View Full Version : Opinions Needed - Decoupling JBL 4345 Studio Monitors

04-06-2012, 11:32 AM
Hi All,

I needed your input in possibly designing stands for my JBL 4345 Clones.

The issue: The 4545 monitors are currently situated in our Sunroom. The Sunroom was actually an addition to our house and was constructed on top of our deck. Hence, the Sunroom is suspended in mid-air. Our deck was torn down and the new floor for the Sunroom was built using 2x12 studs + 3/4" ply + 3/4" hardwood floors. The floor itself is suspended on several 6x6 posts.

Although the floor of the Sunroom feels beyond sturdy when walking on it - the JBL 4345 still manage to send vibrations through the floor. I personally feel this effect takes away from the overall sound - as of the 18" diver is losing its sound. It might just be my imagination, but I feel the speakers sounded "better" when I had them in my basement (i.e., on concrete foundation).

I wanted to know if it would be a worthwhile venture for me to build something similar to the "Sub Dude HD (http://www.amazon.com/Auralex-SUBDUDE-Theater-Isolation-Platform/dp/B003EM17IK)" subwoofer isolation platforms. Or even wood stands like THESE (http://www.jbl43.net/img/woodstands_4345-1.jpg) with spikes to "decouple" the speakers.

Any thoughts would be much appreciated.



04-06-2012, 12:53 PM
Spikes are meant to couple, not decouple.

If you want to prevent your floor from vibrating you have two options:
- put the speakers on rubber or foam pads that will let the speakers vibrate on its own. The pads have to resonate at a low enough frequency to be completely outside of the range of interest (ie < 10hz). You will have to carefully calculate the resulting resonance frequency given the type of pad, the number you use, and the weight of the speaker (ie the weight per pad).
- put the speaker on a very heavy support that will act as a vibration "sink", absorbing any vibration the speaker could produce (the heavier the better). The speaker and the support must be coupled. This is the best approach, but with speakers like yours you will need a really big and heavy support (concrete, rock, sand-filled wood box, etc...). You can also mix the two approaches by putting the heavy support on rubber or foam feet. With the added weight it will be easier to get the resonant frequency of the pads low enough. Here is an example of such an approach, for smaller speakers: http://www.primacoustic.com/recoil.htm

Added benefit: putting the 18" away from the floor should also clean your lower midrange significantly.

Lee in Montreal
04-06-2012, 01:13 PM
I'd be tempted to think that the vibrations felt on the floor are not transmitted from the cabinet itself, but simply by the woofers. Your floor is resonating. Not much you can do.

04-06-2012, 02:51 PM
I'd be tempted to think that the vibrations felt on the floor are not transmitted from the cabinet itself, but simply by the woofers. Your floor is resonating. Not much you can do.

Hey D!

Thing is, if I am understanding all this, when you moved out on the porch (sun room), you changed a lot more than the floor. So, seems to moi, you've got to figure what/where the root cause is. Floor AND wall stiffness, basic dimensions, etc. Start from scratch. And, with the room cantilevered off the back of the house, perhaps you actually could stiffen the whole structure by adding more supporting posts - sunk in concrete preferably (contractor could help here). I'd try the easy stuff first and it would be interesting to take an RTA of the room before you start modifying. Anything you can do to change the excitation mode will help - but first you have to figure out what's moving. Assume you already did the repositioning enclosures drill, etc., tipping back 5 degrees or so. And, of course, all this and 99 cents will get you a coffee at McDonalds. SWAG. Good luck.

P.S. Agree with Lee, doubt the cabs are causing the vibration - too much mass and I assume they are "stiff as hell" (no vibrations). So, seems a basic acoustic driven phenomenon as suggested above. For what it's worth, my rig, which is similar mass to yours, is not spiked to the floor and has thick carpet + foam insulator under the feet (4) - it floats. They DO NOT move or vibrate and the bass is good - although I am not a blaster by today's standards. 80 - 90 db. Room is 20 X 22 with all sorts of weird hallways and connecting rooms - I think it all tends to cancel out. There are some standing waves but, well, welcome to home hi fi.