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It has been a whiles since my last post, and a lot has happened in the mean time.

 

I have had an acoustical consult and done a few THX courses as well in the mean time. Most of the time I have spend on the planing, budgeting and designing the acoustical treatment as it is going to be large scale.

 

The major plan is to build an acoustical front wall 500mm out from the existing front wall and turn this into a large bass trap. Hang up an AT screen and use the area behind the screen for acoustic treatment as well. (nearly all front wall first reflection points end up on screen). I am currently looking for different products to use as porous absorbers. Polymax is one candidate, simply because of the known flow resistance, but the price of it is putting me off. I am currently trying to find out the flow resistance data of the Greenstuff AAB products as these are much cheaper in price.

 

I have been doing a lot of reading/research into helmholz resonators and panel resonators, as the limit of the Polymax porous material isn’t going to solve all my nodal problems. But the potential shear size of these units and their effect on the rooms volume, and their effect on the effeciency (air gap) of the porous traps, has put things on hold. Theoretical, when using a low flow resistance of 3000 over 500mm I could theoretical effectively reach into the 35Hz region. But as I found out reading a bit more, these models/calculators don’t necessarily produce a predictable outcome.

 

In other words, I just been going around in circles, that every time I found an answer, something else pops up and put a dent into it. The consensus on Gearslutz seems to be (also by Ethan Winner) that in small rooms it is better to work with porous absorption. The biggest issue with online research about this subject seems to be, that everyone who seems to know a good deal about the subject, keeps his cards close to his chest. If this is due to, competition, the amount of time and energy invested needed to grasp the subject, or simply due to many variables effecting the outcome I don’t know.

 

Once the front wall trap has been made I will need to put in reflection panels to balance the room. I am looking at a MLS hole sequence like the RPG BAD pattern. Ideally I would need to come up with a custom sequence as I need custom sizes to fit around the screen. I have calculate the % of wholes for the RPG BAD (25%), Artnovion(21%) and Vicoutic (30%) patterns, as this will be useful to balance the absorbtion rastio of max 25% total surfarce area, between front and back wall.

 

Does anyone know how to create/program your own MLS sequence? As I believe there are some people on the forum who have done this and can help with the CNC production of the panels.

 

The second trap will be the ceiling trap with bulk heads along the length of the room. I am planing of using a MLS sequence I found generated on Gearslutz with 18mm thick mdf as reflection slats, to balance the reflection/absorbtion area to +/-40% wich comes down to less than 25% between (timber) floor and ceiling, which allows for the placement of a fabric sofa in the room. I have made the slats out of 5 different heights (11, 18, 30, 42, 66mm) mostly for ecstatic reasons.

 

Obviously everything is theoretical, and most likely is going to need adjustments in real life. I am still toying with the idea of building a riser as this could potential hide a subwoofer for clean looks and will rise the seating area in a better pressure zone. (0.2, 0.32, 0.45, 0.55, 0.68, 0.8 x room dimension)

 

As I have access to advanced 3D modeling software I have build and rigged the room in 3D so that it can visualize, room nodes areas based upon dimensions, best out of the box pressure area’s, early reflection points based upon listen position and speaker placement.

 

One thing that I need to test before starting this build is the influence of the acoustic wall on my speakers. With the wall in place, the speakers will be sitting 55cm from the acoustic wall, which doesn’t sound to pleasing to me if you consider the advise of min distance of 100cm on the Martin Logan forum. Having some BAD panels to play with would be nice.

 

I might have to simplify my setup as well in the end, as I might have to much equipment to access in a tight space.

 

 

HT_10.jpg

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Just look at density for the insulation. Its your best guide for fliw resistivity.

 

Ultratel is super ezy to work with and can be cheap.

 

Quest's Q-perf can be cut easily into whatever shape you need.

 

 

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On 15/05/2018 at 12:56 PM, Primare Knob said:

Does anyone know how to create/program your own MLS sequence?

for 1D you could just use the sequence on Gearslutz (I think Boggy generated it), or I'm sure lots of other options on the googlwebs

for 2D it was @hochopeper on here that was clever enough to generate it

 

Mike

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@Primare Knob, I feel for you mate.  Like me, you would prefer to do it once and do it right but there are few hard and fast rules with room treatment so it may pay to do some experimenting in order to get the best results for the budget.

 

Myself, I am interested in the VPR/CBA absorbers.  Apparently they can be made to be effective from 40Hz to as high as you like, depending on configuration and should not take up much more than 100mm off the wall.  I have some samples of a few types of Australian foam to use in VPR's (the foam is the trickiest design criteria with them) and I also have been lucky enough to find a consultant that has built and tested and installed them in the real world in Brisbane.  He even gave some frames to duplicate when it comes time for me to start building.  I plan to make some one day this year and play around with them in my room to see how useful they are for regulating the decay rate and treating room nodes, but first I have some other stuff to finish building. 

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1 hour ago, acg said:

@Primare Knob, I feel for you mate.  Like me, you would prefer to do it once and do it right but there are few hard and fast rules with room treatment so it may pay to do some experimenting in order to get the best results for the budget.

 

Myself, I am interested in the VPR/CBA absorbers.  Apparently they can be made to be effective from 40Hz to as high as you like, depending on configuration and should not take up much more than 100mm off the wall.  I have some samples of a few types of Australian foam to use in VPR's (the foam is the trickiest design criteria with them) and I also have been lucky enough to find a consultant that has built and tested and installed them in the real world in Brisbane.  He even gave some frames to duplicate when it comes time for me to start building.  I plan to make some one day this year and play around with them in my room to see how useful they are for regulating the decay rate and treating room nodes, but first I have some other stuff to finish building. 

I'd be very interesting in your progress with this. So far, reading about them on Gearslutz, no-one really has been able to make a proper one with good (and tested) results.  I thought that the minimum dimensions of these units need to be at least 1m by 1.5m.

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5 minutes ago, Primare Knob said:

I'd be very interesting in your progress with this. So far, reading about them on Gearslutz, no-one really has been able to make a proper one with good (and tested) results.  I thought that the minimum dimensions of these units need to be at least 1m by 1.5m.

There are a few that have done well with them on GS...but there are lots that don't "follow the recipe" or even bother trying to find the "recipe" for themselves.  Fuchs gives out the "recipe" in his book and the consultant that built them here in Qld followed it and used melamine foam, 1mm steel plate, some folded 1mm steel frames, a soft glue and had them lab tested.  They worked exactly according to the results in the book.  Exactly:  an absorption coefficient of 1.0 down to about 65Hz and 0.4 at 40Hz.  For a 2.5mm thick steel plate,  the book shows an absorption coefficient of about 1.0 at 40Hz:  you need some really thick porous to get to that sort of bass performance.

 

There are all kinds of variables, but basically 1.5m x 1m is the "standard" size.  I would build them bigger than that...floor to ceiling.  There is a mob in Melbourne that has some nice polyester fibre foam (they also have melamine) and I can get Basotect melamine foam in Brisbane.  Steel is cheap to get laser cut and folded for the frames.  I plan to put some kind of diffusion on the front of my VPR's. 

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2 hours ago, acg said:

There are a few that have done well with them on GS...but there are lots that don't "follow the recipe" or even bother trying to find the "recipe" for themselves.  Fuchs gives out the "recipe" in his book and the consultant that built them here in Qld followed it and used melamine foam, 1mm steel plate, some folded 1mm steel frames, a soft glue and had them lab tested.  They worked exactly according to the results in the book.  Exactly:  an absorption coefficient of 1.0 down to about 65Hz and 0.4 at 40Hz.  For a 2.5mm thick steel plate,  the book shows an absorption coefficient of about 1.0 at 40Hz:  you need some really thick porous to get to that sort of bass performance.

 

There are all kinds of variables, but basically 1.5m x 1m is the "standard" size.  I would build them bigger than that...floor to ceiling.  There is a mob in Melbourne that has some nice polyester fibre foam (they also have melamine) and I can get Basotect melamine foam in Brisbane.  Steel is cheap to get laser cut and folded for the frames.  I plan to put some kind of diffusion on the front of my VPR's. 

I should get my hands on some books. I got the general idea that on Gearslutz, not many are following the design principle of the VPR. My understanding so far is that you need a free "floating"/vibrating plate and a foam with a rigid backing that can work as a spring. The function of the frame to me seems more related to a way of hanging the unit, although it does seem to help extending the broadband effectiveness due to edge diffraction.

Which kind of idea are you playing with when it comes to diffusion. There seems to be some designs with a foam mounted on the front of the plate, but not sure if you mount a BAD type panel onto that, that it might start to act as a (second) leaf.

 

It will be nice if the two can be combined successfully within a 100mm unit depth.

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@Primare Knob, you basically have a decent understanding of the concept.  

 

The steel plate is slightly smaller than the foam and the foam is attached to the steel plate with a soft glue and it is also attached to a backing sheet with a glue (not necessarily soft though, it does not matter).  The foam acts as the spring and damper for the vibrations in the steel plate.  The steel plate  should not be square and generally needs to be 1m x 1.5m or larger.  Smaller plates will work but are not as effective because they have fewer eigenmodes (resonant frequencies - the more frequencies as which the plate vibrates the better).  The thickness of the steel plate can be 1mm - 3mm with thicker plates more effective lower in frequency.

 

The perimeter frames can be open or fully closed depending on how much edge diffraction you want to take advantage of for further absorption.  The foam is not connected to the perimeter frame and must be free to move.  The benefit of the frames though is that they can wrap around the front of the steel plate (not touching it of course) and you can attach a nice surface covering such as BAD panels or whatever you like really.  You can add foam in front of the steel plate to absorb higher frequencies if that is required.  Personally, I would use some sort of wood grain diffusion panel (such as a BAD) to give the room a really nice feel but there is almost no limit to what you could do.

 

I think these VPR's are so flexible.  They can attack low frequencies without using too much room and you can add stuff to them or use them in a slightly different way if you need to attack different frequencies.  I can't see a way that I am going to find time to start on mine until next summer, but am happy to help out in any way I can if you want to have a crack at them.

 

Cheers,

 

Anthony 

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21 hours ago, Primare Knob said:

I should get my hands on some books. I got the general idea that on Gearslutz, not many are following the design principle of the VPR.

Books will only confuse you further.     Build the VPR to the recipie.   Don't over think it too much ... but be prepared to build a few, as your materials may not be quite the same.   ie.  build a prototype first.

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3 minutes ago, davewantsmoore said:

Books will only confuse you further.     Build the VPR to the recipie.   Don't over think it too much ... but be prepared to build a few, as your materials may not be quite the same.   ie.  build a prototype first.

There is really only one book that is required...

 

https://www.springer.com/gp/book/9783642293665

 

...but that one is very expensive.  I purchased a download of the chapter on "Panel Absorbers" for about $20 from memory and it gives out the recipe in detail as well as the mathematics.  Good value.

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