Stacking/combining speaker cabinets

[drurb]

A number of folks have asked me, in threads and PMs, to comment on the practice of stacking cabinets so I thought I’d shoot for a comprehensive answer in one place. In general, if one is planning to stack or gang cabinets, the best result will be achieved if they are the same model from the same manufacturer. What follows assumes that one wishes to drive, with a common amplifier, two cabinets that are in very close proximity such that the effective path length from each to the listener is essentially identical.


The Phase Crapshoot

Suppose you have two cabinets from different manufacturers that have, for all intents and purposes, the same sensitivity and the same frequency response. What will happen when they are stacked? Well, it’s not clear. The reason is that, depending upon the relative phase responses of the cabinets, one can end up with constructive (addition) or destructive (subtraction) interference. That is, their combined acoustic output may be greater or less than either alone! Furthermore, constructive and destructive interference will vary across the spectrum. This is especially true when multi-driver systems with crossovers are considered. In such systems, the phase response varies (sometimes wildly) across the spectrum. This is a function of the drivers and the design of the crossover. In fact, it is not uncommon for manufacturers to connect drivers in a multi-driver system out-of-phase with respect to each other because this sometimes smooths the frequency response. (For the technically and hi-fi savvy, I am not considering “time-aligned” crossovers which typically do not (never?) appear in instrument cabinets as there is no real call or need for them). All this means that, when two cabinets of different design are stacked, some parts of the spectrum (frequencies) will be partially reinforced while others will be cancelled. At very high frequencies, because of a combination of near-reflections and their short wavelengths, this does not turn out to be much of a problem. We, however, play bass!!! The wavelengths of the fundamental frequencies with which we are concerned are measured in feet. For these frequencies, the effects described are real and can, indeed, be quite dramatic. (Go home and wire your stereo speakers out-of-phase and sit equidistant from them. Where did the bass go?). Now, if you stack two identical cabinets, you can nearly achieve a 6 dB increase in output because their outputs will, at the very low frequencies, come close to adding perfectly in phase.


Sensitivity Disparities

Suppose you have two cabinets that have the same frequency response but differ in sensitivity by 10 dB. What will happen when you stack them? Well, not much! Assuming their phases add orthogonally (i.e., not in-phase, not out-of-phase, but half-way in between, as in 90°), adding the less sensitive cabinet will increase the output by a whopping 0.4 dB over that achieved with the more-sensitive cabinet alone.

Frequency-response disparities

Now, add to both of the above, the condition wherein the cabinets have different frequency responses. I think it is clear how this, especially in addition to phase disparities, can end up yielding a very ragged, and probably quite undesirable, overall frequency response.


Summary

So here’s the bottom line. If you want to stack cabinets under the conditions described above, your best bet is to get two identical ones. You’ll get the biggest bang for your buck in terms of addition of acoustic power and increase in perceived loudness.

Okay, I can just hear some of you who will note that you have managed to stack different cabinets with a good overall result. More power to you (pun intended). Given all the considerations, you are most fortunate. Enjoy your success! My motivation here was to try and help those who may be considering spending their hard-earned cash on a second (and not identical) cabinet in the hopes that, in combination with the one they own, their sound will be improved.

Finally, I have intentionally not considered the case where one wishes to place two cabinets at very different locations directed at different parts of the room. That’s another story entirely.

[fdeck]

Thanks for the nice treatment of this perplexing issue. Given that I am planning on doing what you suggest (stacking two cabs when I need more volume), it is helpful to know the benefits and pitfalls). In my case, a second identical cab means buying another $48 Eminence driver and some plywood ;-)

[panos]

DRURB, thanks for your useful informations and technical analyses. Really, some of us (including me first) do thoughts like "i like the A cab, i like the B cab, if i get these two and stack them,then i'll have a better result" . But things are not often so.

I was wondering what could be the result of stacking an Epi 110 or 112 (sensivity 100dB SPL, 8ohm) with a Acme B1 (sensivity 90dB SPL, 8ohm). The theory has an answer: Not something great! better to avoid it.

I think that Adrian Cho was lucky to combine the LDS 1x8 with Acme B1 with succes, relative to their sensivity, it leaved me the impression that he didn't check the sensivity of LDS. Adrian, when you'll test again your two cabs together, can you check them in different phases, and tell us if you find constructive and destructive interferences? It could be a very interesting issue.

...and a question: all these interferences are in effect considering a maximum distance between the two cabs? otherwise, how important can be the factor "distance from one cab to another", how it works this?

[drurb]

Originally Posted by panos

...
...and a question: all these interferences are in effect considering a maximum distance between the two cabs? otherwise, how important can be the factor "distance from one cab to another", how it works this?

Well, what I wrote assumes a minimum distance between the cabinets. Now, as you separate the cabinets, you can, of course produce all sorts of changes in the interference pattern for a listener equidistant from the two. If one is pointed toward one group and another toward a second group, it's another story entirely (as I mentioned in the previous post). Then there are the standing wave patterns... This all becomes way to complex and involved for this forum (and perhaps, for me).

[Tbeers]

Why not just stack them and point them in different directions?

[fdeck]

Originally Posted by panos

I think that Adrian Cho was lucky to combine the LDS 1x8 with Acme B1 with succes, relative to their sensivity, it leaved me the impression that he didn't check the sensivity of LDS.

It would surprise me if a 1x8 had more than roughly 93 dB of sensitivity in the lower frequencies. And there are doubtlessly frequencies where the Acme is more sensitive.

[drurb]

Originally Posted by Tbeers

Why not just stack them and point them in different directions?

Well, that's fine but I was under the impression that most wanted to achieve more oomph within a common plane by stacking cabinets.

[fdeck]

Originally Posted by Tbeers

Why not just stack them and point them in different directions?

Are bass speakers particularly directional?

[Adrian Cho]

Originally Posted by panos

I think that Adrian Cho was lucky to combine the LDS 1x8 with Acme B1 with succes, relative to their sensivity, it leaved me the impression that he didn't check the sensivity of LDS. Adrian, when you'll test again your two cabs together, can you check them in different phases, and tell us if you find constructive and destructive interferences? It could be a very interesting issue.

There are not to my knowledge any sensitivity specs for the LDS cabs but my ear tells me the LDS is a little more sensitive than the B-1. As for the phase of the cabs relative to one another, that is something that I can try although my ear tells me that there was no cancellation going on.

[panos]

DRURB,
I have a question:
What will be the result of stacking two identical cabinets relative to their loudness , but one at 8ohm impendance and the other at 4ohm?
which are the problems of this combination?

[drurb]

Originally Posted by panos

DRURB,
I have a question:
What will be the result of stacking two identical cabinets relative to their loudness , but one at 8ohm impendance and the other at 4ohm?
which are the problems of this combination?

Well, what matters is not the impedance, per se, but the output of each, which would be specified by the sensitvity. Let's take an idealized situation. We'll assume the two cabinets have the same sensitvity and the same frequency response. Further, we'll assume that we have an amplifier that is driving the two cabinets in parallel. The parallel setup will produce a common voltage across the inputs of the two cabinets. Now, assuming that the "4-ohm" cabinet is half the impedance of the "8-ohm" cabinet at all frequencies, twice the power will be developed into the 4-ohm cabinet and, under our ideal assumptions, twice the acoustic power will be delivered. So, the sound-pressure level of the 4-ohm cabinet will be 3 dB more than that produced by the 8-ohm cabinet. Finally, assuming all of the above to be true, adding the 8-ohm cabinet to the 4-ohm will produce an overall increase in sound-pressure of 1.76 dB (assuming independent addition of phase). This will yield only a meager increase in loudness.

[Ric Vice]

DRURB,
This thread is very interesting to me because I've always purchased
two of the identical cabinet, when I wanted to achieve a louder sound. I just did it intuitively, I guess because aesthetically it looked better to me, to have two of the same cabinets. Thus I have owned, two Bag End 15" Cabinets, two Bose 802's, two Flite Sound 2x8 Cabinets with tweeters, and recently two 3 way Euphonic Arts VL 108's.

What I have discovered through all these "experiments," is that even when the cabinet's are identical, the end result is a louder system that's usually more prone to feedback. So for me at least, I opt for progressively
larger cabinet's rather than using a matched pair. So rather than use two 108's I'm currently having a LDS 2x8 built. Granted it's more cumbersome, but it suits my needs better without feeding back or turning up.

Ric

[drurb]

Originally Posted by Ric Vice

DRURB,
This thread is very interesting to me because I've always purchased
two of the identical cabinet, when I wanted to achieve a louder sound. I just did it intuitively, I guess because aesthetically it looked better to me, to have two of the same cabinets. Thus I have owned, two Bag End 15" Cabinets, two Bose 802's, two Flite Sound 2x8 Cabinets with tweeters, and recently two 3 way Euphonic Arts VL 108's.

What I have discovered through all these "experiments," is that even when the cabinet's are identical, the end result is a louder system that's usually more prone to feedback. So for me at least, I opt for progressively
larger cabinet's rather than using a matched pair. So rather than use two 108's I'm currently having a LDS 2x8 built. Granted it's more cumbersome, but it suits my needs better without feeding back or turning up.

Ric

Indeed! I would recommend your approach to anyone. Thank you for making this explicit. Because of all the potential problems and pitfalls mentioned, I would always opt for a single cabinet that suits my needs over a pair of stacked ones.

[bobyoung53]

I've been using either a single, and sometimes 2 sealed Ampeg 8 x 10's with SVT heads which can probably easily surpass 120 db for 30 years now and I hear fine or two Ampeg single 15" ported cabs. I have stacked the 15" cabs both facing forward, also had the bottom one facing forward with the top facing the drummer and have also had them side by side on the floor close and far away (10'-15'). They are bassiest and the least clear with both on the floor, clearest with both facing forward, and the drummer hears the bass the best and likes it best with one facing him which is a nice compromise. This all depends on the room however of course, every room is different and I usually experiment with placement. By the way the louder I play the less bottom I use usually if that is what you wanted to know, it cleans up the sound. I like two cabs better anytime I can use them as the sound is much bigger.