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fEARful shelf extension for lower tuning
You can make a removable section to extend the shelf depth if you find that you want a longer-sloped roll-off of bass frequencies that in the case of fEARfuls comes with a lower box tuning. This actually reduces the mid-bass output some bassists interpret as low end - but it also allows greater output at the very lowest frequencies and moves the "unloading driver" threshold that all ported enclosures face even further down. If you typically find your lowest-pitched string a bit overpowering compared to the others, or find you often cut some EQ in the lower mid-bass region this will smooth out the room-induced low frequency "cabin gain" effect.
It's also useful if you are using extreme low tunings (lower than B0) or amplifying instruments with more fundamental content than the electric bass. Acoustic standup basses and synth basses are examples of instruments that often benefit from extended-shelf alignments.
Do notice that this "tighter" voicing is not for everyone - it slightly decreases FREE mid-bass content that many players like, and there is little benefit except when above-mentioned criteria is of concern. fEARfuls are already powerhouses of big low bass without boominess (don't have a mid-bass hump like many cabs and yet reach low without running out of excursion). So be aware of the tradeoffs.
That said, in any fEARful, adding an inch of shelf depth takes the tuning about 1.6 Hz lower. Adding 2" takes it about 3 Hz lower, and adding 3" takes it 4.3 Hz lower. You can see that the amount of LF reach gained per inch of shelf extension diminishes as one adds more. That's due to internal box size getting consecutively a little smaller as the incursion of the shelf area affects gets larger.
The chart below indicates cone excursion with a 450-watt swept sine. The red circle indicates where each cab alignment shown crosses past useful xmax and thus is the top of what I call "the unloading zone". Moving the unloading zone even slightly lower by extending the shelf just a little deeper can be of benefit when a lot of sub-bass content is being input.
Here it is also important to realize that swept sine content is extremely radical compared to the amount of a note's fundamental that is typically output by electric bass - and even most bass synth patches that sound really deep and phat. So. Unless you are actually sweeping sine waves, unloading the cone in a ported enclosure will happen lower yet than the graphs indicate, unless there are extreme EQ boosts being applied at sub-bass frequencies. Knowing this helps. It gives an idea of what's what when choosing adequate high-pass filtering for extreme SPL use, and gives one an idea how far things can really be pushed.
That said, fEARfuls are so ahead of other bass cabs due to woofer choice and box size and tuning that most users really never need to think about it overly. It's really nice not to have to do EQ cuts at bass frequencies just to keep the speaker from farting out or getting damaged when you want to play louder! Then any EQing can be a tonal choice rather than a forced decision.
It's better to check out extension of the shelf by making a 1/2"-thick extension as wide as the current shelf (19.5" for many fEarful models) and creeping upward in depth, in increments. The SHELF EXTENSION can be duct-taped to the current shelf temporarily, or secured with a LIP which is a small strip of material that can screw to both the SHELF and the SHELF EXTENSION. Later the duct tape can be removed and replaced by a sealing glue bead.
A pair of EARS can be screwed or glued to each end of the shelf. Then when the EXTENSION is in place they can be screwed to the side walls of the enclosure from the inside. Make sure the screws are not so long as to penetrate the outer wall.
That said, I've often used thinner plywood for both the shelf (during build) and the latter-added shelf extensions (1/4" thick is good) - and even duct-taped plastic stock in there when just testing. And when going "permanent" I don't bother with ears (I just use glue and a couple of toe-nailed screws) nor with a lip (glue alone is enough).
Never take EXTENSIONS any closer to the inner back panel than the height of the port + the thickness of the shelf (if port is 3" tall and shelf is 1/2" thick, that would be 3.5"). And if damping materials (open cell foam or polyfill batting) stick out too far close to that, they should be tapered down / mashed down with staples or spray adhesive to assure clear, unobstructed flow in the port area.
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