Panoramic Power Response:

Panoramic Power Response:

A Fresh Approach To Loudspeaker Dispersion and Control Room Design:

by Dave Moulton

published in Recording Magazine, September, 2000

What Has Dave Been Up To Now?

About 18 years ago I began fooling around with the high-frequency dispersion characteristics of loudspeakers. As I grew more familiar with these characteristics, and had a chance to listen to loudspeakers with improved high-frequency dispersion, I came to understand a great deal more about how humans “localize” sounds and perceive sonic environments. I also gained some insights into the behavior of the phantom images that are at the center, literally(!), of our stereophonic listening experience.

This has led me to some new ideas about the loudspeaker/room interface, control room design and loudspeaker performance criteria. I’m going to share some of these with you here. But first, some background.

As wide-dispersion loudspeakers become commercially available (this is finally in the process of happening), it is appropriate to consider the implications of this behavior for both stereophony (and surround sound) music production, as well as for control room design.

What’s The Problem?

The dispersion of sounds from loudspeakers varies dramatically as a function of wavelength (and therefore, frequency). As a general rule, low frequencies (long wavelengths) are emitted omnidirectionally and high frequencies (short wavelengths) are emitted in a very narrow beam. Around crossover points, these behaviors are often wildly erratic, due to the fact that the sound is being emitted by two drivers at different points in space with differing dispersion characteristics.

This means that any speaker that has flat frequency response on axis MUST have declining power response as the frequency increases and the wavelength shortens. Much less acoustic power is emitted in a narrow beam of energy at 15 kHz. than is emitted hemispherically at 100 Hz. , when both of these frequencies have the same acoustic sound pressure level on axis as measured by a test microphone in an anechoic space.

We make the perceived power response of the loudspeaker even worse through our practice of using high-frequency absorbent material (such as Sonex, etc.) in our control rooms. This practice is based on a time-honored, if mistaken, belief that high-frequency room reflections obscure the details in recordings. Such treatments usually do little or nothing for low frequency reflections and the result is that we often work in playback environments whose reflected sound fields, by design, really degrade the already poor power response of the typical modern studio loudspeaker monitor.

We have come to accept this state of affairs as “the way it is,” and to accept this loudspeaker behavior as “the way loudspeakers sound.” However, in physical reality, other possibilities exist and loudspeakers can be made to have different behaviors that sound different, and interact with rooms differently.

Meanwhile, we face a difficult conundrum. Our recordings contain information about recorded sound sources and the space (real or artificial) that they were recorded in. At the same time, the room in which we play back the recording has its own ambient character (which, as we’ve noted, is often degraded). We would prefer to not have the details of the recording obscured or degraded by the ambience of the playback space. How can we do this?

There is a school of thought that says we would do best by doing all of our production work in an anechoic environment (i.e. one without surface reflections). In such conditions, power response becomes irrelevant (it is never even detected as such) and room reflections of the playback space have been eliminated.

Sounds ideal, right? Not so, in my experience. I’m one of a very few recording engineers who have actually done some serious critical listening to stereo in an anechoic chamber, and I am convinced that such environments are not appropriate for music production work, for a broad range of reasons. These range from cost to a sound quality that is incompatible with reverberant playback by end-users. See my book, Total Recording, for a more detailed discussion of this.

Over the years, I’ve come to the realization that there is a better way to do this, a way that makes productive use of the playback room and its early reflections. While this new way works best, of course, with both room and speakers optimized, it also works well with any reasonably-behaved speaker, and better and better as the power response of the speaker improves.

What happened is that I realized that humans have no problem at all integrating early reflections of a sound with the direct sound. Further, I found that the really important thing to do to enhance the stereo illusion is to enable us to connect the early room reflections in the playback room to the sounds of the recording, not with “the sound of” the playback room. This calls for two things: loudspeakers that emit “recorded information” consistently in all directions (or, at least, horizontally), and playback room surfaces that don’t alter that information upon reflection.

There are several ways to create loudspeakers that have suitable high-frequency horizontal dispersion – dispersion that yields approximately the same frequency response as on axis, and no more than 6 dB change in total level all the way to 90° off-axis, for 180° total horizontal dispersion. In my experience, ribbon tweeters (such as the Genelec S30 employs) or acoustic lenses such as I have been developing yield such dispersion.

A Case In Point: Acoustic Lens Technology and Panoramic Power Response

At Sausalito Audio Works (one of the places I work), we have created a series of loudspeaker prototype designs using conventional drivers coupled with devices we call Acoustic Lenses. These devices cause the sound energy (and power) emitted by the drivers to be distributed evenly and smoothly across a wide horizontal angle (ca. 180°) and a narrow vertical one (ca. 30°), from approximately 400 Hz. (for a 3-way system) up to greater than 15 kHz.

This means that the power response of our loudspeaker systems is reasonably consistent from 400 Hz. up (below 400 Hz. the long wavelengths dictate that the speaker output will continue to be omnidirectional), and that it is dispersed into the playback room in a way that has proved to be excellent for music playback. We call this performance quality Panoramic Power Response (PanPower, for short). Using these loudspeakers, we can easily work and listen in rooms that can closely and inexpensively resemble conventional end-user playback rooms, with superb results.

Meanwhile, we have found that the use of such dispersion yields some additional sonic benefits as well, sonic benefits that are neither obvious nor intuitive. We get great gains in the solidity and relative depth of phantom images. Depth is particularly enhanced as the speakers are moved out into the room. In a symmetrical room with flat reflective sidewalls, we get surprisingly strong envelopment (sense of surrounding spaciousness) from stereo signals, while maintaining great imaging and timbral quality. We believe that the improvement is general and fundamental, vis-a-vis conventional speaker topologies.

So, how can these things be? What is going on?

About Localization

Our localization mechanism (including the ears, auditory nerves and auditory cortex) is multi-faceted and complex. It excels at identifying and localizing sounds sources in chaotic reverberant environments. It also does quite well at perceiving the environment as well.

The simple models we use for localization – time and amplitude differences between the two ears – don’t really describe how we hear in reverberant spaces. We have an extremely highly-evolved echo-location mechanism that makes use of a variety of sonic behaviors to create a remarkably seamless holistic perception of a sound source, even in very reverberant (i.e. chaotic) spaces. Not only that, but we actually prefer playing and listening to music in reverberant spaces, a preference strong enough that musicians will often refuse to play in dry or non-reverberant spaces. Further, we all unequivocally regard such spaces as “bad sounding” when listening to music.

Because of this, I believe we need to redefine the event we call “a sound.” In any normal reverberant space, we perceive the direct sound energy from the source as well as a volley of early reflections also from the sound source. We identify this group of sound artifacts as a “single sound” by their unique phase-locked relationship. We identify the timbre and location of the source via an integration of the spectra, times and angles of arrival of all such artifacts that arrive within 50 milliseconds of the direct sound.

This is a remarkable feat! Instead of being swamped and confused by the multipath array of 6-30 nearly identical sounds arriving from 6-30 different directions, we instead use this accumulation of sound artifacts to develop an extremely rich perception of the timbre of the sound (we have 6-30 versions of it, remember), its position in the room, our position in the room, and the physical nature of the room (including its dimensions, furnishings and surface materials).

Further, we use different parts of the audio spectrum for different parts of this task. We use high frequencies/short wavelengths to localize (at each ear) the angles of arrival of the various artifacts, and to specifically identify the distance from us in space of the direct sound. Meanwhile, we use low frequencies/long wavelengths (using both ears in combination) to learn about the room boundaries.

We do all this identification and learning preconsciously and through neurological feedback, iteration and cross-correlation. What we actually end up consciously perceiving is a fully integrated, highly processed, edited and developed construct that includes remarkably clear and unambiguous sensations of timbre, position and surroundings of a sound source.

In a general way, then, our perceptions of individual sounds are based on an accumulation of information that piles up over 50 milliseconds. We learn what the sound is, where it is in the room, and what the room is made of.

Perception of Phantom Images and Music From Loudspeakers

In stereo playback, loudspeakers actually function as a phase-locked array of the first two early reflections of sound from the recorded space. From these two “early reflections” we would like to acquire a realistic, sense of the timbre, position and space of the recorded sound source and its environment.

Two loudspeakers alone, in an anechoic space, can’t do this very well. Instead of the rich multiplicity of sound artifacts that our ears desire and use for richer and more complete perception, we are constrained to two versions only of the recorded space. Also, it doesn’t help that all of the relative directional information of all the artifacts that our ears are equipped to detect has been lost at the microphones, which cannot detect such directional information.

Meanwhile, a key feature of stereophony is the existence of so-called “phantom images.” These images are illusions of sound emitting from points in space other than the loudspeakers. Interestingly, their existence confirms a breakdown in our auditory localization mechanism, because they incorrectly identify the location of sound sources (i.e. the sound does NOT appear to come from the loudspeakers, when in fact it DOES).

The whole story of phantom images is a little too complex for this article (again, take a look at Total Recording). As I’ve noted, the loudspeakers function as the first early reflections of a sound whose direct version was not emitted. Our auditory system infers the location of the sound source based on those “early reflections” from the two loudspeakers. Those perceived locations are the so-called phantom images. They are surprisingly life-like and real to us.

The Behavior of a Mirror

A mirror is a reflective surface where all of the energy is reflected back into the space. A feature of mirrors is that we don’t perceive them as surfaces, but rather as windows. We don’t perceive the mirror itself, we perceive the space whose image has been reflected, including three-dimensional depth in the perceived reflected space. The better the mirror is, the better the reflected image. If we degrade the mirror, by the addition of a tint, for instance, or by smudges on the surface, or cracks in the surface, the resulting image is similarly degraded and the “mirror as surface” becomes increasingly apparent.

So it is for reflected sounds, particularly from loudspeakers.

Consider again the volley of sound artifacts that constitute the perceptual construct we call “a sound.” We identify members of the volley via their related spectra and phase-locked qualities. These are spread out over time. When we emit recordings of these volleys from loudspeakers into a playback room, if the early reflections from the playback room walls are similar in spectra, particularly at high frequencies, the ear accepts that these are additional information regarding the recording, NOT the playback room. And it is the high frequencies that especially help us form really solid and palpable phantom images.

Happily, such an illusion supports the reverberant information from the recording as well. This means that the early reflections in the playback room carry not only the direct sound and early reflections from the recorded space but also the reverberance to the listener, in an expanded and enriched way. Spaciousness is enhanced, depth is enhanced, images are enhanced, envelopment is enhanced. It’s a win/win/win/win kind of situation!

What About Comb Filtering?

One part of the mythology regarding early reflections is that they generate interference patterns with the direct sound, resulting in comb filtering and timbral degradation. Interestingly, this problem is a severe one for microphones, but not so for ears. For reasons related to our integration of that volley of sound artifacts, we don’t perceive comb filtering of phase-locked sounds that arrive from significantly different directions. Further, what little comb-filtering we perceive is diminished as the volley of early reflections in the playback room becomes richer. So, in a highly reflective room, comb filtering will be essentially inaudible, while it is generally quite audible even between the speakers themselves in an anechoic chamber, where only two artifacts exist.

So, comb filtering problems exist primarily in the domain of microphones and recording, and not significantly in the domain of loudspeakers and playback, except possibly under highly damped or anechoic conditions.

Why Don’t We Make Our Rooms Totally Reflective?

In theory, this all sounds terrific. So why don’t we make our playback rooms totally reflective? Why, we could just set up a room of totally reflective acoustic surfaces (how about polished marble?). A veritable auditory fun-house!

The problem, of course, is what happens AFTER the early reflections have occurred, AFTER 50 milliseconds have gone by. At that point in time, reverberance starts.

Reverberance is the part of the playback sound event that carries perceptually audible information about the playback room. It commences at 50 milliseconds and can run on for many seconds in a large reverberant room. In small rooms, it usually doesn’t go on for more than a second. Interestingly, the reverberance that exists between 50 and 150 milliseconds especially tends to interfere with clarity and intelligibility of sound.

This is why we need to absorb sound in a playback room. We need to shorten the sound decay to a point where the reverberance of the playback room never gets a chance to build up.

Happily, it turns out that it is extremely cheap and easy to make a small room highly reflective at almost all frequencies for a brief period (50-100 ms.) and highly absorbent after that. Without going into actual design details (I’ve actually worked out a topology based on this thinking I call a Moulton Room), we simply make the end of the room behind the loudspeakers highly absorbent at all frequencies, and the other walls highly reflective. All the energy from the speakers propagates along the length of the room and back and is then absorbed. All early reflections are broadband, and all reverberance is absorbed. It really is almost that straightforward!

What Happens With Conventional Loudspeakers – Why Have We Done It The Other Way For So Long?

Conventional speakers generate early reflections that have rolled off high frequencies. This reduces the effectiveness of the stereophonic illusion, at best. We can get pretty good results, however, by carefully mounting comparatively directional speakers in the absorbent wall. Our lateral reflections are still deficient, and we sacrifice a good deal of perceived depth, but we nonetheless obtain performance on a par or better than other studio topologies.

However, problems do arise with speakers that have extremely poor or erratic off-axis performance. Such off-axis performance is audible to the listener and in controlled listening tests will prove to be offensive to listeners, even when on-axis response is excellent. This is increasingly true as the side-walls become more reflective.

This is one of the reason that heavily damped control room designs are favored by studio designers. They usually have little choice about or control over the monitors that will be used, and need to protect their design topologies for worst-case situations. Floyd Toole, a noted loudspeaker researcher, recently wrote that “the inspiration for the [Live-End-Dead-End® control room topology] appears to have been the need to improve the sound of a then popular studio monitor loudspeaker that misbehaved dreadfully in its off-axis response. The only way to deliver good sound quality was to absorb the sounds that would normally have reflected off the floor, walls and ceiling.”

What About Floors, Console and Ceiling Reflections?

In my discussion this far, I haven’t dealt with the issue of floor, ceiling and console reflections. The presence of these reflections, particularly in terms of high frequencies, seems to be less desirable than lateral or rear wall reflections, for reasons that are not fully understood.

Therefore, it is my practice to install substantial broadband absorption in the ceiling unless it is extremely high. At the same time, SAW’s Acoustic Lenses direct their output over a vertical range of approximately 30°, from 0° horizontal to +30° above horizontal. As a result, the reflective paths to floor, console and ceiling are generally devoid of energy above 400 Hz. This seems to have a strongly positive effect on the sound performance, particularly in regard to console reflections.

The Meaning of It All

It is possible to have extremely inexpensive control room topologies that will work extremely well for music playback, utilizing broadband specular early reflections from the side and back walls of the control room. It helps to have loudspeakers with at least smooth off-axis response.

If you want to go the next step, it will take speakers with Acoustic Lens Technology or similar horizontal dispersion and performance approaching Panoramic Power Response. Such speakers are already in use, on a custom basis, in numerous facilities around the world. We have designed and equipped the new world-class surround facility at The Plant Studios in Sausalito, CA called the Garden, which has been enthusiastically embraced by groups such as Primus and Metallica.

We believe the combination of this acoustical approach to loudspeaker playback and this technology is yielding a new standard, even a new paradigm for high-quality playback of music through loudspeakers.

Happy listening!

Figures:

Graphic showing ALT lenses

Graphic of Response Curves at 0°, 30°, 60° and 90° for The Garden Speakers by Sausalito Audio Works

Photograph of The Garden

figures to be supplied by Manny LaCarrubba