This project has been a long long time in the making. i try to incorporate as much theory and learnings accumulated over the years into the build.
Starting with the LFE channel. i think everyone has heard of the Hoffman iron law when it comes to bass, or subwoofers. There are 3 factors - Size, Low bass extension, and Efficiency. One can only pick two out of the three.
If it is small and extends low, it will very low efficiency. This is the most popular trade off for commercial subs, for practical reasons. Power is cheap these days, relatively.
I chose to go with low bass extension, as well as efficiency. That means giving up size. The LFE subs fill up the entire baffle wall across the front of the room, some 6000 Liters in volume from my rough estimate.
The LFE subs have the following characteristics.
Infinite baffle, IB for short- this is a bass alignment, like sealed, ported, transmission line. IB alignments are basically sealed, but with very large volume. The official criteria is for volume > 10x VAS, but that as usual it is not a hard threshold where one side is IB, one side is sealed. There will always be efficiency gains with larger cabinet volumes.
Subjectively, the sound of an IB is also different. Typically a subwoofer is built in an enclosure, and the optimal volume is determined by the VAS parameter of the sub. One can think of VAS as the volume of air that has the same ‘springiness’ as the subwoofer suspension. A very stiff suspension sub has low VAS, and vice versa.
For a sealed sub with cabinet volume ~ VAS, say 100L, the air spring in the cabinet is ~ equal to the the suspension. One can imagine that that the air spring is non linear, especially at large excursions. The air spring effect is also different on the outward vs inward stroke of the subwoofer. All these non linearity has an effect on the subjective sound. And suffice to say, there is a avid following for IB subs. They hang out at an old forum - Cult of the Infinitely Baffled. https://ibsubwoofers.proboards.com/board/16/loudspeakers
Recently, IB subs seem to become more popular. The main barrier to entry is having sufficient space for the IB. Attics, basements, store rooms, equipment rooms, baffle walls are some of the ways to achieve IB setups. Subjectively, the sound quality of IB bass is 2nd to none.
Single bass array, SBA. This is related to the Double Bass array , but with passive back wall absorption. Theoratically, it generates a plane bass wave and sends it down the length of the room. In my application, my room is not symmetrically rectangular, and i also had to offset the upper right source inwards. The SBA will theoratically cancel the width and height modes of the room, leaving only the length modes to contend with. More on this later.
Double bass array - Wikipedia
Push pull force cancellation Subwoofer drivers ideally have suspension that are fully symmetrical in the outward vs inward directions. Real subs do not have perfect symmetry, and the difference in output from the cone-out direction vs cone-in direction generates 2nd order harmonic distortion. Having one driver flipped basically cancels out this 2HD. This feature is commonly seen on push pull MK, KK and most recently Perlisten subwoofers.
Edit : the difference in the KK, MK and Perlisten subs - they have 2nd harmonic force cancellation due to the push pull, but with the drivers 90deg rotated there is actually no force cancellation between the drivers.
Plenum / Slot loading. This is approaching the more subjective aspect. The idea is that plenum loading the subs is akin to a short horn, and subjectively this provides desirable characteristics to the bass sound. Both 3 and 4 are something that i’ve been wanting to try for the longest time. In my plenum, i have about 3:1 compression, meaning the total opening of the plenum is 1/3 of the combined displacement area of the 2 driver cones.
There is a large impedance mismatch between air and the surface of the driver. If you imagine trying to push air with a flat (subwoofer cone) surface, the air just moves away to the side. Having the air constrained in the plenum improves the impedance match between the air and the subwoofer cones, and supposedly this is a desirable character when one is trying to move (pump) lots of air to generate bass.
3 and 4 combined gives Push pull slot loaded. PPSL. There are various threads, including this : A Thread for those interested in PPSL enclosures | diyAudio