A room reinforces certain frequencies that are dependent on the size of the room. There are a number of them but the fundamental two wall(axial)frequency that will be reinforced, for example, is found with this formula:1130/2L. where L is the length of the room in feet. 1130 is the speed of sound per second in feet.
If your room is 16 feet long the room's natural/resonant frequency between those two walls is 35.31Hz. The room will reinforce that frequency and multiples thereof. You want to avoid/tame this tendency to reinforce a single frequency and not others as it creates an unequal response. You also want to keep the resonant frequencies (and multiples thereof)of the other walls of the room from being the same becaue when this happens the resonant frequencies of each dimension of the room reinforce one another. That is why a square room(15x15)is said to be so bad.
One way to avoid this is to build with the "golden" ratios. Another sometimes used is to splay your walls. When you splay walls you cause the distance between opposing walls to vary slightly and, therefore, the walls do not reinforce just one frequency.
I know Rives(Abstract7) of Rives Audio is going to do this in his new room. Maybe he will pipe in. Others, like F. Alton Everest, think the effects are "nominal."(Master Handbook of Acoustics, p 281-282)
I remain, |
A room reinforces certain frequencies that are dependent on the size of the room. There are a number of them but the fundamental two wall(axial)frequency that will be reinforced, for example, is found with this formula:1130/2L. where L is the length of the room in feet. 1130 is the speed of sound per second in feet.
If your room is 16 feet long the room's natural/resonant frequency between those two walls is 35.31Hz. The room will reinforce that frequency and multiples thereof. You want to avoid/tame this tendency to reinforce a single frequency and not others as it creates an unequal response. You also want to keep the resonant frequencies (and multiples thereof)of the other walls of the room from being the same becaue when this happens the resonant frequencies of each dimension of the room reinforce one another. That is why a square room(15x15)is said to be so bad.
One way to avoid this is to build with the "golden" ratios. Another sometimes used is to splay your walls. When you splay walls you cause the distance between opposing walls to vary slightly and, therefore, the walls do not reinforce just one frequency.
I know Rives(Abstract7) of Rives Audio is going to do this in his new room. Maybe he will pipe in. Others, like F. Alton Everest, think the effects are "nominal."(Master Handbook of Acoustics, p 281-282)
I remain, |
Clueless-good post but I got it the first time ;) j/k
But he is exactly right on, the frequency humps a room with a less then perfect size can impart on a system is very noticeable. I spent some time recently at Rives site and it was really an eye opener and I feel it was possibly the best time I have spent online in a while- very informative and explained in english! The formulas that Rivesaudio gives is VERY helpful and simple to use, I would recommend any one trying to optimize there system read his remarks and follow as closely as possibly. I know Bill E.(Lakefrontroad) is having his room designed and built and he is going all out and it should be really interesting when it is done, no parrellel walls at all. It is amazing what a difference a room can make or what a difference you can make on a room! |
It's standard in recording studios (when they're designed as recording studios that is...not just a bsement with some mics)...not two walls should be parallel or the same length. |
Wooterb, first I should comment on Russ H. HT home theater architect. Just because he designed IMAX doesn't mean he is qualified for small room acoustics--they are very different. The fact is Russ H. is very qualified for small room acoustics, but my point is, don't take someone that has done studios and theaters as an expert in home theater or small listening room applications. Small rooms (less than 8000 cubic feet) for home listening and HT are very different animals--but the tools of the trade are the same. In fact we are redoing some rooms that were done by studio engineers--they were perfect for studios, the reverb times and everything are right on--but they sound terrible for home listening.
Now on to your real question: splayed walls. Two parallel walls will produce room modes. Some of these modes can have very high Q factors (they are narrow band and highly peaked). When you splay the walls you don't eliminate the mode, but you distribute it. The Q factor goes down as does the peak. The more the splay the lower the Q factor, until at some point there is basically very little left of a room mode. Splaying walls if very effective in this matter.
Why don't people splay walls? It's costly. It doesn't work that great for the resale value of your home. It is "unpredictable". The last statement is not at all true, but has been quoted from Alton Everest and others. He also doesn't believe splaying is very effective: calling it nominal--but I disagree here as well. (Everest's books by the way are still excellent--I just don't agree with everything in them) Splaying the walls is very predictable, but the math is much more complicated and not easily described. If you do intend to splay the walls, it's a good idea to work with an acoustical engineer and make sure it's right. I don't recommend most people to go the splayed wall route alone.
A comment on the golden triangle: Most of the previous work in our opinion is not quite right. It evenly weights axial, transaxial, and oblique modes. Oblique modes are almost negligible and axial should have 3 times the weighting factor of the transaxial. We are working on new ratios and new methods that better distribute actual real world results. We are not done with this work yet, and still take each room as a case by case basis.
As to Bill E. room: It will be fantastic. Bill is fortunate enough to be able to spend what is necessary to build this room (in an existing house no less- far more expensive than new construction). There are no parallel walls save one in the rear which is such a small surface area it is negligible. It is designed for 2 channel only, and the difference in design for 2 channel vs HT is pretty great. We are very anxious to get this room complete--and listen.
Please do visit our website at www.rivesaudio.com. It has been mentioned and the tutorial in the acoustical issues section should be helpful. |
As an Architect of high end residential, I've had quite a few opportunities to design/ build and research the room issues. Two years ago my client spent quite a bit of money ($15,000) having an acoustics expert calculate his $200,000 home theater needs. This guy designs home theaters, small scale preforming arts facilities, movie houses and churches.
In home audio our needs are unique in that the scale is very small, reverberation rates, sound enhancement and echo are nominal issues. This could possibly be what Everest is discussing. Our issues are mostly side wall and floor/ ceiling first order reflections (slap echo) and frequency exaggeration.
A very small splay (narrower at the front) will alleviate most of these frequency nodes, but some thought could be put to the amount of splay. Too much could reinforce the projection and act as a "horn" and not enough will not achieve the intended goal. Ceiling and floor sloping can be more dangerous, in that the sound angles off a sloped ceiling (in the same fashion as the walls) could end up directing the reflection bach to the listener (especially with multiple seating locations). Think of a dome, it reinforces all sound to the middle, very bad in our situation. A slope down the axis of the room however could disperse the reflections very well.
The consultant we used had a very elaborate computer program that he was able to model the full spectrum of possibilities for the best solution. He was able to produce an audio track for any position in the room to determine what every seat would hear. There are cheaper programs available that could do some of the same things, just not quite as involved.
The rule of thumb I would use is the ratios spelled out in Everest's books or Robert Harley's book to determine the best ratio and size. If you did splay the side walls, I would use the width recommended as the average (mid point) of the splayed wall. I have been using a 2 ½ degree angle on my designs with very good results. The ceiling I would vault if possible.
As to how important this is, it completely depends on how far your willing to go. If your intending to get every ounce from your system with power conditioning, cables and tweaks, then this makes sense. If your happy with 80% of perfection, skip it, it's not worth the hassle. If you do however pursue this I would recommend you look into what wall materials your using. One layer of drywall over wood studs can give a soggy base response compared to a more rigid construction. You might consider building in a true base trap in the design, what acoustic treatments are you willing to live with, if non then these issues become extremely important.
"Acoustic treatments are a band-aid for problems not addressed during design, not the answer" according to this expert. |
Wooterb, have you look into getting CARA release 2.1. This is supposed to be the best computer analysis software available and should give you some answers before start building. It costs about $50.00. |
To RIVES' response regarding Russ H. of Stereophile HT magazine, If I'm not mistaken, that guy has done a tremendous amount of smaller custom high end home theaters for residential! Infact I believe he's done some of the better more well docummented and publicized custom home theaters to boot. I think he writes for several audio and video mags out there, but I'm only familiar with the Stereophile one, as that's what I subscribe to. Also, thank you for pointing me to your website. I see you dable in this as well.
My question to RIVES would be how many of these splayed rooms have you done on your own? Or for that matter how many regular theater rooms have you done? Exact numbers would be appreciated. Are there any references I could get from your website? Also,how long have you been designing rooms professionally. This would be my first so I want to do it right you understand.
I certainly know Russ claims to have done hundreds of large scale high end custom theater rooms in home environments, costing in the several hundred thousand dollar or more range. He has a website you can contact him through if anyone is interested in finding out more about his services. Although I noticed he charges mega bucks for his services! An well out of my price league currently unfortunately. At this point however I would be more inclined to believe his theories off hand. But I do appreciate the information, and will strongly consider all options.
I guess I'm still stuck with trying to gauge the real practical application advantages of "splaying". It just seems to me that it won't eliminate the modes, but just more randomely scatter my seating options around for best sound in the room. I would like to, if possible, keep my seating arrangements more symetrical looking and uniform, and I'm more inclined to think that a rectangular room will give me a more prdictable table with wich to plot my seating arrangements from, and will probably cost less for me too do. We'll see.
To anyone who's interested, when I was as the Airport newstand the other day, I noticed that this months issue of Home Theater Magazine has an article about "building a custom home theater on a budget". Also I noticed they are doing their theater foom without splayed walls! But then again I noticed they consulted Russ Herschelmann with the room design, as well as two other guys. So I guess that makes sense since he's a rectangular room advocate It appears.
Also, "Cdc" where do you find such a program (CARA release 2.1)? I would be interested in looking at it.
Also, thanks to everyone for posting, the info has been, well, informative.
|
To RIVES' response regarding Russ H. of Stereophile HT magazine, If I'm not mistaken, that guy has done a tremendous amount of smaller custom high end home theaters for residential! Infact I believe he's done some of the better more well docummented and publicized custom home theaters to boot. I think he writes for several audio and video mags out there, but I'm only familiar with the Stereophile one, as that's what I subscribe to. Also, thank you for pointing me to your website. I see you dable in this as well.
My question to RIVES would be how many of these splayed rooms have you done on your own? Or for that matter how many regular theater rooms have you done? Exact numbers would be appreciated. Are there any references I could get from your website? Also,how long have you been designing rooms professionally. This would be my first so I want to do it right you understand.
I certainly know Russ claims to have done hundreds of large scale high end custom theater rooms in home environments, costing in the several hundred thousand dollar or more range. He has a website you can contact him through if anyone is interested in finding out more about his services. Although I noticed he charges mega bucks for his services! An well out of my price league currently unfortunately. At this point however I would be more inclined to believe his theories off hand. But I do appreciate the information, and will strongly consider all options.
I guess I'm still stuck with trying to gauge the real practical application advantages of "splaying". It just seems to me that it won't eliminate the modes, but just more randomely scatter my seating options around for best sound in the room. I would like to, if possible, keep my seating arrangements more symetrical looking and uniform, and I'm more inclined to think that a rectangular room will give me a more prdictable table with wich to plot my seating arrangements from, and will probably cost less for me too do. We'll see.
To anyone who's interested, when I was as the Airport newstand the other day, I noticed that this months issue of Home Theater Magazine has an article about "building a custom home theater on a budget". Also I noticed they are doing their theater foom without splayed walls! But then again I noticed they consulted Russ Herschelmann with the room design, as well as two other guys. So I guess that makes sense since he's a rectangular room advocate It appears.
Also, "Cdc" where do you find such a program (CARA release 2.1)? I would be interested in looking at it.
Also, thanks to everyone for posting, the info has been, well, informative. |
Wooterb: Please read my response more carefully. I said he is VERY qualified. "The fact is Russ H. is very qualified for small room acoustics" My point was that some people--not Russ--who do large room acoustics and studios are not qualified for small rooms. Russ most definitely is--and I would never imply anything differently. As to where you can buy CARA 2.1 software, we sell it on our website, or you can call us directly 800-959-6553. It is a very good package. |
Splayed walls and novice do not = hi- end sound. I agree with Rives. Simple = smart unless you are a pro. To many angles are hard to predict. Worse yet is the correction for an irregular room. I have been trying to work magic in mine for the last 9 years and I have not fixed it yet. Accoustic treatments are only a band-aid and not the cure, I second what Jadem6 said... Personally I will keep searching. Tom |
Jadem 6 said > "If you do however pursue this I would recommend you look into what wall materials your using. One layer of drywall over wood studs can give a soggy base response compared to a more rigid construction."
Make sure your dry wallers are generous in the use of screws. I had one person (who I consider fairly well informed) argue that the choice of stud/drywall and the screw schedule in the typical american room (fairly simple and cheap to implement from the start) was more important than most of the other exotic stuff combined. He also noted that this issue was almost never found in computer programs for room acoustics.
What you learned ones say to this?
Also, Wooterb: Rives does not need a defense from me but I think you came down rather hard on him. He clearly stated that Russ was qualified and given his site and answers here for some time I think he does more than "dable" in it.
I "dable." To tell a professonal that he is a "dabler" is a back of the hand thing. It's nice to have pros in the mix sometimes and I have seen a lot of them chased off forums this way never to return.
Cheers,
I remain |
Hi all:
Immediately below is a quote from my post above.
>>>Jadem 6 said > "If you do however pursue this I would recommend you look into what wall materials your using. One layer of drywall over wood studs can give a soggy base response compared to a more rigid construction."
Me talking.> Make sure your dry wallers are generous in the use of screws. I had one person (who I consider fairly well informed) argue that the choice of stud/drywall and the screw schedule in the typical american room (fairly simple and cheap to implement from the start) was more important than most of the other exotic stuff combined. He also noted that this issue was almost never found in computer programs for room acoustics.
Well I got no response to my post (so here I try again) and since posting it I read Martin Collom's article in the October issue of AudioXpress. The byline of the article is as follow:
"Balance of treble and bass in your speaker system may have more to do with what materials were used in building your house than other factors-which is good news for audio amateurs" Martin Colloms 11/02 AudioXpress p4.
What are the "other factors"? Well how about the Theile-Small parameters which are usually touted as gospel?
Colloms says" "the entire low-frequency scenerio deserves consideration." as they "assume 2Pi space" and "really need rethinking."
He implies that the entire structure of T/S are faulty and that the underlying assumptions are based on the kind of house Theile lived in at the time he did his work....
""Theile confirmed to me that his seminal work on low-frequency alignments and design was based on 2Pi partly because thet studio building where he researched and listened was a relatively "leaky" timber structure."" Collom p 6. (Jadem6 is making sense eh?)
In any event, maybe my friend from my first post was right. The kind of materials chosen for your walls and the screw schedule is more important than, as he puts it, "the exotic stuff"... i.e. TS measurements and computer models.
What you think?
I remain |
Clueless, I too remain clueless as to who (if anyone) actually knows the facts. Here are some random thoughts based on my personal observations and quite a bit of research and continuing education.
Buildings react to the low base signals in what I would call an organic way. Think of how wood responds in instruments and how tweakers have used woods of differing densities to "tune" there rooms. The material itself actually interacts with the sound wave and can take on it's characteristics for that moment. In the case of gypsum board (gyp. bd.) the material is only ½" thick in normal construction spanning 14½" of air over the height of the wall. This is in effect a long drum with a loose skin. The more screws used to fasten the gyp. to the studs may tighten the drum skin some, potentially changing the frequency but the issue remains of a thin skin over a wide space. I would assume more studs, 8" on center would alter the bass response quite a bit, although I've never tried it. The "experts" I've worked with suggest 4 layers of ½" gypsum board (each glued and screwed) as a minimum construction in a church, play house or theater. This is a minimum, they prefer 8" concrete block or poured concrete. The concrete has very little reaction by nature with the bass frequency thus allowing it to pass through. Base waves do not bounce and reverberate off materials like higher frequencies do. If you have ever been late for a concert in a stadium or hall built from concrete, you will know your late because the base is heard from the street. The problem for us with concrete is it reflects the high frequencies, but not a lot worse than gyp. bd. The paper on the surface of gyp. bd will tone some frequencies in a slap echo scenario. For most of us, concrete is not practical unless your listening space is in a basement. Then I would try to make sure both side walls were made of concrete. In an above ground situation with wood studs, the only practical solution is thicker gyp. bd. In the four layer scenario the net result is 2" of gypsum, in that it's glued to each preceding layer it acts as a solid 2" gypsum shell. Gypsum is a softer and a more absorbent material than concrete, but the two are very similar as a basic structure. I'm thinking the gyp may have some properties beneficial or possible a hindrance to the low midrange and upper bass where it may absorb some energy. Would this be released at a later time causing smear? Does a thicker stud wall with insulation help defuse this energy along with base signals? It's also possible to retrofit an existing room by adding three layers of gyp, the problem here is all the trim must be removes and the door and window frames would need to be extended.
Anyway, the idea as I understand it is to make the walls rigid but not created a drum. Once the space is created that has the least interaction to the base signal, we can consider wall materials. Much has been written on the wall coverings. I have used fabric pleated curtains, ceiling to floor on home theater designs. This is much like a movie theater, I think this is an attractive solution that responds to the greatest amount of frequencies due to the pleats. The added benefit of curtains rather than something applied to the wall is it's ability to double it's potential by absorbing not only first order but reflected waves. |
Clueless and Jadem6: You all have brought up important points that absolutely deserve attention. In general, proper construction can be more important than all sorts of esoteric materials. Abundant screws is a must, but the real key is to create layers of varying density material. For this we generally use 2 layers of gypsum sandwiched by a third low density layer. This gets rid of the drum effect because for the drum to resonate it needs one somewhat uniform medium. Layering up gypsum does not get rid of the problem, but it does reduce it by building up mass and by staggering the layers. Another area, that you did not bring up was glass--it leaks bass badly and reflects mids and highs. On top of that--if left as is--it resonates very badly, and is usually a source of serious problems in listening rooms. There are solutions for this too, but I won't go into the details.
The other aspect about computer programs, not having the data to properly measure these things--that is absolutely correct. There is a book published by our illustrious government on noise control. It was published in 1980 and is out of print (and very hard to find). It has tables upon tables of absorption coefficients for most building materials and even sandwiched structures like those mentioned above. Doors, walls, floors--it's there. We use this in all of our calculations. Even with this abumdance of data--the computer can't quite do everything--here's the problem:
You want to retain your bass energy--so you build a rigid wall. Unfortunately, this rigid wall is going to create bass modes with a very high Q factor. So, you could go the opposite way, and have single layer gypsum. You won't have the high Q bass mode, but you will have resonance from the "drum" effect and you will lose and have sloppy bass. So what is the answer? There is no one answer. It depends on room dimensions, furnishings, speakers used, taste of listening, volume of typical listening. If there was one answer--why would people need acoustical consulting services? We could all just follow a simple method and be done with it, but it's not that simple.
The computer and calculations are great tools, they are just that--tools and need be used appropriately and whoever is using them needs to understand their limitations. In fact, we are redesigning a listening room that was designed by someone who had designed a number of studios (that does not make them a bad designer--but in this example it just didn't work). The room, purely from a calculations and frequency measurement point of view is excellent. The reverberation times and frequency response are just what they should be--but the room sounds terrible. I won't go into the details of the problems, but suffice to say--you can design a room that based solely on computer programs and caculations that will sound terrible. At the same time, however, the computer and calculations are necessary to determine if a particular design is going to work--you know if the frequency response and other measurements are off based on these--you have more work to do. But when you do get the right calculations--it doesn't mean you have the best possible room. |
