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Problem with my force canceling 12" or 15" sub design?

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Herman Chigrin:
Hey guys,
I'd like to lay down my preliminary plans for a "force cancelling PA sub" and get your feedback on my implementation of this concept.
It is sort of like placing a pair of ported cabinets back-to-back, with a total depth of 24 to 30 inches.

The concept is simple but has a potential problem (and I don't know if it is a problem) so I would love to hear your thoughts.

First, I will list my "wants":
Scalable, so one or more cabinets, depending on the application: outdoors, or indoor venue such as a bar or a club.
A pair of 12" or 15" drivers per cabinet, one on the front and one on the back.
Ports on the front only
Simple to build in multiples
Mobile cabinet and not too big and heavy
Cabinet depth of 24" to 30", stackable for use in multiples
Needed bandwidth of 30-150hz

Why force cancelling? The reason is many venues are built in such a way that the vibrations from subwoofers are transfers to the building structure. It is problem for DJs and vinyl playback as well as causing the entire venue to vibrate. The goal is to minimize this problem to not have to fix it down the road.

What is the potential problem? Well, here is where this concept may hit a dead end. The issue of the front woofer to back woofer offset that is tied on the depth of the subwoofer, 24" to 30" inches. It will cause a delay between woofers from the listener location. There will probably be a NULL in the freq response but where? What kind of irregularities can I expect in the radiation pattern around the sub? What happens when I use multiples, lets say a stack of 3x3 -- now the distance from front to back woofer is greater than that of single box. Will this create a NULL in the front and rear radiation pattern?
Is it a cardioid type of a NULL?

Most certainly there will be a NULL somewhere, tied to the offset of drivers, but where? Will it be audible in a room? Outside?

This sketch is based on above criteria:

Please let me know your view on this design.

Have a look at cardioid subwoofer deployment/configuration.  You would be better off with both drivers facing front and in multiples just rotate the proper ratio of cabinets to the rear.

Tim Weaver:
What you are searching for is a "Cardioid array". It has been done a few times in a single cabinet, but I don't think they've been super popular so far. Most people would rather buy multiples of a regular sub and when an array is needed you just put 2 facing forward and one facing rear (for example. There are other ways to do this).

Tim McCulloch:

Article by Bennett Prescott, currently the USA rep for B&C loudspeakers.

I'd suggest a search for "directional subwoofer arrays" to find other background and engineering.  EV/Bosch, EAW, JBL (look for the white paper on Forward Steered Arrays)... and others have wonderful resources to get a better understanding of how each method works.

The basic problem that remains is that a directional sub array is not a complete answer to problems with noise pollution from a disco, concert venue, night club or other source of amplified sound.  The amount of rear cancellation is insufficient because you still have a full "pressure zone" inside the room or building, and the entire structure can still resonate.

While I'm a user and promoter of directional subwoofer arrays, they are not a noise control panacea.  You'd best direct money and attention to the architecture and engineering of the venue with an eye/ear to controlling resonance and transmission from the structure itself and mitigating leaks caused by doors, windows, or other envelope penetrations.

Dave Garoutte:
Part of the stated goal is to keep physical vibrations from transferring to the floor, etc.
Short of using 6" of concrete for the cabinet, it will always have some contact transfer.
So, isolate it with sorbothane, or damping springs on a platform, or some such.
The audio sub frequencies will themselves get things vibrating, so I think there's only so much you can do.


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