[Housteau]

I had been thinking about something.  There are quite a few that convert nice sized basements into soundrooms and others that are able to build from scratch.  This hypothetical relates to both situations.

With basements one often needs to cut down the size of the space by installing new walls to create the boundries desired.  I was wondering if it would be a good idea to instead build less solid and more acoustically transparent walls at the lower frequencies?  Lets say that the entire front and back walls of your room would be entirely absorptive of low frequencies, but reflective of the higher.  These 'false' walls would then basically just be the outer membrane of one huge base trap, with absorptive material between them and the actual wall somewhere behind it.  I am not sure what this wall could be made of, possibly very thin wood or other material that would flex and be vented and ported where needed.

Since just about all of the main problem corners and intersections within the room would be taken care of, no further bass trapping would be necessary.  The rest of the acoustic treatments of absorption, or reflection and diffusion would be designed for the higher frequency controls at first reflection points and the like.

Would this be ideal?  What am I missing?

[bpape]

All you're doing in that case is increasing the size of the room (and thereby, lowering the modal frequencies of the space).  Nice try though...  The only way to negate modal issues is to listen outside.

Bryan

[Housteau]

All you're doing in that case is increasing the size of the room (and thereby, lowering the modal frequencies of the space).  Nice try though...  The only way to negate modal issues is to listen outside.

The boundry dimensions, both 'false' and real could be set for the best ratios.  I wouldn't be expecting to eliminate all modal issues, but wouldn't creating such huge bass trapping be the ultimate in controlling the bass issues that were there? 

[bpape]

Well, by doing front and rear walls, you've addressed the room length.  You've not done anything for height or width related issues.

Also, in trying to avoid modes, you've made them lower in frequency which will require even larger, thicker absorbers to deal with  them.  Also, by adding low frequency trapping at a good distance from other boundaries, you potentially will create humps in filtering based on the distance from absorber to hard boundary and the related quarter wavelengths.

Bryan

[Housteau]

Well, by doing front and rear walls, you've addressed the room length.  You've not done anything for height or width related issues.

What it would do is fully treat and basically eliminate all four vertical wall intersections floor to ceiling.  It would also eliminate the wall and ceiling intersection corners front and back.  Those are most of the primary problem areas that most of us deal with using the standard treatments.  But, in this case they would be treated to the maximum possible.

You are right that the width and height would not be directly addressed, but with most of the primary offenders already being eliminated, wouldn't one still be better off this way then going the standard route?  I suppose my example could extend to creating a false ceiling, not too far removed from a standard drop ceiling, and false side walls as well.  Since we are just imagineering here, why not? 

Also, in trying to avoid modes, you've made them lower in frequency which will require even larger, thicker absorbers to deal with  them.

Since there would be say two feet, or whatever one needed between the walls to build the most ideal trapping, would that even be an issue?

Also, by adding low frequency trapping at a good distance from other boundaries, you potentially will create humps in filtering based on the distance from absorber to hard boundary and the related quarter wavelengths.

This is an interesting one here and one I would like to understand a bit better.  Are you thinking it might alter a critical timing that would through off a balance between the higher and lower frequencies and/or create strange peaks, nulls?

If not a room similar to what I am suggesting, what design do you think would be a better option, given the choice?  Mine really isn't so far fetched as it is simply a room within a room.  I am not so sure about construction materials and the like, but I am sure that could be easily worked out.

[bpape]

The absorbent material spaced a distance from a hard boundary can act like a single band EQ in relation to what else it's absorbing. 

While corners are a very efficient place to trap, they're not everything and many times, not the biggest offenders.  Also, think about tangential modes which do not involve the front or rear wall (floor, ceiling, 2 sides)

Another thing is that we generally like to be able to tune SBIR based on what we do on the front wall. 

Also, what would we do for things like Maggies, OB dynamic designs, electrostats, etc?  We really don't want to kill the back wave completely. 

Ah, but what you're suggesting isn't a room in a room.  It's trying to make room boundaries out of something that will pass sound. 

The ideal situation is to avoid parallel surface.  So, slanted ceiling (low in front), splayed walls, etc. To get enough angle to avoid true axial modes takes quite a bit of space - say a minimum of 1" per foot of length on each side.  So, assuming a flat floor, the ceiling would be 2" change per foot of room length.

Another 'ideal' is to build the treatments into the construction. Think of building a closet in the front room corners and filling them with material. 

Bryan

[Housteau]

Also, what would we do for things like Maggies, OB dynamic designs, electrostats, etc?  We really don't want to kill the back wave completely. 

Ah, but what you're suggesting isn't a room in a room.  It's trying to make room boundaries out of something that will pass sound.

I have a lot of experience with dipoles, stats, ribbons etc. and you are right that they like that back wave reinforcement, but that is with the mid and upper frequencies.  My thoughts on this design would allow for that. 

I guess this whole idea comes from an old example explaining low frequency acoustics in small spaces on Ethan's website.  He started by explaining how a speaker behaves in the open air environment, then places it within an enclosed space with increasingly stronger and more reflective (to low frequencies) walls.  He shows how the less passive to low frequencies the walls were, the worse the bass issues within that space became.  I had been thinking of possible ways to reverse, or at least attack that known trend at the root cause by allowing that energy to leave and not return.  The points that you have brought up show that would not be as easy as one would wish.

The idea to build in a closet or hollow corner cavity that could then be filled in with treatment material is basically the same idea, just reduced down to a much smaller scale.  To be honest and given the opportunity to build once again, that would probably be the way I would go, that and the use of non-parallel surfaces where most practical.  I don't think I would be adventurous enough to jump head first into some of the possible issues that you have brought up by creating an entire room as I described.   

[poseidonsvoice]

Housteau,

I refer you to this chapter, especially the section titled 'Summary.' Of course the entire book is very useful as well as he describes the construction techniques.

Best,
Anand.

[Housteau]

Thank you.  I found that very interesting.  I would like to see the actual construction techniques involved.  Do you have a link to that section you can point me to?

[poseidonsvoice]

You need to purchase the book. If you do, say hi to Earl for me. You can also read about everything else he is doing, his speaker designs, etc...here. He is a complete 'no nonsense' kind of fella. In addition, you should also have in your room acoustics arsenal, this book, by Floyd E. Toole of the Canadian National Research Council. Both Linkwitz and Earl Geddes respect this man's work although they may not entirely agree with everything he says.

Still for a neophyte like myself, Ethan Winer, Bryan Pape, Earl Geddes, Siegfried Linkwitz and Floyd E. Toole, are GODS when it comes to room acoustics.

Eat it up, learn and build your reference room. In addition, I feel that George Cardas has some effective ideas with regards to room dimensions, etc...here. Although if you look at Floyd E. Toole says, it really makes NO difference.

And finally here is even more info directly to some papers written by Floyd E. Toole.

Best,
Anand.

[Housteau]

Still for a neophyte like myself, Ethan Winer, Bryan Pape, Earl Geddes, Siegfried Linkwitz and Floyd E. Toole, are GODS when it comes to room acoustics.

Eat it up, learn and build your reference room. In addition, I feel that George Cardas has some effective ideas with regards to room dimensions, etc...here.

Agreed, and that is why I find forums such as this so valuable.  I have learned more in these last couple of years from Ethan, Bryan and the other contributors input here, than I have in all my years involved in this hobby.  If only I could have had what I know now back some 16 years ago when I built my current room, so much would have been done differently.  It has been just fairly recently that I have been able to get most of it's potential working for me instead of against. 

It has been a fun journey that isn't over yet and I know I am fortunate to have what I do.  But, still it would have been nice to have engineered out these issues at the beginning during the building phase, instead of taking years creating different kinds of band-aids.