Combo subwoofer isolation bridge & satellite stand

[4thtry]

Has anyone built or experimented with a satellite/subwoofer bridge similar to the system I describe below?
I did a search on subwoofer bridges but all I could find were threads showing how to electronically bridge stereo
power amps for mono.

I would be very interested in other projects using subwoofer isolation bridges or platforms that also serve as a satellite stand.
Most subwoofer platforms that I have seen only isolate the sub from the floor; not the satellite. Also, I am interested in designs
that place the subwoofer directly below the satellite speaker; not off to the side or behind. Thanks.

Bill S.

Continued next post . . .

[4thtry]

SOME BACKGROUND:

Below is a picture of the project that I am currently working on. The MTM satellites are sitting on top of a small black 8x10" box that, in turn, sits directly on top of my subwoofers. The subs are sealed 1.6 cu ft boxes housing 12" Titanic MK III woofers and SPA 250 plate amps. The satellites are Morel MDT-33 tweeters flanked by a pair of isobarik 4" midbass drivers (RS 40-1022A). Crossovers are 150Hz and 2.1kHz @ 12dB/octave.

They were played early Saturday morning at InDIYana. Compared to other speakers, I noticed that mine sounded a little flat and soft in the midrange & lower treble area. Very pleasant & smooth sounding, but a bit less clarity than most. This was probably due in part to the fact that my drivers were manufactured over 25 years ago. I really can't expect them to compete with modern drivers costing two or three times as much.

VIBRATIONAL PROBLEM:
Another part of the problem, however, might be that I am placing my satellite speakers directly on top of the subwoofers. During play, if I place my hand directly on top of a satellite speaker, I can feel a significant amount of vibrational energy transferring up from the subwoofer, especially in the 40 to 100Hz area.

One solution would be to place the satellite on a regular speaker stand and then move the subwoofer to one side or behind the satellite. I have tried this, but I do not like the way it sounds and would prefer to keep the subwoofer directly underneath the satellite.


Continued next post . . .

[4thtry]

THE BRIDGE:

So, I built a pair of satellite/subwoofer isolation bridges. I made them out of construction grade two-by-fours, inexpensive foam rubber from the local craft store, and a pair of 1-1/8" particle board stair treads. I added some steel angles to make them sturdy and less resonant. I also added some 3/4 by 3" oak "feet" along the bottom sides so that bridge would float on top of the carpet instead of digging in. According to my microphone tests (see below), spiking the subwoofer and floating the bridge significantly reduced the transfer of vibrational energy through the carpet and then into the satellite.

As you can see in the drawing below, I broke the bridge into an upper and lower section. This broke things into two resonant sections so that I could control vibration at different frequencies by varying the amount of damping material placed between the sections. I currently have approximately 1/2" of foam damping between sections, which helps to better control vibrations in the 20 to 40Hz range.

The bridge is almost complete. In the 2nd picture below it is missing the carpet isolation pads at the bottom because I removed them (after making measurements) for staining & polyurethane treatment. I will post a picture of the finished system with satellite & subwoofer in place when I get them completely done next week.

[4thtry]

THE SOUND:

Thus far, I am very happy with the sound. I can definitely hear a difference, even though I cannot measure it using conventional on-axis frequency response tests. With both bridges in place, the midrange & treble sound much clearer, almost as if I turned the subwoofer gain down by 1 to 2dB. Well worth the small amount of time & effort.

I wanted to measure what was happening, so I taped my microphone to the top of a satellite. Then, I played a very low level swept sine wave through the subwoofer alone. I kept the level very low to prevent damage to my microphone. I made one measurement with the satellite sitting directly on top the subwoofer. Then I made another measurement, at the same level, with my bridge system in place (see graph below).

The result: Even with the subwoofer crossed at 150Hz, I could measure a substantial amount of vibrational energy reaching as high as 2kHz on the top of the satellite. Below is graph comparing the vibrational energy of my "bridge" verses "no bridge" set up:

The dB scale is meaningless on this plot, but you can clearly see a significant reduction in vibrational energy from 20 to 100Hz and then from 200 to 5000Hz with the bridge in place. Now, when I place my hand on top of a satellite when playing music with lots of bass, I can only feel a very slight amount of vibration coming from the satellite itself. The satellite cabinets are 1.5 inches thick with multiple layers of damping compound inside, so they are very inert. Almost no vibrational energy is reaching the satellite from the subwoofer.

Please let me know what you think of this technique. Also, can you direct me to other sources of information on this subject.


Thanks,


Bill S.

[[email protected]]

Interesting study Bill. Thanks for sharing. Attractive design as well.

[thekorvers]

I suspect that your sub enclosures are inadequately braced and/or not built sufficiently stout. Perhaps it would be more effective to focus your efforts on improving your sub enclosures.

The ideal enclosure is one that does not vibrate at all and where the woofer simply moves in and out of the enclosure like a piston, without causing any vibration to the enclosure itself. People have built enclosures out of concrete to achieve this. Another way is to use additional parallel walls, spaced apart and filled with sand. If these methods are too cumbersome, then at least use thick walls and very extensive bracing.

If you achieve an enclosure with such minimum vibration, a simple thick rubber mat is the only thing required to absorb any remaining vibrations.

[billfitzmaurice]

I suspect that your sub enclosures are inadequately braced and/or not built sufficiently stout. Perhaps it would be more effective to focus your efforts on improving your sub enclosures.

+1. Even thick walls aren't a necessity if there's sufficient bracing. In addition, it's very rare when the best placement of a sub and main is within the same footprint. At the very least it makes it impossible to eliminate any Allison Effect cancellation notch from the reflection off the wall behind the speakers.

[4thtry]

+1. Even thick walls aren't a necessity if there's sufficient bracing. In addition, it's very rare when the best placement of a sub and main is within the same footprint. At the very least it makes it impossible to eliminate any Allison Effect cancellation notch from the reflection off the wall behind the speakers.

Thanks for the excellent tips. I agree. My sub cabinets are single walled 3/4" MDF only, which is probably inadequate. There is absolutely NO internal bracing; I just stuffed them with fiberglass and called them done. Outside dimensions are 15x15x21.5 inches with a cut out in the back for the plate amplifier.

Since I am almost done with my bridge project, my next project will therefore be to pull out the woofers & plate amps to install a bunch of braces. It should be fairly easy to install some criss-cross type bracing through the woofer and plate amplifier cutouts. When done, I will re-run and post an updated cabinet vibration test to see what kind of a difference this makes.


Bill S.

[billfitzmaurice]

Even a single brace connecting the middles of the opposing panels will be the equivalent of doubling the panel thickness.

[Pallas]

Linkwitz uses a similar bridge on his LX521

[Chris Roemer]

I DID see some isolation feet at AXPONA last year. I THINK I could really hear a difference with them (I THINK).

[billfitzmaurice]

I DID see some isolation feet at AXPONA last year. I THINK I could really hear a difference with them (I THINK).

Most likely you did not. Required reading:
http://ethanwiner.com/speaker_isolation.htm

[4thtry]

Linkwitz uses a similar bridge on his LX521

Thanks, Pallas. Very interesting project & write up.

Bill S.

[4thtry]

I disassembled one of my two subs and put together a bracing plan (see pics & drawings below). My plan is to cross brace every 6-8" or so, once along the 13.5" inside dimension and twice along the 18" inside dimension. Since my box is very small (1.6 cu ft with the big 12" woofer on one end) I will be breaking the vertical top to bottom brace into 4 sections and then press fitting all 4 sections directly into the woofer magnet. What do you think of my plan? I should have it completed in about a week. I will then re-run and post some comparative vibration tests (before & after the mod).


Bill S.

[fbov]

If you want to isolate something from its foundation, the most effective approach is spring isolation. Aim for a resonance frequency that's an octave below the lowest frequency you want to produce. I made an end-table subwoofer like this that doesn't vibrate. My Fc was ~4.5 Hz... Square root of spring constant K, over mass M, with K the constant of each spring, k, multiplied by the number of springs, and M the suspended mass. Just get your units straight, then test it (time 10 bounce cycles; the system's easy to excite by hand).

It gets at exactly what you're trying to do. Commercially non-viable, save for custom work, as the thing's fairly fragile. But it doesn't move a lick when the sub has the room shaking.

Have fun,
Frank