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Thursday, June 14, 2012

Marshall Cab Build (1)

I followed the basic plans for a straight 4x12" marshall stack cabinet.

Only I plan to seal the back up and reinforce it,
and mount the speakers so they are removable from the front.

Thing is, I'd like this cabinet to work as a bass-cabinet too,
and have it go down to about 30 Hz for a 5-string bass.

 I got a few ideas from here:

Always be sure to select a port size that is roughly half the diameter of the driver you will be using. This prevents wind noise in the tube, which is kind of like blowing across the opening of a partially filled bottle, and hearing the resonate hum. I like to use 10 inch drivers, with a port tube size of 3 to 4 inches.
One design I finished recently used 4, 10" Dayton drivers, and used a pair of 4" vent tubes approximately 7 inches long each. The cabinet dimensions were roughly 24" by 24" wide and high, and the depth was about 13". Although the cabinet is quite large and heavy, it has a overall F3 of 29 Hz, which allows a five string bass to really push out the low B string with authority.

 Now the thing is, I've already built my basic cabinet,

which is now 30" x 30" x 12 1/2" outside,
out of 1" thick counter-top coated real plywood.
(someone threw out a 3 1/2 x 7 ' sheet! formerly a table)

Inner dimensions 28" x 28 3/8" x about 11" = 5.026 cubic feet


27.9 cm x 71.9 cm x 71.1 cm = 142627.311 cm3 = 0.142627311 m3 = 142.627311 Liters.

I used the following calculator to see what the standing wave frequencies of this space was:

Oddly, this cabinet looks pretty good, in spite of two dimensions being very close.

I plan to reinforce the cabinet more, and use some deadening sealing foam and stuffing to remove resonances/reflections.

The front panel will be mounted about an inch in
to allow front-mounting the speakers from the front.

I'm wondering how to pick speakers, and/or a porting option,
in order to give a very flat response down to about 35 Hz.


I'm not sure what to do next, or what speakers I should be looking at.
Here are some good comments from the thread-posters:

 "Most bass guitar 4x10 cabs have 1.2 - 1.5 cf per driver so your cabinet is about right. "  The 10 inch drivers allow a lower system resonance (all else being equal).
 A 12" driver requires more cabinet volume than a 10" driver, so, comparitively, the 10" sees a larger cabinet, so the system resonance will be lower, giving more bass extension."
 you already have a ~5 cubic foot enclosure and you need to put some speakers in it.

My friend used a couple of these in some sealed cab for his personal system

Tang Band WQ-1858 12" Aluminum Subwoofer 264-897

Very low Fs of 20Hz, also large xmax of 13mm. Vas is a bit big so thats only two drivers, not 4

 Thing is, the more Xmax you go for, the lower the sensitivity of the resulting driver.

Forget about 30hz. Traditionally, bass cabinets never go that low, and if they do, you will have problems. They often start rolling off around 100. You are hearing the harmonics of those notes in the video, with very little fundamental.

Using a sealed cabinet will make the bass response roll off more gradually as it goes deeper, which will generally sound 'tighter' in a room."


I came up with a real strong front/back support scheme:
The St.Andrews cross-diagonals.

Click the image to open in full size.

This is like the Greek-cross idea, but actually better I think,
because it doesn't divide up the back resonating drum-head
into smaller drumheads.
Instead, the triangular shapes quickly dampen and kill resonances
and turn standing waves into quiet heat.

Also, the cross supports the front panel in five places, including
the front center, the most important zone. I am also toying with
adding 2x4 or 2x2 struts at 12 o-clock, 3 oclock, 6 oclock and 9 oclock,
about half-way from center, to further support the front-panel at its weakest points.

As well, there will be a supporting rim (probably 2x2 (1.5 really)) running the inside perimeter.

There will also be diagonal treatments of the for sides/top/bottom, to triangulize and deaden those.

Finally, I will add heavy felt on the sides and foam the corners
and stuff the cabinet with medium weight pillow/comforter fluff.

What think ye?

Now is the time to make construction suggestions.
The back, sides, front are all 1" plywood covered with counter-top laminate.

My friend at the speaker shop suggested avoiding wheels, for a bass cab.
So I came up with this ingenious idea: Put wheels on bottom (relative to handles)
and put rubber feet on side (flip on side for playing)!
Ta Da! see picture 3.

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