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Acoustical Design Discussion
For the serious technologically minded.
Edition #4 - The Effects of Room Geometry
White Paper
- Each month we look at a different acoustical design or calibration
element. Taken from the HAA Acoustic Design and Calibration Review
checklists, these elements detail the many attributes that define high
end home theater sound.
Acoustical Design Element 26 and 27: Side wall reflections are controlled by distance, absorption, diffusion or realignment.
In small rooms and in situations where speakers or listeners are close to the side walls, the reflections of sound can act to overwhelm the direct sound of the front speakers. For that reason, proper design dictates treatment to absorb or scatter excessively strongly reflections. Treatment can be acoustical panels yet many household furnishings work well to diffuse or absorb sound. Bookcases, drapes, and even some furniture, if properly placed, can solve a harsh reflection problem.
The concept of controlling early reflections (also referred to as echoes) has received a lot of press over the years. Many companies have come to the rescue of imperiled consumers providing everything from special acoustical pillows to wooden abstract masterpieces each designed to conquer these terrible sonic intrusions. How bad are these problems… really? The answer is really bad or really good; is that vague enough for you?
The “really bad” comes from the experiences of the audiophile trying to hear stereo imaging and sonic detailing. A keen listener looks, err listens for a highly detailed and spatially accurate recreation of musical instruments or other sounds. These recreations are presented as a virtual image of the instrument floating in a precise location between the front three speakers or LCRs (Left, Center, Right). These images should be easily localized and focused; they create the linchpin of what is often referred to as the front (or focused) soundstage. Within this front soundstage we hear the lion’s share of dialogue, musical instruments, soloists, and in fact, most of the highly articulate sounds of the recording. (Ed Note. Lest we forget; DVDA and SACD recordings often are designed to recreate this focused soundstage at all angles). The level of clarity and focus (two of the five acoustical goals we use the gauge the quality of the system) must be made based upon how well the front soundstage is presented to the listeners. So what can screw up clarity and focus? You guessed it, among other things, strong reflections of sound from the side walls.
The “really good” interpretation of side wall reflections also touches on the sensibilities of the audiophile and in reality are equally important elements of a properly designed system. OK, I’m on the verge of losing a few of you out there; how can a side wall reflection be both good and bad? The difference between good and bad is in the intensity of these indirect reflections compared to the direct sound from the speaker. I introduced the front soundstage earlier; the naming of the "front" soundstage implies there is a rear and for that matter two side soundstages, and so there are. We refer to the sum of these components of the total soundstage as the surround field or surround-stage. This sonic marvel is not only enhanced by echoes but is mostly created by echoes. In a two channel system the surround field is entirely created by reflected sound. We describe the quality of the surround field as the “envelopment” of the system; the third sonic goal in play here. We now can see the interaction between the first three sonic goals of the system; clarity, focus, and envelopment as a balancing act between the intensity of the direct sound versus the intensity of the reflections. Allow me to reprint the descriptions of these goals as a refresher.
Clarity
Clarity is the prime acoustical goal because its perfection depends on the successful attainment of all other goals. Of paramount importance is dialogue intelligibility in movies, but one must be able to understand musical lyrics, detect quiet background details, and sense realism for acoustical sounds. Elements that affect this goal are varied including equipment quality, room reverberation levels, ambient noise levels, and listener position among others.
Focus
The ability to precisely locate each reproduced sonic cue or image in a three-dimensional space is defined as acoustical focus. Recordings contain many such images superimposed side to side and front to back in every direction for 360 degrees around the listener. A system is said to have pin-point focus if, from the perspective of the listener, each of these images is properly sized, precisely located, and not wandering. Good focus also provides that individual images be easily distinguishable from amongst others within the limits of the recordings quality.
Envelopment
An audio system should reproduce virtual images of each recorded sound presenting the listener with its apparent source location in a three-dimensional space. Each sonic image relates a part of the recorded event and together these sounds compose a wrap- around soundstage that envelopes the listener. Proper envelopment requires that the soundstage be seamless for 360 degrees without interruption by holes or hot spots caused by speaker level imbalance or poor placement. While envelopment requires three-dimensional imaging of all sonic cues, of pivotal importance is the realistic recreation of the ambient sound field of the recorded venue. Focused sounds become more realistic as they move side to side and front to back with the backdrop of the ambient sounds of the intended venue.
A calibrator reviewing the design of a system will pay close attention to the source of strong reflections, and to the overall reverberant sound level. We measure the level of reverberant sound not as a measurement of its loudness but rather by how long reverberations are allowed to bounce around a room. We call this time measurement the reverberation time, most commonly quantified as the RT60 of the room. But such an analysis falls short in small rooms of the sort most home theaters are built into. The reason is the close proximity of the walls to speakers and listeners. Here the sound paths of the first reflections are so close to the direct sound’s path that their intensities would be comparable to each other thus competing audibly. In effect the early reflection can drown out the direct sound. That’s why in small rooms we treat these early reflections as a different sort of acoustical problem. Rather than measuring the RT60 of the room we first measure the intensity of each reflection compared to the intensity of the direct sound. If the reflection’s intensity is high enough, the direct sound becomes less obvious; a bad thing because the direct sound is where clarity and focus reside. The resulting sonic effect is less intelligible dialogue, a loss of detail, and a loss of the very sharp focused imaging we expect. On the other hand, the surround field becomes more enveloping. Our goal now becomes to retain the “good” envelopment without drowning out the focus and clarity provided by the direct sound. Here the differentiation between good reflections and bad becomes clear; a good reflection means it is quieter than the direct sound level yet is still audible. Bad means the reflections are louder than the direct sound. How does one split this hair?
During the HAA ACR or Acoustic Calibration Review process, the calibrator will measure the reflected sound creating an Energy Time Plot or an impulse response. This is a graphic representation of the intensity of the sound versus time. Here we see the first impulse of the sound must be the direct sound. A reflected sound can’t get there first… right? The follow-on impulses are echoes.
Energy Time Graph
Comparing the intensity of the direct versus each of these follow-on reflections is the key. Research has shown this to be a very complicated physical relationship; however research done by Dr. Floyd Toole and Sean Olive at the National Research Council in Canada (now both are with Harman International) has created some guidelines. More research needs to be done on this phenomenon but at least considering the vocal range of sound the following chart paints a useful guideline for good results.
Perception of Sound Reflections
Master Handbook of Acoustics, 4th edition, McGraw-Hill
A less precise yet more easily quantified rule of thumb is that the early reflections reaching the listener within 30 milliseconds of the direct sound should be on the order 10 dB quieter than the direct sound level. This can be easily monitored using the Energy Time Plot I introduced earlier.
Well I’ve come a long way from the highly generalized rule spelled out in Elements 26 and 27 to discussing the intricate relationships between direct and indirect sounds. For the casual observer reviewing a side wall is a matter of understanding the realities of sonic perceptions versus scientific measurements. Reducing the level of early reflections can be as simple as employing those pillows I mentioned at the outset. Of course the results for clarity and focus would be greatly enhanced if we eliminated the reflections by covering the both side walls with pillows. But we can’t forget the third sonic goal; envelopment. Removing the pillows to keep the level of the reflections high enough to be heard yet less than the direct sound is in reality the right course. As the description of these elements implies, there are other ways to skin this cat; diffusion and distance being two good ones.
Since our stated metric is the sound level difference between the direct sound and the reflected sound, how else can we increase this difference? One very useful approach rather than absorbing the sound is to scatter it. This process is referred to as diffusion and is an exciting alternative. It importance is due to the value we place on envelopment as a sonic goal. The end result of scattering an echo is that while we reduce the level of the reflection, we retain the reflection. This is akin to getting your cake and eating it too. The reflection is reduced thus improving clarity and focus, meanwhile the surround field is enhanced with audible yet low level reverberation. What a concept; devices that turn bad reflections into good ones. Now then, what is the catch? Unfortunately there is a catch, in that the frequencies of sound diffused are limited by the depth or unevenness of the diffuser. That means that a diffuser that is only a couple of inches deep will not benefit the lower midrange much. Have faith though; while use of professional diffusers is the best approach, we can employ other devices as well. My favorite is the bookcase. I have noticed over many years of listening that libraries can sound very good. Many household furnishings can be used not only as absorbers of sound but diffusers as well.
The last treatment in the discussion is something that as it turns out can be the most expensive treatment on the market; real estate. Of course I’m referring to the distance between the speaker of listener and the walls. It is amazing what a couple of extra feet between you and the side walls can do for clarity and focus. The added path the reflection must travel reduces the intensity of the indirect sound significantly. The loss is on the order of 6 dB every time the distance is doubled. Using this metric it’s easy to see why some completely untreated walls still yield acceptable results.
A few of you may have noticed that I didn’t yet address realignment in my discussion. Realignment simply means that the wall is splayed or reoriented slightly to direct any reflection to miss the listener. This can be a great tool in a design but it’s not something that I normally recommend for home theaters because the non-rectangular shape can cause other issues acoustically and frankly is not a popular choice in design. Factoring in all of these conditions into a review of elements 26 and 27 requires a good knowledge of the problem and a healthy level of experience.
I am sometimes confronted, during friendly discussions with clients, about the relative importance of one or another "minor" acoustic or setup flaws in their home theater. Does proper setup and calibration really matter that much balanced against other priorities? In the final analysis, just what is missing due to any of these seemingly innocuous misalignments? I think the answer is best explained from my perspective as a music lover relating how I am occasionally teleported in time and space by the majesty of an amazing recorded performance. If never on a quiet evening in your home, a recording has astonished you with its realism and moved you emotionally, which among these few subtle acoustical flaws has robbed you of the experience? If you have not been surprised recently by your sound system perhaps Elements 26 and 27 could be a contributing factor, of course, don't forget about the many other Design and Calibration elements in the mix. Next month another Element.
From "The Home Acoustic Alliance" by Gerry Lemay (Gerry is the Director of the HAA, President of Quest Convergence Systems, and writes the Home Theater Rx column for Home Theater Magazine)
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