@Charles_Tidwell…I’m just wondering because I’ve seen a lot of these posts in the last few months. Twin subs that look to be almost the same size as the speakers at the base of yours. Does it make much difference in the sound? Necessary?
I apologize in advance for the extra long post guys… Sorry. 
Simple, short answer… Yes and Yes.
For the more technical, unusually long answer…
The size of the subwoofer doesn’t matter as much as its capabilities and what it can add to the system overall. Generally, the larger the sub, the louder, cleaner and deeper it can play, but there are exceptions to the rule, thanks to modern technology.
The ML SL3’s utilize a 10" long throw driver in their sealed enclosures that are rated to 30 Hz but are capable of reaching down to around 25 Hz in-room, but at much lower output levels than the rest of the speaker at that point making that output not very useful to the rest of the sound. And if you know about T/S parameters, it’s X-max (excursion) is right around 1 inch peak-to-peak with an RMS powering handling of something like 150 watts, 400 peak. Anything beyond those parameters, the woofer starts moving out of its range, creating non-linearities, cone flex, loads of distortion, and bottoming out. This ends up with horrible sounds, extra heat in the voice coil, damage to the driver or worse, a blown driver.
As you know, subwoofers are designed for such abuse as recreating bass and high volumes. Heavier cone materials, larger voice coils and motor structures, heavy duty suspension systems (spiders, large surrounds), beefy, heavy magnets, higher power handling, heat management, and greater excursion.
The Polk Audio PSW505 subs I was using before utilizes a 12" driver in a ported enclosure tuned to 23 Hz with 300 watts on tap. I believe it’s X-max is in the neighborhood of about 1.5 inch. Looking at the extra half inch of excursion or extra 2 inches of cone area don’t look like much, but when you add that extra cone area with the extra excursion, the Polk’s overall driver displacement is considerably more than that of the 10" driver in the SL3’s. That makes a difference.
In-room response, the Polk subs are capable of a solid 20 Hz, but because they are ported, their response drops off like a brick after that with pretty much zero output beyond that point (even with both subs playing). But within their capabilities, they are able to play those same frequencies as much louder and cleaner than the SL3’s, allowing to fill in where the SL3’s left off, giving a fuller, more full-range sound.
The JL Audio e110 subwoofers I’m now using have a 10" driver (same size as the SL3’s). These drivers are little monsters. The entire front baffle of the enclosure IS the actual driver basket. It’s a solid, thick aluminum square basket in front with the cone in the middle, so when you’re looking at the front of this subwoofer, that entire front end is the driver. Like I said, it’s a 10 inch driver, but it’s over 12 inches deep and weighs nearly 35 lbs on its own. It has an X-max of just over 2.5 inches and has a built-in 1200 watt RMS amp.
Like the Polk’s, the JL’s are rated to 23 Hz, but because they are sealed, they can easily extend lower than that, and when combining the output of both of them with room gain, their useable output reaches down to around 16 Hz. With their insane amount of X-max, their overall driver displacement is much greater than even the Polk’s 12" drivers. And because of their massive motor structures, dual floating spikers, large surrounds, huge voice coils and magnets, they play deep, clean, loud and are very quick to start and stop moving, making them perfect for music.
So in all of those regards, that should answer your first question.
To answer your second question, that’s really a matter of personal preference.
Some people feel that if their loudspeakers reach down to 40 Hz, then that’s all they need. Some feel that if they mainly listen to vocal, acoustic music, they don’t need a subwoofer. Some say that because there’s very little to no information below 30 Hz in music, there’s no need for a subwoofer. Some feel that only one subwoofer is needed while others think at least two are needed. Some think that because their loudspeakers are rated/capable down to 20 to 25 Hz, they don’t need a subwoofer.
I’m in the camp that says no matter what the music, no matter what the speakers are capable of, they ALL need at least a couple of subwoofers.
Of course, this is the point were the nay-sayers start posting up images of that chart that shows various instruments and their frequency ranges, which has absolutely nothing to do with anything. It’s a useless chart when discussing the subject at hand.
Reason #1… No matter what the music is or what instruments are used, there’s always some sort of bass content in the recording whether if it’s from the instruments or the room/venue that it is recorded in. This sort of bass content isn’t musical, but atmospheric, adding a sense of size, space, depth (scale), liveliness to the recording, even if it’s a fully digital, synthesizer recording. This sort of bass content really opens up the sound of your system, giving you a sense of “you are there” so to speak, getting you more involved in the music.
Reason #2… And this is the important one that everyone seems to either ignore or don’t completely grasp. Yes, sure, your loudspeakers may be capable of a solid 20 Hz output in a room, but most likely not in the locations they need to be for proper stereo imaging and sound staging, which means the loudspeakers are usually a good distance from the front and side walls. When the loudspeakers are in these positions, their bass output and extension drop some, sometime a little, sometimes a lot. Either way, the bass output and extension isn’t what it could be, which is where subwoofers come into play.
If you placed those loudspeakers in the positions for optimal bass reproduction, then stereo imaging and sound stage would suffer, probably drastically, but you would be getting really decent bass. Subwoofers allow you to have your cake and eat it too, placing the loudspeakers where they perform best for optimal stereo imaging and sound staging, and the subwoofers in the locations for optimal bass reproduction. It may just be a matter of a few inches or a few feet, but it all adds up in the end and serves up a complete, cohesive, full range sound.
So yes, I feel that subwoofers (2 or more) are definitely necessary for ALL loudspeaker systems.
Then there’s the whole subject of crossovers, crossover points, whether or not to crossover your main speakers or not, etc, etc. I won’t go into that since I’ve already written a book here. LOL
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But… that is where this starts to get interesting. And meaningful. Please do continue. I have my own opinion here of course. 
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I would, but I feel this subject requires its own thread. I don’t want to be hogging up this thread with off-topic jibber jabber.
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And now it does, fill your boots Great topic to discuss in depth.
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I see that. Thank you Carl!
Well, I’ll try to keep this short, but it probably won’t happen. Ha!
So as for crossovers, that too is confusing for some and is fairly subjective.
Some think the best bet is to set crossover points the same as your typical home theater receiver, which means set the crossover point to 80 Hz and don’t think about it again. Bad way of thinking IMHO, especially when it comes to perfect subwoofer(s) and main loudspeaker integration.
Other than the points I maid in my first post, the main goal with adding subwoofers is perfect integration so that you don’t even know there are subwoofers in the system. You want that extra bass extension and output to sound as natural as possibly, and have it sound as though it is coming from the main loudspeakers only. At no point should you be able to tell that there are sub in the system, nor should you be able to locate them by sound. Theoretically, the same should really hold true for the main loudspeakers as well. In a perfect world, if you were to close your eyes and listen to your system, you should be able to paint an image in your head of all the musicians, instruments, the venue, etc, etc just floating in space without the boundaries of your room holding it back. You “should” be able to picture the boundaries of the venue in which the music was performed in whether it be a studio, music hall, theater, church/cathedral, stadium, etc, etc.
So crossovers… Where do you start?
Personally, I get my mains set up and dialed in to where they sound best at my listening position, focusing only on stereo imaging and sound staging only, not paying any attention to their bass output or extension at all.
Then I focus on my subwoofers. I get them positioned to where they too sound best on their own at my listening position, getting great bass extension, punch, detail and least amount of boom possible. It takes a while, especially with more than one sub, but it’s required.
After the mains are dialed in as good as I can get them, I then focus on their bass performance. Of course, their position, your listening position, your room and furnishings all determine how the bass performance is going to be, both for the loudspeakers and subwoofer(s).
Just a side note, I’m a firm believer of allowing the main loudspeakers to remain playing full range, not crossing them over at all, even with small bookshelf speakers. Yes, you lose overall maximum volume output and power handling and demand more from your main amp and your loudspeakers, but I think it’s worth it and have always acquired best results this way, rather than crossing the main loudspeakers over.
Another side note… One other thing I’m a firm believer in is feeding your subwoofer(s) their signal via the main amplifier’s speaker outputs rather than feeding them the low level RCA/XLR signal from your preamp. In my experience, subwoofers always perform better getting a high level signal from the main amplifier than the low level signal option. The controls seem to have better adjustability for some reason and just the overall sound quality is drastically improved, better detail and tonality. You don’t get the issues of hum, RFI, EMI or ground loops either from this type of connection. At least I never have. This route also help your sub(s) to match better with your mains since they are getting the same “sound signature” of your amplifier that your main loudspeakers are receiving. This is probably why REL subs are so highly regarded as some of the best musical subs out there as they recommend being fed a high level signal with those Neutrik connectors/cables that they use from your main amplifier. It makes sense.
This was proven recently when I was still running my Polk subs. When I switched out the Schiit Aegir amps to the Roksan Caspian amp, the Roksan amp has as punchier, slightly heavier bass response than the Aegir amps, and that showed through not only with the SL3’s, but also with the subs, making those subs sound that much better, deeper and tighter.
Anyway, once I see where the loudspeakers are starting to drop off bass-wise is where I roughly set the crossover point on the subs. Once that and the gains are roughly set, I sit back and listen for a bit. From there, I start tweaking the crossover point more, either slightly up or slightly down, while also adjusting the gains and phase if needed.
If the mains start dropping off around 50 Hz, I set the crossover point on the subs to around 50 Hz. Depending on room, sub/loudspeaker interaction, etc, etc, that crossover point may end up upwards of 65 Hz or below 40 Hz. It’s a matter of much listening and tweaking to see what sounds best with best possible blend of the subs and loudspeakers, making for the most even, smoothest transition from one to the other, again, making it sound as one complete sound source; a natural extension of the main loudspeakers.
The same basic idea is true if you decide to cross your main loudspeakers over as well. Of course, this would require an outboard stand alone crossover unit in your system. Again, if your loudspeakers start rolling off around 50 Hz, you may want to set your high-pass crossover point at 50 Hz or a little higher at say 60 Hz on the mains. Then you would set your sub(s) to that same crossover point, tweaking gain and phase as well until you get a good, seamless blend from loudspeaker to sub.
Doing things this way, there’s two things I don’t particularly care for… You’re adding another component into the signal chain, and you’re not getting the full sound signature of your main loudspeakers. But you DO gain more clarity in in your mains, relieving them and your amplifier of the stress of reproducing all of the bass information. Keep in mind though, when you do things this way, you can no longer feed your sub(s) the signal from your amplifier since the amp is no longer getting a full range signal. So at this point, you are having to feed the subs via a low level signal, which may or may not perform as well.
Just as a reference, my SL3’s in this room start rolling off around in the low 30’s, going nearly silent below 25 Hz or so. I started with the subs crossed over at 40 Hz. Through trial and error, I had them up as high as 60 Hz and as low as 35 Hz. I eventually ended at just above 40 Hz, phase is at 0, variable phase (timing) is also at 0, and the gains are barely a third of the way up. While during the tweaking, I was also moving the subs around a little bit more as well. They started somewhere behind and to the center of the speakers, now they are off to the outside edges and about 8 inches behind the plane of the SL3’s woofers. Then placing the subs up in IsoAcoustics iso200sub isolators helped even more.
I hope this sheds some light on the subject.
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Charles, please could you say a little more about the process you use to achieve that? For example, do you do it by ear or by measurement (or both) and in either case what are you listening/looking for?
Just to take this off at a slight tangent, work by Floyd Toole into subwoofer placement showed that two subwoofers, one centrally in front of the listeners, and one centrally behind them, was very good for evening out the room modes, for the particular room he tested. The diagram comes from an article of his published some years ago. The bits in red, green, blue were my additions to the original diagram.
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Just to be clear, the ones you have boxed in red are the “ideal” positionings for that set-up - so two subs are best front and back middle? And what is the frame of reference here - what room is actually being tested?
The diagram comes from this article.
BTW it was published by Todd Welti and Allan Devantier, not Floyd Toole - I originally found it some time ago alongside a number of his articles and incorrectly assumed it was also his.
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Thanks for the thoughtful and very interesting response Charles. 
There are definitely different approaches to this subject. There are just too many details to cover, but will go over some main ones. I fully agree with you Charles on the use of two subwoofers, but surprisingly disagree on some of the other information.
Always 2, 4, 6 or higher number of subwoofers. I use 6 myself. Going from 2 to 4 made an enormous difference. The step up to 6 was more academic in nature. These even numbers are of course to 1) better fight room modes and resonances in the listening space 2) lower the distortion by sharing the work between many identical moving parts.
There is a good reason for the THX-standard for a crossover to be set at 80 Hz. It is sort of a middle ground, but where the positives outweighs the negatives. Also I absolutely, 100%, am an advocate for using high pass filter for the main speakers (and low pass for the subwoofers). We should relieve the mains of the hard task to play deep bass, in order to lower the distortion as much as possible. Membrane and surround moving will undoubtedly cause the distortion to rise. 80 Hz is also suitable because at lower frequencies, it is very hard to locate the separate subwoofers, especially in a normal listening room. Also you need good drivers which are not distorting creating sound in frequencies higher up, which makes them easier to identify.
Of course you also need a crossover good (and steep!) enough for this. My active filter is 30/18 (low/high pass). The built in crossovers in surround receivers are usually not step enough is my experience.
I absolutely prefer a placement in front of the listener, close to the main speakers. 1/4 in of the room width is a good approximation that has worked very well for me. Mine are now standing as towers right next to my main speakers in this position. Another good placement is evenly spread out along the speaker wall. This will fight the room modes very effectively.
Most commercially available subwoofers, also insanely expensive ones, seem to prioritise one thing above all and that is a small size. It is such a shame, as it is near impossible to get true, good, deep bass in a small cavity. Still no matter price range, they seem to be as small as physically possible instead of letting the driver specification dictate the size.
As a reference, I have shared my own solution here previously: Showing (off) your Roon setup - description and photos
Temporary setup and will be fine-tuned in the coming months.
I am looking forward to this thread!
I had 2 JL audio F112 subs, [Sadly one stopped working after a power outage and was too far gone to repair] but going back to one sub after readjusting settings ended up with better bass in my room. This goes against everything I have read about dual subs. They were placed just inside of the main speakers crossed over at the processor… I was very surprised by this.
Have any of you noticed this with your setups?
Wow, that is a big setup with the subs in your photo!
Did you ever try having four in the four corners of your room? i.e. example 10 in the diagram I posted previously? I ask partly because since posting the diagram, I received a copy of Flod Toole’s book Sound Reproduction, and he also comments on having four subs in the four corners. (To be fair, I’ve not read the detail, just looked quickly at the diagrams and some of the commentary.)
No, never tried. I know there are studies suggesting (Harman-Kardon also comes to mind) this as some sort of optimum placement, but I have never heard natural sounding bass when having some of the bass modules/subwoofers behind the listening position. It starts to sound more in the head to me so to speak.
I know it can be a way of fighting issues with resonances, but this room is fairly good in this regard. Have had it measured by an acoustic consultant to exactly look if such measures would improve anything, but we decided to keep them as is. There will be a large bass trap built in the back of the room though.
One reason for having them all on one wall is to simply have one wave-front of sound reaching you in the listening position (well, academically speaking as there are sound waves appearing everywhere from all directions. But let’s say the first wave reaching the listener, upon a hit on the bass drum).
These are now placed about 1/4 of the room width in from each wall.
I have multiple subs, JLA F112 V2 in the front right corner and a JLA e110 in the left rear. Sub/size difference because of price and availability, and location restricted by furnishings, room obstacles and my spouse (it has to be integrated and out of the way). 1800 watts from the F112 and 1200 from the e110 does add a lot of bass in an 18 by 25 space and it does take some tuning.
I read everything I could find on sub integration before the initial purchase but had to settle for the best I could do. One of the articles dealt with timing, amplified sound from the speakers being ahead of the sound from the sub/amps. You can’t speed up the subs response but you can slow the rest of the speakers down. The article suggested running the room/speaker configuration process then adding an additional 12 feet to the subs distance. This allows the subs to get the signals fractionally faster than the speakers and give the sub/amps time to process. You can vary the distance ether way to adjust the timing but JL subs have a built in adjustment to slow their response. So in this case they get the signal slightly ahead of the speakers then use the built in function to slow them to match. It’s not perfect but the ability to tune the sub response to match the speakers works well enough for me.
They make a difference in the sound, but is a 100% personal taste difference (you’ll be way out of the natural sound definition).
If you are at a point where you ask yourself if you need or no to add a sub to your music system, you should first look at other issues with the system (room, speakers/amp/source synergy and/or standalone performance).
Full disclaimer, I’m in the “best sub for music is absolutely no sub at all” club (and I’m enjoying a lot the electrostatic speakers)
Fascinating - I have two systems; one at home and one in my office. Both suffer from poor placement due to room restrictions. Specifically on both set ups the left speakers are somewhat in a corner, while the right speakers are in relative free space. On both systems the sound is skewed to the left. It is very frustrating!!! Neither of my setups have balance controls. Per one of the suggestions above I just checked the sound levels (in my office) using an app on my phone. Decibel levels are +/- the same on both speakers. At least I’m not going mad!!!
Moving the speakers is not a option. Is there anything I can do room treatment wise(or any other method) to help the situation?
Cheers.
Ian
Thank you
Well, it is the corner placement that causes this. If it is flush in the corner, that is a the theoretical +9 dB compared to in free space (+3 dB for adjacent surface). Probably it is not possible, otherwise I am sure you would have done it, but get it out of that corner.
Thank you Martin - but I cannot move the speaker any further. It is approximately 18 inches (50cm?) from the back and side wall while the right speaker is about 10 feet (3meters?) from the side wall and about 6 feet (2meters) from the rear wall.