I moved from one flat in my building to a different one five weeks ago. The circumstances were auspicious, what with my landlord installing a dedicated spur for me, the floor being solid oak ‘glued’ to the hard surface underneath, and room dimensions fairly generous at 4.10 m by 5.10 m, suspended ceiling height ca. 2.65 m. The entrance is not a door, but an opening about three door widths; a narrow hallway adjoins (1 m by 3.something).
At two in the morning I installed my system – given the above, the power down effect should be negligible. But I can say that the disappointment was great: My one-week old SBLs sounded pretty lifeless, with no bite. Overall bass response was limp, feeble, meagre.
The seals were checked, speaker positioning altered, but considering *how* bad the system sounded, room acoustics seemed to be the main culprit. This went on for days with me being extremely unhappy, when one night I stood in the hallway and realized there was a airy noise coming from holes in the ceiling which had been drilled for spotlights that hadn’t been installed yet. I stuck a pair of socks in each hole and presto! everything started sounding much better. Obviously the suspended ceiling had acted as a Helmholtz resonator. A test CD showed that there was still a rather gargantuan peak at 80Hz and a bit of a dip at 125Hz. I then took a chance and had the suspended ceiling filled with insulation material (not cheap), which again brought about an improvement in the bass response.
I can now listen to and enjoy music properly again, but the peak and the dip remain. I also have some absorber foam thingys to compensate for a lack of furniture, and adding more of those showed that the room really does need and profit from that kind of treatment, but still the 80Hz peak makes some bass notes buzz horribly.
Has anyone encountered (and rid themselves of) anything similar? Maybe one of those home-made tunable tube traps could do the trick? Also, the pitch of the notes on the test CD increases in increments of third octaves (or so it says), i.e. 63Hz, 80Hz, 100Hz, 125Hz, 160Hz – does anyone know of a test CD with smaller increments in that range?
(Apart from that, in my system the SBLs were a huge eye-opener – simply stunning.)
Thanks for any ideas.
Thomas
Posted on: 15 August 2001 by Steve Toy
quote:
The entrance is not a door, but an opening about three door widths; a narrow hallway adjoins (1m by 3. something)
There lies your problem, IMHO. The standing waves must be horrendous with that *alcove* in the equation. In my last abode, where my system was in my bedroom, I tried setting it up in the living room with a similar *alcove* separating it from the dining room. It sounded bloody awful.
I soon moved my kit back upstairs...
If you are stuck with your system in that room, get some doors/glass panes installed in that...
It's always a nice day for it, have a good one 
Steve
Posted on: 16 August 2001 by Thomas K
James, Steven,
Thanks for your replies.
I have tried loads of different positions, starting out with the speakers on the short wall. They are now on the long wall (5.10 m), 6 ft tweeter to tweeter and 5 ft from each side wall – should be plenty of space to breathe. I might try again, but perhaps …
Maybe Steven is right about the alcove. I’ll try and get some plasterboard to temporarily close the opening and see what the effect is.
Thomas
Posted on: 16 August 2001 by Martin Clark
Thomas,
Forget the alcove as the source of the problem, it's your ceiling.
Suspended ceilings are notorious for this kind of effect, becasue with the cavity above they form a resonant panel absorber. Filling the cavity with insulation as you have done is a good move, and works by reducing the 'Q' of the resonance. The real solution is more stiffness (more fixings at regular close spacing into the floor/structure above) and more mass.
Adding mass, say a second layer of plasterboard at right angles to the first, bonded on and fixed as above, will render the effect negligible. The resonant frequency of the plane will drop toward the second octave of the bass, where there is comparitively little energy to excite it (whereas 80-120Hz is the realm of the kickdrum), the extra mass and stiffness will mean that movement is deadened completely. In any case, what remains as a little structural absorbtion in the deep bass will be a good thing for good perceived in-room balance, and your neighbours upstairs will thank you!
Covering up the alcove is opening a second can of the same kind of worms. Gross.
Moving the speakers side to side may make only minor difference in balance but will not cure it; the problem is a resonant phenomena - and the amount of energy in the room to feed it is the same.
I've outlined the basics on the Acoustica site under 'soundproofing'. BTW there are more pages to be posted shortly, including a speaker placement method we are having some success with.
Good Luck!
martin clark
Posted on: 17 August 2001 by Thomas K
Martin,
When the ceiling cavity was filled I had a look inside – there are heavy rafters, approx. 7x7 inches, every 3 or 3.5 ft, so that is pretty rigid (maybe not enough, though). Interesting what you say about ‘pushing’ the resonant frequency down.
Thanks for the acoustica link - I remember looking at your site about a year ago and thinking "Oh those poor sods with bad room acoustics ..."
Thomas
Posted on: 17 August 2001 by Martin Clark
quote:
– there are heavy rafters, approx. 7x7 inches, every 3 or 3.5 ft, so that is pretty rigid (maybe not enough, though).
Exactly that - the board is not rigid enough. For comparison, a new plasterboard ceiling would be fixed at 12-15 inch centres. 3 feet will make the plasterboard finish very lively, because the material relies on stressing its cardboard surfaces for any strength in bending...
Adding a second layer of plasterboard with the supports you describe would be no problem, so long as the layers where also glued together. However whether it is a reasonable solution in a rented property is a very different matter. The old solution to such problems used to be 'pugging' or laying a 2inch layer of sand on top of the ceiling. If the joists where spaced normally you could introduce a small amount of sand stealthily through a few carefully drilled holes; but don't try it here - you'll probably end up wearing your ceiling!
Martin
Posted on: 17 August 2001 by Thomas K
Martin,
It is rented property, and seeing that I’ve already spent about 500 quid on having the ceiling filled, I’d like to keep costs down. The ‘plugging’, as you say is probably not an option – there’s already enough weight resting on top of the plasterboard between the rafters (I still picture someone finding me buried under 10 cubic meters of the shredded paper used as insulation material).
I don’t know if it would help to just attach some 2x4s perpendicular to the rafters. It would certainly look pretty dorky …
Would you recommend a tube trap tuned to kill 80Hz (I know that’s treating the symptoms instead of the cause)? The suck-out at 125Hz I may be able to tolerate.
Thanks for the exhaustive replies.
Thomas
[This message was edited by Thomas K on FRIDAY 17 August 2001 at 13:20.]
Posted on: 17 August 2001 by Martin Payne
quote:
The old solution to such problems used to be 'pugging' or laying a 2inch layer of sand on top of the ceiling
This is a new one on me!
Would this be a viable solution in general to bass problems caused by too-flexible ceilings?
I'm willing to try anything to try to kill the excessive bass in my room!
Is this likely to cause any structural problems?
cheers, Martin
Posted on: 17 August 2001 by Martin Clark
Thomas,
quote:
I don’t know if it would help to just attach some 2x4s at 90 perpendicular to the rafters. It would certainly look pretty dorky
Yes, but it is probably similar in cost and upheaval to adding a second layer of plasterboard.
As for tube traps... the 80 Hz peak is only half an octave below the 125Hz suckout and the tuning is never that sharp, so you risk losing more bass; a Helmholt resonator approach would probably be more successful, if difficult to DIY. However, the tube trap is something you could have a go at yourself for not much money - this has been covered pretty well over at the Tweaker's Asylum
Martin,
Even if the bass problems are caused by too-flexible ceilings...generally, no. The real purpose is to add mass, increasing isolation from rooms above and the damping is a secondary benefit. It works out to be a considerable distributed load and the ceiling needs to be designed for it, or you would soon have quite a mess to clear up. An additional layer of plasterboard is a safer bet; it is lighter, and with a little adhesive between will give the ceiling 'constrained layer' damping.
regards,
Martin C
