Flutter Control Angle

Soundman2020 asked in another thread wrote:

I've been meaning to ask you another question about angles, André: Do you have any idea where the RFZ 6 degree / 12 degree splay angles come from? Any mathematical background on that? I guess there must be, and probably an equation or two to go along with it, but I haven't been able to find much. It just seems to be one of those things that everybody repeats, but nobody gives the underlying reason.

I have never seen any development of RFZ minimum angle criteria. This is understandable considering the wide range of room sizes and geometries. For flutter control it was empirical. Considering how long it has been since the problem was first addressed, this does not surprise me. Rettinger made reference to 10°* in Acoustics and Noise Control, which is almost 40 years old. Psycho-acoustic knowledge was primitive compared to today. On a similar vein, do you know how Rettinger's 10° became 12° or where the 12° came from? :?: Obtusely, Angular Andre *Wes' post later in this thread jogged my memory. Michael refered to one in ten splaying, not 10°. Thanks for the memory jog Wes!

Auto Industry Engineers?

Wouldn't the height also be a factor? To reduse flutter - an 8ft room may need 12degrees but a 10ft room may only need 8degrees. Sorry I have not read the papers on the subject. lil

OK; this is dangerous! Look like I stirred up a hornets nest, and it's going to suck up some more of my time to answer my own question! And starting to get confusing. This guy recommends splaying 7% (yes, percent, not degrees, but percent). http://www.sparling.com/SparAdmin/arts/ ... %20BHJ.pdf Over here they argue that it is not "12 degrees" but "1 in 12". Then Wes Lachot says that it should actually be 4.7 degrees... http://recording.org/studio-constructio ... fused.html Here, someone says that it should be 5 degrees: http://www.gearslutz.com/board/studio-b ... shape.html ARRGHHH!!!! Time to study again... Maybe some re-design..... By the way, there is also an intriguing quote by Eric Desart in one of those threads, saying that it actually is OK to average the dimensions of slightly non-rectangular rooms when calculating ratios and modes, within reason, but understanding that it won't be accurate.... Fun and games! :) - Stuart -

Alton Everest suggested 5%, or more ;)

And this guy at Auralex says anything LESS than 8° or MORE than 15° is "wasting your time" http://www.acoustics101.com/angled_surfaces.asp Go figure! I think I'll just make my walls of Silly Putty, then I can push them around to see what works... :) - Stuart -

"Before we get into some suggestions there, let us point out that, in many cases you are going to be treating your room(s) with Auralex acoustic products." For me, that knocks that reference out of the collective.

For me, that knocks that reference out of the collective.

Yup! Good point. :) - Stuart -

Soundman2020 wrote:

OK; this is dangerous! Look like I stirred up a hornets nest, and it's going to suck up some more of my time to answer my own question!

I think that part of the problem is that the question is not clearly defined, or at least adhered to. You started with splay angles for RFZ zones. This thread has morphed (incredibly quickly) to eliminating flutter echo. Two distinct, acoustic objectives. Related, but distinct. Andre

Soundman2020 wrote:

.... This guy recommends splaying 7% (yes, percent, not degrees, but percent). http://www.sparling.com/SparAdmin/arts/ ... %20BHJ.pdf Over here they argue that it is not "12 degrees" but "1 in 12". Then Wes Lachot says that it should actually be 4.7 degrees... http://recording.org/studio-constructio ... fused.html Here, someone says that it should be 5 degrees: http://www.gearslutz.com/board/studio-b ... shape.html ..... - Stuart -

Stuart, It depends on what the purpose of the angle is. Some times, you just want the walls to be non-parallel to avoid flutter echoes. Other situations are much more critical, like the side walls of a control room, where you're trying to guide the sound waves in certain directions, to create a reflection free zone. These two examples have almost nothing to do with each other. In the case of flutter echos, 5 degrees is all it takes, and I mean one wall splayed 5 degrees from parallel, not both walls. Just build a room and try it, and you'll see. That's what I did, many years ago. And by the way, it doesn't make a dimes worth of difference whether it's exactly 5 degrees, or a 1/12 slope, which is 4.7 degrees, roughly. (I often express angles as whole number ratios, because it's sometimes easier for builders to get them right when they're communicated that way.) Control room side walls are a completely different animal, and require much steeper angles, if a reflection free zone is what you're after. --Wes

Thanks, André and Wes! So it's starting to make sense. To summarize: General rule of thumb for flutter echo is a 5 degree angle (which is roughly the same as 1 in 12). Wes, is it safe to say that there isn't really a maximum limit angle here, as claimed by one of the websites I linked to, which said that anything over a certain angle (15°) does not work? That doesn't sound right to me. And the general rule of thumb for Reflection Free Zone, then, is that the angle needs to be calculated for each specific case, based on the speakers, room geometry, etc. Am I doing fine so far? - Stuart -

Soundman2020 wrote:

General rule of thumb for flutter echo is a 5 degree angle (which is roughly the same as 1 in 12). Wes, is it safe to say that there isn't really a maximum limit angle here, as claimed by one of the websites I linked to, which said that anything over a certain angle (15°) does not work? ...for Reflection Free Zone, ...the angle needs to be calculated for each specific case, based on the speakers, room geometry, etc.

+2! That's a plus one for both points, summed up excelently. Andre

Great! :yahoo: Thanks again; André! So that leads to the next question: How does one go about calculating the angles for an RFZ? Is this just a matter of sitting down with pen and paper, tracing out rays from the speakers and bouncing them off the room geometry, to see where they go? Or is there a better way to do it? Maybe some software somewhere to help? - Stuart -

I was responsible for the 12 degrees angle back in the Studio Building and Display forum at Homerecording.com back in 2003. I used 6 degrees in the plans in The Recording Manual back in 98. When I was building Left Bank in 2000 I had a ceiling where half was at 12 degrees and the other half at 6. The 6 degrees still produced flutter echoes when you stood under it and clapped your hands so I've used 12 ever since.

John Sayers wrote:

I was responsible for the 12 degrees angle back in the Studio Building and Display forum at Homerecording.com back in 2003. When I was building Left Bank in 2000 I had a ceiling where half was at 12 degrees and the other half at 6. The 6 degrees still produced flutter echoes when you stood under it and clapped your hands so I've used 12 ever since.

Thanks John! Andre

Soundman2020 wrote:

How does one go about calculating the angles for an RFZ? Is this just a matter of sitting down with pen and paper, tracing out rays from the speakers and bouncing them off the room geometry, to see where they go?

Yes, that is it in succinct terms. Of course it is humorous to read it being called "just a matter of sitting.." with many people terrified of even room mode calculations. Using some form of CAD program makes it much quicker and easier to do. Andre

AVare wrote:

On a similar vein, do you know how Rettinger's 10° became 12° or where the 12° came from? :?: Obtusely, Angular Andre

Is there an answer coming for this or is it another question? I am sitting here...waiting ;)

John Sayers wrote:

...When I was building Left Bank in 2000 I had a ceiling where half was at 12 degrees and the other half at 6. The 6 degrees still produced flutter echoes when you stood under it and clapped your hands so I've used 12 ever since.

Interesting! I don't doubt that one bit. At the same time, I've built rooms where it was easily demonstrated that 5 degrees was enough to control flutter. I'm sure there were plenty of variables that came into play in both cases... There's a bit more to the story, for anyone who's interested. Flutters are arguably only bad when they become unmasked, like when two of the three dimension of a room have been heavily treated, but one has not. This happens quite often with the ceiling being the untreated dimension. Since flutter from the three dimensions normally mixes to create ambience (just like multiple echos are combined to create ambience in electronic reverb units), when you remove some of those echos from the mix, the others stop sounding like ambience and start sounding like short repeating echoes, or flutter. We tend to like long repeating echoes, but not short, robotic ones. The truth is that there is a transition as you move from strict parallelism to seriously splayed walls, with the flutters becoming softer with each degree of splay. What is objectionable under certain circumstances, with a given treatment regimenm may not be objectionable under different circumstances. It's hard to give a one-size-fits-all answer to something like this, for this reason. One other tidbit of experience: It takes a couple of feet of parallel surface to create flutters. So treatments don't have to cover entire surfaces to take care of the problem. -Wes

I'm sure there were plenty of variables that came into play in both cases...

definitely Wes - the distance between walls being one of them.

John, when you say 12 degrees, you mean each wall is angled 6 degrees to form a 12 degree difference? or 12 degrees each wall?

each wall is angled to form a 12 degree difference - But! - in the area we constantly work in which is small rooms, it works, whereas as Wes suggested, in bigger rooms it doesn't need to be that great and lower angles will also work.