Not Your Average Subwoofer (Multiple subwoofer cabinets)

[fredygump]

My background is also in pro audio sound reinforcement, Dave and I started in the same era building our own systems. He provides loads of good information in his videos and interviews.
Sound waves are sound waves, and frequencies are frequencies, and because of long wavelengths compared to room dimensions, low frequencies behave differently in small rooms than large rooms or free space.

Having a cardioid (shaped heart-like) pattern makes a subwoofer cardioid.
There is no mention of a cardioid pattern in the W371A user manual, and no free space measurements- in fact the manual states "This product is designed for indoor use only."
The Genelec W371A in combination with a top cabinet and the GLM (Genelec Loudspeaker Management) provides 3 separate functional locations to provide "Null Steering" or "Continuous Directivity Matching" within the crossover overlap region. The three different locations, and their three distances to boundaries are key elements to the GLM.

Again, the directivity characteristic of the Genelec W371A as provided in the manual is not that of a cardiod subwoofer, the patterns show "Null steering" or "Continued Directivity" in the crossover overlap region down to around 60 Hz, the "sub" region from 25-60Hz is an omnidirectional pattern not affected by processing.
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Yes, I agree "active cancellation" and "null steering" are somewhat synonymous, though not the same as a cardioid pattern.

Art

It has been awhile since I posted, so it's probably time for an update. A lot has happened since I posted last, and now I'm drafting a final design.

The subwoofer/ low frequency design has stayed the same, but it is now a full range speaker with the addition of 12" coaxial drivers (B&C 12fhx76) drivers.

The 2 prototype cabinets have been working quite well when I EQ them the way people commonly do for multiple subwoofers in home theater. I was questioning the effectiveness of this technique in a previous post, but with both cabinets, and with a little practice, I have managed to get a result that I'm pleased with. In the main listening positions I'm measuring a +-3db response from 20hz to 100hz.

I don't know what is happening acoustically to get this result. But what I do know is that I am getting the result I was hoping for, so I'm calling it a win. I am getting the effect of multiple subwoofers as championed by Floyd Toole, but without having a bunch of subwoofers spread around the room.

The technique I am using to adjust each individual driver is to measure a sweep of 1 driver, referenced to 1 tweeter, and then time align each additional driver, all referenced to the same tweeter. I'm using the alignment tool in REW, and then using delay and/or phase inversion to get the smoothest result I can get. There isn't a huge amount of EQ required.

I upgraded from the Dayton 408 DSP to the Peavey 48e processor. The Peavey has helped a lot because it has parametric EQ on the input, and it has a much larger range of delay. (They Dayton did up to 8ms, which isn't enough.)

The input EQ is handy. I can fix some room issues common to multiple speakers on the input side, which frees up the output EQ for fine tuning.

And the inputs have high and low pass filters, which allows me to create stereo high frequency inputs (A & B) and stereo low frequency inputs (C & D). The high frequency is 100hz to 20khz, and the low frequency is 0-100hz.

On the outputs I can selected combinations of these inputs. So the rear subwoofers are both just C & D (summed). The front woofer is B, C, and D on right, A, C, D on left. Then the coaxials drivers are A or B.

So the front woofer is combining the summed subwoofer signal with the stereo signal, allowing it to extend up above 100hz. It's crossed at 200hz right now. The B&C (mid) woofers drop off pretty quickly below 200hz, so I do need the woofer to extend up to meet it.

I hope some of that makes sense. The big picture result is that I achieved my goal with the prototypes, and I'm moving on to building shiny, carbon fiber wrapped speakers!