Sean
Ex Member
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The principle is simple. In a bandpass enclosure, the output comes from the port(s), not the woofer cone(s). The woofers in this case act solely to vary the air pressure within each respective chamber in a manner that creates sound waves within the port(s) as their contained airmass resonates. In that case, the shape of the woofer cone, and direct radiation pattern from it, is not critical as it would be in a direct radiator application. (you couldn't mount a woofer backwards in a sealed enclosure with the woofer exposed and expect it to perform the same way, since the cone shape in that case governs the sound dispersion) All that matters in a bandpass enclosure is the volume of air moved at what speed, which is identical regardless of which way the cone is travelling.
Clamshelling two woofers together as you would do (twice) in a quad load configuration increases accuracy, since the woofer surround presents a slightly different resistance to cone motion in the forward direction than it does in the reverse direction. Using two woofers in this manner averages the non-linearity of this effect.
Having two woofers (or pairs of woofers) installed in separate baffles adds an airmass (and additional capacitance) between them which is driven in sympathy with the woofer cones, acting as a buffer which lowers the effective resonant frequency of the drivers. By doing this, depending on the woofer specs, you might see increased performance by installing both woofers cone-up (hence increasing the center chamber volume), despite the fact that doing so lowers the volume of the lower chamber, increasing its tuning frequency.
The best thing to do if you want a rough idea of how your woofers will perform is to model them in WinISD or similar software as a 6th order bandpass enclosure, using 2 isobarically mounted drivers. To model the quad load, just halve the Vas of the driver. The response you will see will be a curve with two very pronounced peaks, which would not be a nice enclosure to listen to, but the HWK center chamber acts to smooth out that response a bit and increase the bandwidth. By looking at the 6th order bandpass plots of your drivers in the various HWK configurations, you can get an idea of the best configuration for your driver, keeping in mind that the end result should be less peaked than the graph shows. Once built, EQ can clean up the rest.
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