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Cardas' "Golden Trapagon"
Old 31st December 2019
  #1
Gear Head
 

Cardas' "Golden Trapagon"

Hi forum,

I'm intrigued by the idea of building a listening room without any parallel surfaces, so naturally I've been lead to read about Cardas' "Golden Trapagon" - however:

1) I cannot find any measurements made in such a room. Can anyone perhaps provide a link to an article or post, showing some data to describe the acoustical characteristics?

2) How come this room concept have a parallel front and back wall? Why not make the front wall angled instead of vertical, for example, and thereby eliminating ALL parallel walls? - Perhaps in an angle corresponding to the angle of the ceiling, which rises towards the back of the room..?

Happy new years to all and thanks in advance for your replies!

B.r. Jon
Old 5th January 2020
  #3
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Quote:
I'm intrigued by the idea of building a listening room without any parallel surfaces,
That would be a mistake. Symmetry is very important for accurate stereo imaging, and to create a clear, solid soundstage. Without room symmetry, it's reaaaaalllly hard to get acoustic symmetry. If you don't have acoustic symmetry, then your left ear is hearing a different acoustic response from what your right ear is hearing.

Look around ad designs and photos of word-class studios and listening rooms, and you'll notice something: they are all symmetrical, and most are rectangular, or based on rectangles.

Unfortunately, it's a common misconception, widely repeated around the internet, that making your walls non-parallel is a Good Thing, and will solve all the acosutic issues in your room. In reality, that is not true. There's nothing wrong with having a purely rectangular room as the basis for a control room or other critical listening room, then treating as needed for the design concept being used. Making your walls non-parallel doesn't help much acoustically, but it DOES make things a hell o a lot more difficult to predict. For example, all the room mode calculators you see out there on the internet assume that you have a rectangular room. If your room is not rectangular, then you cannot use those to accurately predict the modal response.

Quote:
1) I cannot find any measurements made in such a room. Can anyone perhaps provide a link to an article or post, showing some data to describe the acoustical characteristics?
That would be nice, wouldn't it? Having actual results from one of those would be great! But as far as I know, you are right: there isn't much testing done on such rooms. Probably for two reasons: 1) Nobody actually sets up their rooms like that. 2) The few who do, and then test them, are so disgusted by the results that they don't dare publish much...

Quote:
2) How come this room concept have a parallel front and back wall?
Because that's the way the vast majority of studios are built! Angling the front and back walls upsets the symmetry, makes it really hard to predict results, messes with the acoustic response, and doesn't look good either.


Quote:
Why not make the front wall angled instead of vertical, for example, and thereby eliminating ALL parallel walls?
Because there is no need to do so. Wes Lachot often angles the VISIBLE front walls in many of his designs, but the ACTUAL room walls are still vertical... and parallel. I have done the same in some of my designs, but you do need to do it very carefully: it can cause more trouble than it's worth if you aren't careful with the calculations and predictions. Tilting the front surfaces means you have to raise the speakers higher, which produces it's own set of problems (unwanted reflections from the desk and console, comb filtering, a frequency response notch in the mid range, etc.). Best avoided.

When deciding if you really want to go with that fictitious concept you mentioned, ask yourself why nobody ever seems to build professional studios that way? Most pro studios these days are built based on the RFZ, NER, or similar design concepts... there's a reason for that...


- Stuart -
Old 5th January 2020
  #4
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best wishes Stuart

https://rbdg.com/recording-mastering/


https://holonor.com/#references

// and Un// all is possible.
Old 7th January 2020
  #5
Gear Head
 

Quote:
Originally Posted by Soundman2020 View Post
That would be a mistake. Symmetry is very important for accurate stereo imaging, and to create a clear, solid soundstage. Without room symmetry, it's reaaaaalllly hard to get acoustic symmetry. If you don't have acoustic symmetry, then your left ear is hearing a different acoustic response from what your right ear is hearing.

Look around ad designs and photos of word-class studios and listening rooms, and you'll notice something: they are all symmetrical, and most are rectangular, or based on rectangles.

Unfortunately, it's a common misconception, widely repeated around the internet, that making your walls non-parallel is a Good Thing, and will solve all the acosutic issues in your room. In reality, that is not true. There's nothing wrong with having a purely rectangular room as the basis for a control room or other critical listening room, then treating as needed for the design concept being used. Making your walls non-parallel doesn't help much acoustically, but it DOES make things a hell o a lot more difficult to predict. For example, all the room mode calculators you see out there on the internet assume that you have a rectangular room. If your room is not rectangular, then you cannot use those to accurately predict the modal response.

That would be nice, wouldn't it? Having actual results from one of those would be great! But as far as I know, you are right: there isn't much testing done on such rooms. Probably for two reasons: 1) Nobody actually sets up their rooms like that. 2) The few who do, and then test them, are so disgusted by the results that they don't dare publish much...

Because that's the way the vast majority of studios are built! Angling the front and back walls upsets the symmetry, makes it really hard to predict results, messes with the acoustic response, and doesn't look good either.


Because there is no need to do so. Wes Lachot often angles the VISIBLE front walls in many of his designs, but the ACTUAL room walls are still vertical... and parallel. I have done the same in some of my designs, but you do need to do it very carefully: it can cause more trouble than it's worth if you aren't careful with the calculations and predictions. Tilting the front surfaces means you have to raise the speakers higher, which produces it's own set of problems (unwanted reflections from the desk and console, comb filtering, a frequency response notch in the mid range, etc.). Best avoided.

When deciding if you really want to go with that fictitious concept you mentioned, ask yourself why nobody ever seems to build professional studios that way? Most pro studios these days are built based on the RFZ, NER, or similar design concepts... there's a reason for that...


- Stuart -
Hi Stuart,

Thanks a lot for the thorough answer, but to clarify:
I'm NOT suggesting an asymmetrical room. It should definitely be symmetrical down the length of the room to ensure that the stereo imaging is consistent. Something like what's shown in my first attachment - room.png.

Secondarily, the ceiling should slope down towards the front of the room, as shown in attachment number two - room_side.png.

My QUESTION is then: How about sloping the front wall too? As shown in attachment number three - room_side_v2.png.

All of these non-parallel walls are of course there to avoid various artifacts in the sound, which I'll abstain from trying to explain, as I'll probably get some details wrong, which will then derail replies in here..

Again, thanks!

B.r. Jon
Attached Thumbnails
Cardas' "Golden Trapagon"-room.png   Cardas' "Golden Trapagon"-room_side.png   Cardas' "Golden Trapagon"-room_side_v2.png  
Old 7th January 2020
  #6
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avare's Avatar
 

Wrong. Soundman explained it well. What artifacts? Every space has modes, regardless of how non-rectangular.
Attached Thumbnails
Cardas' "Golden Trapagon"-car-modes.jpg  
Old 7th January 2020
  #7
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Quote:
Originally Posted by jonlanghoff View Post
Hi Stuart,

Thanks a lot for the thorough answer, but to clarify:
I'm NOT suggesting an asymmetrical room. It should definitely be symmetrical down the length of the room to ensure that the stereo imaging is consistent. Something like what's shown in my first attachment - room.png.

Secondarily, the ceiling should slope down towards the front of the room, as shown in attachment number two - room_side.png.

My QUESTION is then: How about sloping the front wall too? As shown in attachment number three - room_side_v2.png.

All of these non-parallel walls are of course there to avoid various artifacts in the sound, which I'll abstain from trying to explain, as I'll probably get some details wrong, which will then derail replies in here..

Again, thanks!

B.r. Jon
The non parallel wall avoid the flutter echo. The flutter echo can be treated by absorption.

The use of non parallel or absorption depends of the level of absorption you need, the sound esthetic (dead or live room) and of course the possibility to do.
Old 7th January 2020
  #8
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avare's Avatar
 

Quote:
Originally Posted by dinococcus View Post
The non parallel wall avoid the flutter echo. The flutter echo can be treated by absorption.

The use of non parallel or absorption depends of the level of absorption you need, the sound esthetic (dead or live room) and of course the possibility to do.
Why are you ignoring difraction and diffusion?
Old 7th January 2020
  #9
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Quote:
Originally Posted by avare View Post
Why are you ignoring difraction and diffusion?
Here It is not trendy and i have a bad experience with the diffusion so I'm still struggling to overcome the trauma.
Old 8th January 2020
  #10
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Quote:
It should definitely be symmetrical down the length of the room to ensure that the stereo imaging is consistent. Something like what's shown in my first attachment -
Ahh! OK. That wasn't clear from your initial post: I thought you were talking about a design where no two walls are parallel, and angled at random. What you propose for the side walls is sort of similar to Newell's version of the NER design concept, but it wastes space and isn't necessary to angle the entire wall. The RFZ design concept is better in this sense, as it only angles some surfaces at the front of the room: the ones that actually need it, not the entire wall.

Quote:
Secondarily, the ceiling should slope down towards the front of the room, as shown in attachment number two
Same applies as above: You CAN do that, but it wastes space and reduces room volume. Room volume for small rooms is critical: you want as much air volume as you can possible get inside the room, and angling the walls or ceiling reduces that. Therefore it is better to NOT angle the walls and ceiling, and only angle some surfaces inside the room to reflect sound where you want it to go. So instead of angling your ceiling, just hang hard-backed clouds. It's also a lot easier to adjust the angle of a cloud, that it is to adjust the angle of the entire ceiling, if it turns out that you got it wrong, and there's an unexpected artifact! Tilting the loud an extra degree or two is dead easy. Tilting the entire ceiling a degree or two extra is a bit harder...

Quote:
My QUESTION is then: How about sloping the front wall too? As shown in attachment number three -
Ummmm that would have your speakers aimed towards the ceiling! (assuming you are going to flush mount them, as in the majority of studios today).

As I mentioned, Wes Lachot does tilt the front wall of many of his studios, but he tilts it the OTHER way... and even then, it's not the actual wall that he tilts, but rather just the speaker flush mount baffles and other surfaces. The room wall behind those is still vertical. I think you'd find it hard to get a structural engineer or building inspector to approve a load-bearing wall that is tilted way off from vertical...

I also mentioned that in some of my studios I have done the same as Wes: tilting some surfaces at the front. But it has to be done very carefully, since doing that creates ADDITIONAL artifacts that were not there before, so you have to design the entire room such that those artifacts are not going to be a problem. For example, if you tilt the wall, then you have to raise the speaker up higher and tilt that own too, but raising the speaker up too high creates psycho-acoustic issues that can lead to things like a narrowed sound-stage, or inaccurate perception of frequency response, or reflections form the desk and console, or comb filtering, or a real notch in the mid-range frequency response (in addition to the perceived shift in high-mid and high frequency response). etc. I have found that tilting the front wall by more than about 5° makes these issues unacceptable (Andre put me onto this issue many years ago, and suggested 7° as the limit, IIRC, and he was dead right: even at 5° the issues are a big deal). In practice, I never tilt walls more than 4.5°. I wont go into the reasons WHY I do that sometimes... I only want to point out that it does not solve any acoustic problems, and can create new ones that would not have been there for a vertical wall.


Quote:
All of these non-parallel walls are of course there to avoid various artifacts in the sound, which I'll abstain from trying to explain, as I'll probably get some details wrong, which will then derail replies in here..
What artifacts do you think you'd be avoiding? In reality, angling and tilting room walls does not eliminate artifacts: it just moves them. For example, it does not change the modal response, it does not get rid of any modes... at best, it might move some modes to a slightly different frequency, but that's about it. Contrary to popular belief, angling walls does not eliminate modes, and even if it did that would be a BAD thing, not a good thing! The entire problem with small rooms is a LACK of modes! It's not that are too many, but rather that there are not enough! Eliminating some of them (if it were possible) would make the modal distribution problem even worse, not better. In fact, the only way to eliminate room modes, is with a bulldozer: knock down some of the walls! But that' a rather drastic solution...

As Dinnocus mentioned, the ONLY artifact that you can eliminate by angling or tilting walls, is flutter echo. But you'd have to have rather large angle, greater than about 10° or so, in order to achieve that. And you can treat flutter echo much more easily (and ore cheaply) with absorption or diffusion on the walls at key places.


Summary:
- For a control room, the best shape for the actual boundary walls, floor and ceiling, is rectangular.
- There MIGHT be a need for some angled surfaces within that shell, depending on the design concept that is being followed.
- Then treatment is done as needed to control the artifacts as much as possible.
- Non-rectangular rooms do not solve or eliminate any acoustic artifacts (apart from flutter echo).
- It is far easier to predict what most of those artifacts will be, for a rectangular room.
- Predicting for a non-rectangular room is a LOT more complex. If you can predict it, then you can design treatment for it in advance, then test the room "before" and "after" each piee of treatment goes in, to confirm that it is working as intended, and did not create any new, unexpected artifacts of its own.


- Stuart -
Old 8th January 2020
  #11
Gear Head
 

Again, I'm blown away by the level of commitment in your reply..

Quote:
Originally Posted by Soundman2020 View Post
Ahh! OK. That wasn't clear from your initial post: I thought you were talking about a design where no two walls are parallel, and angled at random. What you propose for the side walls is sort of similar to Newell's version of the NER design concept, but it wastes space and isn't necessary to angle the entire wall. The RFZ design concept is better in this sense, as it only angles some surfaces at the front of the room: the ones that actually need it, not the entire wall.
No no, my OCD couldn't cope with an asymmetrical room like that!

Quote:
Originally Posted by Soundman2020 View Post
Same applies as above: You CAN do that, but it wastes space and reduces room volume. Room volume for small rooms is critical: you want as much air volume as you can possible get inside the room, and angling the walls or ceiling reduces that. Therefore it is better to NOT angle the walls and ceiling, and only angle some surfaces inside the room to reflect sound where you want it to go. So instead of angling your ceiling, just hang hard-backed clouds. It's also a lot easier to adjust the angle of a cloud, that it is to adjust the angle of the entire ceiling, if it turns out that you got it wrong, and there's an unexpected artifact! Tilting the loud an extra degree or two is dead easy. Tilting the entire ceiling a degree or two extra is a bit harder...
You have a very good point about not reducing the volume of the room by putting up permanent angled walls/ceiling, but rather use stuff like clouds and the likes of this to also increase flexibility - you're absolutely right about that. I didn't consider the importance of these aspects!

Quote:
Originally Posted by Soundman2020 View Post
Ummmm that would have your speakers aimed towards the ceiling! (assuming you are going to flush mount them, as in the majority of studios today).
Just some more clarification here:
1) I'm not building a studio, but rather a relaxation/listening room for me and my record collection!
2) I have considered to go for on/in-wall speakers to avoid the 1st front wall reflections, but I've decided to stick to free-standing speakers for future flexibility with regards to positioning and swapping out equipment.

Quote:
Originally Posted by Soundman2020 View Post
As I mentioned, Wes Lachot does tilt the front wall of many of his studios, but he tilts it the OTHER way... and even then, it's not the actual wall that he tilts, but rather just the speaker flush mount baffles and other surfaces. The room wall behind those is still vertical. I think you'd find it hard to get a structural engineer or building inspector to approve a load-bearing wall that is tilted way off from vertical...
I get your point, but I would initially have had the room built rectangularly and then put up drywalls to obtain the tilted angles. (Notice the use of past tense at this point..)

Quote:
Originally Posted by Soundman2020 View Post
I also mentioned that in some of my studios I have done the same as Wes: tilting some surfaces at the front. But it has to be done very carefully, since doing that creates ADDITIONAL artifacts that were not there before, so you have to design the entire room such that those artifacts are not going to be a problem. For example, if you tilt the wall, then you have to raise the speaker up higher and tilt that own too, but raising the speaker up too high creates psycho-acoustic issues that can lead to things like a narrowed sound-stage, or inaccurate perception of frequency response, or reflections form the desk and console, or comb filtering, or a real notch in the mid-range frequency response (in addition to the perceived shift in high-mid and high frequency response). etc. I have found that tilting the front wall by more than about 5° makes these issues unacceptable (Andre put me onto this issue many years ago, and suggested 7° as the limit, IIRC, and he was dead right: even at 5° the issues are a big deal). In practice, I never tilt walls more than 4.5°. I wont go into the reasons WHY I do that sometimes... I only want to point out that it does not solve any acoustic problems, and can create new ones that would not have been there for a vertical wall.

What artifacts do you think you'd be avoiding? In reality, angling and tilting room walls does not eliminate artifacts: it just moves them. For example, it does not change the modal response, it does not get rid of any modes... at best, it might move some modes to a slightly different frequency, but that's about it. Contrary to popular belief, angling walls does not eliminate modes, and even if it did that would be a BAD thing, not a good thing! The entire problem with small rooms is a LACK of modes! It's not that are too many, but rather that there are not enough! Eliminating some of them (if it were possible) would make the modal distribution problem even worse, not better. In fact, the only way to eliminate room modes, is with a bulldozer: knock down some of the walls! But that' a rather drastic solution...

As Dinnocus mentioned, the ONLY artifact that you can eliminate by angling or tilting walls, is flutter echo. But you'd have to have rather large angle, greater than about 10° or so, in order to achieve that. And you can treat flutter echo much more easily (and ore cheaply) with absorption or diffusion on the walls at key places.
I probably had the impression that the angled walls/ceiling would address more issues than just the flutter echo. If that's all to obtain from angling surfaces (besides from the room looking cool), then I'll for sure go for something more conservative. The actual scenario is that I'll build an extension to my house, so I'm more or less free to pick dimensions as I wish, perhaps within something like 3.5 meters high, 5 meters wide and 8 meters long. What would you go for in this case? - Should I just stick to regular golden ratio dimensions or do you have another suggestion?

Quote:
Originally Posted by Soundman2020 View Post
Summary:
- For a control room, the best shape for the actual boundary walls, floor and ceiling, is rectangular.
- There MIGHT be a need for some angled surfaces within that shell, depending on the design concept that is being followed.
- Then treatment is done as needed to control the artifacts as much as possible.
- Non-rectangular rooms do not solve or eliminate any acoustic artifacts (apart from flutter echo).
- It is far easier to predict what most of those artifacts will be, for a rectangular room.
- Predicting for a non-rectangular room is a LOT more complex. If you can predict it, then you can design treatment for it in advance, then test the room "before" and "after" each piee of treatment goes in, to confirm that it is working as intended, and did not create any new, unexpected artifacts of its own.

- Stuart -
PS: My girlfriend says "thank you" for discouraging me here..
Old 12th January 2020
  #12
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Quote:
Originally Posted by jonlanghoff View Post
Again, I'm blown away by the level of commitment in your reply..
Thanks! I try to be helpful... sort of "paying it forward"...


Quote:
Just some more clarification here:
1) I'm not building a studio, but rather a relaxation/listening room for me and my record collection!
Studios and listening rooms both fall into the category "critical listening rooms". The specs are the same, assuming that you are talking about a high quality listening room, (apt for the perfectionist...). Designing the acoustics or both of them is very similar, even though the layout is usually different.

Quote:
2) I have considered to go for on/in-wall speakers to avoid the 1st front wall reflections, but I've decided to stick to free-standing speakers for future flexibility with regards to positioning and swapping out equipment.
That's fine, but flush mounting speakers is not just about front wall reflections! (more commonly known as "SBIR"). That's just one of many problems that flush mounting will fix. There are several others: smoother frequency response, flatter power response, no corner diffraction, no edge diffraction, better stereo imaging, better bass extension, less low end distortion, etc. I often tell my studio clients that flush-mounting their speakers is probably the best singe thing they can do to greatly improve the overall acosutic response.

That said, depending on your speakers, it might not be feasible. Audiophile speakers often have strange shapes and forms, some of them have drivers poking out the sides, back, bottom, and top, in addition to the front, and that makes it either impossible or extremely hard to flush-mount. If you let me knw what speakers you will be using, I can give you a better idea of whether or not they are candidates for flush-mounting.

Quote:
I probably had the impression that the angled walls/ceiling would address more issues than just the flutter echo.
Actually, it's a rather common thing! Many people think that angling walls is the solution to fix a whol bunch of things, but in reality it isn't. Unfortunately, all over the internet you see comments about this, saying that you HAVE to angle your walls to get good acoustics... but with no actual reasons, logic, or acoustic knowledge to back it up. Everybody just "knows" that you have to do that... Sigh!

Quote:
If that's all to obtain from angling surfaces (besides from the room looking cool), then I'll for sure go for something more conservative.
Smart move!

Quote:
The actual scenario is that I'll build an extension to my house, so I'm more or less free to pick dimensions as I wish, perhaps within something like 3.5 meters high, 5 meters wide and 8 meters long.
If you are free to go with any (reasonable" dimensions and shape, then you are one luck guy! That's fantastic! A lot of home studio builders would get green with envy from having that freedom, and a whole bunch of audiophiles, too!

There's a document that lays out the ideal acoustics for critical listening rooms, that goes by the cryptic title "ITU BS-1116.3" You can download it here: usfeul studio building documents About half way down that list, I think. Take a look at chapters 7 and 8. You can ignore the others, since they are not relevant to your room, but those two are. (the document actually lays out the conditions for performing research on audio: it's just coincidental that they also happen to provide the single best description of what a "critical listening room" should be like! That is widely regarded as the best set of technical specifications for such rooms, and is used in the design and construction of rooms all around the world.) One of the things defined there, is the size that such a room should be: 20m2 floor area is the minimum, 60m2 the maxim (for sterero: slightly bigger for surround). Also the room volume: the paper does not state it specifically, but indirectly it defines the optimum as around 100 m3. Your room would fit in VERY well with those specs, with 40m2 floor area and about 140 m3 volume. That can be a really, really good listening room. Or studio control room!

Quote:
What would you go for in this case? - Should I just stick to regular golden ratio dimensions or do you have another suggestion?
Ahh, yes.. room ratios.. Another of my favorite subjects to rant about!

In fact, you cannot use the Golden Ratio for acoustics here, because there is no such thing! Once again, contrary to popular belief, there is not one single ratio that is best. Rather, there are a small bunch of good ones, and an even larger bunch of bad ones. None of them is "perfect" o "golden", for the very simple reason that such a concept is actually impossible in a small room. Let me explain:

Modes. Rooms have them. A "mode" is just a singled, fixed way that the air inside the room can resonate, if it happens to be "triggered" by a certain frequency. Each mode depends on just one thing: the dimensions of the room. Whenever a sound wave can fit in exactly between two walls, because the wavelength of the wave matches that distance, then you have a "mode". There are three different types of mode: If the mode forms between two walls at opposite ends of the room, then it is an "axial" mode (because it forms on one of the three major axes of the room: front-back, left-right, or up-down). If the mode happens to use four walls to form, then it is a tangential mode, and if it needs all six walls to form, then it is an oblique mode. Any of those modes will start ringing out if you happen to play a note that coincides exactly. For example, if you have an axial mode at 35 Hz between your side walls, and you happen to be paying music where the bass guitar hits a C#, then that mode will be triggered, since they coincide. For that to happen, your room would need to be 4.5m wide, since 4.5m is the distance (wavelength) that corresponds to 35 Hz. The problem here is that, when the bass stops playing that note, the room can carry on playing it! Because modes are resonant. The mode "stores" energy at that frequency, the releases it again over time at that same frequency. So the note can play on for a while, after the speaker stopped creating it. Clearly, that's not a good thing! You want to hear the REAL music playing, not the room playing!

There's no need to go into the theory of that, as long as you understand that for each set of dimensions, there will also be a set of "modes". It is possible to predict what those modes will be, mathematically, for any given set of dimensions.

So, we already figured out that if a mode is triggered, that's not good. Thus, part of the treatment of your room will be aimed at preventing that from happening.

However, modes also do something else, which isn't quite so intuitive to understand: the lowest few modes of the room define the entire acoustic "shape" of the room! They define what the overall frequency response will be. ANd even if you damp them so the don't ring too much, they still have an effect on that frequency response curve for your room.

So, you must be thinking "So we need to get rid if the modes then!". Actually, that isn't possible (except by using a bulldozer, to remove the walls of the room... ), and even if it were possible, it would still be the exact wrong approach. Because the problem in small rooms is not that there are too many modes, but rather that there aren't enough! You need more modes, it less.

Think of it this way: each mode provides one single "peak" in the frequency response of the room. If you had a room where there was only one mode, then the frequency response for the entire spectrum would be flat, except at that one point, where there is a mountain that messes it up. If you add a second mode, then you now have two peaks on your flat line. And as you add more modes, you get more peaks. But if you keep on going, adding more and more, then you soon get to the point where adjacent peaks start to blend into each other, and the curve smooths out a bit at that location where two peaks are close... but there are still valleys at other places... And as you add yet more, the curve starts to smooth out more and more! If you could add an infinite number of modes all over, then the curve would be totally smooth and flat again... just higher than it was before.

With that simple mental picture, you can see that the problem is a lack of modes. For the upper end of the spectrum there are thousands of modes, and the curve is smooth, but at the low end you can see the individual mountain peaks, and that's the problem.

But what if you could somehow bring a few of those mountains closer together, so that the peaks blend into each other? Not completely, of course, but just slide one mountain closer to another, thus smoothing out the valley in between.

It turns out that you CAN do that, because each mode depends one one thing: room dimensions! So if you can adjust the room dimensions, then you can move some of those mountains around, and smooth out the valleys.

That's what "room mode analysis" is all about, and "room ratios". It's finding the best set of dimensions to arrange your mountains so that the peaks are not to far apart, not too close together, and the valleys between them are smoothed out.

That is the goal of finding the best "room ratio" for your studio: to get the smoothest low-frequency response, which in turn implies the smoothest overall response, since the low end modal issues also govern the rest of the spectrum.

Now for the next problem: there are not enough mountain peaks to go around! As you move one down the scale a bit, to get it close to it's neighbor down there, you are also moving it AWAY form the next higher one! And you only have maybe a dozen or so to pay with, maximum, so there's no way you can use just a dozen mountains to fill in the entire landscape! So you just do the best you can, with the few mountains that you have, and get them as smooth as possible.

As you can imagine depending on the overall size of the room, there might be a lot of ways of arranging the modes, or only few. For very large rooms, there is no problem at all! There are more than enough mountains: Thus, large rooms do not have modal issues. But small rooms do. The smaller they are, the fewer modes they have, and the messier the frequency response is. Thus, for any given size range of room, there are only a few possible arrangements that are are good. And what is "good" for one size of room, night be bad dor a different size room.

Now you can see why acousticians laugh at Cardas and his "trapagons" (which have no meaning nor definition in mathematics anyway): there simply is no "one size fits all" arrangement of speakers in a room, or walls, or anything else.

In reality, many acousticians and scientists have done extensive research on this problem, and come up with sets of ratios that are good for different shaped rooms. As you start looking into this more, you'll come across names like Sepmeyer, and Louden and Boner. They all found ratios that are particularity good, mathematically and musically, so there are things like Sepmeyer's third ration, and Lounden's firfth ratio. Then another guy came along, called Bolt, and he figure out that if you draw a graph of all possible ratios, with "room length" on one axis and "room width" on the other axis, then all of these good ratios happen to fall within one area of that graph. Obviously, that is not refereed to as the "Bolt area", and it looks like the graph below...

Basically, if you check your room ratio against that graph, and yours falls inside the Bolt Area, then most likely it is a good ratio: It is not the "best" ratio for your room, because the is no such thing! But it's a good one.

Now, if you want to go into even more detail, then you can use a "Room Mode Calculator" to figure out the best dimensions for your room. Here are the two best ones, that I use all the time when designing studios, home theaters, and other critical listening rooms:

http://www.bobgolds.com/Mode/RoomModes.htm
https://amcoustics.com/tools/amroc


Those give you a LOT more than just the ratio. They also provide a wealth of other valuable information about your room, that is very useful when designing it.

So, to answer your question after that long sidebar: No, don't use the Golden Ratio (because there isn't one), and don't use an fictions "trapagon" ratio either (because that also doesn't exist).

Be careful of the unicorns, leprechauns, cyclops, and other mythical creatures out there in Internet land! Even though they don't exist, they can still hurt you!

Quote:
PS: My girlfriend says "thank you" for discouraging me here..
She is very welcome! Lets see if we can encourage you onto a better path, that she might approve of even more...


- Stuart -
Attached Thumbnails
Cardas' "Golden Trapagon"-bolt-area-graph-beranek.jpg  
Old 14th January 2020
  #13
Gear Head
 

Thumbs up



Thanks a lot for the layman's terms descriptions of room modes - they helped me be to visualise the concepts very well!

Quote:
Originally Posted by Soundman2020 View Post
That's fine, but flush mounting speakers is not just about front wall reflections! (more commonly known as "SBIR"). That's just one of many problems that flush mounting will fix. There are several others: smoother frequency response, flatter power response, no corner diffraction, no edge diffraction, better stereo imaging, better bass extension, less low end distortion, etc. I often tell my studio clients that flush-mounting their speakers is probably the best singe thing they can do to greatly improve the overall acosutic response.

That said, depending on your speakers, it might not be feasible. Audiophile speakers often have strange shapes and forms, some of them have drivers poking out the sides, back, bottom, and top, in addition to the front, and that makes it either impossible or extremely hard to flush-mount. If you let me knw what speakers you will be using, I can give you a better idea of whether or not they are candidates for flush-mounting.
I thought that flush mounting was a studio/mixing thing, rather that a listening room thing, but if so I might of course consider it! I currently use the Audiovector Si1 Avantgarde compact speakers, so it shouldn't be impossible to do, I think..

https://www.hifiengine.com/manual_li...ctor/si1.shtml

I'm also considering to get a pair of electrostatic speakers at some point, such as a pair of Quads, perhaps even stacked - in which case flush mounting will probably not be feasible..

Quote:
Originally Posted by Soundman2020 View Post
Basically, if you check your room ratio against that graph, and yours falls inside the Bolt Area, then most likely it is a good ratio: It is not the "best" ratio for your room, because the is no such thing! But it's a good one.

Now, if you want to go into even more detail, then you can use a "Room Mode Calculator" to figure out the best dimensions for your room. Here are the two best ones, that I use all the time when designing studios, home theaters, and other critical listening rooms:

http://www.bobgolds.com/Mode/RoomModes.htm
https://amcoustics.com/tools/amroc
Other than Bolt, I've now also stumpled accross Bonello, who suggest that the amount of room modes should increase with each third octave. This seems very obtainable within the dimensions I mentioned earlier (800 x 500 x 350) when I mess around with the second calculator you linked. Bonello seems like the most manageable way to select a specific set of dimensions when one is pretty much free to choose. Would you agree?

Considering what you've written then 680 x 500 x 300 cm seems to work pretty well, for example:
https://amcoustics.com/tools/amroc?l...&h=300&r60=0.6

It falls within the Bolt area, it has a volume of 102 m3 and has a Bonello "sequence" of 1-1-1-1-5-7-10-25-39.

Any suggestions for tweaking these dimensions further? Perhaps considering other stuff than Bolt, Bonello and volume..?

B.r. Jon
Old 14th January 2020
  #14
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Quote:
Originally Posted by jonlanghoff View Post
... The actual scenario is that I'll build an extension to my house, so I'm more or less free to pick dimensions as I wish, perhaps within something like 3.5 meters high, 5 meters wide and 8 meters long. ...
A convenient way to compare different ”famous” ratios is this chart from Andrey Smirnov. (LINK)

E.g If you adjust your dimensions to fit Trevor Cox black areas and you are still compliant with all the others.

Best
Old 13th February 2020
  #15
Gear Head
 

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Hi, I've done some adjusting and drawing now!!

So, if going with these room dimensions:
Length: 640cm --- Width: 450cm --- Height: 300cm

- then both Bolt, Bonello and Cox would be pleased, and it would look something like what's shown in the attached file.

Now for the acoustic treatment. I'm thinking something along the lines of a LEDE room with a dead front wall and a live/diffusing rear wall.

Front wall:
Floor to ceiling rockwool-ish insulation (sorry Dennis Foley) of e.g. 50cm thickness.

Rear wall:
The same absorption implementation as on the front wall PLUS five embedded N23+-2 QRDs, with a range of 500-6880 Hz, according to QRDude.
Is it sensible to go for a design frequency of 500?
Should I go for a lower prime with the trade-off of getting a lower HF cutoff?

Side walls:
Some sort of absorption for first reflections - broadband? - Or perhaps something tuned to the room modes..? The frequencies 26.80, 38.11, 46.59, 53.59 and 57.17 Hz are the first modes.
Free-standing versus wall-mounted? (This will be a listening room, not a living room, so I'm free to choose..)

Ceiling:
I'm thinking of a custom-made large suspended cloud absorber, as thick as possible, covering the area between speakers and listening position.
Anything to be aware of?

Floor:
I'm playing around with the thought of making a "pit" in the floor, fill it up with absorbant material and covering it with some sort of grid to make it traversable. Of course placed between speakers and LP, as with the cloud absorber.
Haven't been able to find examples of this done before. Is there a reason for this?

Listening position / speaker placement:
I'm not entirely sure what to go for here - I usually go from "the rule of thirds", but since the minimum listening distance to the rear wall diffusers are 2 meters (with this suggested design), then I might push forward the listening position a little bit as shown in the drawing.
Would this be the correct approach?
Or should I rather get diffusers that demands a lower minimum listening distance, with the tradeoff of a higher design frequency?

Thanks, all!
Jon
Attached Thumbnails
Cardas' "Golden Trapagon"-floorplan.png  
Old 13th February 2020
  #16
Gear Head
 

And other comments are very welcome too, of course!
Old 13th February 2020
  #17
Gear Head
 

Amroc and Cox screenshots attached.

NB: The yellow dot is on-top of a black spot in the Cox diagram.
Attached Thumbnails
Cardas' "Golden Trapagon"-cox.jpg   Cardas' "Golden Trapagon"-amroc.jpg  
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