Open Backed Cabs - Do Dimensions Matter?

jaymenon

With closed back cabinets it is evident that the internal dimensions matter (standing waves, resonance and all that).

However, with a Fender style open-back cabinet, do the dimensions of the cab matter?  With the same amplifier / head, does varying the size of the enclosure make a difference to the tonality?

Expert opinions most appreciated...

ICBM

Yes, very much so.

If you compare something like a Mesa Mark IV which was available in ‘standard’ and ‘widebody’ sizes, there’s a big difference - the wider cab is much deeper-sounding and less directional.

I also recently re-cab’ed a Blues Junior into a 5E3-size cab, and there was a huge change in tone. OK the new cab was also pine, but I think the difference was much larger than that alone would make. 

p90fool

Yes, absolutely. I was running a pair of identical heads into old Fender combo cabinets with identical speakers and the cab which is one inch shallower and an inch and a half narrower has much less bass than the other. 

http://i65.tinypic.com/30tp01e.jpg

To further ICBM's Blues Junior example, I have one as an extension cab and it just refuses to produce anything other than a boxy rasp regardless of which speaker is in it or which amp I'm using.

It's now become a bass practice amp, for the simple reason that my neighbours won't ever be able to hear it!

ecc83

It is practically a "cut and try" situation. How any particular drive unit will react in a given enclosure depends upon many factors. Size and shape of box naturally but the cone mass/suspension compliance couples to the air (both inside and outside the box!) and determines the low frequency response. The amplifier will also influence this because although most valve guitar amps have buggerall damping factor (output Z) some do have a bit!

Paper cones are also notoriously difficult to QC and so what works in one box with ONE V30 might not be so good with another.

"Bigger" is generally "better" (especially the front to back dim') but beware that for closed back bass cabs, bigger means lower ultimate LF power handling.

Dave.

jaymenon

I found a post with useful info on the gear page by a gentleman called Khromo - reprinted below, since it's rather interesting.

The great news is, it's almost impossible to build an open back cabinet "wrong".

Leo Fender proved this by building great sounding amps in open back cabs. How did he do it? He built them as wide and deep as the chassis they would house, and just tall enough to accommodate the chosen driver(s). It wasn't intelligent, cutting edge design, and it wasn't dumb luck. It was just the nature of the open back cabs: unless you make it way too small, it will work fine, for the frequency range guitar players are concerned about.

If those cabs had been designed by an acoustic engineer, they never would have used horizontally aligned drivers in the bigger amps! Nobody does that, except guitar players!

(Although Fender wasn't completely oblivious to smart cab design. He did bless us with tilt-back legs! And we Princeton and Deluxe fans will never forget him for that! Thanks, Leo!)

The open back cab is primarily preventing the waves generated by the rear of the cone from canceling or otherwise negatively interfering with the waves generated by the front of the cone to any great degree. Those waves eventually mix and interact based to a great extent on the reflective surfaces behind and around the amp. Boxes that are too small will tend to fail here, and that is why a lot of guys warn against trying to go too small, but otherwise the open back cab design is just not really critical.

Don't get wrapped around the axle with a lot of bologna about the size and shape of the openings, or the dimensions of the box. We've built dozens of cabs, brought in guys with documented Golden Ears to test them, and found that where an open back cabinet was placed in a particular room made a lot more difference than the dimensions of the box or the size or shape of the opening in the back. 

Put it in a corner, you get one result. Start moving it different distances or angles from the wall behind it, and you get different results. Put it in a different room, it sounds different. The room, and where you place the cab, plays an enormous role in how an open back cabinet sounds to you.

In a more constructive vein, two things that will make a difference when you're planning your open back cab are the angle of the driver and the height of the driver from the ground (or think of it as the distance from the driver to your ear). Tilting the baffle back 10 to 15 degrees can make the cab a lot easier to hear, unless you are planning on lying down in front of the cab, or having it up on a stand or shelf. Getting the driver up off floor level in a taller box has a similar but less dramatic effect. Twelves beam like mad, and you want to build your cab with this in mind.

The fashionista in me likes to build 1x12's at least 20" wide or so, so most heads will "fit" on top without lapping over. Two cabs I'm currently building for a sweet tweed Twin chassis measure 22" wide, 17 1/2" high, and 11 3/4" deep. These are larger than many guys would build, but the width was dictated by the head they will be used with, the height was necessary to get the driver several inches off the ground, and the drivers will be front loaded, so they are a little deeper than a similar cab with a rear mounted driver. Since each cab will only house one driver, the weight will be reasonable in spite of the size. Glue on some crocodile pattern vinyl, and the women will be swarming all over me! Whoa!!!

One will have the baffle angled back, and the second cab will have the same footprint as the bottom of the angled cab, so when I use two, the drivers will be vertically aligned, with the top one angled up toward my deaf ears.

Don’t mount the driver in the center of the baffle, avoid random cab resonances, don’t make it too small, and don’t cut any fingers off with the table saw. You will be fine! Have fun!

p90fool

Sounds like "Khromo" has never had a BJ.