Its the room, stupid!!!

Posted by: Ron Toolsie on 18 September 2001

Almost 3 years ago I relocated my DBLs into a 'bonus' room, located above a 3 car garage. The
previous installation had the DBLs against an outside wall with the floor joists beneath them firmly reinforced with floor jacks that braced them to underlying poured concrete. Here the DBLs provided the most visceral and palpable presention I have heard from a music system. In contrast the current installation seemed far lighter in bass (NOT a trait of DBLs) and required the 52s volume pot to be turned up to 2 o'clock for any subjective sense of power and scale, at which point it started to become more than a little 'shouty' and still lacking in the entrail-rattling extension. It didn't take long for me to find out where the missing sound was going... it was exiting through the floorboards to enter the garage, where most of the bass was to be found. In effect the underlying resonance chamber was sucking much of of what the DBLs outputed. The back wall was also vibrating quite alarmingly with palpable pressure changes.
Yesterday I (finally) addressed some of the acoustical deficiencies by having a large amount of cellulose (not fiberglass) insulation blown into the space between the flooring of the room and the underlying sheetrock that formed the roof of the garage. Previous to this there was large amounts of empty space (its a 20x30 foot enclosure) that was noticeably compliant even when walking across the floor. Anyway.. before I turned on any sounds I spent a few minutes walking across the newly reinforced flooring and was greeted with a reassuringly firm 'feel' beneath my feet.

The Sound The improvements were immediately obvious. Even at 'only' 11 o'clock setting of the volume knob there was a far greater sense of power, control, dynamics and extension. Far less smearing and with it an attendent increase in articulation and gestalt. A rather unsubtle pervasive midrange 'honk' now seems very tonally neutral. And when the volume increases, the room no longer sounds overloaded and shouty.. instead it just gets louder while retaining all the delicacy. Fundamentally this is a larger degree of improvement than a Hi-to-Supercap upgrade, and significantly cheaper too ($700).
The next phase is to screw onto the backwall several sheets of thick seven-ply boards making full use of the studs behind them. When painted over (in possibly a different but sympathetic colour to the walls) this should hopefully be a very effective way of bracing the all-important DBL-loading wall and should allow even further gains.
This sort of reminds me of looking at an expensive projection screen TV in a room with an unshaded window. The solution is to not buy a far more expensive and brighter projector.. it is to put a shade on the window!
Those of us living in Stateside where the floors and walls are notoriously flimsy would be well served by attending to the easily-remedied structural integrity of the room before attempting to buy the equivalent of a brighter projector.
I would welcome any opinions/alternative suggestions as to how I can firm up the back wall. Blowing insulation is not so easy as holes have to be drilled between each and every stud (which is very messy indoors- in a garage this is less of a concern)and I somehow feel that hefty boards screwed in at 18 inch intervals into the studs would be fundamentally better.

Ron
Dum spiro audio
Dum audio vivo

Posted on: 18 September 2001 by David Dever

Gee, Ron, if you lived this far north, you'd have needed that insulation just about the moment when you noticed the condensation on the amplifier cases, typically mid-October..

Glad to see you got that sorted out, as 11 o'clock on the volume pot is much kinder to DBL tweeters!

Take care,
Dave @ NANA

Posted on: 18 September 2001 by Martin Clark

The easiest way to stiffen up the back wall is to add some mass to it - fit a second layer of sheetrock (plasterboard) on top of the present layer. Bond in place, at 90degrees to the existing layer, with drywall adhesive to get some damping between the two; you'll only need a few screws into the studs to hold in place while the adhesive sets.

I think I've covered this on the acoustica soundproofing page; if not, well, I'll update it one of these days....

Posted on: 18 September 2001 by Mike Hanson

I had a similar problem with bass transferring to the floor in my new room. I solved it by using an extra isolating platform under the speaker stand. I've attached a simple diagram to give you an idea of what I've done. I'll probably replace it with a Mana Soundstage or two at some point in the future, but it's working well enough for now.

My biggest problem at the moment is a room resonance, which I'll try to solve by tweaking the position of the speakers. It happens only in a small range of lower frequencies, but it's rather annoying when the room lights up.

The room has a 3-foot-high crawl space below it, which has a concrete floor. I'm considering putting some type of jack under the joists where the speakers sit, which might help things further. Vuk had suggested this at one point, and your post today reminded me of it. Also, the floor has fibreglass batons between the joists. I suppose I could enclose them, and then blow them full of cellulose insulation.

I've also not decided what to do with the windows. Currently I'm using the metal, horizontal blinds that the previous owner passed on to me. They're horrendously ugly, and I fully expected them to be very detrimental to the sound. However, their repeated curved surfaces seem to help with dispersion (compared to the bare window), and they don't rattle. (I haven't decided whether they add harshness.) Therefore, I don't know whether should stick with blinds (metal, vinyl, or cloth; horizontal or vertical), or go for big heavy curtains.

Thanks for your "report from the front", and good luck on your future room tweaks.

-=> Mike Hanson <=-

Posted on: 18 September 2001 by P

Mike

I get the impression from your diagram that you don't sneeze too often?

I have visions of your speakers gliding across the floor at the merest hint of a disturbance!

Ron - Nice post.

I have recently moved to a new abode where the walls are particularly flimsy and I've been pondering over this very same thing. I went outside for a smoke the other night and the bass was definitely "Out There" IYKWIM.

I'm contemplating cavity wall insulation.

Anyone in the UK done this in a new build house?

Cheers

Pete.

Posted on: 18 September 2001 by Allan Probin

quote:
I'm contemplating cavity wall insulation.
Anyone in the UK done this in a new build house?

All houses built since about 1994 (pehaps slightly earlier) must have cavity wall insulation of some sort in order to pass building regulations (I built my current house shortly after this). What many commercial new-build houses seem to be doing is installing a plaster-board lining separated from the outer wall by wood batons. Why ? I suspect its a quick and cheap way of avoiding traditional plaster-over-brick.

If this is what you have behind your speakers you might want to consider removing it and having the wall plastered profesionaly. Alternatively, what about that expanding foam available in large aerosol cans, injected into the wall behind the speakers at points inbetween the batons. Although I have a suspicion that if the foam begins to set at the leading edges before its stopped expanding there is a danger that the plasterboard will be pushed off the wall.

Allan

Posted on: 18 September 2001 by Mike Hanson

quote:
I have visions of your speakers gliding across the floor at the merest hint of a disturbance!

They're actually fairly stable, and take a fair bit of horizontal force to move them. Of course, you wouldn't want to run headlong into them, but I don't think you should try that with any speaker.

Once I decide where their final resting place will be (due to the resonance problems), I may change the ball footers to spikes.

-=> Mike Hanson <=-

Posted on: 18 September 2001 by Thomas K

Hi Ron,

Nice post, as always! Yes indeed, I shudder to think how many people are out there not getting the best out of their system because their room is not up to par acoustically.

After moving three months ago, I thought I had descended into Room Acoustics Hell. No amount of speaker shuffling or any of the other usual remedies helped, so I had the suspended ceiling filled with cellulose. After that the music still wasn’t “there”, but Martin Clark encouraged me to try the speaker shuffle again, and this time it worked magic (I think the hollow ceiling had to be sorted out before anything else could be attempted). May I just say that Martin was extremely kind and helpful – he’s the man!

I think one of the reasons many people are in neglect of room acoustics is that they upgrade box after box without ever having heard the full sonic potential of their system (at any point). Those who are not too experienced, like myself, have to have heard their system in a benevolent room to be able to recognize a deficient one. The same seems to apply to speaker positioning (as James Jong has pointed out in the past).

Thomas

Posted on: 19 September 2001 by Martin Clark

A few bits and pieces:

P - Alan is right, your house will already have cavity wall insulation, and adding more will be no help. The sound is travelling through the fabric of the house, mostly the windows at a guess.

Insulation - fibreglass, cellulose, mineral wool - is not the universal panacea for acoustic problems. All that such 'insulation' does is selectively deaden the vibration of the air in the airspace in which the insulation lives. If you want less bass to be transmitted, you simply need more massive room boundaries. Have a look at the Acoustics FAQ, section 4.3.

Allan is correct, plasterboard is used because it is a rapid finish - no wet trades and no drying time required - which makes it cheap in terms of time. You might need to be wary of removing it and the studs though, they might be structural - an increasing number of newbuild houses in the UK are essentially wooden framed houses with an external brick skin veneer for appearance sake. Nothing wrong with the principle, but very awkward from an acoustic control standpoint.

Mike - I wouldn't worry about the blinds messing up the acoustics - the width and spacing of the blades is too small (compared with audible wavelengths) to have any dispersive effect except at the very top of the audible range. Heavy curtains, spaced at least 4inches off the glass would start to provide damping, but don't sweat it: you say they don't rattle and, as I've posted before, glass is only as reflective to sound as a painted wall.

cheers,
MC

Posted on: 19 September 2001 by Mike Hanson

quote:
the width and spacing of the blades is too small (compared with audible wavelengths) to have any dispersive effect except at the very top of the audible range.

It is only in the upper ranges that I heard a difference. Thanks for reaffirming my observations.

As I said, I do have a problem with a particular range of bass frequencies. After trying the speaker positioning to get it right, I might try rigging up some cloth over the window to see if it would help. I might also try a homemade tube trap to see if I should bother to get a "real one" from ASC.

-=> Mike Hanson <=-