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Thursday, June 14, 2012

Marshall Cab Build (2)

  Now supposing I were to put just two 12" bass speakers in this cabinet.

The question then becomes, just where should they go?

The most significant alignment has to do with the distance from the floor.

I found this interesting bit of info on distance of speaker to floor:

Optimum frequency response curves in the bass range.

a. The influence of the floor reflection

Depending on the woofers' height over the floor, the contribution from the floor will be in phase with the direct radiation from the driver up to a higher or lower frequency. Some examples are shown below.

Height: 70 cm (29 in") > 250 Hz (1st cancellation at approximately 500 Hz).
Height: 30 cm (11.8") > 600 Hz (1st cancellation at approximately 1200 Hz).
Height: 10 cm > 1800 Hz (1st cancellation at approximately 3600 Hz).

Below this frequency the level will be elevated approximately 2-6 dB compared to higher frequencies. To be more specific, the level is elevated 6 dB when the reflection is in phase with the direct radiation and, on an average, it is elevated 3 dB in the higher range where the reflection appears in random phase.

Both the 6 dB increase at low frequencies and the 3 dB increase at higher frequencies can be less depending on how elastic the reflective surface is (for low frequencies) and how absorbing it is (for high frequencies). Therefore, the interval is 2-6 dB.

b. The cavity effect of the room
Real rooms contribute with more bass increase than if they had been a "theatre box" open in the front (this is not a "Voice of the Theatre" speaker box, but the place you sit in a theatre). In practice, there are of course many variations between different listening rooms, but it is possible to draw a "typical room curve". From it you can also see some typical floor modifications.

Click the image to open in full size.
Comments: The influences of the standing waves in the room are not included in these curves for several reasons. Firstly, they belong to the listener's acoustic in the "theatre box". Secondly, there are no "general standing waves"; all rooms are unique. It should also be pointed out that a room-adapted speaker will have an inverted frequency response compared with these curves. You can also see that you do not have to worry about the floor reflection if the woofer is placed low to the ground and the midrange is placed high and the crossover point is placed between the floor knees of the two drivers. You should, however, see to it that the woofer has 2-4 dB less output in its free field response, without the help from the floor.

Height: 70 cm > 250 Hz (1st cancellation at approximately 500 Hz).
Height: 30 cm > 600 Hz (1st cancellation at approximately 1200 Hz).
Height: 10 cm > 1800 Hz (1st cancellation at approximately 3600 Hz).

(the upper position in my 4 x 12" cab is about 23", or 55 cm) >= 700 Hz?)

Judging from this tidbit, it looks like I could fine-tune
the response just a bit further,
by mounting the speakers high enough from the floor,
to say reinforce at the low B or just above it on a 5-string!

I don't know how he is calculating this,
but it looks like an approximate curve could be extended to say 40 Hz, and maybe the speaker would still be inside the cabinet!

Oops nevermind: actually it looks like I'd have to position the cabinet about 2-3 feet off the ground!

In the lower position (about 7" from floor) it looks like maybe a 5kHz boost!


Here is my build update:

Pic 1 shows me adding a permanent divide between the two sides.
This allows one 12" bass speaker to drive each side, and have its own box.

The divider is also 1" plywood from the same sheet of laminated tabletop I got free.
This makes the two sides totally independent, and also allows lots of glue-surface
and screw-down wood to sink into to hold both the front and back panels rigid,
and attached to each other across the center.

Pic 2 shows 2x2 (really 1.5 x 1.5" pine/spruce along the top and bottom edge
for sealing and screwing down the front plate(s).

Note also the chunk of 2" plywood triangle in the center back,
to give the screws on the back panel something to bite into.

Pic 3 shows a detail of some triangular glued supports to keep the sides and back connected.

Pic 4 below shows how a half-panel can be easily substituted to make the cabinet bass/guitar combo,
or for comparing different guitar speaker-pairs.
Panels can be put in vertically or horizontally, so for instance,
I only have to cut 2 12" holes, one in each panel to have a complete bass cabinet.
Extra panels are easy to make, for swapping in and out!

You will notice the rabbit has expressed his usual dim view of speaker projects.
In fact he is quite unimpressed with the slippery top, although it is water-proof,
and probably rabbit-proof too.

Pic 5 shows the bottom panel (same as top). They receive some stiffness from the side supports, but tapping various sides and compartment walls will reveal rattles, weak gluing, unusual resonance problems and/or areas needing more stiffness/support.

In this case, the bottom and top are still sounding a little like
the "Three Stooges Pop", so I'll need to dampen those two panels at least.
The side panels are stiff as a board, from the cross-divider.

I estimate I lost about a half a cubic foot of internal space adding these supports,
but I also gained most of it back by simply letting the 3/8" plywood front-panel supports to raise the front-panel forward over 3/8" (i.e., 3' x 3' front = 9 sq ft x 3/8"). 

Because the front panel at the moment is 1" thick,
there is plenty of room for front-mounting speakers
and also sinking them into the top with a ridge/lip to make them flush with the front.

All the wood I got free so far, within a block and a half of my house.
The plywood tabletop I carried home, and probably would have
had to carry it the same distance to and from my car if I had bought it...

The other wood is from a dumpster bin at a house-reno down the street.
I got plenty of 3/8" plywood scraps and 2x2" strips, and 2x4 end-pieces,
and old (real) 2x4s too, for fixing my garage, which they were tossing.

My cost so far:

handles $9 each = $18.00 (Speaker Shop)
wheels $5 each = $20.00 (Home Depot)
screws $5?

One circular saw and extra fine blade $100.00 (however, I'll be using this for more projects!)

Also, I may buy a plunge-router (used at pawnshop) and blade to do speaker cutouts.
(estimate $50?)

Call it $150 in parts/tools, and a couple of days fooling around with son building it (priceless!)

They are selling a ready-made carpeted box with handles (no wheels) out of chipboard (no supports),
for about $160. So I'm still ahead, and its a great DIY project!
I would have had to do the same amount of reinforcing to the premade cabinet, and add wheels...

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