drurb
09-14-2005, 08:05 AM
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.
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.
