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Old 28th April 2002, 10:47 PM   #1
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Default Designing a Linkwitz Transform sealed sub

After reading about the extraordinary properties of the Bag End subwoofers, I have spent considerable time thinking about designing and building a sealed-box subwoofer with the frequency response of a ported box, but with the transient response of a small sealed box.

This idea has taken several forms:

1) I own a Blueprint 1001 woofer (one from back when they had the double-stack magnets in the motor). Currently, is powered by 150 watts, and it provides very low frequency response in a ported box of my own design, but it has very muddy transient response. This may be in part because of the long distance that the back wave of the driver must travel to reach the port's opening. The port is about 15" long, 12" wide, and 1" high. The opening is 1"x12" at the front of the box. This also does not have very good control over the driver
cone motion, and I have been able to bottom this out with a 20Hz tone at 75% volume. My idea is to build a 12" cube for my woofer. I imagine that with a 250w amplifier and a small sealed box, I can use a LT circuit to make this act like my big ported box, only with the immediacy of a sealed box.

2) I will be going off to college in a year and a half, and I want to be able to get my lows, even when constrained by dorm room space and noise restrictions, so I imagined using an 8" woofer with 230w max power handling and a LT circuit in a 10" cube, powered by my 150 watt amplifier, to get down to 30 Hz.

3) I have heard some incredible things about the M&K studio refrence subwoofers (with their push-pull 12" woofers). Things like transient response that hits one in the sternum. That and the fact that this is a unique design all add up to making me want to build one. I'm not sure what the push-pull woofers really do that make them so much better than just two side-by-side forward-facing drivers, but I want to build a sub like this with an LT circuit and either a pair of Dayton Series II 10" woofers or a pair of Dayton classic 12's.
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Old 28th April 2002, 11:32 PM   #2
CHRIS8 is offline CHRIS8  United States
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aled-box subwoofer with the frequency response of a ported box, but with the transient response of a small sealed box.

Since frequency response controls transient response, EQing a sealed box to mimick a ported alignment would result in the identical transient response as the ported alignment.

Now, if what you want is extended response(f3) of a ported alignment, but a shallow LF cutoff of a sealed, then the LT will provide deep response with superior transient decay response. BUT NO FREE LUNCHES! You will reduce power handling, inversely proportionate to the extended response! Bottom line: You will not achieve the same SPL of a ported box under normal circumstances.

a ported box of my own design, but it has very muddy transient response. This may be in part because of the long distance that the back wave of the driver must travel to reach the port's opening.

Despite some of these 'experts' claiming the 'long distance' in a large enclosure causing 'loose' sound, this is simply NOT true. Regardless of size of the box, the entire internal volume of the box is resoanted in a linear manner, coupling to the vent. The air in the enclosure IS a spring. Does a spring that is 10" long not follow the laws of physics 'as well', as compared to a spring that is only 2" long? NO! A long vent, however, can produce it's own problem.....pipe resonance. This would be pretty obvious though, and would not usually be subject to a 3" or 4" diameter 15" long vent. Your problems are likely do to room modes(standing waves) and/or an improperly tuned enclosure. Try temporarily adding a few inches to the port lenghts and see if response is stabilized. IF not, then try your subwoofer outside in an open area...your problem is likely room modes.

But...

Do you have YOUR measured TS paramaters for this woofer? I can not(nor you) do accurate CAD modeling of box response without them. The factory supplied specs are likely not accurate enough to get reliable data. You need the TS data to get in the right ballpark to begin with. If you using factory specs, you need to then measure outupt of the subwoofer in an open area, or a near field response in a large room to see if the fr curve is inline with the predicted response.

Dayton Series II 10" woofers or a pair of Dayton classic 12's.

Neither one of these woofers is suitable for sealed alignments, much less LT sealed alignments. The excursion of these woofers(5.5mm, and 4.2mm) is unacceptable, as only very small amplitudes could be achieved with these drivers.

The benenefits of push-pull applications is dependant on the particular woofer being used. This configuration allows the motors of each driver to linearize the non-linear +/- force cteristics of the drivers, by opposing each other. Improvements can be drastic with 'some' woofers, depending of course on how theyreacted with + vs. - movement, compared.

-Chris
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