Twin cab arrangements

[mcgraham]

Hi Alex

I run a Berg IP and EX112 rig. Other than the IP having a tweeter and being powered, they are identical. I tend to stack these, one on top of the other with normal orientation and front panels aligned to be flush with one another.

I was wondering about other twin cab arrangements... e.g.

immediately adjacent side-by-side, front panels in the same plane
side-by-side but with distance between them, front panels in the same plane
side-by-side, one firing at 90' to the other (opposite rear edges touching) etc

and what their advantages/disadvantages might be? In particular, benefits to room coverage.

Thanks for your time,

Mark

[Stewart]

Generally drivers closest together, vertically arranged.
Gives least interference (leading to comb filtering), and best dispersion from side to side.

[alexclaber]

Hi Mark,

I'm in the midst of writing a comprehensive user manual for the various Barefaced cabs which will cover this in more detail but here's some stuff to be going on with. When you place two speakers side by side they couple below certain frequencies and act as one large speaker with equivalent dimensions to the dual speaker array. This has two effects: Firstly the efficiency of the system is increased due to improved coupling with the surrounding air. Secondly the dispersion of the system changes so that the off-axis response roll-off shifts down by an octave. This coupling and dispersion shift is what really matters with bass guitar amplification.

At higher frequencies the drivers act as two separate sound sources and depending on your distance from the sources the waveforms either constructively reinforce or destructively combine. This causes the oft mentioned comb filtering but it isn't a particularly big deal with bass guitar due to the signal content at these higher frequencies being fairly non-critical. Furthermore the combined direct and reflected sound we hear reduces the impact of comb filtering further.

Looking at your three examples, a vertical stack has reduced vertical dispersion and exhibits consistent comb filtering I he horizontal plane. A side by side pair turns that round by 90 degrees so narrower horizontal dispersion and consistent comb filtering in the vertical plane. A side by side outward angled pair will make the horizontal dispersion worse at lower frequencies but improve it more at higher frequencies but will suffer from worse comb filtering. A cross-fired pair will perform better but without a tweeter on both cabs this still isn't a good approach due to the HF ending up further out of whack.

One of the great things about a vertical stack is that it reduces floor reflections, improving coherence out front. It does mean that if you stand too close to the cab you'll miss out on upper mids but this can be remedied by offsettif the top cab by pushing it back a few inches (as the Midget+Compact does when it's stacked normally) or by tilting the while stack upwards slightly (not just the top cab).

It's easy to try all this stuff for yourself by playing around. Differences will be most obvious outdoors or in heavily furnished rooms (or indeed in a small venue packed with punters).

Edited July 17, 2010 by alexclaber