Pleasing everyone
This is all fine and dandy, but what do we build if we want EVERYONE who walks into our room to say, "This sounds good. I can work here."? Drawing upon our collective experiences as recording engineers, a lot of good published acoustical research and some very strong anecdotal evidence from our work with ultra-wide dispersion loudspeakers, my partners and I think we have pieced together a good bit of the "sounds good" puzzle.
Let's start with the obvious. Loudspeakers and rooms form an acoustic transmission path. They both affect what we hear and how we perceive it (two VERY different,but related, things). Loudspeakers need to have low distortion and flat frequency response, and to be free of audible resonances. They also need to have good off-axis response. At the very least, rooms need to minimize standing wave problems and not have excessive reverberation times. On these basic points there is little, if any, disagreement.
However, (and this is not yet widely appreciated in the recording industry) loudspeakers ALSO need to have horizontal coverage angles of 140° or more across the audio bandwidth. Meanwhile, control rooms should have hard side walls to generate lateral specular reflections that make full use of the off-axis acoustic information and power generated by such loudspeakers. Control rooms also need to be very well damped at the front and on the ceiling. Non-specular diffusion (such as quadratic residue diffusers like RPG diffusers) should by and large be avoided for such applications, particularly on the rear wall. (No offense to the diffuser makers here. Diffusers are great devices, with lots of good uses.)
If we take a quick look at the photos of studios in this magazine, we are not likely to find many pictures of control rooms that look like this. That's OK. Studio design is what it is today as a process of evolution, and the room designs are generally based on common-sense approaches to the apparently obvious solutions to some perplexing acoustical problems.
One of the most basic of these approaches has been what I like to think of as the "pseudo-anechoic" approach. The basic principle here is based on the assumption that the signal emitting directly from the loudspeaker is some sort of "acoustic truth," and that it would be best if we could listen to just that signal and nothing else. This seems like a reasonable approach. However, it turns out that we need an anechoic chamber to do this, and such chambers turn out to be unsatisfactory as control rooms for a variety of reasons. So we rationalize a bit and allow as how we do need some reverberance maybe, but not "early reflections." Some of us even characterize those reflections as "acoustical distortion." So we attempt to suppress those reflections, or we use quadratic residue and similar diffusers to convert those reflections into "reverberance."
Now this flies in the face of some 7,000 years of acoustical experience (the first "auditorium" was built around 5,000 BC), experience which suggests quite compellingly that reflections and reverberance are ESSENTIAL when it comes to listening to music. Musicians refuse to play without reflections and reverberance, as a rule, and the general acceptance of listening to music in reverberant spaces (i.e. rooms) is, well, universal.
If this is so, how can early reflections be thought of as "acoustical distortion?" The answer is that reflections are distortion, insofar as measurement microphones are concerned. That's why we need anechoic chambers to make measurements with microphones. However, for human hearing, such reflections aren't distortion at all. Instead, those reflections are an important central part of the acoustical sound that us humans enjoy so much.
So, the specular early reflections that come from hard side walls in the control room provide the listener with full-spectrum, phase-coherent information that our ear/brain uses to localize the loudspeakers and their related phantom images, as well as to help determine the timbre of the sound. (Believe me, this is not snake oil. This is an essential part of how humans perceive and localize sounds in rooms.) The more closely those reflections match the direct sound in terms of frequency and phase response, the better we can perceive all the timbral, timing and ambience cues in the recording that the loudspeakers, especially in stereo or surround arrays, are playing back. This may seem a little counter-intuitive, but with strong, full-spectrum lateral reflections, phantom images become more stable and "palpable" and the depth of the stereo sound-field increases, with strikingly more resolution of the ambience and reverberance of the recording.
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
