Stereo Subs - Do they need to be identical?

I'm thinking of getting a couple of subwoofers to mate with my MBL 121 monitors. Generally, I've seen setups where people use two subs of the same brand and size. I'm wondering whether it can make sense to use two different sizes. For example - a Venoyne DD12 and a DD15. I'm likely going to go with Velodyne DD series or JL Audio.

In particular, I note that the big Velodyne 1812 uses an 18 inch and 12 inch woofer, with suppsed benefits in terms of speed (12 inch) and extension and depth (18 inch).
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Because you are are going to be using the subs, for more than just the bottom octave with your monitors...I would go with two of the exact same subs. You ear may pick up on any tone differences between the two subs.

If you were only going to use the subs for the lowest octave (20-40hz)...I doubt it would matter if the subs were an exact match...those freq's are well below what your ear can hear.

I will have to weigh in with an opposing view. I have had good luck using 2 totally different subs in the same system. YMMV, but I can see no reason not to try it, especially if you already own one sub and are adding a 2nd.
I don't think there would be a problem using different subs as long as you are crossing them over low enough but I would go with the same size myself. I don't see any merit to the smaller sub being faster theory. First of all a small sub has to move further to produce the same amount of bass as a larger sub therefore negating the speed advantage. In addition bass is slower because it is a longer frequency. If it were fast it wouldn't be bass. A bass drum is larger than a snare for a reason.
I have found no correlation between size and performance in subs. My subs have 10" and 12" drivers and go very low. The idea of a big slow cone for a woofer was prevalent when I got into audio in the early 60s. Then you had the Electro-Voice with a 30" woofer and the Hartley with a 24" woofer. 15s were common in 3 way systems. As speaker design has progressed woofers have , by and large, grown progressively smaller. Now the $33,000 Avalon Diamond , which may be the best speaker in the world, uses 2 7.5" woofers. It is the overall design of the sub that counts , not the diameter of the woofer. I would not worry about mixing different sizes, AS LONG AS, you are crossing them over sufficiently low. I use 22 Hz on mine. If you could hear a difference between them then you would be crossing too high, you should NEVER be able to hear a sub, just feel them.
There is a reason subs can be smaller now but you still can't defy the laws of physics. A small sub in a small box requires much more cone excursion than a larger design. For a small sub to equal the output of a large one it takes much more power and also equalizing to go as low. The driver also has to capable of much greater movement. Computer aided design and servo control has been a blessing when it comes to sub design. Also digital amps allow high power with low heat and low price. Crossing over as low as posible is the secret to seemingly fast bass response not the size of the sub.
If you can "hear" the sub, as opposed to "feeling" it, then different sizes should be discernible to your ears as differences in tone or volume. Therefore, I would prefer to go with identical subs. If you do use non-identical subs, I think that the crossover frequency should be quite low so as to avoid hearing any differences. Keep in mind that the open E string on an electric guitar is somewhere in the mid 30's in terms of Hertz. I wouldn't want to crossover any higher than that.

As far as sub speed goes, my experience is that the higher the crossover frequency, the slower and more sluggish it makes the main speakers sound. How fast a sub is depends on a lot of design aspects in addition to cone size so I wouldn't feel comfortable making any generalizations. Speaker designers can tell you a lot more than I can on this point so I'll defer to them.
I would say that... it depends.

In my opinion the goal is to get a smooth in-room response in the bass region, and a smooth (perceptually seamless) blend with the main speakers. Depending mainly on the frequency response characteristics of the subs and their low-pass filter, their positioning, and the rolloff characteristics of the main speakers (whether or not they are high-pass filtered), it's possible that in some situations identical subs would give a better blend, and in some dissimilar subs would give a better blend. In general, I would expect dissimilar subs to be a bit smoother. Let me explain why:

In my opinion, most subs are not designed with room gain correctly taken into account. Here's a link to a graph that I think was generated by Martin Colloms, depicting "typical" room gain (roughly 3 dB per octave below 100 Hz):

So if you take a sub that's "flat" to 25 Hz (anechoic), in-room it may well be +6 dB at 25 Hz - which will sound heavy and boomy.

Now suppose you have two subs, one of which is flat to 40 Hz, and the other flat to 25 Hz. The summed response of these two subs may well come fairly close to approximating the inverse of room gain.

If you start out with subs whose anechoic frequency response is approximately the inverse of room gain, then I think two such subs would be smoother in-room than two dissimilar ones.

Another factor to consider is where the subs will be placed, and the slope of their low-pass filter. Placing the subs far apart (but not in corners) is usually beneficial if smooth bass is the goal, but the farther the subs are from the main speakers the more important it is that they have a steep-slope lowpass filter so that lower midrange energy doesn't come through loud enough to give away their location. I personally favor highly asymmetrical placement, but if the crossover is high and/or the slope shallow that might not work well.

Using two different size subs can be very helpful in evening out room nodes. Place them at opposite ends of the room. With monitors as mains, I would use two identical subs. Proper integration is way more than twice as difficult, but the effort is worth it.
>There is a reason subs can be smaller now

1. Solid state amplifiers technologies which generate less waste heat so you can run modest sized (and less expensive) heat sinks and stick them in a small box like class D (switch between full on and off so the output device is only dissipating heat when it switches) or G/H (switch between output rails to limit power dissipation) amplifiers.

2. Better understanding of math and computer modeling. 4th (3dB) and 6th order (I haven't looked at bandpasses) designs can be more efficient over a limited (real sub-woofers don't cover much over an octave) bandwidth and we don't need to rely on trial and error to build ones with relatively flat response.

3. Better motor designs help some. You can get more force for a given current + power to make drivers work in smaller boxes. It just might take 40 pounds of driver to get there.

>but you still can't defy the laws of physics. A small sub in a small box requires much more cone excursion than a larger design.

Nope. Output is purely a function of the air you move. With the air spring formed by a box and driver cone increasing in stiffness as size goes down, it just takes more force, current, and therefore voltage + power to get there.

At a fixed frequency, a given driver surface area will require the same excursion to reach a certain output level whether in a .5 cubic foot box or 4 cubic foot box; although 100W may be more than enough in the 4 cubic foot box while 800W isn't (thermal compression is a much bigger issue) in the half cubic foot box.

>For a small sub to equal the output of a large one it takes much more power


>and also equalizing to go as low.

You can get there with a combination of mechanical parameters and equalization, although if you've pushed the resonance down through a lot of mass the sub-woofer is going to be inefficient through it's entire operating range (at the top, power requirements are dominated by the current it takes to accelerate the moving mass). If you want a small box it's better to have a sub woofer that's inefficient in the last octave where there isn't much musical content but 10dB better in the second octave.

>The driver also has to capable of much greater movement.

Same movement either way. Only a resonant device (port or passive radiator helps). You'll need a more expensive passive radiator (driver without a motor) because of space constraints in a small box.

>Computer aided design and servo control has been a blessing when it comes to sub design. Also digital amps allow high power with low heat and low price. Crossing over as low as posible is the secret to seemingly fast bass response not the size of the sub.

It's the transfer function. The same transfer function (amplitude and time domains; distortion spectrum within reason; where the power response is coming from at high frequencies because of HRTF) sounds the same regardless of how you got there

You can get "fast" bass from a lack of low to mid bass. With power response summing +3dB over a single speaker when you feed the same signal to a stereo pair at higher frequencies and 6dB at low. Sound sources spaced out from a wall have nulls at their quarter wave length spacing (70Hz for 4' to the front wall). A low cross-over means you have the SBIR null in the main speakers' pass band and are no longer getting the artificial boost at low frequencies.

Of course, neither "fast" nor "slow" bass is what you get with real music like a dude going to town on an upright bass in your favorite corner bar. Real bass just is.
As you go towards small ported subs you tend to get more harmonic distortion thermal compression, lower overall SPL output and poor transient response. It is just physics and heavy helpings of compromise. Larger is better when it comes to subs. Sealed box usually has the best transient response even if output SPL is correspondingly lower.

Two subs or one - I don't think it matters that much - for best sound I would not recommend to place a stereo sub next to each main speaker. Definitely an asymmetrical placement will be better.
Again, if using monitors, stereo subs next to them is definitely the general recommendation. And it most assuredly does matter that much!
Again, if using monitors, stereo subs next to them is definitely the general recommendation. And it most assuredly does matter that much
The reason I suggest to avoid stereo sub placement like you describe is to
avoid quarter wave rear wall cancellation in the bass that you always get from symmetrical placement of full range speakers. The great advantage of a sub is that you can reduce this problem with an asymmetrical placement.

I agree that stereo subs next to speakers is definitely the "general recommendation" though. With so many free standing full range speakers out there, I suspect most people don't worry about these details or are blissfully unaware. No wonder there are so many room acoustic bass problems and speaker demos never sound the same twice and some companies have special "setup" technicians trying to figure the best sounding compromise in placement of large full range speakers.
Following the advice of mfgrs who know much more than I has worked too many times to count. Perhaps you are using monitors too close to the back wall. In theater applications asymmetrical is the way. Please try what these folks (mfgrs) recommend first before dismissing it based simply on theory.
Thanks guys for all the responses - a lot to think about. I have a key, related question. My MBL monitors have a low pass filter that cuts the bass off at 49hz - that's the lower limit of the bass.

Were I to introduce two subs to the mix, where would you recommend I mark the upper crossover point for the subs? And, would it be worth introducing some external crossover to cross the MBLs even higher (not force them to go as low as 49hz). Finally, even then, would some room correction unit be a welcome addition (like the Copland DRC) - maybe I'm straying too off topic here, so would need a seperate thred, but I would like to resolve these questions together and they seem tightly related. Overall, I'm looking for absolute seamless integration for two channel music. I had thought of getting 12inch subs because I may move to an apartment in the near future and I want to make sure I don't overpower the room.

To answer one of the questions above - my MBLs are about 4 feet from the back wall and three feet from the sides - its a large open plan room. I hope to upload updated system photos later this weekend (after I tidy the room ;-) )

i do not see a tv or projector. is this a 2channel music set up? if so skip the subs you really don't need them unless you are looking for an exagerated bottom end. subs very tuff to set up with out affecting the mids and upper end and not being an exagerated lower end. first class home theater system, yes(for action movies only), 2 channel music only NO! my thoughts anyway. could move that tt off the ground though.
There is much to be said about a simple system but 49 hz just isn't great for rock or classical. A sub crossed over at around 50hz with a steep slope should be very non directional and integrate well in your system.
Shadorne, I think it depends upon the distance from the rear wall and the construction of that wall. Frequencies below about 45 hz or so pass through typical sheetrock walls and so no cancellation takes place. Therefore speakers more than 6 feet or so from this wall will not be affected by 1/4 wavelength cancellations. If the wall is reinforced then it is still O.K. if the speakers are more than 8 feet or so away as the loss will be attenuated by typical " room gain" below 35 hz or so. If 6 or 8 feet is not practical then try pushing the speakers very close to the rear wall and use thick absorption behind them. ( thick enough to absorb 125 hz) If this is not practical then yes nonalignment is a good idea. My bias is that the same frequency (crossover region) coming from two different sources at two different distances always creates phase issues and its own cancellation issues as well as slurred imaging in frequencies high enough to be directional. A lower crossover of course limits the damage of the last issue but not the first two. Each case is unique and the only way to know for sure is to experiment which in my experience takes a lot of time and patience and in any given case can lead to unique solutions. JMHO - Jim
In some cases, the quarter-wave cancellation from the bounce off the rear wall can be taken advantage of to tame a peak.

As for asymmetrical placement, here's a link to a brief informal paper by one of the leading researchers in psychoacoustics, Earl Geddes. He's comparing four subs in corners vs four subs asymmetrical, so it's not exactly what's being discussed here, but might be worth taking a look at as it illustrates one of the advantages of asymmetrical placement. The thing to look at is how much the peaks and valleys are reduced for each of the three listening positions in the asymmetrical case (represented by different color lines):

Funnily enough the link I gave above came from a manufacturer...

The link also shows up to a 20 DB SPL hole in the bass in some cases due to quarter wave cancellation and a succession of harmonically related cancellations further up in frequency (a comb filter)...

Furthermore one might wonder why studios often soffit mount large full range speakers ?

These same studios often leave small (less than full range) near fields out far into the room on the meter bridge.

Perhaps there is some logic ....perhaps is is all just aesthetics...

Here is another speaker designer's comments on the reasons for soffit mounting speakers.

Read and draw your own conclusions. Dismiss the science if you like...
Here is the link to calculate the wall bounce from reflections. If you use stereo subs then you can assume the same thing will happen up to a couple of octaves above the crossover (when the effect starts to become negligible compared to the many other room effects/anomalies)
Yes science (theory) and experimentation are necessary to get you to where you need to go. Theory is a good starting place. Mounting into the wall (soffit) is an option and can be tried but you also have to factor into the equation the room length and how this placement will specifically fit into that. My point is that theory is just a starting point and that when you really get into this particular project you have to do your own listening and your own measurements because the variables in any given case are so numerous as to render any given prediction based soley on theory ( even very sound theory) woefully insufficient. - Jim
Dear Outlier and friends: IMHO it is better to have to identical subs it does not matters which is the crossover frequency.

There are IMHO many subjects on the subs integration and maybe many things to know about, here are some links that can be interesting to read:

There are to much to " explore " on the subs subject ( a complex one ) and there are a lot of misunderstood about, IMHO I think that till today the last word on subs is/are not writing yet.

Regards and enjoy the music.

Thanks - Earl's paper proves what I said - symmetry in the bass is likely to
give you stronger (more coherent) peaks and suckouts than an asymmetrical

I would add that Aldavis point about phase is a real concern. I'd prefer to have
the timing of the primary sub signals to arrive at the listener all at the correct
time. So in practice this limits the amount of asymmetry that can be achieved
(to the left or right side wall at a similar distance to the listener as the mains).