Sub Isolation or Coupling

[gvstark]

I will be runnding dual ultra-12 subs in the basement theater area. They will be behind acoustically transparent screen on floor, but there is a 9", sand filled stage in front of the screen. Thus I plan to build risers for the two subs. Should these be anchored to the floor to couple the subs with the floor? Or would it be better to just put legs on the stand, fill it with dense foam to try and isolate it from the floor?

Any advice would be appreciated.

Thanks,
Greg

[jlafrenz]

The sub should come with spikes so I would use those either way. Since it is a basement, you will be on a solid concrete floor and it won't make as much of a difference compared to a wood or upstairs floor. Risers can still provide benefits though. Here is a link with tons of information on risers and the topic.

forum.blu-ray.com/subwoofers/103410-diy-subwoofer-risers.html

[Wideawake]

Why do you have a sand filled stage in front of the screen? What are the dimensions of this stage? Can it be moved or removed? If not then you will need to raise the subs.

I’m not a big fan of using spikes unless you have carpeting and the spikes are able to dig into the substrate below the carpeting and the padding in order to provide a good grip. Even when used in this manner, I would want the spike to be just long enough so that the bottom of the sub / speaker cabinet is in contact with and rests on the carpet so as to provide maximum surface contact and therefore stability.

I believe that to get good clean bass it is important that the cabinet be made as stable as possible. Using spikes on solid floors reduces the cabinet surface area that comes in contact with the floor. I would remove the spikes and put a non-slip rubber mat between the cabinet and the floor.

If you’re going to have to raise the subs because of your sand filled wall, consider using concrete bricks to do so. Place a non-slip rubber mat on them and place the sub, without any spikes or other raisers, onto the mat.

Raising subs for the sake of raising them might not really help to ameliorate room modes since bass wavelengths are so long that a few inches in height here or there will likely not make much of a difference. If you manage to increase the distance from the floor, you decrease it by the same amount from the ceiling. Where’s the advantage?

One school of thought is that, for bass, you should aim to get early floor reflections since reflect it shall and there is nothing you can do to avoid that. Following this philosophy, it is recommended that the distance from the floor to the bottom of your bass driver should be less than the radius of the driver. So, for a 12” driver, it should be less than 6”.

Having said all this, your best bet is to try various things for yourself and to incorporate whatever sounds best to your ears. Everything else is merely theory and conjecture. However, if you’re using open baffle speakers or subs, do not raise either one off the floor since you will end up short-circuiting the bass altogether.

[jlafrenz]

One of the reasons to use spikes is to get the sub up off the floor. If you are using spikes, why would you still want the sub to rest on the carpet/floor? This kind of defeats the purpose of the spikes. Spikes should have no problem offering stability.

[Wideawake]

Jun 11, 2010 17:27:28 GMT -5 jlafrenz said:

One of the reasons to use spikes is to get the sub up off the floor. If you are using spikes, why would you still want the sub to rest on the carpet/floor? This kind of defeats the purpose of the spikes. Spikes should have no problem offering stability.

Why would you want to raise the sub from the floor? If at all, the floor can help to transmit the really low sub-20Hz frequencies to you more effectively and you can use this to your advantage to "feel" the low frequencies better since you can't hear them anyway. That's why they have buttshakers and other such devices on the market.

Regarding the use of spikes for isolation, let me refer you to this article that really resonated with me. If I'm not mistaken, JMilton first posted the link somewhere on the forum. I found it using a Google search.

www.audioholics.com/education/loudspeaker-basics/speaker-spikes-and-cones-2013-what2019s-the-point

[jlafrenz]

Jun 11, 2010 21:25:28 GMT -5 Wideawake said:

Jun 11, 2010 17:27:28 GMT -5 jlafrenz said:

One of the reasons to use spikes is to get the sub up off the floor. If you are using spikes, why would you still want the sub to rest on the carpet/floor? This kind of defeats the purpose of the spikes. Spikes should have no problem offering stability.

Why would you want to raise the sub from the floor? If at all, the floor can help to transmit the really low sub-20Hz frequencies to you more effectively and you can use this to your advantage to "feel" the low frequencies better since you can't hear them anyway. That's why they have buttshakers and other such devices on the market.

Regarding the use of spikes for isolation, let me refer you to this article that really resonated with me. If I'm not mistaken, JMilton first posted the link somewhere on the forum. I found it using a Google search.

www.audioholics.com/education/loudspeaker-basics/speaker-spikes-and-cones-2013-what2019s-the-point

I don't think that there will be that much energy transmitted via the concrete. It is a different situation in a non-basement or lower level floor. If there is energy transmitted, it can also work negatively. Instead of transmitting that energy into the floor, it should be transmitted into the room. I am very familiar with the article you linked to. Spikes do couple the sub to the floor, but are also designed to transmit as little energy into the floor as possible. As far as sub platform, it is to isolate the sub from the floor so as that the energy is not transmitted into the floor. Yes, buttkickers are popular, but there are lots of subs out there that have no issue allowing the user to feel the bass. The counter argument for products like bass shakers is that products such as the Auralex SubDude exist. There are products that are designed for either situation. It really just depends on the application and user.

[gvstark]

The stage was more of an aesthetic thing that we added. It was filled with sand to avoid any resonance. It is the width of the room (13.5 feet) and arced so that is is about 18" into the room on the sides and almost three feet into the room in the center. So I do need to raise the subs and do not want to remove the stage since I rather like it.

The area behind the stage will be carpeted. So cement bricks with a nonslip rubber pad on top would probably be very stable and provide a solid base. The other alternative would by a DIY riser like suggested in the blu-ray.com link made of 2x4's, furniture legs, and a plywood top. Both options are relatively inexpensive so I could compare the two.

[Wideawake]

Jlafrenz, you're right in that concrete will not transmit a lot of low frequency energy but a wooden floor will. Also, energy from bass drivers is transmitted into the room, whether or not it is transmitted into the floor. One cannot direct it to transmit through the floor and not into the room.

I'm not entirely convinced that spikes help to mechanically isolate the sub from the floor but even if they were able to do so, there is a trade off in that the sub cabinet becomes more unstable on spikes than without them. This is pure physics. The greater the surface area that comes into contact with the floor the more friction is generated and this is what stabilizes the sub. Therefore you are likely to loose some of that bass definition that you are attempting to preserve by attempting to isolate the sub from the floor. Rubber discs might be a better alternative to using spikes if you are really determined to achieve mechanical isolation.

I experimented with the placement of the bass drivers in my open baffle speakers. I found that having the drivers closer to the floor provided fuller and richer bass. There was less smearing of the low frequencies. I don't claim to have golden ears. I don't hear differences in various cables and power cords and such, so for me to come to any conclusion about SQ the difference needs to be clearly audible.

As I mentioned in my first post in this thread, you should do what sounds best to you. A lot of audio tweaks and such are subjective. Some will hear a difference and others won't. Even if the perceived difference is imagined, it is real for the person experiencing it. As you have rightly pointed out, there are products designed for either situation and the choice to use one over the other is really based upon the user and his preferences. There is no right or wrong.

[Wideawake]

Greg, what is the flooring comprised of in front of the stage? Where will your front speakers (L/C/R) be placed and will you be raising those too? Would you be able to post a picture of the room?

I'd be very interested in how these two options for your subs compare. Do let us know.

[jlafrenz]

Spikes are used to couple the sub to the floor and not isolate it. They couple it with minimal energy transfer. As far as stability, I still feel spikes can maintain this. Many spikes are adjustable so that it can level out the speaker so that it has a solid foundation. I do understand your point about more contact with the floor and stabilization. It is a valid and useful point. While rubber will isolate, it also is not perfect. This comes down to the type of rubber used, size and weight of the speaker. If the rubber is too soft as the driver moves the whole speaker may move. As pointed out in the Audioholics article (and physics), every action has an equal and opposite reaction. As that driver fires forward the whole speaker may move backward. It is now working against itself. If the speaker is more stable, this is less of an issue and can impact the low end of the sound. I think we both agree on stabilization here, just different ways to achieve it. Using the proper rubber discs or feet will also help eliminate this.

As we both have stated, it really depends on the application or intended use. Both have their pros and cons. One may work with the other will not for one person and be totally opposite for another. The situation and the listeners ears are still the best way to determine which is best.

[gvstark]

Here is a picture of the front wall. As you can see it is still under construction. The floor in front of the stage will be carpet.
There are three shelves for ERM 6.3s to go behind the screen. Subs could go on either side of center speaker or I could try stacking them. If I do stack the subs, what goes between then?
I will move forward with both options and do a comparison and let you known my impressions. As you can see I have a lot of setup to do still so it may take me a while.

[jlafrenz]

Either side of the center would work just fine. If you want to stack them, I believe that the Emo subs come with rubber feet that you could use. If not or if that is not steady enough, then I would use some poster tack. Make a one inch ball and place it in the corners and then stack the other sub on top of it.

[Wideawake]

The front of the room looks very nice. Can't wait to see the finished look.

For stacking you might also want to try using the non-slip rubber pads used under rugs placed on floors. Remove the rubber feet and place the sub cabinet over the other sub with the grip pad in between. In fact try the grip pad for both subs, whether stacked or not.

BTW, have you considered that you may need to move the subs out from behind the screen if your room dictates it? Do you have an alternate plan?

[gvstark]

It has occurred to me that the subs may need to come out from behind the screen for optimal performance. However, I really do not have a back up plan. There are some options that could work if it comes to that. The screen wall was a good compromise with the wife; I get a lot of equipment, but it gets hidden away.

[roshi]

I don't know if you even need to raise the subs at all... low frequencies are non-directional, so it's not like they are going to hit the stage any less if you raise the subs by 9". In fact many concert halls, etc have their subs under the stage, to hide them from sight.
As far as isolating or coupling. Personally, I think that isolating is the better route, since coupling results in resonances that you might not like, probably less due to the concrete floor however. Personally, I have had a good experience with the Auralex Subdude, it really helped clean up my sub and make sound less muddy and more accurate.
Overall, I would suggest just try everything out. The nice thing about having a set-up like yours is the fact that you can do whatever you want behind that screen and nobody will ever notice, except for the better acoustical experience.