very adaptive. We CAN work in rooms constructed like this. BUT, we struggle, and have these ongoing debates about rooms and loudspeakers. One guy loves this room and hates that loudspeaker. Another guy hates this room and loves that loudspeaker. Whatever. We all manage to adapt to a particular playback system in a particular room. Once we have learned "the sound" of "our particular" system, and have learned to produce predictable recordings with that setup, other sounds often seem unnatural and the readjustment process can be time consuming and unpleasant. (This is exactly how the NS10s-on-the-meter-bridge phenomenon works, and this is why some engineers end up mixing to pain cues.)
Now, here's what happens in The Plant's new mix room, "The Garden," with the weird-looking loudspeakers. These monitors emit sound in every direction like any other speakers, to be sure. However, there is NO low pass filtering of their output in the horizontal plane until we are over 90° off axis. The directivity is reduced in the vertical plane only. The reflective and non-diffusive side walls of the room provide full-spectrum lateral reflections that aid in the localization process. The rear wall is absorbent at low frequencies and spectrally reflective at higher frequencies by use of a large cylindrical reflector that directs energy back toward the side walls of the room. The front wall is a very efficient broadband absorber. The room has a constant decay time of less than .2 sec from 250 Hz all the way to 8 kHz. It doesn't sound the least bit overdamped. Normal speech in the room sounds normally ambient. With the ultra-wide dispersion monitors, the sweet spot is ridiculously huge. The key word to describe how all this sounds with music playback is
clarity. It's almost like not listening to speakers at all (indeed, in some controlled double-blind listening tests of these ultra-wide designs using professional listeners, one listener stopped the test, allowing that she got confused and thought she was listening to a live player, for an unprecedented first time in her testing experience!). Everything in the recording is utterly revealed, but without sounding analytical. As a matter of fact, playback sounds extremely musical and entertaining - and very "clear." Those are not adjectives that are usually used to describe a single playback system, but they describe The Garden and its monitors well.
"How do mixes travel?" you may ask. In fact, speakers of similar design have been in daily use at a small post-production/mastering facility in New England for several years now and the answer has consistently been, "Very well, thank you." Experienced engineers that hear the room immediately recognize that, however different all this looks, the mixes made with these ultra-wide dispersion speakers will travel to other environments extremely well, without significant problems, and that has proved to be the case.
Please keep in mind that none of the stuff I've described here negates most basic criteria for good room and speaker design. However, one of my pet peeves is that many people regard listening room acoustics as an exercise in damping or diffusing reflections. This is a questionable practice in general. Properly managed, full-spectrum reflections provide "good data" for our auditory system to use in processing the information provided by the recording.
In conclusion, I need to stress that the ideas that I've put forth here are not all that new, nor are they necessarily all my own. My partner David Moulton, for instance, has developed a very simple, very effective low-cost control room topology suitable for all types of speakers using this reasoning. There are plenty of people who recognize that wide dispersion loudspeakers and rooms like the one I've described here are an excellent approach to room design. What is unique and special here, and of importance to the professional audio community, is that these ultra-wide dispersion loudspeakers are now installed in a world-class room that can take full advantage of them, and this is in one of the foremost mainstream pop/rock recording studios in the world. In an industry filled with lemmings, this is no small achievement. One thing is for sure, though. So far the voting is unanimous - it sure "sounds good!"
Manny LaCarrubba is the president of Sausalito Audio Works. He wishes to thank his partner David Moulton, from whom he appropriated much of the information for this article.
COMMENTS
Jan 21, 2005 10:32 AM
To the Editor of Mix Magazine:
While Manny LaCarrubba's ultra-wide-dispersion loudspeakers may or may not "sound good" ("The Wide-Dispersion Listening Space," November 1999), his conclusions about control room design are a misapplication of basic acoustical principles.
Mr. LaCarrubba would be correct that lateral reflections from a room's side walls are important for localization cues, if the loudspeaker were a guitar or a singer or a violin. But it's not. The information necessary to form a sound image (a.k.a. the localization cues or "room sound") is already contained within the audio signal, owing to the complex set of reflections that have combined at the microphone in the recording space, whether it's happening right now on the other side of the glass or during a session forty years ago. If the reflection information already present in the sound you're trying to reproduce is convoluted by early reflections in the control room, you can no longer say with certainty whether what you hear is actually in the signal or just a consequence of the peculiar characteristics of the listening space.
Yes, anechoic spaces are "unnatural and unmusical," but that's due to our perceptual inability to reconcile what we hear in the playback of recorded sound with what we hear when we talk or turn our heads or move around within the space. Anechoic spaces are also very unforgiving in allowing the listener to move outside a narrowly defined "sweet spot" without giving up all sense of spatial image. That's why acoustical diffusion is useful in a control room, and why "robbing the reflected energy" of its directional and temporal information is precisely the point.
Being able to tell exactly where the loudspeakers are as you listen to music is not a goal in optimizing a listening environment. Quite the opposite - a well-defined image (particularly in a multi-channel system) should not seem to emanate from discrete monitor locations. The importance of lateral reflections to a listener's sense of envelopment and spaciousness has been well known in the acoustical community for decades, and they are essential in any performance space. In a properly designed control room or listening space, however, early reflections and comb filtering are not "good data." Confusing these two room types does not help to further the science of control room acoustics.
Richard Schrag
Russ Berger Design Group
Dallas, Texas
Jan 21, 2005 10:33 AM
As Manny LaCarrubba's partner, I have a few comments in regard to Richard Schrag's letter.
Loudspeakers have a GREAT deal in common with other musical instruments - from an acoustical standpoint, they are essentially the same device, and subject to the same physical rules (except for one quirk I'll get to in a minute). Much of the information contained in Mr. Schrag's "complete set of reflections that have combined at the microphone" has actually been lost at the microphone, which cannot detect that "complex set of reflections" except as a comb-filtered two-dimensional map of pressure over time.
If we were considering the behavior of a mirror, we would probably not say that "reflection information already present in the image we're trying to reproduce is convoluted by reflections of the mirror." The optical mirror may fool the range finder on a camera, just as an acoustic mirror will fool a microphone. However, neither our eyes nor our ears are bothered much at all, unless the mirror is tinted, discontinuous or both. Our auditory system is extraordinarily well equipped to make good use of acoustical reflections, and does so with ease, integrating them with the direct sound artifacts in a way that microphones cannot approach.
The unmusicality of anechoic spaces has NOTHING to do with the specifics of playback of recorded music, as Mr. Schrag implies, and the issue has little to do with control rooms, which are not anechoic and never have been (except in a few special cases - Hidley, Newell, et al). End-user environments also have never been anechoic. Under existing conditions, then, it is inevitable that there will be early reflections involved with loudspeaker playback. We cannot pretend that low-pass filtering of those early reflections is the same as eliminating them a la anechoic treatments. Our traditional loudspeakers, as a function of their inherent directional behavior, cause such lateral low-pass filtering of early reflections. The process of "deadening" walls with absorbent materials increases the severity of such filtering.
Mr. Schrag suggests that accurate localization of a loudspeaker is undesirable. Not so. If we are going to get stereo to work reliably, it is essential that we should perceive a unique signal sent to a single loudspeaker as coming from that loudspeaker. The particular unique quirk of loudspeakers that I alluded to above is that they may be operated in phase-locked arrays. When that is done and a common signal is sent in parallel to emit, phase-locked, from two points in space simultaneously, this defeats our auditory localization system and causes us to perceive phantom images. This is, of course, the basis for stereophony. What is actually happening from a perceptual standpoint is quite interesting. The loudspeakers are no longer perceived as the source of the sound, but as early reflections of a "phantom" source whose direct artifact was not perceived, and whose presence and position is inferred by the auditory system based on those perceived early reflections. They are, psychologically speaking, the first early reflections of the sound. If they are supported by subsequent early reflections from the room that are spectrally and temporally accurate, the phantom image becomes that much stronger, palpable and precise. This is why we call such reflections "good data." And, the more accurate those reflections are, the MORE data about the recording and LESS data about the playback room will be transmitted via those reflections.
How much stronger, more palpable and precise? Manny LaCarrubba was being modest when he said the speakers "sound good." One of our country's leading mastering engineers characterized the imaging of the speakers, in a large reverberant control room, as "stunning." A senior, highly experienced and extremely successful loudspeaker designer felt the phantom image was so real he had to physically verify, with apologies, that the center channel was turned off (it was!). A professional, highly experienced test listener, doing blind testing, asserted that for the first time in her experience she thought a live player might have been substituted for the loudspeakers! Numerous audio professionals and musicians have said they believed that live performances were occurring as they first entered the playback room. Numerous audio professionals have characterized the speakers as "the best I've ever heard."
Mr. Schrag suggests that the early reflections of the playback room mask the reverberant artifacts of the recorded signal(s). Happily, the time bases of the two are both different and generally complementary in both acoustic stereophonic recording and multitrack production, so that the volley of early reflections in the playback room supporting any particular playback artifact is pretty much complete by the time that a single "reflection" of that artifact occurs in the recording of the performance space (ca. 30 ms.). The net result of this effect is a positive one, where a multiplicity of early reflections in the playback room carry direct sounds, early reflection and reverberant artifacts of the recorded sound to the listener, in a wonderfully comprehensive and rich way that our auditory system is well-suited to assimilate.
The real problem, in our opinion, is the playback room's reverberance, that wash of increasingly uncorrelated and unintegrated reflections that occur after the Precedence Effect (with its integration of early reflections into a coherent perceptual construct) has decayed. This begins to occur at approximately 40 ms. after the direct sound arrives at our ears. It is this reverberant sound from the playback room that we believe muddies up the playback of recordings, masking details and obscuring images, and that we should try to avoid in order to obtain "transparent" playback.
The design solution? Support accurate specular early reflections for 50 ms. or so, and then employ broadband suppression of all reflections and reverberance after that. It turns out this leads to an easy and dirt-cheap control room design topology (I call it a "Moulton Room") that works quite well with almost all types of speakers. It works extraordinarily well with wide-dispersion speakers.
The problem with the incoherent diffusion created by quadratic residue diffusers and similar devices in this application is that such diffusers convert direct energy into early reverberant energy. This creates a reverberant wash in the playback room precisely during the time period when we should be integrating early reflections as part of our localization.
In closing, I don't think Manny and I are confusing control rooms and performance spaces at all, and I DO think we are furthering the science (and art!) of control room design.
Sincerely,
David Moulton
Moulton Laboratories
Sausalito Audio Works
