[Housteau]

I still find it so interesting that often there can be views and opinions regarding areas of audio reproduction that can be so different from each other.  In some cases these views are a complete 180 from the other.  Just today I read a case in point on room acoustics where the author was describing the structure of an ideal listening room.

To paraphrase his point briefly, he basically states that the room walls should be as rigid as possible (concrete would be ideal) to capture all of the acoustical energy allowing the best possible bass response.  The one other article I can quickly recall which also supports this idea was written by J. Gorden Holt many years ago.

Through my research using forums such as this and from other acoustic studies and articles from those ‘in the know’, such as those posted on the Real Traps site, I have learned a different set of acoustic principals that deal with low bass issues.  I now know and believe to be self evident that all room wall boundaries are bad for reproducing bass frequencies, as those sound wave reflections create both the peaks and dips present in our listening rooms.  The ideal listening space would then be completely non-reflective to those lower frequencies, while allowing the higher frequencies the needed reflections to properly define that acoustic space.  That in turn would produce a bass response as flat and correct as the speaker itself would permit.

I don’t really see a debate over what the problems are and what causes them.  I see more value in discussing how to deal with those problems, because with this I can see the legitimacy of various different methods of attack.  Most of us have an existing room to tame, which can require different solutions.  Others have the opportunity to build a nice dedicated listening room that can incorporate bass solutions within the design.  I was fortunate enough to be able to design and build my own room years ago, but unfortunately I had followed some poor acoustic advice at the time.  Thankfully not everything I did was wrong (my dimensions are very good), but it could have been done a lot better with what I know now.  Over the years I ended up adding in a lot of treatments and continue to have some issues that could have been avoided.

So, if building a boundary less room would be ideal for the bass, how could this become even a semi-practical reality?  At some point there would need to be a defined wall of a certain rigid structure that would support the higher frequencies that would determine that acoustic space.  Where is the tipping point?  At what frequency should reflections be allowed to roll in and then into into full reflections?  I can see that wall structure itself would act at this tipping point in a similar way an electronic x-over works in a speaker.

A room such as this would need to be a room within a room I would think.  I can picture the space between the inner and outer room being designed to capture and hold as much bass energy as possible, to prevent it from reflecting back into the listening area.  Such a room would still need internal treatments for the higher frequencies and for that tipping point with reflections, but ideally it would provide the best (flattest) bass response possible.

Building a room within a room isn't a new concept as I often see this being done by those enclosing larger spaces, say within a large workshop, a standard room with bad dimensions, or a basement.  So, this wouldn't necessarily need to be built from scratch.  Even if it was just one or two walls of the room built in this way, that might be enough to really transform a good listening space into a great one. 

Any thoughts?       

[Ethan Winer]

Wow, you totally get it. 

he basically states that the room walls should be as rigid as possible (concrete would be ideal) to capture all of the acoustical energy allowing the best possible bass response.

This is so easy to prove false that it's not even worth discussing. The only time I recommend rigid walls is when sound isolation is needed. This of course requires more, not less, bass trapping inside the room to get the same end result.

The ideal listening space would then be completely non-reflective to those lower frequencies, while allowing the higher frequencies the needed reflections to properly define that acoustic space. That in turn would produce a bass response as flat and correct as the speaker itself would permit.

Exactly. My "fantasy" ideal room has cardboard walls with the outdoors on the other side.

So, if building a boundary less room would be ideal for the bass, how could this become even a semi-practical reality?

It probably can't become reality due to zoning laws and the practical need to support a roof and keep the rain out. So a more practical solution is to design treatments that give the same effect.

At some point there would need to be a defined wall of a certain rigid structure that would support the higher frequencies that would determine that acoustic space.  Where is the tipping point? At what frequency should reflections be allowed to roll in and then into into full reflections?

I'd say the tipping point (crossover frequency) is around 300 Hz.

A room such as this would need to be a room within a room I would think.  I can picture the space between the inner and outer room being designed to capture and hold as much bass energy as possible, to prevent it from reflecting back into the listening area.  Such a room would still need internal treatments for the higher frequencies and for that tipping point with reflections, but ideally it would provide the best (flattest) bass response possible.

You don't really need a room within a room just to get lots of absorption at bass frequencies with less at mid and high frequencies. If you follow my company's site you know we build bass traps with a semi-reflective membrane on the front surface to achieve exactly this balance. As an experiment I took this concept to the extreme, which is the basis for our Hearing is Believing video. And when building treatments into a newly built room you could do similar using very thick rigid fiberglass with a thin reflecting (or diffusing) surface applied in front.

--Ethan

[NekoAudio]

So, if building a boundary less room would be ideal for the bass, how could this become even a semi-practical reality?

You could attempt to build a giant anechoic chamber, or buy a large parcel of land and set everything up outside. I imagine the latter is actually semi-practical, depending on where you live.

[TRADERXFAN]

I think there is more to it...
Read through this, specifically 4.2 about the "spaciousness" quality of perception as described. I think this would argue that actually the ideal room would be very large, and concrete walled.

Another idea to deal with modal problems, caused by small room boundaries (high schroeder transition frequency of the listening space) is the use of multiple subs, used to average out the room modes... 

http://www.gedlee.com/downloads/Chapter%204.pdf

-Tony

[Housteau]

As an experiment I took this concept to the extreme, which is the basis for our Hearing is Believing video. And when building treatments into a newly built room you could do similar using very thick rigid fiberglass with a thin reflecting (or diffusing) surface applied in front.

That is a good simple and instructive video.  What sort of material would be rigid enough to act as a wall, while passing most bass energy through and reflecting the midrange and trebble?  I wonder what 1/8" plywood over a standard stud wall would do?  I can see someone easily creating a false wall in the front and/or back of a longer room to shorten it up a bit.  While personally I like the look of acoustic treatments within a listening room, there can be too much of a good thing and I like to engineer problems out in place of adding in band-aids if possible.

[Housteau]

I think there is more to it...
Read through this, specifically 4.2 about the "spaciousness" quality of perception as described. I think this would argue that actually the ideal room would be very large, and concrete walled.

Another idea to deal with modal problems, caused by small room boundaries (high schroeder transition frequency of the listening space) is the use of multiple subs, used to average out the room modes... 

http://www.gedlee.com/downloads/Chapter%204.pdf

Thank you.  that is a good read.  Does the author get into the specifics of building these bass absorbing walls?

I am semi-familiar with the multiple sub idea, but that is more of a band-aid to correct the room issue, as in the addition of room treatments.  The first attempt to control what the room does I would want to be in the construction of the room itself.

Does this author get into phase issues of the music itself?  I read somewhere by someone that while he believes in the values of bass absorption does not like passive bass absorption, as it hurts the overall phase relationships of the original musical waveform somehow.  I can't remember the specifics, but it seemed to make sence at the time.

[FullRangeMan]

Hi,
A room inside a room seems great to get a low noise floor, a important acoustics item, specially If your amp are flea power.
Personally I prefer real wood on floor and walls.  About the room shape, the best project for a music room I see is the Golden Trapagon, as seen in the Cardas site, a handsome room shape, fully golden ratio.
I think TubeTraps or Sonex foam are unnecessary in a GoldenRatio room.
Gustavo

[Ethan Winer]

That is a good simple and instructive video. What sort of material would be rigid enough to act as a wall, while passing most bass energy through and reflecting the midrange and trebble?  I wonder what 1/8" plywood over a standard stud wall would do?

If the wall was 1/8 inch plywood only, that will pass most low bass to the other side. Likewise, thin window glass passes bass to the outdoors. I never tested material type and thickness versus transmission, but I know the data is out there and I've seen some of it. Testing this is simple enough using a loudspeaker, microphone, and room measuring software like REW. Of course, the thicker / more rigid / massive the material is, the lower the "transition" frequency. And transition is the right word because it's not a sharp cut-off from reflecting to passing through. Just like a speaker crossover.

--Ethan

[Ethan Winer]

I read somewhere by someone that while he believes in the values of bass absorption does not like passive bass absorption, as it hurts the overall phase relationships of the original musical waveform somehow.

That doesn't make sense to me because the amount of phase shift you'd get from any passive bass trap is never audible. In fact, phase shift isn't audible unless it's a very large amount, like thousands of degrees, which never occurs naturally. The only way to create enough phase shift to hear requires extensive digital signal processing. Earl Geddes has his own section at the What's Best Forum, so you might ask there for clarification. Earl and I have posted in the same acoustic threads there several times, and I never heard him say anything like that.

--Ethan

[jtwrace]

Earl Geddes has his own section at the What's Best Forum, so you might ask there for clarification. Earl and I have posted in the same acoustic threads there several times, and I never heard him say anything like that.

--Ethan

http://www.whatsbestforum.com/forumdisplay.php?177-Geddes-On-Audio

[gedlee]

Earl and I have posted in the same acoustic threads there several times, and I never heard him say anything like that.

--Ethan

Yea, I don't think they were quoting me, or it is a misquote (happens quite often actually.)

I like the walls rigid at HF, but not at all at LFs.  This takes some doing to achieve, but it can be done.  Too much absorption at HFs is a major problem in most small rooms.  Makes them sound dead and unappealing.  But at LFs absorption is the only way to smooth out the modes so it is a necessity.

[steve2701]

 I guess I'm one of the (very lucky) guys that has been able to build such a 'room within a room'. A couple of years ago I needed a new office at work & this project morphed into a dream that I had harboured since a teenager - my ultimate 'music room'.
While it still has a floor (it's on the first floor, supported by huge girders) and 4 'solid' ish walls (ultra lightweight blocks that are highly insulating to heat) and a roof that is , well, a huge bass trap. I learnt a ton via Ethan and his forum on music player forums (hi Ethan ) and discovered while it's not difficult to do - you certainly dont want to make an anechoic chamber (yuk) you want a room that retains life yet has a flat as possible responce.
If you succeed in making a room out of which all bass escapes and zero reflections you are going to need one hell of an amp or hugely efficient speakers,(preferably both) as you are not going to have any boundary re-enforcement whatsoever. I ended up with circa 1500k of 'bass traps' and a room that still retains life in music - and is a great place to be.
I seriously considered building the 'golden trapgon' but decided against it on two counts - they are very difficult to actually build, and secondly - nigh on impossible to decorate! Mine ended up being 6m x 5m with a 4.5m height. My 'blog' of the whole build is still there on Ethans forum I think - probably on page three or 4 now..

[Housteau]

If you succeed in making a room out of which all bass escapes and zero reflections you are going to need one hell of an amp or hugely efficient speakers,(preferably both) as you are not going to have any boundary re-enforcement whatsoever.

That is something I didn't consider.

[TRADERXFAN]

Thank you.  that is a good read.  Does the author get into the specifics of building these bass absorbing walls?

I am semi-familiar with the multiple sub idea, but that is more of a band-aid to correct the room issue, as in the addition of room treatments.  The first attempt to control what the room does I would want to be in the construction of the room itself.

Does this author get into phase issues of the music itself?  I read somewhere by someone that while he believes in the values of bass absorption does not like passive bass absorption, as it hurts the overall phase relationships of the original musical waveform somehow.  I can't remember the specifics, but it seemed to make sence at the time.

Yes he does. There are a lot of suggestions in constructing a home theater room from the ground up in his book from which this chapter was taken. I really recommend someone get it, if they are going to be building a room.

Here is a link to a discussion about technique given here in audiocircle by Geddes.
http://www.audiocircle.com/index.php?topic=56687.msg511881#msg511881

The white paper referenced was changed a bit, it used to mention drywall.
http://www.gedlee.com/downloads/Philosophy.pdf

-Tony

[Housteau]

That is something I didn't consider.

The room I built eventually ended up with quite a few large bass traps.  They did bring down the level of the bass relative to the higher frequencies as it became smoother overall.  Since my system is actively triamped (below 70Hz, 70 - 280Hz, and above 280), it was easy to adjust the levels through the crossover to compensate.  My bass amps have good power reserves and the fact they only reproduce part of the bass spectrum for each set of monos helps out as well.  But, for a room built to extremely limit bass reflecting back into it, I'd bet that would take considerably more. 

[TRADERXFAN]

I saw your post in another thread with pictures about using the boxes of insulation to try and tame your issue.
With a smaller footprint to do the same/maybe better, would be to make panel traps as described on Ethan's site. This is the same type of absorber as the drywall suggestion.  You would need to tune them with selecting the proper density to air space for frequency area that you need. You may have found a different area to work for these than the center though, not sure...

http://www.ethanwiner.com/acoustics.html#better%20traps

-Tony

[Housteau]

I saw your post in another thread with pictures about using the boxes of insulation to try and tame your issue.
With a smaller footprint to do the same/maybe better, would be to make panel traps as described on Ethan's site. This is the same type of absorber as the drywall suggestion.  You would need to tune them with selecting the proper density to air space for frequency area that you need. You may have found a different area to work for these than the center though, not sure...

http://www.ethanwiner.com/acoustics.html#better%20traps

-Tony

That is a great link to a wealth of information all organized in easy to understand segments.  I actually refer to it quite a bit, and others to it as well.  My thoughts for doing what I did instead of a panel trap came out of the loction this treatment needed to be installed.  It was right between the speakers at a point where I would want to have either absorption, or diffusion.  A flat panel trap had neither that I could see.  My solution gave me a mix of both.  Although the 703 is still in the boxes, the top is removed facing into the room.  The oriental screen covering is flat on the front, but angled on both sides.  The wood frame creates windows through to the 703 and reflection and scattering allong the frame.   

[BobM]

A wealthy friend who used to be an audiophile once tried constructing something similar to what you describe for his home theater room. Now, unfortunately I didn't hear it much beyond a brief demo, so I can't comment too much on the sound.

He constructed a lot of wooden baffles on the side walls and ceiling, kind of like those outdoor patio's with wooden beams above them to diffract direct sunlight. Something like this picture, but also around the walls. I guess he might have overdone the defraction/absorption thing all things considered.



In any case, he wound up tearing it apart some time later saying that it acted too much like an anechoic chamber. Nothing sounded natural. It was overdamped probably.

Just a thought that this kind of thing can certainly be overdone. I do like your idea that the ideal balance would allow high frequencies to reflect, creating a natural air and dispersion, while allowing bass to be absorbed where necessary and flatten natural room nodes. The bane of all of our rooms, no doubt.

[JLM]

I have a listening room proportioned ala Cardas and found that six GIK 244 panels at the first reflection points on front/side walls plus front corners did very little (speakers rated below 30 Hz).  Years ago I took my previous system into a 20,000 cu. ft. chapel where it sounded simply marvelous.  So my votes are for as big as possible and of the proper proportion.

The Cardas trapagon would be a freaky room to live in (a visual conundrum).


JMO but the true room within a room is the speaker cabinet in your listening room.   

In that case shape and stiffness is also highly debated, but IME the non parallel/stiff camp makes the most sense.  So why not in your listening room too?

[Housteau]

IME the non parallel/stiff camp makes the most sense.  So why not in your listening room too?

It makes sense to me too, but I have attended several lectures by Jim Smith, who wrote the book Getting Better Sound, that is set against it.  He mentions that of all the installations he has done with rooms having non-parallel walls only one worked out.  He said that the others were disasters.  Apparently what may look good on paper may not always translate into reality that well.  It could be that you cannot just use random dimensions for the angles?