Down-firing Sub makes floor rattle

Place an appropiately sized slab of granite or stone underneath the sub. This can be kitchen countertop material - You can sometimes find off-cuts that a stone shop might sell inexpensively. That or a thick wood slab, like a butcher block, though I've not compared the two - I've only used the stone version which did work fine for me (I had wide-plank wood floors that were very old and tended to rattle otherwise).

Marco

marco, i have tried 1 slab of granite, about 1 inch thick, but that still didn't work...what thickness should work? thanks

I think I've got some bad news for you. The vibrations that are being transmitted to your floor are more likely than not airborne, even that may seem counter intuitive. A down and dirty check - put your hand on the sub when its making the floor 'rattle' and see if the sub itself is producing a lot of vibrations. If not you might get some better help by shoring up the floor. JMHO.

Newbee has good point. Ultimately everything will rattle. A plywood sheetboard somewhere could use some extra screws perhaps? A lose air duct mounting? A heavier carpet or underlay may help too. Also try moving furniture around or put you hand weight on different things (touch windows/doors) - it might not be just the floor.

Yeah, you're really not going to be able to solve this, other than balancing the sub out properly in the first place. Basiclally, down-firing, front-firing, slab, no slab, or even suspending the sub in mid air for that matter, isn't going to do anything about the airpressure and bassmodes in your room from being excited!!! Basically, unless you do something to your floor, it's going to respond accordingly. That's how bass works.
conversly, people think if they move their sub out from the walls, that it won't be as boomy in the room next to it. WRONG!!! That's not how it works.
Acoustics are what they are. You'd need to, like I say, just make sure you're sub is balanced out in level with your mains/the rest of the system (other than sub being way louder than the mains), and add some mass to your floor!
Good luck

thanks for the advice. i will try some more things...wouldn't front-firing the sub be better in my case?

1 more thing....the manual says the crossover on sub should be set at 70% of main speakers lowest frequency...so if my speakers go down to 56 hz, the crossover on sub should be set to 39hz...does this make sense? that leaves a huge gap between where mains leave off and sub picks up...i am assuming martin logan knows what they are doing but this doesn't make sense to me....... what do you guys think?

Yes, it makes sense. You sub rolls off gradually above 39hz, your main speakers roll off gradually below 56hz, and the combined energy from both speakers between 39 and 56 hz will equal the out put on either speaker alone at the roll off frequency. BTW, this assumes that the crossover network you are talking about is the one built into the sub. An outboard crossover network not specifically made for your sub might have different slopes and require different frequencies.

Shadorne's point about other sources of rattle and ways to cure it is right on. I had a framed picture on the wall rattle and I wasn't even being overwhelmed by the bass. Thought it was in the electronics/speaker. Put some soft materiel between its bottom edge and the wall to fix it.

Flrnlamb is also correct about the sub's drivers position not being particularily relevant.

update........lowered the crossover to 39hz, huge difference, sounds much better...also, kept it downfiring, but put the spikes on, also definite improvement...now just trying to fine tune the volume...gonna listen like this for awhile, then try to front fire...see what happens.....i really do like this sub and may just get another for stereo subs...what do you think?

An Aurelex foam platform should do the trick. This was very effective in taming the 160 yr. old sprung floors under my Vel DD-15.

http://www.auralex.com/sound_isolation_gramma/sound_isolation_gramma.asp

Turn the gain down.

And that's exactly the reason why for music one wants to get a front-firing sub with side radiators, something like Totem Thunder, for example.

Alex, when I was shopping for subs, Velodyne Tech Support told me that, assuming proper physical decoupling (slab, spikes, platform, etc.) due to long wavelengths there is no difference in room vibration caused by downward vs. forward radiator.

There's alot of great feedback above from everyone.

As an additional option, you might wish to consider grabbing a single Adona AV45AS amp stand; they work amazingly well as a subwoofer isolation and elevation platform. Depending upon whether you buy 6", 12" or 18" uprights (legs) and where you lock the x-frame into the uprights, you have a large degree of control over the separation of the sub and it's down-firing port from the floor.

The port then would be firing at a granite surfaced platform which would allow the wave to have minimal interference as it travels out of the port and then the many feet out in your room where sub-bass waves actually begin to materialize.

I have my REL Britannia B1 on an AV45AS with amazingly clear results (IMHO). Send me an email privately and I can share pics and other info with you if you like. Paul at Adona is also a great guy to deal with and has a vast degree of knowledge that he loves to share.

Check out http://www.adonacorporation.com/av45as.html

At $300 US their reasonably priced, look great and deliver real sonic benefit; I use them throughout my system. They're also modular so you can start small and over time, if you like, grow your setup.

Room vibrations, that I understand, as in interacting with room acoustics. Floor vibration - that's a different story, it seems. Decoupling, proper placement etc, I am sure, could help to minimize the effect of the down-firing sub rattle. Personally, I've never had good luck with those, in real life environments. Floor, furniture...everything tends to vibrate more with down-firing subs,that's in addition to the usually less musical, overbearing bass quality. IMO, of course

hey, thanks for the info....i am getting very close to getting this sub sounding very, very good with pretty much seamless integration......i have been playing with the phase, which i just thought should be at 0 because the sub is in line between the speakers...while it gives me the most bass, it is not the best bass...i think this has been most of the problem i have been having....i have neen trying 90 and 180......180 degrees seems to be the ticket...all of the problems i have been having are mostly gone....i am now getting good tight, deep, bass, with no overhang...

zephyr, i do like those stands and may look into it

I had the same arrangement with my mains and sub alignment and began at 0 degrees phase. After much testing and test relocating of the sub, I found the original position was the best (to remove a room mode or two) with 180 degrees yielding the best, most musical and appealing bass...My REL B1 is front-firing however it does have a floor-firing port.