"Completely dead" rooms

[damonrpayne]

mas:

I don't know enough to disagree with you!

[damonrpayne]

Also, mas, do we all agree on our working definition of "small room". The "less than 1500 cubic feet" metric has been thrown out. For my own selfish purposes, I'm more in the ~4400 cubic ft range.

PS, there's not many of us in here, but this is by far the most interesting part of the forum right now.

[damonrpayne]

RT60:

I have read RT60 defined as the time needed for sound to decay not a fixed amount ( the 60db you quote), nor to a specific dB level, but rather "Time to decay to the level of the ambient noise in the room", is this false or incomplete?

[mas]

RT60 is the measurement of the decay time of a well-mixed reverberant sound field well beyond Dc.

Specifically, RT60 is defined as the apparent reverberation time for 60 dB decay in seconds.

A small acoustic space is not defined in terms of volume, it is defined by its characteristics, the fundamental characteristic being the lack of the above!

This is critcal to understanding small room acoustics! While we may use the term reverberation, we use it as slang in a small acoustical space! What we incorrectly refer to as a reverberatnt field in a small acoustic space, is actually a semi-reverberant field!

This is perhaps the most critical and the most fundamentally misunderstood aspect of acoustics - especially in the 'small' room! We must understand the fundamental limitations. Once we completely understand this, we can then begin to study the ramifications of this!

Look at the comparison of the ETC and the waterfall diagrams of a small and large room. Also, check out the next post to see an additional slide.

A reverberant field has strict conditions, which if not met, is NOT a reverberant field! A small acoustical space is defined as a space that will not support a reverberant field!

Now, there are things we can do to create a semi-reverberant sound field that more closely approximates a reverberant space, and a few neat tricks via the use of coupled spaces; but it is important to realize that we never achieve a truely reverberant field in a small acoustical space!

Put bluntly: If we don't get this, it is pointless to go further.

How is that for an apocalyptic (yet accurate) pronouncement!? ;-)

Once we completely understand the nature of a reverberant and a semi-reverberant sound field we can then move on to how they are treated and see just what occurs.

LgVsSmRoomReverbSpaces.Lg.pdf

[mas]

Here is a suggestion that many will find sacreligious!

Its time to put the Everest Master Handbook... away!

It serves as an adequate survey course by presenting many of the up and coming technologies that were to transform acoustics up through the mid-80s. But he neither presents them with completeness nor does he present the full ramification of that which he introduces. Its time to move into the research and findings of those whose diagrams and buzzwords he quotes.

In other words, he points to many of the developing ideas, but he does not adequately present them. Its time to find a new, more complete textbook. The Master (it hurts to use that term!) Handbook has served its purpose, it has served as a finger pointing to the new science of acoustics. But you must move beyond the survey course and delve more deeply to appreciate the significance of the resultant research.

FundDifBetweenSm&LgReverbSpace.Lg.pdf

[PrestonTom]

.......

Its time to find a new, more complete textbook. The Master (it hurts to use that term!) Handbook has served its purpose, it has served as a finger pointing to the new science of acoustics. But you must move beyond the survey course and delve more deeply to appreciate the significance of the resultant research.

..........

Obviously, a reasonable explanation of the newer approaches can not be provided in a piece meal fashion via this forum.

For those of us who are interested, what review chapters or papers are you suggesting?

-Tom

[DrWho]

Hey mas, I hope you don't mind a return reply []

I think I understand what you mean, but as I read your statements, I cannot avoid reacting with a bit of playful incredulity:

"I'd just use my own ears and decide what flavor I like more - using the technical crap to facilitate the process."

Literally, then this entire discussion is moot. Just do what you want and what feels

good. I reject this radical sense of its being either purely

subjective or its purely objective! If they don't correlate, and you

are not at least aware and concerned about that, then you are off the

path. And I like the concept of 'optimization'.

The choice in music is what makes everything purely subjective

because the treatment of the room is directly dependant upon that

decision. The playback acoustics for say a stereo mic pair, IDE (or

whatever that in-ear crap is), or close-mic'ed multichannel recording

are all going to be different. And even then the end-user may prefer a

slightly different sound, despite what might be considered appropriate

for the application. I'll come right out and mention that I prefer a

dry sound - whatever that means []

That said, someone skilled in the ways of acoustics will probably be

better at objectively steering the sound to his own tastes. I guess I'm

saying the target final goal is a subjective issue - not to belittle

the huge advantage measuring brings to the situation. It's great to

have a process, but if you don't know where you're going it's a

complete waste of time.

"It really shouldn't be the responsibility of the average

consumer to purchase crazy fancy measuring equipment nor hire

"professionals".

Its not! But isn't that what someone is in effect doing when they

spend ~$7500 on a pair of Klipsch speakers????? The only difference is

that they are ignoring the room in which they will be placed! And the

"crazy" equipment and the "Professionals" are simply in another

location. Of course they could easily go buy a set of generic wonders

from Best Buy, or of course I am wondering more than ever why some like

to complain about Bose - who has chosen to cater to the "average

consumer"!

Not everyone is capable of or even wants to learn all the technical

mumbo jumbo. Heck, I'd be pissed if someone could learn everything

about acoustics in a few months - I have to bloody pay to learn this

crap! [:@] I'd argue that the typical audiophile you describe uses more

of the artsy fart side of their brain and prefers not to think about

the numbers. They just want it to sound good and don't really care

about the physics. Blasphemy yes, but that's how it is. I wouldn't

expect an art enthusiast to know how to paint - even if a knowledge of

painting would enhance their enjoyment.

My comment about "professionals" has more to do with the vast

majority that don't have a clue what they're doing - and will royally

screw people in the process. A completely side issue, but an important

one, and a category I hope I never fall into [] If these companies

have no problem screwing over churches I'd hate to see what they'd do

with a lessor moral dilema! Maybe I'm just bitter...

But then the room is not mass produced in some plant with

standard designs! It is a rather unique element and the technicians

must go to it, as , at least the last I checked, it was rather awkward

to bring the room to a processing facility.

Most of the people interested in acoustical treatment are already

building their room's from scratch. Just like the "ideal room ratios",

how hard would it be to pre-engineer half a dozen rooms that people can

just hand to their builder. Ideal? Probably not - but that's where the

engineering finger print comes in and calls it close enough. It's

definetly going to sound better than a concrete 19x19x19 room []

But I hope that you will be as quick to remind the rest of

the forum that these fancy speakers and cables and SPL meters and RTAs

and crossovers, as well as the rest of the menagerie of pieces and

parts whose total price is more than several hundred dollars can also be considered just a tad bit ridiculous!

...

I just fail to understand how some can spend justify efforts in

expending absurd amounts of money on subtle improvements quickly

approaching or exceeding the point of diminishing returns, and on the

other hand dismiss real improvements amounting to orders of magnitude.

Heck, bring it to the amplifer/preamp stages as well [] Speakers

and source material are a harder argument to make, but sometimes you

can bring it there too...

"Someone needs to sit down and standardize what

constitutes a good listening environment. And then people can just

build to the predetermined standard and know they will achieve good

sound - and the studios can mix for those environments too."

And given that far too many engineers are having great difficulty

simply mixing the DIRECT signal, I will thank then to forget trying to

tell me what 'approved' listening environment I am supposed to use! I

will assume control over that! And considering the myriad types of

'legitimate' listening spaces there are, who will determine this? And

are we to omit speakers (monpole/dipole. plnaer, electrostatic, horns,

and the myriad types of loading configurations, full range, 2 way,

3-way, more-way), amplifiers and analog and digital, tube and solid

state, etc.? Must the rooms be dedicated, multi-function, strictly

music, home theater, or mixed use, cinder block, drywall, ICF, tile,

carpet, hardwood flooring, windowed or 'dark', timber framed or log,

etc.from this 'standard' as well? After all, we already have the

saleman's tool called THX! I am rather shocked that you, of all the folks here, would make this statement.

Perhaps I'm just frustrated with the impossibility of being

objective...all those variables are choices made by the listener and

have their own compromises that need to be dealt with differently.

But in a controlled acoustical situation I'd argue the differences

wouldn't be very large - assuming of course acoustical spaces optimized

for each of the different topologies. I've never actually heard every

topology out there in a perfect environment and I'm sure if I did that

I'd be recommended to try a better room or be accused of being deaf - I

wouldn't mind being deaf though...I'd probably enjoy more music that

way.

Not to mention...I assume that you are pursuing your degree

simply for your own altruistic pleasure, as, by your reasoning, I would

wonder why anyone should expect to pay you the wages you will

expect! Why should the average consumer purchase products from a

company who has purchased "crazy fancy measuring equipment" and hired

superfluous "professionals" such as yourself? Besides, anyone can mix

(can't they???)...just go to the average show for ample evidence! ;-)

;-) ;-) ;-P

Sure, if you like, you are welcome to guestimate to your heart's content.

But there are those who are concerned with a more accurate

prediction and correlation of the subjective experience with objective

tools. No, we don't know it all, and that is what makes acoustics so

exciting compared to many other disciplines where there are very few

practical unknowns. But the discipline has advanced at an incredible pace over the past 35 years, and most of this has been made possible precisely by the time domain models these 'fancy tools" make available.

lol, I'm 100% for the use of measuring equipment - you should check

out some other threads; it's a most unpopular topic [] I am all too

aware of how powerful measuring can be when interpreted correctly.

But that's the crux of the matter...how many "experienced" audio

engineers ran around with their RTA and Graphic Equalizer to "flatten

out the system"? Then put yourself in the shoes of a young kid trying

to explain to older experienced professionalIs that their methods suck

[]

I would be very wary of pushing the inexperienced to interpreting

their own results and then making treatments based on a limited

understanding. Heck, it might push a youngen like me to actually go out

and apply my limited understanding and screw over the person paying me

to do the job - and then I'm no better than the "professionals" I so

despise. I guess that's why I offer to do everything for free []

So in light of those two things, I think experimentation and shooting

in the dark is absolutely vital. There are plenty of tricks all over

the place for tuning things by ear - and they work quite well too. Not

perfect, but well. One of my favorites is dialing in the phase on a

subwoofer - you first reverse the polarity on the mains and then adjust

the phase until you hear as little bass as possible - and then you

revert the polarity on the mains back to normal and you're now in

phase. Sure, measuring equipment makes it easier to know when it's

dialed in, but you can get pretty close with just your ear. And then

going a step further, sometimes it's advantageous to be slightly

mis-aligned with the phase, which can be determined by ear, but not

only by the phase measurement (and this can be verified by measuring

other attributes, like the frequency response). An overly simple

analogy, but I think it can be applied to the realm of acoustics - just

as it is applied to all the other realms of audio.

I'm sure my opinions will change, but right now I see measuring as a

great tool for suck removal. We can't determine when something is going

to sound perfect, but we can certainly detect and correct for something

that sounds bad. It seems a lot of your comments about room acoustics

is along the same lines - you just have more experience with the common

suck factors that need to be removed []

So ya, them's a lot more words but hopefully it puts my perspective

into focus...now it's time to go to class and pretend to learn

something useful []

[mas]

Doc, Thanks for the reply! I was really scared that you hadn't been able to 'see' the big grin on my face intended when I selected your post on which to comment! And I was afraid I had (unintentionally) offended you!<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

I too just have a moment to comment, but a few obvious things should probably be noted.

There is a middle ground between subjective preferences and objective verification.

And objective tools can provide a large degree of correlation between objective behavior and subjective experience. To mention that they should be used by those who not only understand the concepts, but who can also understand what the measurements 'say', I think should be obvious!

And to say that some may like a subjective experience that defies all objective measure...well, just listen to most rap - the music for those who can't play music - for validation in that realm! (And if you like rap, please don't bother to write a rebuttal! I find rap a waste of time and substance just like you like it - so I guess neither can validate their subjective tastes!)

Can a few rooms be standardized? Sure! But first you have to designate the criterion that is desired. And that ranges from types of equipment, to the intended use, to the philosophy of the treatment. Everything involves a choice! My problem with this is not a standard given a set of criterion - that is easy! - what is difficult is to address the demand for freedom in all degrees on one side and a standard on the other!

One only look as far as interconnects , bi-wire, SS vs. set vs. tube wars that rage seemingly endlessly here. Most of the issues are moot to me. I can appreciate a few of the 'irreconcilable' formats with no conflict, and I reject several outright, but I don't feel compelled to spend too much time positing one side over another.

And the same with some of the acoustical environments.

But we do need to acknowledge that we are involved in an interesting area of science, where dramatic advances are being made 'as we speak', unlike many disciplines, where the major leaps occurred 100 years ago.

One need not have any idea what is happening. They can simply go to a store and listen to whoever puts on the best marketing display and take it home. And I suspect they will be happy with it, especially if they are convinced it is the best option for their money.

But my perspective is not the same as a casual consumer who simply wants a system to playback movies and to listen to ABBA, Britney and whoever wins the latest round of American Idol that the wife will accept.

After all, I am an over-educated crazy who has chosen to study this area in far more depth and to hang around with too many who are actively 'moving and shaking' this industry and the discipline. And I have feelings too!!! (Can you see the tear rolling down my cheek? ;-) But what impresses me is the advancement of the science where a correlation can be made between not only an improved subjective experience (as after all, the listening is the ultimate goal!!!) but also of the measured parameters that assist us in understanding the parameters that are identifiable factors, both to better understand the process as well as to facilitate replicating &/or implementing the improved subjective experience in other venues at other times. And this data is emerging at a very fast rate - ahead of most of the archived general texts.

So, yes there are valid trends and new processes and understanding that are driving the industry. And no, they are not often found in the general music or home theater section at your local audio/video store - even an exotic emporium such as Magnolia (hey, give me a break as I try to have fun making fun of what I consider to be gross pretense! And I guess I need to get on my running shoes in order to avoid the folks with torches coming to tar and feather me! ;-)

These advances are manifesting themselves in the more rarified pro environments where the gifted professionals are given a little more room to experiment, and also in the select mid-level rooms where these same principles are implemented in a more stream-lined manner - meaning that the budget is not unlimited, but the focus is in not compromising the acoustic principles. And it is in these mid-level rooms where the challenge is for me.

Edit: But the vast majority of the techniques employed in those rooms are almost all available to a home listening room. With a little perserverance and a little ingenuity and initiative, one can have a state of the art home listening room without breaking the bank! Now, I can't guarantee that it will be free, but I can suggest that, depending on your willingness to 'get involved'**, it can be less than the current price for a 'room in a box' the major retailers and websites are offering.

Oh, and to suggest a book for reference. If I had to recommend ONE book currently available, it is easy! It would be Sound System Engineering by Don & Carolyn Davis. Don & Carolyn have been the lightning rods for the most elite of the acoustic minds for the past 40 years. And the vast majority of the technologies referenced in Everest's Master Handbook come from Davis et al - from Russ Berger, P. D'Antonio, Heyser, Keele, Patronis, BHowse, Prohs, Klepper, and the active collaboration of these and far too many more to mention! And this group (and MORE!) have actively collaborated!** (Oh, and PWK was an active member of this group as well!)

But within ~ the next month, a truly exciting event will transpire, and that is the release of the NEW edition of Sound System Engineering, co-written by Don Davis and Dr. 'Gene' Patronis. This should be the latest compendium of the latest work to arrive off the presses, and it promises to be the most comprehensive approach and summary of the last 30 years of radical transformation of the acoustics industry. I can't wait!

**If anyone desire more information about this, PM me. It is so much easier to chat in an interactive context via Skype than over a BB format. And allot of ideas can be more easily shared and confusion alleviated and avoided in this manner!

Oh, and by the way Doc, I hope you learned allot in class today! ;-)

[DrWho]

Apart from being introduced to Smith Charts (way cool), I didn't learn too much today. I did however apply for an internship at Bose! I wore my PWK to the interview just in case...[]

I might take you up on that skype offer Mas...I'm dying to read some of the more recent research. I think I'll save up for that new edition of Sound System Engineering - I was going to purchase the old one a couple months ago, but decided I'd rather eat instead [] Any idea on when it will be released? And how does one go about getting into the group of people in that list? Looks like a bunch of old farts that get together for tea on Saturday mornings? []

[mikebse2a3]

Also, mas, do we all agree on our working definition of "small room". The "less than 1500 cubic feet" metric has been thrown out. For my own selfish purposes, I'm more in the ~4400 cubic ft range.

PS, there's not many of us in here, but this is by far the most interesting part of the forum right now.

Just to make it clear when I mentioned the 1500 cubic foot room I wasn't trying to give a definition of what a small room acoustically was but instead was trying to point out that rooms with this kind of cubic footage or less will have dimensions that will exhibit reflections that are very high in Amplitude and Very Early in Time to the listener(and are a real challenge to get the best sound out of in my experience and as the room's dimensions begin to get larger the Time Delay of the early reflections can naturally begin to better be used to your advantge.) and the use of diffusers if not placed properly can still send back problem early reflections also. The ability to test and verify the chosen placement of absorption and diffusion is very important to reach your final goal.

mike tn[]

[mas]

The diffusion of the Lre signals should not be a great problem.

With the diffusion, the energy of the reflection is greatly diminished by virtue of its distribution, unlike a focused 'undiffused' specular reflection which will essentially be at unity (ignoring the small losses). An easy way to imagine this is to visualize a flashlight such as a Maglite, and imagine the intensity of the beam as you adjust the beam from a high Q focused beam to a low Q diffused flood. The brightness (focused energy) is diminished with the increase in diffusion.

This diffused signal will then encounter additional diffusive surfaces further distributing the energy and allowing for a well behaved exponential decay in gain/ energy. If the diffusor you are using returns a specular reflection that is sufficiently focused and high in energy, the problem is the quality of the diffusor.

Any late energy returned to the listening position is generally outside the ITD/ISD or below the acceptable energy threshold.

Only occassionally, and usually as a result of anomalous structures, or the insane concave surfaces so popular with architects in large rooms such as churches, you may encounter a refocusing of the signals that manifest themself as an additional 'high gain' specular reflection that exceeds the exponential decay slope in the Lre region, This can generally, especially in a small room, be easily addressed with a very surgical application of absorption. But again, this problem is very rare in a small room.

With measurement tools such as an ETC, such problems literally leap out and scream to be addressed. Simply holding a piece of absorptiuon such as Sonex (not an endorsement!!) at differing angles to each side of the measuring microphone as you take additional sweeps will quickly isolate the path of the offending signal, and it is easy to extrapolate to a spot to surgically treat the anomoly and repeat the sweep to verify the results. {With PEQ from TEF, you simply move the cursor to the point in the ETC and it will resolve the exact 3 Space coordinates of the reflective path, thus allowing you to quickly mount a laser pointer into a transit placed at the microphones position and go to the exact spot the first time and address the issue. A great savings in time (etc.!) when you are dealing with large aditoriums that might otherwise require MANY trips up and down a slow scissors jack or up into the catwalk system. And it is even worse if you are in a gymnasium or church that has , say, very high ceilings and no available means to access the surfaces!)

ConcatSmallRoomResponseCharacterisitcs.pdf

[mikebse2a3]

The diffusion of the Lre signals should not be a great problem.

With the diffusion, the energy of the reflection is greatly diminished by virtue of its distribution, unlike a focused 'undiffused' specular reflection which will essentially be at unity (ignoring the small losses). An easy way to imagine this is to visualize a flashlight such as a Maglite, and imagine the intensity of the beam as you adjust the beam from a high Q focused beam to a low Q diffused flood. The brightness (focused energy) is diminished with the increase in diffusion.

This diffused signal will then encounter additional diffusive surfaces further distributing the energy and allowing for a well behaved exponential decay in gain/ energy. If the diffusor you are using returns a specular reflection that is sufficiently focused and high in energy, the problem is the quality of the diffusor.

Any late energy returned to the listening position is generally outside the ITD/ISD or below the acceptable energy threshold.

Only occassionally, and usually as a result of anomalous structures, or the insane concave surfaces so popular with architects in large rooms such as churches, you may encounter a refocusing of the signals that manifest themself as an additional 'high gain' specular reflection that exceeds the exponential decay slope in the Lre region, This can generally, especially in a small room, be easily addressed with a very surgical application of absorption. But again, this problem is very rare in a small room.

With measurement tools such as an ETC, such problems literally leap out and scream to be addressed. Simply holding a piece of absorptiuon such as Sonex (not an endorsement!!) at differing angles to each side of the measuring microphone as you take additional sweeps will quickly isolate the path of the offending signal, and it is easy to extrapolate to a spot to surgically treat the anomoly and repeat the sweep to verify the results. {With PEQ from TEF, you simply move the cursor to the point in the ETC and it will resolve the exact 3 Space coordinates of the reflective path, thus allowing you to quickly mount a laser pointer into a transit placed at the microphones position and go to the exact spot the first time and address the issue. A great savings in time (etc.!) when you are dealing with large aditoriums that might otherwise require MANY trips up and down a slow scissors jack or up into the catwalk system. And it is even worse if you are in a gymnasium or church that has , say, very high ceilings and no available means to access the surfaces!)

Hi mas

I agree with most all that you have been saying but my point is to caution about how a person is using a diffuser and especially if he thinks he is just going to buy some diffusers and just place them in his room. Let me give an example of what I'm talking about when I say that you have to be carefull that your chosen placement of a diffuser doesn't send back reflections to high in amplitude and early in time to the listening position. The fact that the diffuser is of a sufficient high quality and suppresses the specular reflection well isn't the problem but instead it is the placement that is the problem and will require attention to these scattered reflections from close room walls ,ceilings and floor in relation to the listening position. This is also why I'm saying that to not use testing like with the ETF program it is going to be hard to optimize a listening room.

Lets say your using the RPG Skyline Diffuser at the first reflection points in a room. The example room will have dimensions of 12' (w) x 15' (d) with 8' ceiling(1440 cubic foot). The speakers are on the 12' wall and the listener has to sit 12' back from the front wall which means the back wall is only 3' from the listener. So just looking at lets say one of the diffusers placed (although diffusers placed anywhere on the back wall in this example could cause audible problems because of the proximity to the listening position) on the back wall at the mirror image of the left speaker. Even though the diffuser is going to suppress the specular reflection from the left speaker it will send it's scattered energy ( yes lower in energy but still probably not low enough) to the ceiling/floor and side walls which in this example are still very close to the listener and some of these diffused reflections will be arriving say by 10ms and possibly of high enough amplitude to cause audible problems. So yes these reflections will have to be investagated and treated if the diffuser is going to be left in this position. So yes it can be said that the diffuser is placed to close to the listening position but in rooms along these dimensions compromises will need to be made and my point for people who might want to use them in smaller rooms(which I would encourage) is that they can be of good benefit even in small rooms but If you aren't willing to Test were you are placing them then you might be disappointed in your results and especially considering the cost of these type of treatments. All normal domestic listening rooms need some form of diffusion but they need to be used properly.

mike tn[]

[mas]

I m not sure of the real cause of the debate here, except as one of emphasis. And I fear we are doing a cute but futile dance that differs only in subtle emphasis and may serve to confuse others. But I guess that is OK if we have gotten the word out to folks that a small acoustical space is one that is lacking a true reverberant field!<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Of course, the placement of absorptive panels is critical. They are used to define the ITD/ISD gap and to (but only necessary in rare occasions) address anomalous high gain specular reflections that might remain in the Lre field.

The proximity to the back wall is not necessarily a problem. We are not concerned with a diffuse field eminating from the diffusors as long as the total time to reach the listening position is outside of the ITD/ISD. And then we concentrate on the density and gain of the exponential decay of the diffused semi-reverberant field. The time of travel from diffusor to listening position is not critical, provided the energy is diffuse. It is the total time of travel relative to the Ld, provided it is greater than the Haas kicker that effectively ends the ITD/ISD.

Diffusive panels are much less critical in their precise location, and except for their cost, they do not have the potential detrimental effect due to overuse as absorption does. Additionally, you want to allow for a specular reflection in the form of a Haas kicker to terminate the ITD/ISD. So you may have to introduce an appropriate reflective surface to provide this signal if it is not readily present in the room to be utilized.

But in any case, aside from the placement of bass traps and possibly a Haas kicker reflective surface, you will want to use diffusion on the entire rear wall! Properly designed diffusive panels do not support the reflection of focused specular waveforms. That is precisely their purpose! I guess I am a bit confused (but it is not a critical issue) as to why there is the assumption that diffusers are not adequate diffusers?

I would add a caveat here. I would not be using the generic 'room in a box' products from so many of the 'we can tune your room by mail based upon a picture and a credit card number' establishments that seem to flourish. Maybe I have been spoiled by using diffusion products from a few of the proven leaders in the field (or designs from these same folks) - as I do not use what cannot or has not been objectively verified by real measurements. (And trust me folks, I delight in cost saving do it yourself designs based upon verified proven concepts!)

{But then I guess I should move to Santa Fe or Vail and open a very exotic 'crystal recharging center' utilizing very exotic secret techniques requiring at least a week of intensive therapy, as I have the credentials in physics, if I could only stop laughing long enough to process their credit cards and assume custody of their 'little gems' ;-)) And I could sell my exotically colored directional wire as well! }

Quadratic diffusion has proven itself extremely effective in this regards. But again, if there is a problem, surgical use of absorption can always be employed to deal with anomalous specular reflections. This has always been maintained.

Regarding the use of diffusion on the rear wall immediately behind a listening and/or mix position, one need only look at the application pictures for their use in project rooms and studios. Please note the use of diffusion on all of the back walls and on the ceilings! Parts are from <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />Davis' Sound System Engineering (pp. 220, 221, 223, 231, 232) while parts come from RPG. In addition Pat Browns SynAudCon Theater is also included, and I can assure you that ALL of these examples are verified by measurements and exhaustive proof of performance assertions! But please do not think you can or want to simply copy these examples...

But I don't think we are fundamentally disagreeing, except in terminology and perhaps process. My issue is perhaps a procedural issue. It is simply that where you seem to indicate that diffusion can be 'misused', I would disagree and say that absorption, while indeed necessary to define the ITD/ISD, has a much greater potential for overuse, while diffusion is a necessary but much more benign threat. In other words, one use should use absorption with care for specific purposes, but you are pretty safe to go crazy with diffusion. Additionally, the use of adequate diffusion normally negates the need to use absorption to deal with high energy specular reflections in the Lre semi-reverberant sound field ;-) (But additional absorption remains a viable tool if it is needed!)

But we have traveled far a field from the CRITICAL issue of what constitutes a reverberant field, and why it is that a small acoustical space does not have a reverberant field, but has only a semi-reverberant field and how we treat it!!!

ConcatDiffusionExamples.pdf

[mas]

Mike, rather then confuse others here with a rather arcane digression, give me a holler on Skype and I think we can easily resolve our 'spat' ;-))

Do take a look at the first panel of the attachment in the last post for an ETC of the QRD and PRD diffusors and how the specular reflection is both diffused, resulting in a denser Lre, and how the gain is also reduced due to the more diffused energy field.

Normally, you want this energy in a small room!, as the room already has a very short Lre semi-reverberant field. And both absorption and diffusion are valuable tools. What you do not want are the hard focused reflections that exceed the exponential rolloff of the Lre and which are audible. Instead you want to harness that energy, but in a more diffuse (spread over time) sound field of lower intensity.

But one thing I think we both agree on, with a good ETC measurement, dealing with any of these issues becomes an easy hit and run proposition, as you know exactly what you are dealing with and you simply have to identify the method which is most effective in achieving the desired result. In fact this discussion is much more difficult than dealing with the real problems, provided one has the ETC measurements! So, if anyone else is reading this and has become confused, please ignore the issue. It is trivial relative tot he larger issues. And again, if anyone is stumped and want additional info or explanation, see below...

I am available to chat with anyone, should one choose, on Skype and/or Yahoo Messenger (WHEN it works! - as YM v7.5 and 8 are absolutely worthless as it conflicts with nearly everything and either appears to hang the machine in its competition for resources and constantly is logging off and trying to log on as it holds resources again! Until several months ago yahoo was gold, now I seldom launch it, while Skype now stays up continually) Just PM me and I will gladly provide you with the username and /or a phone number to use skype to chat and to speak by voice.

[WMcD]

Again, I love this exchange of ideas and the work people have put into explaining it.

Let me point out some linguistic problems that seem to muddy the issues. And mas can correct me, of course.

If we say that "small" rooms do not have revererant fields, people ask themselves, "what it is a small room?" Apparently, physically, we are talking somewhere around 1500 cubic feet. That is a cube about 11 feet on a side; people can noodle with their own dimensions. It does seem that a lot of our listening rooms fall into that range. I'll guess that rooms with twice as much volume have that same problem or we are getting to semi reverberant.

The statement that small rooms do not have reverberant fields at first reading means they can not (or never) have reverberant fields. I think the real message is that they can, if we apply diffusion. Which is the point.

Then there is the statement that "small" rooms by definition are those that don't have reverberant fields; implying that rooms with reverberant fields because of treatment are not "small" any more. Fine, but you can see that big acoustically and small physically, are the goals and starting points repectively.

The Live End Dead End concept is making a physically (dimensionally) small room . . . big (acoustically) in two different ways to achieve the time response we want. We're using two different treatments to create a big room.

The dead end lacks all reflections because the absorption makes the front walls go out to infinity (a bit of an overstatement) and we get a clean signal from the speaker. That is the first part of the time response.

The live end makes walls seem more distant because diffusion is setting up delayed-from-the-speaker-listener AND very complex semirandom distributions. This is the second half of the time response.

I know mas has been saying all of this. But as I describe, the "small room" term is being used in different ways in similar statements.

mas, thanks for beating us over the head with this. It is necessary.

Gil

[mikebse2a3]

Mike, rather then confuse others here with a rather arcane digression, give me a holler on Skype and I think we can easily resolve our 'spat' ;-))

Do take a look at the first panel of the attachment in the last post for an ETC of the QRD and PRD diffusors and how the specular reflection is both diffused, resulting in a denser Lre, and how the gain is also reduced due to the more diffused energy field.

Normally, you want this energy in a small room!, as the room already has a very short Lre semi-reverberant field. And both absorption and diffusion are valuable tools. What you do not want are the hard focused reflections that exceed the exponential rolloff of the Lre and which are audible. Instead you want to harness that energy, but in a more diffuse (spread over time) sound field of lower intensity.

But one thing I think we both agree on, with a good ETC measurement, dealing with any of these issues becomes an easy hit and run proposition, as you know exactly what you are dealing with and you simply have to identify the method which is most effective in achieving the desired result. In fact this discussion is much more difficult than dealing with the real problems, provided one has the ETC measurements! So, if anyone else is reading this and has become confused, please ignore the issue. It is trivial relative tot he larger issues. And again, if anyone is stumped and want additional info or explanation, see below...

I am available to chat with anyone, should one choose, on Skype and/or Yahoo Messenger (WHEN it works! - as YM v7.5 and 8 are absolutely worthless as it conflicts with nearly everything and either appears to hang the machine in its competition for resources and constantly is logging off and trying to log on as it holds resources again! Until several months ago yahoo was gold, now I seldom launch it, while Skype now stays up continually) Just PM me and I will gladly provide you with the username and /or a phone number to use skype to chat and to speak by voice.

What Spat mas?[] I'm glad to see discussions of this caliber because all can learn much needed knowledge from them and this is an area were new understanding is coming rapidly compared to other areas of audio especially.

I pretty much agree with all that you have been saying. I definitly am against the over use of absorption in most listening rooms and agree it is often and easily misused. I also agree that diffusion seems to have less chance of severly screwing things up but I do believe it would be wise for anyone looking into buying diffusion products to educate themselves about the appropriate use of them and I do believe they can be overdone in your average residential listening room especially if the room boundries are close to the listening position. As a general rule though I would rather see most people invest in diffusers over absorption given most room's problems/needs.

This Book: http://www.rpginc.com/aboutrpg/pdfs/rpg_book.pdf

Acoustic Absorbers and Diffusers

Theory, Design and Application

by Trevor J. Cox and Peter D'Antonio (of RPG)

In Chapter 2: Applications and basic principles of diffusers

It opens with " In many respects, the right and wrong places to use diffusers are still being defined. There are places, such as rear walls of large auditoria, where there is a general consensus that diffusers are a good treatment to prevent echoes and better than traditional absorptive approaches. The case for using diffusers in some other places is less clear cut, and until further research is undertaken, some applications of diffusers are going to be based more on precedence and intuition, rather than scientific fact. Having said this, much has been learned in recent decades which can help to insure that diffusers are used where they are needed."

In this chapter also is mentioned: " Unfortunantely, in most critical listening rooms, it is usual for sources and receivers to be in the near rather than the farfield, as the far field is outside the room. Consequently, listeners should be positioned as far from scattering surfaces as possible. Precedence has shown that it is best if the listener is at least three wavelengths away from diffusers. Since diffusers used in listening room applications have a lower frequency limit of roughly 300-500Hz, this means a minimum distance of 3 meter is recommended. In some situations this distance may have to be compromised."

This is were just saying place diffusers on the rear wall (most of the examples you mentioned still have the diffusers placed a proper distance from the listener) which could be to close to alot of domestic users could actually give rise to colorations of the sound due to the listener being to close to them. Now putting them on the rear wall might still be a good option and what someone wants to do but you will need to deal with controlling the returned energy from the diffusers otherwise coloration and imaging problems most likely will happen.

I honestly don't want to confuse/discourage people and I firmly believe all listening rooms should and would benefit from diffusion. I just don't want people to naively think that they can just buy alot of diffusers($$$$) and their problems will subside. Buy and Place them wisely and you will be well rewarded.

Anyway I believe I've said enough and I don't want to get the thread off track as you said.

mike tn[]

mas , I will PM you for a phone number and would like to give you a call if that is OK.

[mikebse2a3]

Gil said:

If we say that "small" rooms do not have revererant fields, people ask themselves, "what it is a small room?" Apparently, physically, we are talking somewhere around 1500 cubic feet. That is a cube about 11 feet on a side; people can noodle with their own dimensions. It does seem that a lot of our listening rooms fall into that range. I'll guess that rooms with twice as much volume have that same problem or we are getting to semi reverberant.

-----------------------------------------------

Gil I guess mas is correct and just by me mentioning an example number like 1500 cubic feet confusion has been raised.

My example was only to give people a visual example and a heads up that rooms of this volume and even less are a real challenge to deal with because the dimensions of rooms with these volumes places the listener and speakers very close to the room boundries where acoustical treatments will be used and In my view/experience they are a real challenge to treat acoustically and especially without some assistance from programs like ETF and others. As Room Dimensions/Volumes increase then acoustical treatment/placement starts to become more predictable because Time/distance starts to work more to our advantage over a much wider range of frequencies and among other things this is part of the reason larger rooms on average will outperform smaller rooms all else being equal.

mike tn