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PostPosted: Sun Nov 11, 2018 3:51 pm 
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Quote:
I did a bit more digging and apparently, panel absorbers are used behind some sections instead of the porous absorption to treat modal issues
I figured something like that would have to be the case. There's no way you can treat modal issues with a bunch of thin wood planks poking out from the walls, and a bit of insulation behind it. It seems there's a LOT of stuff that the "paper" isn't saying.... Hmmmm... :roll:

Quote:
But I am surprised by how few rooms have been built with the design.
I'm not surprised! :) When folks plan to spend a lot of money building their studio, they usually do the research, like you are doing, and come to the same conclusions you are arriving at: this is not a viable method for treating a control room.

Quote:
... and some more detailed pics of the construction process
But curiously, it has no actual acoustic measurements of the final outcome.... I wonder why? :)

If you look around the forum, most members here are not scared to show their results... They do REW tests, and happily post the graphs that show how their rooms REALLY work. As a studio designer, I'm also not scared to show the results I get from rooms I have designed (either completely or partly). You can see that on many threads here on the forum (Steve's thread, Studio Three Productions, and many others). When you have a design concept that works, and has been proven to work, then why would you NOT want to show the results? It makes you wonder why there's no published final results on either the Mk1 or Mk2 versions....

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Providing I can design my room to work for both my live/tracking purposes and mixing (mixing's still secondary)
It can be done, yes! But don't forget that your room size sets the limit for what you can achieve: the dimensions put limits on room response. There's no way you can make a broom closet sound like a concert hall! :) OK, so that''s two extremes, but you get the point. The bigger a room is, the better it can sound, and the less treatment it will need to get there.

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I built the design on a trial of AFMG Reflex and it does seem to have good diffusion and scattering. I used this online calculator to generate the slat depth .
Well, yes, that "calculator" does indeed give you a set of wood plank sizes... but there is no indication at all of how it PERFORMS!! In other words, no details of what the diffusion and scattering coefficients are, nor the lobing, nor the power spectrum, nor the spatial, timing, or phase changes, nor what the cut-off frequencies or cut-off angles are... nor anything at all, in fact! In fat, all that calculator does is to generate a random sequence of numbers that you can use to make different plank sizes, but with no regard to acoustic results whatsoever. On the other hand, if you use a generator for a Schroeder diffuser, or a Skyline diffuser, based on QRD or PRD principles, then you DO get all of that information, so you do know how it will work (QRDude, for example, does that, even though it is very simple).

Also, something I should have mentioned before but it slipped my mind: if you leave air gaps between the wood planks, then it is NOT a diffuser! Sorry: it just isn't. It will scatter, yes, but it will NOT diffuse. No way. Anybody who claims the contrary does not understand how diffusion works. The wells MUST have a solid bottom, and be sealed air-tight to function at all. In order for diffusion to work, the incoming wave does two things: first, it bounces off the bottom of the well and gets sent back out at a different angle and with a different timing and phase (think about it.. it HAS to do that), and second it sets up resonance in the well if the incoming wave happens to be in the correct frequency range, because it is a sealed well. If there is no bottom to the well, just empty air, then NEITHER of those can happen! Hence, it is not a diffuser. It is just a "scatterer". It would only work as a diffuser for the case where the planks are stuck tightly to each other, sealed, with no air gaps. In that case, it is something like a Schroeder diffuser (1D), but it still would not work like a true Schroeder diffuser, because it does not follow a mathematically sound numeric sequence that produces smooth diffusion and flat power spectrum! It produces RANDOM diffusion in that case, which is NOT desirable, because it is not even, smooth, balanced, or "diffuse". They can call it whatever they like, but it simply is not a diffuser if the wells have no bottom. Wood planks separated by air spaces are not diffusers. Plain and simple.

If you want to understand this yourself, then get the book "Acoustic Absorbers and Diffusers: Theory, Design and Application" by Cox and D'Antonio. It is THE definitive book on the subject. Highly recommended. These two guys basically created the entire industry when they defined the theory of what diffusion is, building on the original work of Manfred Schroeder. The math can get a little heavy in places, but the descriptions and examples are crystal clear.

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It would be very beneficial if the Author did test it in a lab. I'm not sure of the cost of those though!
I'm not sure that they want to! :) Getting it tested might reveal that the emperor has no clothes... :)

Quote:
Looking at what's available in my area I'm probably going to use this: Superglass Superwall 36. It is a glass wool and has a density of around 20kg/m3, giving approx 9000 Rayls for GFR,
Looks good to me! That should work just fine, and would be good for low frequencies in general

Quote:
9000 Rayls with a 150mm depth and 15mm air gap has pretty good good absorption
Yep. Although such a thin air gap doesn't really make much difference. You could get slightly better performance by re-arranging that, as 50mm up front, 15mm air gap, then 100mm up against the wall. Not a huge change, but worthwhile. Same materials in a better relationship.


Quote:
Hmm... I hadn't thought about the Haas time here. I was under the impression that a diffuse reflection was sufficiently reduced in intensity to negate this.
Are you saying that even if a reflection is reduced by 20db or even 30db below the direct sound it wouldn't be sufficient if within 20ms?
That's correct, yes. That is what I'm saying: a "diffuse" reflection by itself guarantees nothing. Firstly, it isn't really diffuse (see below) and secondly any specular reflection that arrives within that window (which is nominally 20ms, but can be longer... it varies with frequency, and from person to person.. probably more like 30 ms) will cause psycho-acoustic effects, such as messing up your ability to accurately determine directionality, and messing up your perception of frequency response. (Those two are actually opposite sides of the same coin: they go together, because of the way your ear and brain work).

The entire point of the 20db/20ms thing is that it is the ITDG (or ISDG if you prefer) that lasts for 20ms. That's pat of the RFZ spec: According to that, during the first 20 ms there must be NO reflections at all, at any level, then AFTER that delay the diffuse sound field is supposed to arrive, at least 20 dB down. Which is why it is impossible to implement a true RFZ design in a small room: the rear wall has to be at least 10 feet behind the mix position, implying that the room has to be at least 14 feet long, but that applies to the surface of the acoustic treatment on the rear wall, and the location of the soffits on the front wall, so for realistic speakers, soffits, and rear wall treatment, the room needs to be at least 20 feet long (6m).

Plus, there's another issue: Notice the the "diffuse field" is supposed to arrive after that 20ms delay... but in a small room, there is no diffuse field! A room has to be very large before there can be a true statistical diffuse reverberant field. In acoustics, "diffuse reverberant field" means a sound field where, for any location in the room, there is an equal probability of receiving sounds coming from all directions at the same level. So the chance of that -20dB signal hitting your ears should be the same for sounds arriving from the rear of the room, or the front, or the floor, or the ceiling, or the side wall, or any other direction. But with a typical small control room, that is not possible: the sound will reach your ears from a specific direction first, that you can easily identify: the initial reflections after the ITDG will come from behind you, after bouncing off the rear wall, and BEFORE they can hit the front wall, side wall, ceiling, floor, etc. It takes a long time to establish a true diffuse reverberant field, since sound needs to bounce around through many, many reflections, all over the place, before the field can be even and smooth in all directions.

In other words, it is impossible to have a true diffuse reverberant field in a small room, and therefore it is impossible to have a true RFZ in a small room. It has to be large. Very large. In a small room, you only get a true diffuse reverberant field about an octave above the Schroeder frequency for the room.

In other words, even though your room is a decent size, it still will not have a true reverberant field in it. Very few control rooms do, so don't feel sad about that! Your situation is better than most... :)

So, the best you an do is to design the room so that first sounds arriving from the rear wall are as diffuse as possible, which implies NO specular reflections at all, and true diffusion (not just scattering) that is as smooth, even, and balanced as possible. That's why the rear of your room needs to be as absorptive as you can make it, especially for low frequencies since they are never going to be diffuse, no matter what you do, and also you have the issue of modes, and SBIR related to the rear wall.

This is why I would not put such large areas of solid, hard, reflective surfaces at the rear of the room.

Quote:
Can you explain a bit more about how it would affect SBIR?
SBIR from the rear wall is the biggest single problem in most small rooms. Period. That first phase-cancellation null is going to be at a very low frequency, and it defines the acoustic "signature" of the entire spectrum, since this is a comb-filter issue, and it starts very, very low. A hard, solid, flat surface on the rear wall will create a huge SBIR problem. There are two ways to attack that: 1) Deep absorption. Very deep (hangers plus other stuff has a decent effect), and: 2) large broken/angled geometric surfaces, to help "break up" the wave a bit. There's nothing else you can do: you cannot diffuse low frequency sound, because the diffuser that you'd need would be larger than the entire room, and would have huge wells, many feet deep, and many feet wide. You can't use a resonator to absorb it because SBIR is not a resonant issue in the first place. You can't diffract it, because any device large enough to do that would be bigger than the room. You can only absorb it to a certain extent, and do some low-level "scattering" by putting very large, solid, heavy objects in the way, with large geometric shapes (for example: a "poly-slat" device covers the entire rear wall, from corner to corner and even then is only about one quarter wavelength of the lowest issues...).

Quote:
If this diffuser allows low bass frequencies to pass through it into the 200mm of insulation behind it, and also has a variable helmholtz effect. This surely would be as effective, if not more effective, against low frequencies than just 200mm of insulation alone?
Helmholtz resonators have zero effect on SBIR. It is not a resonant problem, so there is nothing to "resonate". SBIR is purely phase cancellation, so resonant devices have no effect on it. It does not respond to panel traps, membrane traps, slot walls, perf panel, or any other form of resonant device, since they are all based on there being some type of "ringing" or resonance. With SBIR, there is none.

Also, 200mm is nothing at all for SBIR. Typical frequencies are way down in the 20, 30, 40, 50 Hz range, where wavelengths are many METERS long. For example, if you have an SBIR issue at 45 Hz, the wavelength is 7.6m. You need a thickness of at least 7% of the wavelength to get a useful effect for normally-incident sound, so 532mm would be just enough to have some effect. If you look at Steve's room, there's 510mm of depth to his rear acoustic treatment. In the case I mentioned, the front face of the rear wall treatment (what you see) is 989mm from the rear corner. Even the poly-slat walls are 265mm deep at the thickest part of the curve. Rear wall treatment has to be DEEP. VERY deep.

So no, a few thin vertical planks with 200mm of insulation behind is not going to do much for rear-wall SBIR.

Quote:
I meant because the entire rear wall was treated with the PRN air transparent diffusers then there wasn't any room left for variable-acoustic devices,
... and that's what I was talking about! My point exactly.... by using those ineffective devices across the entire rear wall, you leave no place for REAL acoustic treatment, and certainly none for the variable devices, that you need..

Quote:
and the front wall has the flush mount baffles,
True, but that doesn't mean that you can't make them variable in some way... :) I can think of at least three ways you could do that, simply... :lol: 8)

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so only the side walls were left for these devices.
And the ceiling? You are completely neglecting the single most important surface in the room for tracking most instruments. Control room ceilings need to be mostly absorptive, but musical instruments general sound better when the ceiling is diffusely reflective... or even partly specular...

Quote:
The original concept from 11 years ago got quite a bit more of a look at by the community.
It got a lot of interest, due to its quirkiness, yes, but once again, how many rooms were actually built like that in the end? And where are the acoustic test results that show that it works as advertised? 7 years is a long time for people to test this out, and build a whole bunch of rooms... it if really works... It is one thing to produce nice computer-generated projections of how the things work, but it's another thing entirely to actually test one in real-life. Speaking from experience, I can tell you that most rooms don't actually perform the way that the mathematical models predict. There are just too many variables to be able to model them completely beforehand: it's far easier and faster and cheaper to just do basic prediction, build the room, then analyze it ad treat it for the way it ACTUALLY turned out, rather than the way it should have turned out. So I have little confidence that the MR rooms that were built actually did end up as predicted. Not to mention the HUGE expense of building one....

Quote:
It seems over at Gearslutz they have a lot of people who want to use LEDE and Newell designs, no one really does much else these days.
I'm not a big fan of the Newell concept of non-environment rooms, and certainly not of true LEDE rooms either! From that point of view, I agree with John Brandt: It is a huge waste of space, for practically no benefit, to build a gigantic non-symmetrical outer shell, then a symmetrical inner shell. I very much doubt that the guys at GS are actually building true LEDE and NER rooms. What they build mostly is much closer to RFZ, in reality. And what some of then refer to as LEDE rooms are NOT true LEDE rooms either! Most contemporary designs abandoned that approach a couple of decades ago. A true Don and Chip Davis LEDE room is unpleasant to work in, and fatiguing. It sounds unnatural. There is nothing like it in nature, nor in real life experience. The human brain is not accosted to it. Yes, it works as advertised, and yes, you can produce good mixes in one, but it's not a nice place to work, since your brain has no points of reference. The entire front-end of the room is totally dead, and the entire rear end is totally reflective: the only place in nature that sounds vaguely close to that would be if you stand just outside the mouth of a very shallow cave, just a couple of meters deep, and listen to music coming from in front of you.... not a common experience! :lol:

On the other hand, if you look around at the most successful rooms built over the past couple of decades, RFZ shines through as being the best.

Quote:
Also most of the guys who know their stuff over there are professional designers/sell products so they tend to have their own ideas and not want to use someone else's, or they keep their designs close to their chest.
On the other hand, nobody here on this forum wants to sell you anything! John strictly forbids that. No acoustic products can be promoted or offered here by their manufacturers or reps. The only time you might see a positive mention of a product here is by people who have actually bought and used that product in their own rooms, but have no desire or intention to sell it to you. No advertising is allowed here: nobody is out to sell you anything. And neither do we hide the "secrets" of how we design and build rooms: it's all out in the open, freely shown, freely discussed. And is also proven to work, very well, with a strong, clear track record, and demonstrated, measured, published results. No secrets.

So who do you trust more? The guy who wont tell you how to do it, and wants to sell you something? Or the guys who are happy to show you how to do it yourself, for free, and walk you through it, for free, and will only charge you something if you decide that you don't want to spend the time learning and designing yourself, preferring to hire someone else to do it for you... Which makes more sense? Which do you think is more trustworthy? :)

Quote:
I am interested in testing the effect of the diffuser design in my room.
Honestly, from my point of view, your room is too small to use numeric based diffusion when you are mixing. Probably OK for tracking, but not for mixing. So anything you do on their with diffusion, you need to be able to cover-up or disable in some way for mixing.

Quote:
I'm thinking of building 1 or 2 smaller modules using his method taking measurements.
Not trying to be obnoxious, but how on Earth would you test your devices??? :roll: :?: :!: :shock: Do you happen to have an anechoic chamber, and half a million dollars worth of test gear at your disposal? If not, then you can't test them. You would have no idea if the devices you built would even be working at all, nor if they are producing a flat power spectrum, or lobing, or frequency, phase, or timing distortion. You might think that you can just listen to the devices and see how they "sound" to your ears, but you can't. The issues are not usually audible (unless the device is terribly bad!), but the psycho-acoustic effects are very real, and would cause you to not produce good mixes, and have strange "sounds" in your instruments when you track, that you can't really identify, but just notice that things sound "off". It would be especially noticeable from large instruments, that radiate sound from their entire body, such as guitars, violins, drums, brass, woodwinds, etc. Since each part of the radiated sound would hit a different part of the diffuser, it would come back at you with a different power level, different phase, different frequency, and different timing, but not diffusely: your room is not large enough to allow the field from a diffuser to fully integrate and merge into an even, smooth field. So you would not be able to detect if your devices are working, or not working, yet, you'd notice that some of your recordings just sound a bit "odd", and be unable to figure out why they sound like that...

You can't test diffusers yourself. You need equipment, training, and an anechoic chamber.

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It's obviously not laboratory accurate but it would be interesting to test the concept,
Yes but no! :) See above. You are right in that it would not be a laboratory test, and therefore not accurate, so I don't see the reason to bother!

Quote:
When I get to my measurements stage I'd love to have your input Stuart, and maybe guide me in the best way to test this device (assuming you're interested in the results yourself and have some free time).
I'd LOVE to do that, absolutely! But I doubt that you can create the conditions that we would need to even be able to test them. Testing absorbers is pretty simple. Testing Helmholtz resonators is easy too. So is testing membrane traps, and panel traps, and perf-panel traps, and slot walls... Finding reflections in a room is also fairly easy, and so is figuring out modal response, and SBIR.... but diffusion is an entirely different subject. You need motorized mics that move across a curved frame around the diffuser while they make hundreds of measurements, or many dozens of mics set up in a curve around the device, and you need a place where you can do that without the room itself interfering. The you need the equipment, and software, and expertise to analyses all of that, and graph it, and make sense of it...

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I've been looking over some of John's posts from the past and rethinking my rear wall,
:thu:

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Lower rear corners, deep slat helmholtz tuned to 40-100hz. Then measure the effect, if it's good, place the same device in the top rear corners too.
First, if you tune a device to "40-100Hz" then most likely it is not a Helmholtz resonator! That's a HUGE range of frequencies: more than one octave! Helmholtz resonators are high-Q devices, covering just a few hertz of bandwidth. You can detune them yes, and cover a broader range, but the then Q is really low, and the device is ineffective. Modal issues are also high Q, so they need high Q devices to deal with them, of you plan to attack them with resonance. You would need an array of several devices, each tuned to one modal issue... but then you also need a large surface area and large volume for each frequency... (rule of thumb says that you need about 1% of room volume to successfully treat a modal issue. Let's say you have 20 issues that need dealing with... are you willing to give up 20% of your entire room volume to do that?)

Second, by putting the devices in the corners, you are taking up the prime locations where porous absorption is most effective....

I would not go with this plan.

You said you had looked at many of John's rooms: how many of them did you see with huge diffusers in the upper and lower rear corners? :)

There's a reason why nobody builds rooms like that... :)

Quote:
Middle (vertically) rear corners superchunk at listening height, for reflections.
That's upside-down and back-to-front! Super chunks reach maximum efficiency and performance in the top and bottom rear corners of the room. You could leave out the middle section of a superchunk and not greatly damage the performance. But leaving out the top and bottom is like leaving out the entire thing!

Quote:
Middle of rear wall, 200mm thick absorption panel.
In front of the absorption a full height polycylindrical absorber that is cut down the middle and swings on hinges.
This part I do like. I would make it 500mm and use hangers, but I do like the concept of the split poly.

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When open the rear absorption panel is exposed and the "inside" of the poly "doors" exposes more absorption.
Closing the poly covers the absorption to add more liveliness to the room.
:thu: The concept is good.

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I've thought about how I will physically contruct this. Here's my idea:
Look for "flexible plywood". It's a special type of plywood, manufactured so that it bends very easily in one direction without cracking. Often used by carpenters to make curved furniture. IT can take a pretty tight curve, very easily:

Attachment:
flexible plywood4.jpg


Attachment:
Flexible Plywood-DR.jpg


- Stuart -


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PostPosted: Sun Nov 11, 2018 11:51 pm 
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Thanks for your extremely detailed and thorough response Stuart.

Soundman2020 wrote:
But curiously, it has no actual acoustic measurements of the final outcome.... I wonder why?

If you look around the forum, most members here are not scared to show their results... They do REW tests, and happily post the graphs that show how their rooms REALLY work. As a studio designer, I'm also not scared to show the results I get from rooms I have designed (either completely or partly). You can see that on many threads here on the forum (Steve's thread, Studio Three Productions, and many others). When you have a design concept that works, and has been proven to work, then why would you NOT want to show the results? It makes you wonder why there's no published final results on either the Mk1 or Mk2 versions....


It does make you think...

Soundman2020 wrote:
Well, yes, that "calculator" does indeed give you a set of wood plank sizes... but there is no indication at all of how it PERFORMS!! In other words, no details of what the diffusion and scattering coefficients are, nor the lobing, nor the power spectrum, nor the spatial, timing, or phase changes, nor what the cut-off frequencies or cut-off angles are... nor anything at all, in fact! In fat, all that calculator does is to generate a random sequence of numbers that you can use to make different plank sizes, but with no regard to acoustic results whatsoever. On the other hand, if you use a generator for a Schroeder diffuser, or a Skyline diffuser, based on QRD or PRD principles, then you DO get all of that information, so you do know how it will work (QRDude, for example, does that, even though it is very simple).


Sorry, I actually used two programs: the AMFG reflex program does give details of how it should perform including simulated lobing:
AMFG Reflex, the calculator I linked was just to generate the sequence to put into reflex.

Soundman2020 wrote:
Also, something I should have mentioned before but it slipped my mind: if you leave air gaps between the wood planks, then it is NOT a diffuser! Sorry: it just isn't. It will scatter, yes, but it will NOT diffuse. No way. Anybody who claims the contrary does not understand how diffusion works. The wells MUST have a solid bottom, and be sealed air-tight to function at all. In order for diffusion to work, the incoming wave does two things: first, it bounces off the bottom of the well and gets sent back out at a different angle and with a different timing and phase (think about it.. it HAS to do that), and second it sets up resonance in the well if the incoming wave happens to be in the correct frequency range, because it is a sealed well. If there is no bottom to the well, just empty air, then NEITHER of those can happen! Hence, it is not a diffuser. It is just a "scatterer". It would only work as a diffuser for the case where the planks are stuck tightly to each other, sealed, with no air gaps. In that case, it is something like a Schroeder diffuser (1D), but it still would not work like a true Schroeder diffuser, because it does not follow a mathematically sound numeric sequence that produces smooth diffusion and flat power spectrum! It produces RANDOM diffusion in that case, which is NOT desirable, because it is not even, smooth, balanced, or "diffuse". They can call it whatever they like, but it simply is not a diffuser if the wells have no bottom. Wood planks separated by air spaces are not diffusers. Plain and simple.

If you want to understand this yourself, then get the book "Acoustic Absorbers and Diffusers: Theory, Design and Application" by Cox and D'Antonio. It is THE definitive book on the subject. Highly recommended. These two guys basically created the entire industry when they defined the theory of what diffusion is, building on the original work of Manfred Schroeder. The math can get a little heavy in places, but the descriptions and examples are crystal clear.


This IS interesting. I am familiar a bit with the theory of diffusers. I hadn't thought about that though. The sound wave passes into the deeper well, bouncing of each adjacent well until it hits the bottom and continues back up and out at a new angle, with the time shifted slightly depending on the depth of the well and it's neighbours.
Looking at the air transparent diffuser design, theoretically a sound wave passes down a well, hits the bottom and as there are 5mm air gaps either side of each well, some high frequencies will pass through these tiny gaps and be absorbed behind.
Shouldn't frequencies that have a wavelength greater than 5mm simply not "see" the gaps at all, and reflect back up the well as if it wasn't there?
I think I will download reflex again on another PC for a new trial to test the design.

BTW Stuart I editted my post without realising you already commented. I had actually added a link to show that a picture of the Mrk 1 design was used in the third edition of "Acoustic Absorbers and Diffusers: Theory, Design and Application" under their "Hybrid Diffusers" section. (Pic in the book). I would be interested to know what the book says about the design (It's quite expensive just for curiosity at this point ha!), if anyone owns the third edition and can check page 72. :lol:

Soundman2020 wrote:
Looks good to me! That should work just fine, and would be good for low frequencies in general


Soundman2020 wrote:
Yep. Although such a thin air gap doesn't really make much difference. You could get slightly better performance by re-arranging that, as 50mm up front, 15mm air gap, then 100mm up against the wall. Not a huge change, but worthwhile. Same materials in a better relationship.


Good idea!

Soundman2020 wrote:
That's correct, yes. That is what I'm saying: a "diffuse" reflection by itself guarantees nothing. Firstly, it isn't really diffuse (see below) and secondly any specular reflection that arrives within that window (which is nominally 20ms, but can be longer... it varies with frequency, and from person to person.. probably more like 30 ms) will cause psycho-acoustic effects, such as messing up your ability to accurately determine directionality, and messing up your perception of frequency response. (Those two are actually opposite sides of the same coin: they go together, because of the way your ear and brain work).

The entire point of the 20db/20ms thing is that it is the ITDG (or ISDG if you prefer) that lasts for 20ms. That's pat of the RFZ spec: According to that, during the first 20 ms there must be NO reflections at all, at any level, then AFTER that delay the diffuse sound field is supposed to arrive, at least 20 dB down. Which is why it is impossible to implement a true RFZ design in a small room: the rear wall has to be at least 10 feet behind the mix position, implying that the room has to be at least 14 feet long, but that applies to the surface of the acoustic treatment on the rear wall, and the location of the soffits on the front wall, so for realistic speakers, soffits, and rear wall treatment, the room needs to be at least 20 feet long (6m).

Plus, there's another issue: Notice the the "diffuse field" is supposed to arrive after that 20ms delay... but in a small room, there is no diffuse field! A room has to be very large before there can be a true statistical diffuse reverberant field. In acoustics, "diffuse reverberant field" means a sound field where, for any location in the room, there is an equal probability of receiving sounds coming from all directions at the same level. So the chance of that -20dB signal hitting your ears should be the same for sounds arriving from the rear of the room, or the front, or the floor, or the ceiling, or the side wall, or any other direction. But with a typical small control room, that is not possible: the sound will reach your ears from a specific direction first, that you can easily identify: the initial reflections after the ITDG will come from behind you, after bouncing off the rear wall, and BEFORE they can hit the front wall, side wall, ceiling, floor, etc. It takes a long time to establish a true diffuse reverberant field, since sound needs to bounce around through many, many reflections, all over the place, before the field can be even and smooth in all directions.

In other words, it is impossible to have a true diffuse reverberant field in a small room, and therefore it is impossible to have a true RFZ in a small room. It has to be large. Very large. In a small room, you only get a true diffuse reverberant field about an octave above the Schroeder frequency for the room.

In other words, even though your room is a decent size, it still will not have a true reverberant field in it. Very few control rooms do, so don't feel sad about that! Your situation is better than most...

So, the best you an do is to design the room so that first sounds arriving from the rear wall are as diffuse as possible, which implies NO specular reflections at all, and true diffusion (not just scattering) that is as smooth, even, and balanced as possible. That's why the rear of your room needs to be as absorptive as you can make it, especially for low frequencies since they are never going to be diffuse, no matter what you do, and also you have the issue of modes, and SBIR related to the rear wall.

This is why I would not put such large areas of solid, hard, reflective surfaces at the rear of the room.


That's a good amount of info stuffed into a few paragraphs, thanks Stuart!

Soundman2020 wrote:
SBIR from the rear wall is the biggest single problem in most small rooms. Period. That first phase-cancellation null is going to be at a very low frequency, and it defines the acoustic "signature" of the entire spectrum, since this is a comb-filter issue, and it starts very, very low. A hard, solid, flat surface on the rear wall will create a huge SBIR problem. There are two ways to attack that: 1) Deep absorption. Very deep (hangers plus other stuff has a decent effect), and: 2) large broken/angled geometric surfaces, to help "break up" the wave a bit. There's nothing else you can do: you cannot diffuse low frequency sound, because the diffuser that you'd need would be larger than the entire room, and would have huge wells, many feet deep, and many feet wide. You can't use a resonator to absorb it because SBIR is not a resonant issue in the first place. You can't diffract it, because any device large enough to do that would be bigger than the room. You can only absorb it to a certain extent, and do some low-level "scattering" by putting very large, solid, heavy objects in the way, with large geometric shapes (for example, a "poly-slat" device covers the entire rear wall, from corner to corner and even then is only about one quarter wavelength of the lowest issues...).


That is also some good info there! Can you link me to the "Poly Slat" device you are talking about? I saw on what I thought was a device that seems to match your description, but by saying "covers the entire rear wall corner to corner" you threw me off :lol: Isn't that a corner control room, so has no rear wall? Ha! I'll look into how I can break up these sound waves.

Soundman2020 wrote:
True, but that doesn't mean that you can't make them variable in some way... I can think of at least three ways you could do that, simply...


Ha! Indeed, but the front wall is already reflective so I don't have a need to adjust it to make it more live :mrgreen:
My main focus of my variable acoustics is to bring more liveliness back into the room, and add some scattering in the process for a nicer tracking environment.

Soundman2020 wrote:
And the ceiling? You are completely neglecting the single most important surface in the room for tracking most instruments. Control room ceilings need to be mostly absorptive, but musical instruments general sound better when the ceiling is diffusely reflective... or even partly specular...


I am very interested with the ceilin, but am trying to wrap my head around a method of changing these, without needing step a ladder each time. My current idea involves wide louvre, almost like blinds on a conservatory ceiling, but fins a lot wider. I'll work on a sketchup model.

Soundman2020 wrote:
On the other hand, nobody here on this forum wants to sell you anything! John strictly forbids that. No acoustic products can be promoted or offered here by their manufacturers or reps. The only time you might see a positive mention of a product here is by people who have actually bought and used that product in their own rooms, but have no desire or intention to sell it to you. No advertising is allowed here: nobody is out to sell you anything. And neither do we hide the "secrets" of how we design and build rooms: it's all out in the open, freely shown, freely discussed. And is also proven to work, very well, with a strong, clear track record, and demonstrated, measured, published results. No secrets.

So who do you trust more? The guy who wont tell you how to do it, and wants to sell you something? Or the guys who are happy to show you how to do it yourself, for free, and walk you through it, for free, and will only charge you something if you decide that you don't want to spend the time learning and designing yourself, preferring to hire someone else to do it for you... Which makes more sense? Which do you think is more trustworthy?


I agree with you there!

Soundman2020 wrote:
You can't test diffusers yourself. You need equipment, training, and an anechoic chamber.


I had a feeling that would be the response :lol:

Soundman2020 wrote:
I'd LOVE to do that, absolutely! But I doubt that you can create the conditions that we would need to even be able to test them. Testing absorbers is pretty simple. Testing Helmholtz resonators is easy too. So is testing membrane traps, and panel traps, and perf-panel traps, and slot walls... Finding reflections in a room is also fairly easy, and so is figuring out modal response, and SBIR.... but diffusion is an entirely different subject. You need motorized mics that move across a curved frame around the diffuser while they make hundreds of measurements, or many dozens of mics set up in a curve around the device, and you need a place where you can do that without the room itself interfering. The you need the equipment, and software, and expertise to analyses all of that, and graph it, and make sense of it...


Ahh if only...

Soundman2020 wrote:
First, if you tune a device to "40-100Hz" then most likely it is not a Helmholtz resonator! That's a HUGE range of frequencies: more than one octave! Helmholtz resonators are high-Q devices, covering just a few hertz of bandwidth. You can detune them yes, and cover a broader range, but the then Q is really low, and the device is ineffective. Modal issues are also high Q, so they need high Q devices to deal with them, of you plan to attack them with resonance. You would need an array of several devices, each tuned to one modal issue... but then you also need a large surface area and large volume for each frequency... (rule of thumb says that you need about 1% of room volume to successfully treat a modal issue. Let's say you have 20 issues that need dealing with... are you willing to give up 20% of your entire room volume to do that?)

Second, by putting the devices in the corners, you are taking up the prime locations where porous absorption is most effective....

I would not go with this plan.

You said you had looked at many of John's rooms: how many of them did you see with huge diffusers in the upper and lower rear corners?

There's a reason why nobody builds rooms like that...


I was basing the helmholtz on a design John mentions he uses in a post from 2015 here, top post on the page.
John mentions the principle:
Quote:
It's a vertical slot resonator where the depth is great, the slots are deep 6" - 8" (150 - 200mm) and the gap between slats is 1/4" - 1/2" (5mm and 10mm)
It is tuned to 30 - 100hz range.

In this control room it solved the typical bass buildup problem at the couch.


There looks to be 3 slat depths here. So if some are 200mm, some 175mm, and some 150mm they should affect different frequencies, and by varying the gaps and including some insulation inside, there should be a reduction over a wider range of frequencies. And also adds a little non-numeric diffusion.

The reason I chose the corners is because to target the low frequencies requires a lot of depth that I don't really have elsewhere in the room (Plus this is where John said he places them in the post. I left the listening height position out though, because the reflections from the helmholtz would be directed back to the mix position.

Soundman2020 wrote:
This part I do like. I would make it 500mm and use hangers, but I do like the concept of the split poly.


Cool! I'll work on a new design then.

Soundman2020 wrote:
Look for "flexible plywood". It's a special type of plywood, manufactured so that it bends very easily in one direction without cracking. Often used by carpenters to make curved furniture. IT can take a pretty tight curve, very easily:


I was going to look for this type yes. Any particular thickness you would use for this purpose, specifically? My design probably won't have much of a panel resonator effect as I need to add the bracing to the back of the poly to hold it in the correct curve, due to it being split, so it's mainly there as a reflective surface. When it's open though I need the frequencies that won't be sufficiently absorbed by the 100 - 150mm of porous absorption within the poly itself, to pass through it and therefore use the air gap behind it, bounce off the wall and come back through again.
Have you got a link to a calculator/formula that will give me the transparency of my plywood at certain frequencies?

Thanks again,
Dan


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PostPosted: Wed Nov 14, 2018 8:58 am 
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Ok so update on my rear wall design. It's looking pretty high tech ha!

I've greatly increased the depth of the rear wall treatment. I've added hangers and extended the absorption right up to the corner treatments.
In the corners I've removed the Helmholtz resonators from the top corners and added hangers to the top two thirds of the rear corners,
leaving the tuned devices on the bottom.
I've added a space coupler type device jut behind the middle split poly to break up some sound waves and direct the sound towards the absorption.
I've added polys that swing on hinges from against the wall, to cover the rear corner absorption.

These 3 polys should have noticeable affect on decay time and limit mid-high frequency absorption when closed.

Attachment:
rear wall new.png


Attachment:
Rear wall new design closed polys.png


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PostPosted: Sat Nov 17, 2018 5:24 am 
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Quote:
the AMFG reflex program does give details of how it should perform including simulated lobing:
I may be wrong, but I thought that that program assumes a hard back for all numeric diffusers. I didn't know it was possible to specify that the "wells" are open to the air on both ends.... how do you do that?

Quote:
Looking at the air transparent diffuser design, theoretically a sound wave passes down a well, hits the bottom
That's the thing: There is no bottom! What "bottom" did it hit? There's only air on the other side.

Quote:
Shouldn't frequencies that have a wavelength greater than 5mm simply not "see" the gaps at all
The well is a resonant cavity: If you leave ANY gap at all, then it is not sealed, and therefore not resonant. Here's an experiment you can try: Get a coke bottle or beer bottle, and blow across the top, so you make it act like a Helmholtz resonator, producing a tone. Now drill a large hole in the bottom of the bottle, at least 5mm radius (be careful! Drilling glass is tricky, as it can shatter easily...). Now blow across the top again, and see if you can get it to resonate... good luck with that!

Also, if you check Cox and D'Antonio's book on diffusers, you'll notice that in many places they mention how several reports of QRD's that did not work were traced to tiny gaps in the walls of floor of the wells... just small cracks. The highlight in at least three of four places that I recall, how it is that poor construction of Schroeder diffusers and skylines, with unsealed walls or wells, wrecks the properties of the diffuser, forcing it to not diffuse well and to act as an absorber instead.

Quote:
there are 5mm air gaps either side of each well,
In other words, 5mm spacing between the "planks". Right. Once again I refer you to Cox and D'Antonio, who repeatedly recommend that the wells of Schroeder diffusers should not be narrower than about 5 cm (not mm! CENTIMETERS), because narrow wells have a major problem with the boundary effect: the thin layer of air directly adjacent to the walls of the well acts differently from the rest of the air, since it is the "boundary" layer, that is sort of "stuck" to the walls. It does not participate in the diffusion process but DOES act as an absorber, so Schroeder diffusers with narrow wells (narrower than about 5cm) start to exhibit high absorption rates, and less diffusion. The narrower the well, the worse the problem gets. 5mm wells would show this effect very much: most of the air in the well would be in boundary effect, and thus the devices would be more of an absorber than a diffuser... assuming that it even had bottoms in the wells! But even without the well bottoms, the boundary layer is still there, so I would assume that it absorbs more than it scatters.

Quote:
some high frequencies will pass through these tiny gaps and be absorbed behind.
Have you ever tried listening at an old-fashioned key-hole, in a door? It's probably about 5mm wide. Try it. I think you'll find that you can hear a hell of a lot more than just the highs! :) You seem to be confusing holes with solids: While it is correct that a solid object will only affect sound waves that are smaller than the wavelength, it is NOT true that a hole in an object will only allow sound to get through if it is smaller than the wavelength. If that were true, then the bass reflex ports on your speakers, only a few cm wide, would not work: no bass would ever come out of them... :) And the 8" woofers in your studio monitors would not be able to produce any frequency below about 2 kHz... because after all, a speaker is basically just a hole in a box....

Quote:
Shouldn't frequencies that have a wavelength greater than 5mm simply not "see" the gaps at all, and reflect back up the well as if it wasn't there?
If your assumption were correct, and only frequencies with wavelengths less than 5mm could get through a 5mm gap, then only tones above about 80,000 Hz would get through... In real life, everything gets through.

So no, any frequency can get trough small holes. Some frequencies better than others, true, they still get through. Try listening to the gap under your door, which is probably only 4 or 5 mm high, and see if you can hear only frequencies above 80 kHz,... 8) Or open the window in your car by just 5mm, and see if you can hear the diesel truck running next to you, or not...

Quote:
I think I will download reflex again on another PC for a new trial to test the design.
Don't forget to tell it that the well has no bottom! You MUST indicate that the wells are open to air at BOTH ends.... If you don't do that, then you are modelling a Schroeder diffuser, not one of the strange devices in that paper...

More comments on the rest of your post later...


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PostPosted: Sat Nov 17, 2018 9:04 am 
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... continued later...

Quote:
I would be interested to know what the book says about the design (It's quite expensive just for curiosity at this point ha!)
They do mention it, yes, but they refer mostly to the dispersion properties, not so much the diffusion properties. They do not deal with in in the section on Schroeder or QRD based diffusers, but rather in the section on hybrids. They call it a "slatted hybrid device". They say: "When the device is just a regularly spaced set of slats, then at best a coefficient of 0.2 is achieved.". They also mention that the slat depths follow a QRD sequence that gives even dispersion, which is not the case for the Mk2 version: It uses a random sequence. The photo that you posted shows a graph that compares several different versions, with different sequences, and as you can see, none of them provides any real diffusion below about 2 kHz. So at best they provide some dispersion for high frequencies, but none at all for mids and lows. The Mk1 version graph is similar to (d) in that photo. The coefficient only rises above 0.5 for frequencies over roughly 5 kHz, so basically just the upper end of the highs.

Thus, if you need a device that disperses well for frequencies over 5 kHz, then this is an option.

Quote:
if anyone owns the third edition and can check page 72.
I do have the third edition, and I'm not sure what you want me to look at on page 72. It's all about stage enclosures in large performance halls, and more specifically shell surfaces. It speaks mainly of reflecting sound back to the musicians, so they can hear themselves play. What specifically did you want me to look at there? Maybe you menat page 58, which shows Gerge Massengberg's control room at Blackbird studios? That's VERY different from both the Mk1 and Mk2 styles. That is, indeed, true diffusion, but taken to the extreme that the room becomes "ambechoic". Not anechoic, but totally reflection-free from all directions, while still retaining ambience. I'm not sure how that would be related to the MR concept, apart from it also using long wooden thingies poking out the wall... But arranged as a true diffuser.

Quote:
That is also some good info there! Can you link me to the "Poly Slat" device you are talking about?
Sure: viewtopic.php?f=2&t=21368&start=111

Quote:
I saw on what I thought was a device that seems to match your description, but by saying "covers the entire rear wall corner to corner" you threw me off
It does cover the entire rear wall, from corner to corner! :)

Quote:
Isn't that a corner control room, so has no rear wall?
Right. It is a corner control room, so from that point of view I suppose you could say that it has no rear wall... Rather, it has two "rear-side" walls that meet in the middle, exactly behind your head. So if you are sitting at the mix position facing forward, then it is everything behind you, basically. From that perspective, it is the "rear wall", and the glass doors off to your left and right are the "side walls".... Sort of!

Quote:
but the front wall is already reflective so I don't have a need to adjust it to make it more live
Who said you had to make it more live? :) Maybe making it more diffuse would be useful.... and no, it will not be your entire front wall that is reflective: that would be a mistake: just as unpleasant as the original LEDE rooms, but in reverse... Rather, you will have absorptive areas above and below your speaker soffits, which could also benefit from variable treatment...

Quote:
My main focus of my variable acoustics is to bring more liveliness back into the room, and add some scattering in the process for a nicer tracking environment.
... And therefore large, flat, solid, rigid, smooth, reflective surfaces would probably not be what you want... such as soffit baffles, for example... :)

Quote:
I am very interested with the ceilin, but am trying to wrap my head around a method of changing these, without needing step a ladder each time.
I once designed some "flip-over" ceiling modules for a small live room that needed to be variable. Absorptive on one side, diffusive/reflective on the other. You could reach up with one hand and flip them over in that room, since it had a fairly low ceiling, but for a high ceiling it would have been simple to do with something like a broomstick, or a mic stand....

Quote:
There looks to be 3 slat depths here. So if some are 200mm, some 175mm, and some 150mm they should affect different frequencies, and by varying the gaps and including some insulation inside, there should be a reduction over a wider range of frequencies. And also adds a little non-numeric diffusion.
Not necessarily true! With a slot wall (or perf panel: same principle), if the open area is less than about 5%, then each slot acts more or less on its own, so you can have a wall that is tuned to several peaks with reasonably high Q, but if the open area is more than about 10%, then the entire wall tends to act more like a single device, tuned to the average frequency, with lower Q. There's no hard transition point between the two, at say 7.238%, but rather it's more like a gradual move away from individual tuning towards single overall tuning. 1% coverage is almost totally individual, and 15% is almost totally single tuning. Above about 20%, there's no tuning at all: it's just a bunch of reflective slats with absorptive gaps between them.

Quote:
"It's a vertical slot resonator where the depth is great, the slots are deep 6" - 8" (150 - 200mm) and the gap between slats is 1/4" - 1/2" (5mm and 10mm) It is tuned to 30 - 100hz range. In this control room it solved the typical bass buildup problem at the couch."
Exactly. Assuming that is square section timber (it looks like it), then the slat width is 150-200 mm, and the slot width is 5-10mm, so the pen area is oin the range 5/200 to 10/150 = 2.5% to 6%. So yes, they are tuned, and will have individual resonant peaks. Depending on how John designed that, the resonant peaks very likely overlap considerably, so the device really will work in the 30-100Hz range. In fact, that's very similar to the device I designed for that corner control room: viewtopic.php?f=2&t=21368&start=27 .

But those devices (John's and mine) are nothing at all like the Mk2 air thingies. They are both just normal Helmholtz resonators, with tuned cavities behind tuned slots...
Quote:
I was going to look for this type yes. Any particular thickness you would use for this purpose, specifically?
6mm (1/4") usually works well.

Quote:
My design probably won't have much of a panel resonator effect
It's not supposed to! A poly-cylindrical device is supposed to be mostly diffusive.

:)

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PostPosted: Sun Jan 06, 2019 10:30 am 
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If anyone is interested I've designed a PC case that I'm looking into building to keep my PC quiet in the studio.

Let me know your thoughts!
viewtopic.php?f=16&t=21828

Dan


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PostPosted: Sat Jan 19, 2019 8:05 am 
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I'm getting ready to have my mini split AC installed as soon as wall boards and ceiling modules are in place, and just wanted to get peoples opinions on placement for the inner unit.

My original design was to place it high in the center of the long wall opposite the door (I can't have it above the door as the lintel is in the way of the pipes).
This seemed like a good idea as the unit would have less distance to "throw" the conditioned air in front of the unit.

My newer idea though is to place the unit on the front wall high up between the two speaker baffles. The benefit of this is that it's less than 1m from the ventilation inlet, so it can heat/condition the air as soon as it enters the room. Hopefully reducing a cold draft.

My AC guy said either would be OK, but having the unit near the inlet may make the unit have to work harder and the heating less apparent as it has to heat cold air straight from outside, and not air that has had a little time to warm up.

So I'm not really sure.

The downside of having it on the front wall is that the "cone" of conditioned air may be narrowed by it being set back a little compared to the face of the speaker baffles. And I'm planning on having the unit installed before building my speaker baffles (they are of course already designed) so can't easily place the unit flush with the speaker baffle surface.

Does anyone have any opinions/reasoning on this?

Dan


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PostPosted: Sat Jan 19, 2019 9:17 am 
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For that type of layout, I normally put the mini-split evaporator at the top middle of the rear wall, with the fresh air supply register a little above the intake. Then I have the stale air exhaust register at the front of the room, between / above the soffits. That pretty much guarantees decent mixing of incoming air, decent mixing throughout the room, and only removing the warm, moist, stale air at the front.

Your duct system should also be recirculating most of the air through the room, and replacing about 20% to 40% of it with fresh outside air (while of course also dumping the same amount of stale air back to the outside world). So the air coming in through the register above the mini-split will already have a mix of room air plus fresh air, and that's what will be going into the mini-split, mostly.

That's the way I do it, anyway...

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PostPosted: Sat Jan 19, 2019 11:53 am 
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Hi Stuart,

Thanks for the response. It's a simple extract to outside and inlet. This gives 6 room changes per hour. (I may replace this with an externally mounted HRV in future). The minisplit will be circulating and conditioning the air. Your placement is similar to my second idea but reversed, with mine having inlet at the front instead of rear of the room.
I think I'll have to add the correctly dimensioned mini split to my sketchup model to see how it interacts with my speaker baffles.
Dan


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PostPosted: Mon Jul 01, 2019 6:53 am 
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Hey guys, progressing steadily on my construction.

I'm looking at my treatments again to try and find a way to construct the rear wall efficiently and be able to fit more variable acoustic devices in, without being too expensive.

I think I'm settling on a big set of louvres. Shutting them will block off most of the absorption at ear height (standing and sitting), and should have a big effect on the decay rate. I think I'm going to scrap my "split-poly" design due to difficulty in building it.

For my acoustic hangers I've settled on an alternative to Homasote in the UK that should work fine. It's called sometimes called just "insulation board" and other times "softboard". It's a low denisty fibre board and is 5-6 times cheaper than Sundeala board (the thing most similar to Homasote over here). Here's a link to it Softboard it's quite readily available too. It has a density of around 376Kg/m3, so is not too dense 8). I'll have to see how easy it is to work with, and work out how well it can hold screws. I'll have to buy one to make a test hanger.

I'll get an update of my design from Sketchup posted on here as soon as I can.

Dan


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 Post subject: Re: Garden studio UK
PostPosted: Mon Jul 01, 2019 8:11 am 
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Quote:
I think I'm settling on a big set of louvres. Shutting them will block off most of the absorption at ear height (standing and sitting), and should have a big effect on the decay rate.
Louvers can work, but do be careful with that! When they are closed, you will have a flat reflective surface that reflects a lot of mostly highs and mids, but allows some lows through to be absorbed. That's fine. But when they are open, they are still reflective in the highs and mids! In fact, you now have TWICE the area of reflective surface (because both the front AND the back of each louver is now exposed to the room), and if they are all at the same angle and with the same spacing, you effectively have a diffraction grating. That does a lot of things, some of which you might not want. And since you have evenly spaced, evenly sized objects that are significantly large with respect to some frequencies, and arranged in a repeating pattern, you will have reflects some frequencies with lobing patterns, to a certain extent.

In general, try to avoid repeating patterns of things that are all the same size and spacing. They create "lobing". Try to go for random spacing, or random sizes, or both. Even better is to use a QRD or ORD or binary sequence, to minimize lobing and produce an even power spectrum, just like you would for a diffuser. I realize that it's not going to be easy to do that with a set of louvers, that have to close up to create a solid surface.... Which is one of the reasons why I don't use louvered devices. I've thought about it, but found that the implementation gets to be a bit problematic in practice.

Quote:
I think I'm going to scrap my "split-poly" design due to difficulty in building it.
Why would it be difficult to build? Here's a similar design I did for a client in Germany a few years ago, along with the finished device:
Attachment:
CR-Large-Rear-Wall-Polys-01-closed.jpg


Attachment:
CR-Large-Rear-Wall-Polys-01-open.jpg


Attachment:
CR-Large-Rear-Wall-Polys-03.jpg


Attachment:
CR-Large-Rear-Wall-Polys-04.jpg


Attachment:
CR-Large-Rear-Wall-Polys-05.jpg

It's not quite the same as what you are trying to do, but close enough. In this case, the polys cover storage space and an equipment rack, rather than bass traps, and only the top section opens (the bottom section is fixed in place). But the concept is the same. It worked very well to, In addition to providing some nice smooth broadband diffusion down to lowish frequencies, without changing the decay times too much, it also killed a huge SBIR dip I was getting from that wall, at about 70 Hz if I recall correctly.

Quote:
For my acoustic hangers I've settled on an alternative to Homasote in the UK that should work fine. It's called sometimes called just "insulation board" and other times "softboard". It's a low denisty fibre board and is 5-6 times cheaper than Sundeala board (the thing most similar to Homasote over here). Here's a link to it Softboard it's quite readily available too. It has a density of around 376Kg/m3, so is not too dense
IT looks like it should work fine!

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 Post subject: Re: Garden studio UK
PostPosted: Mon Jul 01, 2019 10:45 pm 
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Thanks for the reply Stuart.

With my split poly, it would be a bit difficult to maintain the correct poly face whilst cutting it down the center. I had a plan to do it, but I think it won't be as effective for variable acoustic effects as the louvres.

I'm trying to reduce the diffraction grating effect by placing some vertically and others horizontally.

I may use different width timber, if I can figure out how to rotate them with the same control rod.

I'm still going to be using polys but just fixed ones on the side walls.

Dan


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 Post subject: Re: Garden studio UK
PostPosted: Sun Jul 07, 2019 7:23 am 
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Hi all, I'm getting close to how I want my rear wall to be I think.

I've sorted out how I can use different width louvres and still have an easy opening/closing system: the swivel hinges will be offset, so the the louvres overhang the same distance to the rear wall. This way a rod/string attached to the back of the louvres will be able to adjust them all, I had to make the louvres not overlap each other to achieve this. This means there is a 90 degree motion on every louvre regardless of the width. Adjusting one louvre by hand will rotate them all.

I used a Qr diffuser sequence to alter the width with the fin width set as my timber thickness (18mm). This means that when the louvres are open there is less periodicity. I used N5 + N5 + N7 (too long so just chopped one out, so only 6 louvres) + N5. Not a real diffuser by any means, but better than all the same width.

I've also added some deep helmholtz to the bottom corners to aid with low frequencies that are problematic (will tune them when measured).

Here's the latest Sketchup images:
Attachment:
July 2019 Louvres Closed Scaled.png


Attachment:
July 2019 Open Louvres Scaled.png


Let me know what you think.
Dan


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