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Discussion Starter · #1 ·
I recently replaced the 3 speakers across the front - I like their performance other than there's some boominess in the bass.


Both the dealer who positioned the speakers and an independant acoustic expert say the platform itself is vibrating and causing the boominess.


The platform measures 15' w x 6' L x 1' high..construction consists of 2x10" s..filled completely with safe & sound insulation. A single sheet of 5/8" plywood was screwed down then covered with an underpad and carpet.


I have learned the hard way why filling with sand is the recommended way.


:)


I am prepared to remove everything and fill the platform with sand but am looking for other effective solutions that are not as messy..


I spoke with an acoustic material company who recommended simply putting an acoustic mat between the platform and the speakers. This would mean removing the spikes from the speakers nad have them sit on the mats. One opinion I got said this would make the problem worse.


A couple of other ideas I had were:


1-Using patio slabs - place em on the platform and then speakers with spikes on top of the slabs..


2-leave everything as is but replace the plywood with a 1/2" concrete board product


Any opinions as to whether any of these 2 ideas would be good?


Any other simple solutions?


Thanks
 

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The latest Home Theater Builder mag. has a very detailed article on how to build the stage correctly. I don't have it in front of me but if I remember right do this:

Put the acoustic mat UNDER the stage.

Fill stage with sand.

Glue and screw 3 layers of plywood on top of stage.
 

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Did you have the same stage and arrangement with your old speakers? Is there a significant increase in lfe response with the new system?


You mentioned that you had an acoustician check out the problem. Did he tell you what the problem frequencies are? What are your overall room dimensions?


If the resonance is caused by a mechanical coupling between the stage and speakers, then isolation like patio blocks or acoustic blocking material, or a combination may help. If the stage is coupling to a room mode, then the isolation is unlikely to help and reinforcing the stage is the fix.
 

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Discussion Starter · #4 ·
thanks rgladiator..


I ultimately may do it but it's radical surgery..


Chris,


"Did you have the same stage and arrangement with your old speakers?"


Yes..


"Is there a significant increase in lfe response with the new system?


yes - there always was some boominess but I assumed it was source related..changing speakers has brought the issue to light..


Went from nautilus 802's to watt Puppy 7's which have way more punchier bass..


"Did he tell you what the problem frequencies are?"


No..he did everything by ear....I will be adding a bass trap in each of the 4 corners which should improve things considerably especially for bass in the back row seats..you can clearly hear more boominess in the back row


"What are your overall room dimensions?


15'W x 24'L x 9'H


"If the resonance is caused by a mechanical coupling between the stage and speakers, then isolation like patio blocks or acoustic blocking material, or a combination may help. If the stage is coupling to a room mode, then the isolation is unlikely to help and reinforcing the stage is the fix."


You got me here..I have no idea which one may be the cause..


I should try the patio slabs 1st as these are pretty inexpensive..


another idea I had was leave the platform as it..but pour cement in a form under neath the speaker areas only - and thereby isolate the speaker completely from the platform..basically make a concrete speaker stand tied into the concrete floor under the platform.
 

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John, I just responded to your other thread with same title.


Tom
 

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Hi John, nice to see that you like the the move to the Watt Puppy 7's.


Well it appears you have some good ideas above, if your problem in fact is the stage. However if your problem is the room modes than there is nothing that you can do to the stage that will help your problem (unless perhaps the vibrating stage itself has picked up on the room modes).


Correct me if I am wrong, but when I was over your place I remember you saying that your sub sounded boomy?? If that is the case then your speakers may have picked up on the same room modes.


In any case I have some equipment that can measure room modes etc. If you want I can come down to your place and we can test your system. Let me know.


Regards

Frank
 

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Discussion Starter · #7 ·
Sorry Tom,


I didn't realize the topic was posted twice.. I'll copy your reply here and see if the other can be deleted..


Yesterday 08:03 PM


tomrhyne

Senior Member


Registered: Dec 2001

Location: Greensboro, Georgia

Posts: 452

John, I don't have the experience on "fixing" this situation; however, I know what Dennis Erskine recommends and what I did: dry sand and 2" of plywood (3/4", 1/2", & 3/4" tongue and groove) screwed and glued to the 2 bys. There may be some other alternative, but this is a tried and proven solution.


Good luck!


Tom
 

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Discussion Starter · #9 ·
Tom,


One of the reasons for starting this thread was to share my experience..


I first started on AVS in this particular forum gathering ideas for the new room..


"3 sheets of plywood'? "sand?" overkill I thought.. Surely one sheet of plywood and insulation should be good enough..


Guess not..:)


Frank,


I'd *love* to get your input to see what your analysis shows using measuring equipment


Especially so if it indicates the platform really is not the problem..on heavy bass tracks I can feel the platform vibrating with my hands - I have no idea how this translates to affecting the performance..


The dealer is convinced it's the platform - when we 1st set them up we were able to tighten the bass a bit by moving the speakers both forward and in from the side walls..


For this initial setup we used strictly 2 channel - the sub was left on but very very little info was coming from the sub..


I though listen to only multichannel and feel the sub is integrating fairly well. When I switch between telling my preamp I have a mono or no sub, I find no sub makes the bass a bit more boomy


The sub is a B&W ASW4000 located on a side wall roughly in the middle between the main and side speaker..I haven't done much experimenting moving the sub around other than about a one foot variation in all directions..as it's big and heavy, I'm also limited to where it can go due to platforms and seating locations..


Dennis,


Mains were B&W Naut 802's and now are Wilson watt Puppy 7's..there's quite a significant difference in bass performance between the 2..


Guess I could wait till the bass traps are put in before ripping up the platform, however I'd rather address the platform before the traps and fix the problem assuming it is a problem..
 

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Ah...so, you're running your L/C/R as Large, not using bass management, and the sub is doing LFE?


First, clue...moving the speakers made a change to the bass reponse.


Now, as I understand it, the only change you made to your room was a swap of 801's for Puppy's. Is it also true the consultant works for the dealer that sold the Puppy's? (not as an employee...but who brought him/her in an paid the fee)?


Based upon what measurements or analysis did the conclusion "platform problem" come from?
 

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To echo Dennis (as if he needed it) - if all you changed were your L/C/R speakers (not the sub) then you should not be experiencing a huge difference in LFE response.


Do you have your processor set to "small" for your mains? Or, have them otherwise crossed over at 80hz? Maybe you've just got way too much bass flying around in there because your new speakers reach down farther than your old ones and they should be limited to > 80hz...


P.S. - OTOH now, maybe we have a thread to link to the next time someone asks "do I really need to build tha stage that way" :)
 

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When adding sand into the stage frame, do you line the openings in the frame with plastic? What keeps the sand in place???
 

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Discussion Starter · #13 ·
Quote:
Originally posted by Dennis Erskine
Ah...so, you're running your L/C/R as Large, not using bass management, and the sub is doing LFE?


First, clue...moving the speakers made a change to the bass reponse.


Now, as I understand it, the only change you made to your room was a swap of 801's for Puppy's. Is it also true the consultant works for the dealer that sold the Puppy's? (not as an employee...but who brought him/her in an paid the fee)?


Based upon what measurements or analysis did the conclusion "platform problem" come from?


Mains are crossed at 30 hz , center at 40hz..only change was the speakers.


There's no connection between the dealer and the guy who I had come by for a 2nd opinion..neither used any measuring devices...my honest hunch here that neither one knows from experience what affect the platform is having..


My very limited knowledge and gut says it is not the platform but a standing waves issue which the bass traps should correct..I say this because we can change performance by simply moving the speakers as little as one inch..
 

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Small Room = If you can move 2 feet or less and the sound changes, it's a small room.


The optimal location for the LCR speakers is absolutely not the optimal location for frequencies below 80 Hz. Change your crossover. (This won't take money from your pocket.) I am presuming you have set the reference levels and delays for each channel already.


Before you go to the expense of bass traps or any further 'fix', determine exactly what, and where, the acoustic issues are for your room. To do that bring in someone with HAA certification or from the Acoustics Guild to conduct an analysis. You'll then have specific solutions for specific problems. You may or may not require broadband bass absorption and/or frequency specific bass absorbers and/or parametric EQ.


For the HAA, you can email Gerry Lemay at [email protected]

I can provide you contacts within the Acoustics Guild.
 

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Discussion Starter · #15 ·
Thanks Dennis,


I have done all the calibration - volume and time alignment etc - but have not experimented with different crossovers..


Guess I have something to do today..:)


I checked out your HAA link..we even have a dealer in town..Guess it would be smart to have him come by..


Do you think he'll be able to determine what affect if any the platform has?


funny thing is I think the room sounds great..at least I thought so until people mentioned the platform..


My room is really based on your posts in this forum..42" high panels on 3 sides - full panel wall behind the screen. I was able to get used to the sound during the time the acoustic panels were being made. Adding the panels made a HUGE improvement!. I was expecting that bass traps would be required..


After the traps I figured that would be it..I have an MC-12 and rumours are surfacing that Lexicon will finally have full room EQ'g available at the end of this year..
 

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You know I would be more inclined to believe it was the walls before the platform but then I've always been a bit dubious of the whole "platform effect." What is your wall construction around your subs? Brian
 

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Brian:


I agree. Walls are the first culprit...and there are more of them than a stage. That being said, a stage, with a subwoofer on it can have horrible adverse effects to the bass unless properly constructed to avoid resonances, vibrations and very, very tightly anchor the sub.
 

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Discussion Starter · #19 ·
I did some experimentation raising the xovers to 80 hz..it does reduce the boominess somewhat so I'll leave em there for now..


Like Brian, I too suspect it's not the platform but the walls and room..


The sub sits on a carpet covered concrete floor..


The wall construction is:


*1/2 layer of soundboard

* layer of 1/2" drywall

*glue

*5/8" drywall


then 1" thick acoustic panel - 42" high around 3 side walls and full front wall


My thinking is that the low bass is not leaving the room and hanging around too long - the boom is most noticeable when you sit in the back row against the back wall..


The bass traps in each corner should greatly reduce the boom..
 

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Quote:
My thinking is that the low bass is not leaving the room and hanging around too long - the boom is most noticeable when you sit in the back row against the back wall..
It will be 'boomy' against the back wall. That's the boundary. That's exactly where all the length modes are the highest.
 
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