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Bass Cabinets

My quest for the perfect stage sound has led to the fabrication of many speaker cabinets over the years.
The "end-of-a-tree" midrange cab is my current favorite, deliberately made over-sized in the hope it could handle more bottom-end, and it has more than served its purpose. Originally intended to be the top end of a bi-amped stack, I have nevertheless used it for a full-range cab, partly because the 2 x 12" bass cabinet it is paired with
a/ doesn't match it enough to do it justice,
b/ has been called a washing machine,
c/ is simply too big.

My fellows watch me struggling with these boxes, thinking "why doesn't he just buy one"? And the answer is,of course, that the quest for a no-compromise sound is not an easy path to follow. There will be many obstacles to overcome, not least fending off doubts like "maybe I should just buy one".

Speaker Cab

Bright drivers

Memory lane takes me back to a covers band I joined, to "fill in" on accompaniment and singing, where my instruments went straight to p.a., leaving me in dire need of a personal monitor. It needed to be small: I was already lugging four instruments and a bag of spaghetti (not my favorite film). It would need to sit under my mixer, about one metre from my head, powered from my old trusty Marshall bass amp, the 3520. I picked two Eminence (that name again!) Alpha 6 speakers, and put them in as small a box as I dared, with two token ports but no real tuning effort. I was very pleased with the result, my impression was that the top-end was almost as clear as if I had used horn tweeters.

rounded baffle 4 x 6inch cab design

The mid-range cabinet

Moving on a few years, the box got thrown away as an unimpressive build, but I still had four Alpha 6s. Strange how they seem to start pining for a home after a while, so I had to get to work designing somewhere for them to live, and doing it properly this time!

Their new home would exploit to the full the bright top-end sound, but also lend as much to the bass response as was possible. With a resonant frequency around 200Hz, the speakers had to be in a large (80-litre) tuned cabinet to get the range closer to around 100Hz. The spherical baffle idea was to get the (very directional) high frequencies dispersed around the room; though I was about to learn that birch-plywood is very hard, having reckoned on needing eight layers of 18mm board to achieve the curvature.

But, complaints aside, the result is very enjoyable. Though the speakers might complain audibly if they get too much bass, the set-up generally works well in smaller venues without a bigger speaker involved to handle the bottom end.
One more experiment I'd like for this cab is to plug in a Marshall JCM800 and then hear how the top-end of a Strat compares to a 4 x 12" cabinet.
And so to the woofer, the sub, the very bottom.

first sketch of the cab design

Divide and conquer

The bass end of the spectrum is notoriously difficult to reproduce cleanly, and the 'loudspeaker' has a reputation for being the most inefficient component of the electrical chain; with the task of converting electricity back into air pressure.
If the efficiency of a speaker could be improved, say, from 10% to 20% by a great design, would it double the perceived quality of the sound? Ask a hi-fi buff; suffice to say that we can partially compensate for the lack of volume in a speaker cabinet by increasing its headroom, and of course, the quality of the speaker itself. By plugging the amplifier into a high-rated, and highly rated speaker, one is assured that there will be less overload, less distortion, less struggling.
So one might consider a bi-amp set up, as is prevalent in hi-fi systems. The principle has evolved to allow terribly inefficient speakers to do what they do best; to project sound in the frequency range they find comfortable.

Heavy set

You can download speaker cabinet calculators, punch in your chosen speakers, input the Thiele/Small parameters, fine-tune the cabinet dimensions, add ducts or ports, until you have the bode plot of a bass response fit for royalty. No-one else in the world will have a bass cab like this, you pronounce... since not many bass players have the size of car it will require to shift it.
Bass cabinets, just like double basses, are a compromise. They are too small for the task we want them to perform. And generally, we are glad of it, for the convenience of taking up less space, the grace of transferring from the car without removing any paintwork, the boon of maintaining a healthy spine.

Power to weight

Similarly one can choose a larger amp and keep it well below its operating limit, to ensure enough drive current is available for the speaker. Both of these efforts have a cost in weight however. A large bass amp has a huge transformer to supply that drive current, though advances in switch-mode power supplies offer a lighter future. And a high power speaker needs a powerful magnet, so I opted for drivers with neodymium magnets. These units are not (yet) available in super high power ratings so I have doubled up, with two 12" Celestions hoping to shrug off the washing machine jibes in a new compact home.
It's not really too big, it's the size it needed to be at the time; but now I want it to be smaller! And to look good underneath the mid-range cabinet, of course.

Being smaller compromises the bass response, but if I can get a good response from the 4 x 6", then there must be room for tweaking the big cabinet, as I see it. Even if I am headed for the same stomping ground as the "bass combo", the worst of all short-cuts!

More to follow...