Room Acoustics - Large Room and Small Room

[mas]

Hi folks:

Many have wondered and a few have requested sources of more detailed information regarding the fundamentals of acoustics that apply to a home theater or home audio listening environment.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

To this end I am going to be posting textbook information regarding both Large Room Acoustics (LRA) and Small Room Acoustics (SRA). (source: Sound System Engineering, by Davis) Note, the completely new edition authored by D. Davis and E. Patronis, Ph.D should be out hopefully as we finish this examination.

Please ignore the yellow highlights in the attachments, as they are meaningless. ;-)

Seeing as how most acoustics are framed within characteristics typical of Large Room Acoustics, there seems little way to avoid addressing these issues. Indeed, an understanding of these characteristics is important to a solid overview of the field of acoustics, most notably the concept of reverberation.

As you will see, it is important to distinguish between the two. Much of what many of you have learned (and what is taught in Everests Master Handbook of Acoustics) is classical Large Room Acoustics, which unfortunately have significant limitations when applied to Small Acoustic Environments.

Hopefully with a good introduction and understanding of the concepts we can then proceed into the realm of Small Room Acoustics, where it should become apparent that we are dealing with an environment where the conditions necessary for Large Room Acoustics are both abbreviated and violated. And hence, while an understanding of the Large Room Acoustics is important, equally important is the understanding of how these compromised characteristics manifest themselves in the Small Room environment.

So, if I may suggest, with regards to the Large Room Acoustical environment, do not worry too much about the details regarding the derivation of the formulas such as the reverberant field. As is usual in the sciences, they follow the usual dialectic pattern. And unless you are dealing with Large room design and application (and yes, there is a mathematical definition distinguishing the Large and Small room) you will have no need to directly apply the equations. And if you do this seriously, you will (let me say 'should' ;-) be using equipment that makes this easy! But it is important to understand their definitions and thus to understand their limitations with regard to SRA spaces. What is most significant is to understand what characterizes a reverberant field and more importantly, to come to see why this concept is not directly applicable to a Small Acoustical Space, (a category into which all of the home audio and theater spaces discussed on this forum -with Very rare exception- fall), and how our approach must be modified. Thus we will be unlearning and modifying many heretofore inappropriately applied principles.

So it is my hope to present these topics in a non-fragmented coherent manner allowing for a systematic approach to the pertinent topics and to their derivation.

I hope that we can keep the comments and discussions focused and pertinent. There will be plenty of opportunity to discuss specific applications after we have covered all of the material, as it makes no sense to delve into applications at this point without having covered the complete topics, as we will be lacking the missing portions that will continue to qualify the topic.

After presenting the basics of Small Room Acoustics, I also have an article by Don Davis that further addresses the LEDE concept in detail.

There is one important point that I would like to present ahead of time in this letter regarding the LEDE. LEDE is not a static concept. Since its introduction approximately 40 years ago, it has undergone a rather radical evolution. Unfortunately, the initial description/definition of its being reflection free and anechoic on one end, and diffuse on the other end, persists and is predominant, as a quick Net search will quickly reveal! While this is the commonly accepted definition, it is unfortunately obsolete! The big change involves the Dead End, in that it is no longer anechoic! And with improved materials, this aspect seems to be ever diminishing - (not in its effectiveness, but rather in its treatment while simultaneously becoming more effective in achieving the original goal. Thus I hope to present some information regarding the LEDE as it exists today. I will wait until after the presentation of the SSE chapters to present this. But I hope that we can mitigate a debate over the merits of the LEDE, some of which are presented in early form in the chapters, until after a presentation of the more detailed and more up to date information is presented.

Thus I hope to present some information regarding the LEDE concept as it exists today. I will wait until after the presentation of the SSE chapters to present this. But I hope that we can mitigate a debate over the merits of the LEDE, some of which are presented in early form in the chapters, until after a presentation of the more detailed and more up to date information is presented.

Ultimately, we will have to address the issue of instrumentation and measurements. I know that many do not wish to deal with this realm, and I can completely appreciate this! I would like to minimize this schism if we can. After a discussion of the concepts I would like to focus on how a room can effectively be treated.

But herein is the rub It is possible to create a cookie cutter list of steps and suggestions for an ideal room. And I do mean ideal. Unfortunately, these become worthless except as a statement of concept in most real rooms. The result being allot of time and effort, and most importantly, allot of money spent, only to have moved the furniture around without effecting an optimal result.

Without real room measurements, room modes and the elements defining the parts of an LEDE room (especially as currently modeled) become increasingly difficult to identify and surgically treat. The use of measurements has resulted in a continuing refinement of the model facilitated by the increasingly surgical identification and treatment of acoustical anomalies.

As we move closer to the current iteration of the LEDE approach, I think you will see how the measurements become the focus, not because they are a substitute for the real response, but simply because they are able to very accurately quantify and illustrate the real phenomena. There correlation is both amazing and the representation very clear. Thus, they not only serve to identify the specific issues to be addressed, but they are able to provide immediate feedback as to whether the action taken is effective. Ultimately, there is no way to escape the fact that these measurement systems have been both the source and the means by which modern acoustics has become enabled. And with low cost tools becoming available, an understanding of how they can be effectively employed is an important aspect.

Thus, it would be nice to remain focused on the material presented as we go along (the work is split into 3 parcels for each chapter). If we can remain focused on clarifying the presented material while resisting the tendency to have the discussion fly off into myriad tangents, I think we will all benefit, especially those on the site who are being introduced to this for the first time.

And if I may be so bold, there is allot of information to be unlearned (or at the very least, to be rearranged! ;-) So may I suggest that even the more experienced should not assume that they understand the material. In many cases, they may have as much to unlearn as the less experienced have to learn!

Once we have addressed the details of the presented material, then we can start a follow-up thread on the implications and practical applications of the ideals presented. And may I suggest that we pursue this in modular form modal, absorption, and diffusion. Then, AFTER we have addressed the basics, we can move to many different configurations and options for their treatment and mitigation.

Below is the proposed itinerary.

Depending upon the responses and the questions that may arise, I am proposing a tentative timetable of 1 week for each segment. It is hoped that this will allow folks to both review the material and to still have a personal life outside of work.

Tentative Itinerary:

SSE The Acoustic Environment Pt. 1

Pt. 2

SSE Large Room Acoustics Pt. 1

Pt. 2

Pt. 3

Pt. 4

SSE Small Room Acoustics Pt. 1

Pt. 2

Pt. 3

LEDE Room Presentation (<?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />Davis), including modifications to the concept as presented in the text.

Thereafter:

Follow-up discussions on Modal, Absorption & Diffusion treatments

Discussion of how the measurements can be used to facilitate room treatment

I hope this proves useful and of interest.

An addendum:

I fear that I may be presenting too much in one small bite in this post, but please allow me to also add just a little more with an important caveat:

I know that it may seem boring to go through all of the material. Besides, many of us figure that we already know it - after all, we are more informed than most...right? ;-)

Well, I will tell you right now, that if you choose to skip parts of the material, that you will not only cause yourself unnecessary grief, but it will become readily apparent in any discussion in which you chose to contribute! The concepts are cumulative, and an understanding of each part (not the formulas! ...but rather the concepts!) is very important. If one were able to pick and choose, I would have simply have presented the highlights and ignored the rest. (And, believe me, I began to try to do just that, mistakenly thinking that I could omit the less imporant aspects, only to have those very aspects come back as necessary qualifications when any questions regarding the 'main points' surfaced!)

So as Gil very aptly mentioned in another recent thread - this is a bit of a "forced march". But I sincerely hope that you will not find it as unpalatable as the label implies! ;-)

So, please give this a chance, and hopefully we can develop a pretty thorough presentation and discussion that will provide folks with a solid understanding and a basis to take your listening experience to the next level.

And as always, please feel to PM me with any questions or comments!

Mark

[mas]

Week1

Here is Large Room Acoustics, Part 1 of 4.

[mas]

Week 1

Here is Large Room Acoustics, Part 2 of 4.

[gaspr]

Hi mas

Thanks so much for taking the time to do this. Although the

material seems a little overwhelming at times, I believe it is

important for us to learn as much as possible about this topic.

Just noticed that the latest edition of "Sound System Engineering" is

now availble at amazon.

Garth

[DrWho]

So these pdf's are from the "Sound System Engineering" book? I'm definetly going to be getting myself a copy of the new one. Thanks for posting all this - it's nice to have everything in the same location.

[mas]

Let me go ahead and post the remainder of the Chapter as well...then folks can proceed at their own pace with the reading...

Here is Large Room Acoustics, Part 3 of 4.

[mas]

Here is Large Room Acoustics, Part 4 of 4.

[DrWho]

One thing I've always wondered when reading these "text books" is how all these equations are actually used in real life. I would love to sit down with a sound system engineer sometime and walk through the design stages and see how all the variables are actually used. Isn't a lot of this already built into the EASE software? It's one thing to conceptually understand it all, but to put it into practice and start optimizing is a totally different level.

[mas]

EASE and CATT-A do incorporate allot of the factors into their algorithms - and actually have the potential to go far beyond, as with an exact AutoCad model you can also specify specific surface characterisitcs for all surfaces as well as detailed speaker data. But addressing the strengths and limitations of modelling and auralization is another ball of wax. They can be used to great advantage if one is aware of their limitations. Unfortunately such discussions seem to all too often degenerate into emotional arguments over whether they are used correctly rather than what their capabilities are presuming they are used responsibly. And I would be the first to concede that any tool, if misused, is unfortunate and something to be avoided. Hence the need to understand this information.

You have hit on a fundamental issue that I alluded to in the intro...

An understanding of the concepts behind the equations are critically important, as they provide you with an appreciation of the factors that are being evaluated (and also ignored, depending upon the model!) But as far as sitting down and actually attempting to calculate the reverberant characterisitics for a complex Large Acoustical space with a high degree of accuracy... that is an exercise best left to graduate student slave labor for inclusion in a textbook serving simply as an example that such an effort can be attempted! Of course such examples of this would then, most likely, be followed by measurements of the room presenting a much more accurate and useful measure which are quickly attainable and are able to be used to affect real actions in real time. The results of these subsequent actions can likewise then be measured and ascertained quickly, easily and with great acuracy! Additionally, the measurements can be made in many different locations addressing a myriad of additional localized factors that the generalized equations that treat the space as a statistical average do not.

EASE can be a very valuable tool for Large Room design. Once constructed, TEF** becomes the measure of the room, as well as the arbitor over the various treatment options as it can provide almost instantaneous proof of performance feedback for each treatment.

That is why I mentioned that the measurements take center stage in room treatment. You are dealing with a real room, not a concept. And the measurements take into consideration a great many interactive factors that the idealized equations do not. Thus a solid grounding in the concepts will serve you well in providing a basis for variables that you will benefit from being aware, but the measurements will serve as your compass as you navigate the space. (For example, I defy anyone here to quickly attempt to calculate the effect of a coupled space of indefinate dimensions upon the response of a room...quick, pull out your calculator.... [] )

As we get into Small Acoustical spaces (and LEDE in particular), I hope that you will see how the measurements are used, and hopefully appreciate how the tools make life much easier and much more accurate and pleasant! ;-)

By the way, this textbook is written by those who have not only done the tedious research, but they are the ones, who with a very solid conceptual foundation, use the time based tools (and a variety of other tools! After all, you use the one most appropriate for the task!) routinely in the real world! It is also why many tend to default to using the measurements rather than spending an inordinate amount of time trying to calculate abstract idealized values that are rarely encounters in REAL spaces...

A classic example is the recent foray into room modes! The calculators can give you a ballpark idea for an idealized space - but the real factors and complexity of a real room with its doorways, alcoves, variations in dimensions, coupled spaces (adjoining rooms!), etc. render the calculations, at best, a rough guide. There are a great deal of other real world factors that contribute to the actual room modes varying from the calculations. So rather then spend a great deal of time with a calculator, it is easier to take a measurement of the real room and ascertain the real response and address it specifically. In this manner you address the real problem in a surgical manner without allot of extraneous effort. So I guess you could say that the more knowledgeable you become, the more quickly you choose to focus on the actual problem, achieve a real solution actually acomplishing the desired goal, and go home! ;-)

Oh, and one more item that might bring home the point. The advances made possible over the past 30 years have NOT been due simply to the use of more formulas! (I will qualify this with those such as pioneered by Manfred Schroeder - that arose from and complimented the results that were achieved based upon measurements!) But the major advances have been made possible by the actual measurements and the detail afforded by them! With this increased understanding of the 'real', as opposed to the assumed generalized statistical abstractions, we have been able to achieve a much more detailed understanding of acoustics. And from this our models have likewise become more accurate.

**Other time domain based tools such as SMAART, EASERA, MLSSA, ETF, etc., are also useful, but they do not offer the full array of tools that TEF does...nor the noise immunity that TDS offers (note: EASERA does offer an optional TDS module). Thus, for discussion, my references will default to TEF, as it is the ultimate Swiss Army Knife for acoustical measurements.

[DrWho]

Man, if only my profs would buy into that argument... []

Would you happen to have any information on where to purchase these tools? I am a bit confused if only one comany makes each type or if there are multiple models to choose from. Some quick google searching doesn't really yield any decisive answers.

I also wonder if any of the labs at school have any of these devices...if not, I would like to get some - and then I won't have to purchase them myself [H]

[mas]

Oh boy...you have hit on another excellent, if politically incorrect problem too often valid today! Pardon me while I editorialize a bit, as we have not yet gotten into discussing the actual acoustics issues...

My first question...it is one thing to teach, it is another thing to have actually done that which many have only read about.

I wonder...how many large acoustical spaces have your recalicitrant instructors tuned? How many small acoustical spaces? How many large SR soundsystems have they designed? And how would they know that they have achieved the level of results they assume they have? I think you will quickly find why so many are either ignorant of, or intimidated by, some of the new tools.

It requires a level of knowledge, skill, and proficiency that exceeds simply memorizing the various reverberent sound field equations!

And unfortunately, unless you have had the privelege of studying under someone such as Dr. Patronis at Georgia Tech, John Prohs at Ambassador College, or Doug Jones at Columbia College, or have taken acoustics at MTSU in their RIM program, or have taken a course at Syn-Aud-Con, few have. (Note, I am leaving out quite a few schools that I know have them.) They have simply read books. And to date, Sound System Engineering has been the most comprehensive and up to date book detailing the adventure that has ocurred in the last 30 years - written by the lightning rod of the 'new science'. So, my first question would be, if you are taking a course in acoustics, what textbook(s) are they using?

Take a few moments and peruse this link:

http://www.prosoundweb.com/install/commentary/kc/sac/doncar.shtml

- and please do read through it! - you just may be a bit surprised to see the names mentioned of just about every single 'whos it' in modern acoustics mentioned as a member of the consortium - from Dick Heyser, to Don Keele, Peter D'Antonio, Gene Patronis, Russ Berger, Jim Cary, Bob Todrank, John Prohs, Sam Berkow, Bruce Howse, Dr. Anhert, and the list goes on!!!!! Quite a distinguished alumni! And while Don and Carolyn Davis are two of the most generous and gracious people you will ever meet, please don't let their modesty fool you! The collaborative effort that is the organization known as Syn-Aud-Con has in large measure been responsible for the majority of the advancements of which we speak!

Syn-Aud-Con came into existence specifically due to the problem to which you mention!

And in Syn-Aud-Con, you had the opportunity to not only learn from the best, you had the opportunity to actively associate and to participate in the discussion and the research that was actively going on.

So, with any luck, you will have the opportunity to study with someone who has had the benefit of someone who has been active in Syn-Aud-Con. If not, well, I hope they have read allot of books...but I wonder which ones if they are not disciples of the revolution that Dick Heyser began with TDS.

As far as the devices go, they are distributed by a variety of sources. But they are specialized tools. You are not going to find them in Home Depot. ;-P

And they are not necessarily cheap, but neither are they expensive compared to the basic tools required to enter into most any professional trade!

Goldline now distributes TEF (originally a division of Techron/Crown). Renkus-Heinz (and Anhert ADA) distribute EASE and EASERA. MLSSA is distributed by DRA Labs, and EAW distributes SIA SMAART.

Obviously even these folks, who are often quick to refer to Everest's Master Handbook of Acoustics, have not taken the time to note his Preface to the Second Edition. I will quote a portion of it:

"When the first addition was published, the application of computer techniques to acoustical measurements was in its infancy. In this edition, another new section has been added to describe an important new measuring device (The Techron TEF). This instrument is placing in the hands of knowledgeable audio persons a simple, yet very sohisticated device, which through simple displays makes available energy, time, and frequency relationships that were previously unattainable. The field of room acoustics is being revolutionaized before our eyes, and every alert student of the audio arts must become aware of what is happening."

Indeed!

Why do some schools who profess to teach acoustics think that they can do so without employing the proper tools necessary to do so? And why do professors who do not have skills with the proper tools teach the subject material? Onee aspect is that they are not teaching how to do it, but are teaching about it. And that is fine given that this is the desired scope. But if they are trying to teach applied acoustics or if they are atempting to provided a more intensive acoustics program, I personally do not see how they do so adequately without empolying the current tools of the trade.

Addendum:

One possible suggestion (having myself spent some time in that rodeo called university physics instruction) - many will allow you to attend the Syn-Aud-Con seminar for credit under an independent or off-campus study umbrella. Now they may require you to do something in addition to the seminar to demonstrate that you actually learned something - after all, most schools value one's ability to regurgitate facts more than one's ability to creatively reason while utilizing the knowledge- but you might consider a 'do-able' project prior to submitting such a request.

Another desierable option would be to intern or to co-op with a practicing acoustician or firm who actively employs the latest tools and techniques.

OK...enough of my ramblings...

Let's address your questions about Large Room Acoustics!!!

[mas]

Let's address your questions about Large Room Acoustics.

May I suggest that you simply walk along with the calculations in the book, simply to get a handle on some of the variables and the factors that the equations take into consideration. It is not intended to be a heavy duty math class - indeed, all of the calculations are presented in simple algebra, without any calculus...rather amazing for a science that has moved into the time domain - and is thus fundamentally modelled as a function with respect to time!

You should also be gaining a basic conceptual understanding for the basic definitions such as the frequency dependent definition of a large acoustical space, and the conceptual definitions of reverberation, critical distance, mean free path, as well as various factors effecting reverberation, and you should begin to become familiar with the graphical signal response depictions such as (especially) the Definition of Sound Fields with respect to time (figure 8-29), as well as the impulse and doublet response, figures 8-12a,b, and the 3D waterfall response such as in figure 8-12c, and the ETC response as pictured in figures 8-15 & 18.

Do not try to memorize the equations! If you should ever need them, you now have the resource to look them up! []

We will be dealing with the ETC response in much greater detail in the future, so you will want to at least be comfortable in identifying the various aspects of the general display. You should focus primarily on the the Definition of Sound Fields with respect to time (figure 8-29) while then being able to identify the corresponding components in the real responses.

To give you a hint as what to expect in the next section...What you are going to discover is that, while an understanding of the classical definitions are very important, we are going to be presenting exceptions to them. Thus it is important to understand the basis from which we will be deviating.

[mas]

I am rather surprised that no one has yet commented on the immense size of the room required to qualify as a Large Acoustical Space!!

For a 30Hz response, the room must be 251,117 cuft!!!!!

Does anyone have a listening room (where you are not employing a subwoofer as the volume would increase!!) larger than this?

[]

[DrWho]

I didn't quite understand the formulas for calculating the size

required for a certain frequency. The pdf came out a bit fuzzy and I

was having a hard time distinguishing between subscripts (throughout

the whole thing actually). But if I'm reading it right, the room must

be 565,000 cubic feet for a 20Hz reproduction system...[]

Another thing I found rather ammusing is that the article spent a lot

of time explaining how Q and Sa were both controllable factors that

affect the critical distance and the reverbation characterstics of the

room. I got a bit worried at first when they started off introducing

RT60 and never specified the directivity of the loudspeaker source. I

would have thought it a bit intuitive, but the older math clearly

didn't show it. I have no doubt the old gurus were aware of it - they

just didn't have the means to quantify it (and they were probably too

busy with the implications of their new formulas to worry about it). Or

did I completely miss something?

Figure 8-33 struck it home too...I've finally got some more credible

information (basically perty pictures) to take back home and present to

the music pastor who doesn't understand the critical distance and gain

before feedback issues with the choir mics...or really any mic in

general.

Anyways, I would anticipate that critical distance issues are going to

be far different in the "small room." (from page 200) Dc determines:

1. Maximum acoustic seperation
2. Ratio of direct-to-reverberant sound
3. Required directivity of the mains
4. Required room characteristics for use with a specific loudspeaker

I think the last two will be the bigger issue since we all love our

Klipsch speakers here [] I would imagine that well controlled lower Q

(wide dispersion) speakers will be the ideal scenario. But I think the

concept of Dc is kinda skewed in the small room...maybe it gets

redefined? Or an entirely new variable is created? Modal systems have

gotta throw it way off (especially in the idealized hypothetical world).

[mas]

I think you can see that no one here will likely be dealing with what qualifies as a large acoustical space!!!

The directivity of a loudspeaker is addressed in the 40 pages comprising chapter 6 of the text - nothing has been omitted in the book - but then I fear too many are already put off by what I have thus far included - and we have only begun! If there are any significant omissions, let me know and I will try to provide sufficient background, as they are indeed dealt with in depth in the text.

I will note that the ability to calculate Q will not be of benefit for the vast majority, as it is not a trivial calculation. Typically you will need to rely on the manufacturer's provided specs. This makes the specs provided for programs such as EASE very important - and it also renders the silence deafening in how many manufacturers yet do not provide them.

I can try to include it as an addendum later if desired (as the first third of the text providing additional groundwork has already been 'ignored'!), but for what we will be dealing with, you will find it of little use. Likewise, you are going to find that RT60 and Reverberent spaces are applicable only to large acoustic spaces and they will become abstractions in that the definitions are not satisfied when applied to small acoustical spaces. And while the directivity, Q, of a speaker is a significant factor, it is much less of an issue in a small room where you do not encounter overlapping coverage areas such as are common in a large acoustical space due to the use of arrays. (Oh, and actually, a horn typically has a higher Q (more focused) dispersion than a dynamic cone speaker[])

But again, trying to apply any of this to a finished product at this juncture misses the point, as there is much more territory we have to cover. And I suspect that there are going to be a few surprises in store, and more than a few apple carts upset before we are finished!

Right now, it is important only to understand the concepts and the definitions based upon the concepts presented as they relate to Large Acoustical Spaces.

For now, please forget small acoustical spaces! You will want to note the defeinitions for the various sound fields in the chapter on the Acoustical Environment.

You will pass through the looking glass soon enough!

And regarding missing information...

In the next two posts I will be posting an additional chapter on the Acoustic Environment, as it addresses a few prerequisite definitions that are necessary to understand ideas that I mistakenly thought were in the two chapters on Large and Small Acoustic Spaces. As I mentioned in the intro, everything is cumulative...there is really no way to jump in without the prerequisite knowledge of the component parts. []

[mas]

The Acoustic Environment Pt1

This info should have been presented prior to Large Acoustical Spaces and contains definitions and concepts necessary to understand both the Large and Small Acoustic spaces.

[mas]

The Acoustic Environment Pt2

Please note the pages beginning on p.157, The Indoor Environment. You will be referring to the diagrams on p.162 and 163 in the future.

Please try to resist the temptation to imply anything yet ... We will get there! []

[mas]

Removal of a duplicate post that previously errored out and did not display at all...

This environment is ...well, let's just say it is interesting.

[8-)][*-)][]

[DrWho]

Btw, my textbook is the Fourth Edition of "Fundamentals of Acoustics", written by Kinsler, Frey, Coppens, and Sanders.

The head professor for the course mostly has experience with imaging

devices and other biomedical research. The extent of his audio work

involved slamming car doors (gotta make it sound like an expensive

door) and making TV's quieter. He puts such insane emphasis on the math

that I had to drop the course and I'm now retaking some math courses so

that I can be adequately equipped for the insanity. Apparently an

engineer isn't useful unless he can pull green functions and taylor

expansions out of his butt...[] The thing that makes it harder is

that we make some assumptions to make the math "easier" - and they

result in solutions that don't make perfect sense when you consider the

real world. I hear the other professor that teaches the course actually

designs sound systems and approaches it from a more conceptual angle -

I bet you can tell which section I'll be signing up for []

I don't want to belittle the math and all that, but it's a bit

excessive when you're working on a problem that takes 4 pages of work -

and you're not allowed to use a calculator. Last I checked, you get

fired in the "real world" for doing things by hand.

Anyways, I wanted to add one more comment on the pdf's....One thing

that gets mentioned quite frequently is the effect that audience has on

the acoustics. I know the Foellinger Great Hall down here on campus is

supposed to be an acoustical work of art that employs seating that

simulates the acoustics of the human body to keep things constant -

regardless of the size of the audience. This kind of approach is rather

expensive and so one of the things I've been pondering about is the

implementation of some DSP to help counter-act the difference between

bare-hall sound check and the full-hall show. Sometimes the difference

can be quite staggering and you end up spending 2 or 3 songs to get

things sounding right again - which kinda defeats the whole point of a

precise sound check in the first place...Yes I know, DSP isn't the

perfect tool, but anything to get you closer is worthwhile. In venues

I'm familiar with I'm able to anticipate the change, but it usually

still takes a song before things sound right.

On a completely side note - I've heard a few performances in the

Foellinger Great Hall and though so many claim it to be a near

acoustically "perfect" venue, I've never come away with that

impression. I must confess though that I'm yet to hear a full

professional symphony perform there...

http://www.kcsa.uiuc.edu/tours/tourWeb/GreatHall.html

Intelligibility has always been a suck fest and the orchestra always

seems so small....it's nothing like my experiences with the CSO in

downtown Chicago. And I don't think it can all be blaimed on the quality of

the performerance...

[mas]

In most engineering programs, 'acoustics' translates into noise level analysis and vibration control. And while these are valuable disciples, they are not exactly what we are concerned with here!

Likewise, the math can be a very valuable tool - but it is only as accurate as the model it assumes. But if one does not understand the model, its sources of error, or attempt to build a model that more closely correlates to the complexity of the real world, it risks becoming an exercise in character building - much like the current state of room mode calculators and computer programs such as RPG's room optimizer which assumes ideally configured rooms. This is not to denigrate such programs, but rather to acknowledge their limitations as we recognize the need for much more detailed and accurate room models that accurately define the space, along with a great many variables that must accurately be addressed, as well as the immensity of the computing power required as the complexity of the model increases. And with this need for more accurate and complex models we reach a point of diminishing returns if we have at our disposal extremely powerful and sophisticated measuring tools that can simply tell us what is actually happening in the real acoustical space - thus avoiding all speculation and limitations of an assumptive model that is but an (albeit detailed) approximation of the real space.

I am reminded of an extremely sophisticated weather prediction program running in the labs at Lawrence Berkeley when I was there in the mid '90s. This amazing program was able to analyze current conditions and to precisely predict the next day's weather - an importance issue that all too few recognize the importance of for security reasons. The irony is that it took two days to run to completion. So, despite its incredible complexity and elegance, the most timely and accurate predictor of the next day's weather was still to stick one's head outside the window.

Likewise, we have to distinguish between designing a space from scratch, and that of analyzing an existing space. An appreciation and an understanding of the fundamental variables is critical in either case. This knowledge can put you in the ballpark with an astute design, but after all is said and done, you are still left with a real space that deviates to some degree from even the most sophisticated model. And in analysing an existing space, the sophistication of such tools as TEF render the idealized models moot. And this is where the theoretical model of acoustics must give way to the applied model.

We will soon see that small space acoustics is a special case where the conditions necessary to satisfy classical large space acoustic behavior are not satisfied. And in this mixed mode environment we need to modify our methods and adapt to a more complex and less well behaved set of conditions and variables. The road will become more bumpy from here on...

When designing a high gain stage monitor in the early '90s for a prominent SR company, we called Jim Carey to assist us in using the TEF to analyze its performance. What always made me smile was Jim's initial query "are we buying or selling?" And to me this in many ways sums up many of the real issues facing us. By choosing the windowing we were able to smooth the response ever so slightly, making the response look simply wonderful, or we could expose a level of detail that would make even the most optimistic designer shudder. Just what are your goals?

In our case, I find myself bearing an appreciation for the theory, as it provides an understanding of the variables that must be considered. And it also provides for an understanding of the sources of error, thus providing the means to analyze a less than optimal situation and to assess which parameters stand to have the greatest impact upon the system.

And it is for this reason that I too look to the new generation of test equipment to provide a much more detailed image of the real space.

And yes, it is a balancing game. Either extreme offers risks and problems. Thus using our theoretical knowledge to postulate a reasonable course of action, balanced by the sanity check of a properly conducted measurement session, provides a powerful system of checks and balances.

So, let us try to deal with the concepts here.

I am curious to know where folks are with the material. Should I shut up and let you take a look over the next week? I suspect the concensus is for me to shut up in any case...but I am curious to know how many are actively looking at this material. I don't want to spend so much time that too many become bored, nor do I want to rush those who have other obligations and who might benefit from just a little more time to read and digest the information.

Please PM me and let me know!

In any case, you need to have a good understanding of what a reverberent space is. In a large acoustic space we have a well behaved space, but we need to understand the main concepts before we plunge into the small acoustic space where almost everything becomes a grey area. there it will be important to know of that which we will deviating, as the differences will become that upon which we will focus.

From here on out I guess we should proceed with a discussion of just what comprises a reverberent space, as well as a quick discussion of just what a few of the variables and relationships are in a large acoustical space.

So please post your questions and those concepts which need clarification stated in terms of a Large Acoustical Space - but please do not try to apply them to a small room or home theater!

Also, take a look at the diagrams... are they clear or do they too need clarification?

Please let me know. As we will do whatever is necessary to answer your questions. And feel free to PM me if you don't want to post your questions publically - I will not identify anyone who has questions or is experiencing difficulty with any of the material. Remember, the only dumb question is the one you don't ask and hence to which you do not get an answer!