What size subwoofer to get?

This question is for 2 channel stereo use 50/50 rock/classical music. I have floor standing speakers rated down to 35hz on axis and instead of getting expensive larger speaker want to look at adding "powered" subwoofer to slightly extend bass. Three models are available:
10" woofer 30hz
12" woofer 25hz
15" woofer 22hz
All these models have adjustable crossover frequency cut-off
50-150 as well as adjustable volume. What size is best to get, my concern is getting too much bass that is hard to dail back and becomes overwhelming. Or is it better to get largest subwoofer and not worry, volume control will be able to effectively tailor sound and control bass?

This is my first subwoofer so need some general advice from members who use them, thanks.
In most cases I recommend getting 2 lesser subwoofers over 1 larger one. It is much easier to place and avoid any major peaks and nodes in the room. These peaks are generally what is perceived as boomy and ill defined bass. If you use 2 you can smooth out the subwoofer to room response. The size of the subwoofer usually determines how low the unit will go. If you are looking for those last octaves, you will likely choose one of the larger ones. The other word of caution, particularly in 2 channel, is integration. Some subs sound great with speaker A but very poor with speaker B.
How big is your room? Do you have a tv or anything between your speakers? What are your main speakers-brand?What spl's do you listen at? Wood or carpet on floors?
I believe you are approaching your dilemma from the wrong angle. The size and quantities of subwoofer do not matter one bit unless you are already buying the absolute best. Why would anybody want to purchase two mediocre subs when they could have instead afford to purchase one good to excellent sub? The world is full of mediocrity. Isn't this what the 'high-end' is all about?

The quality of sound of the sub (just like a set of speakers) and the ability to fine tune the sub to your main speakers means everything.

Bad subwoofers come in all sizes. Good subwoofers come in all sizes.

Like any other piece of equipment, hopefully you are shopping for the best subwoofer you can find in a given price range.

Assuming that, I will make a few other assumptions and/or comments:

o You are looking for a sub that will faithfully reproduce down to 18Hz or lower. There is too much musical information can be found in the 18Hz to 30Hz range to purchase a sub and still not be able to reproduce it. In my opinion, a subwoofer is not a subwoofer unless it can go down to 22Hz or even below that.

o Since your floorstanders go down to 35Hz, you are looking for a subwoofer that you can dialin the cutoff filter somewhere around that same frequency. A subwoofer with a cutoff at 50Hz will most likely give you some kind of mid-bass hump with an over-emphasis in the 30Hz to 50Hz region. You may not be able to compensate for this hump.

o You are looking for a subwoofer that is musical. That is, it reproduces the lowest octaves with accuracy, definition, and speed. Not just flapping in the wind like an American flag. Again, size does not necessarily determine this. There are very bad sounding 8inch subwoofers and very good sounding 18inch subwoofers and vice versa.

o In my experience and opinion, in the lower budgets say $1200 to $2000, you need to be looking at 12inch, 15inch, or 18inch subs, because they stand a greater chance at faithfully reproducing the 18Hz-35Hz range. Usually, one must spend more than that to get a good 8 or 10 inch sub to accurately perform in that same frequency range.

o One extremely musical subwoofer is the Bag End Infrasub 18inch subwoofer which sells new for $1600 that is reported to reproduce all the way down to 8Hz. Now that's a sub. I've had this unit in my home for about one month. I had no idea that an 18inch sub woofer could be so tight and so musical. Almost too tight. But it was difficult to dial-in properly.

o I stepped down a notch and purchased a Triad Platinum 18inch sub ($2000 new) that is still quite musical but allows me to tailor it to my room and floorstanding speakers just a bit more.

o These two 18inch subs I mention replaced an 8inch subwoofer ($1400 new) that would knock my house off it's foundation, but had absolutely no musical definition to it whatsoever.


Slightly different advice:
1. A floorstander's low frequency (in this case 35Hz) can be expected to be its -3dB point (unless you're quite close to a wall). The crossover on a sub is its -3dB point. Therefore, a crossover point an octave above the low frequency spec point of your speaker is a good place to start. In your case that is 70Hz.
2. Though there are many design factors in subs, the amount of air moved by a large sub is much larger than a small one, requiring less excursion (often meaning less distortion) to move the same amount of air.
area = 3.1416 * r squared
10 -> 79 sq in
12 -> 113
15 -> 177 (two 10s)
18 -> 254 (two 12s)
3. Finally, a phase control as well as a volume control is useful in getting floorstanders and sub to sinq.
Good luck.
Stehno is absolutely right. The cone size is not important. Since your current main speakers go to 35Hz you'll need a sub that can crossover below this level (unless you filter the signal going to the main speakers).
My main speakers go to around 35Hz and I'm crossing at 27Hz to avoid boomy mid-bass !

My suggestion would be to look for a used REL strata. It crosses very low (down to 25Hz), it's adequately powerful for all but the largest room, and if you don't like it or ever want to upgrade they hold their value and are easy to resell.

IMHO Better no sub than a cheap sub for 2ch music.
I'm using a Rel Strata III with Magnepans with excellent
results. The Rel is very tuneful, with a highly adaptable
crossover network. If will go down to 18Hz flat - how
much music is below that? I also agree that two smaller
subwoofers is usually better than one larger unit.
Stehno hit it on the head.

Rives also brings up some important points. One sub IS harder to place than two. On the other hand, two well designed commercially built subs can be quite expensive.

Just keep in mind that a vent of any type ( ported, passive radiator, slot loading and yes, even transmission line ) will have slower transient response ( reduced impact, less definition, increased ringing, poorer decay, etc... ) than a well designed sealed box would offer. The sealed box will also TYPICALLY have less problems with interfering in the upper bass since there is no leakage through the port or passive radiator "talking". Placement should also be easier since you only have to worry about room reinforcement / nodes from one radiating surface ( unless you have multiple drivers ).

The major drawback to sealed woofers is that a good one will be quite large. Having said that, two good quality sealed subs that are offer large box volumes would be VERY hard to beat if properly set up.

As to the Bag End, it is equalized to produce 8 Hz, but only at very low volume levels. As driver excursion and spl is increased, the bass processor rolls off the extreme lows so as not to muddy the more prominent notes slightly above this range. Kind of like a smart "loudness" control that one can find on receivers, etc... The biggest problem with Bag Ends is the construction of the cabinet, which is not as rigid or damped as it should be. It also takes a pretty good amount of power to really make this thing sing, which is true of almost all subwoofers. Sean
How important is control of the rollof slope of the subwoofer? I had a REL (Q150e) that allowed me to control both the Xover frequency and the rolloff slope (6, 12 db, etc). That seems like a great advantage - is it really???
System-matching and your music-HT bias are critical. I use a Paradigm Servo-15 to supplement (not crossover) my B&W Nautilus 802's for the sub-50hz information on some (not all) music listening. The Servo-15 is an excellent HT sub and a good music sub. However, it's still too slow for the fast 802s on some music. Several knowledgeable "golden ear" audiophiles I respect have independently told me that there is nothing better than the Vandersteens for music (preferably using 2 or more). They each use 3 8" drivers and are very fast. I understand a cheaper alternative is NHT Sub Two's (2 10" woofers each), which are out of production. Though the Paradigm Servo-15 is an excellent compromise for music and HT, it just illustrates that you should be biased in favor of which is more important to you.
Peter, I would think having the additional ability to pick different roll off slopes would allow you to better blend the sub especially if you have a dip or a hump at or around the crossover frequency.

A question to Rives. I've been thinking of building two subs to be powered by a Bryston 4B in stereo mode for music and dual mono from the LFE output for HT. You mentioned that having the ability to vary the placement of two separate subs independently can avoid some of the limitations of only using a single sub. Is this assuming they will be reproducing identical mono signals? If the two are placed in radically different schemes how will this affect their ability to operate in a stereo set up? Thanks.
As I concur with the above advice I would suggets the Vandersteen Sub which used should run $750.It has an execllent crossover which is is accessed through a direct hook up to your speaker posts and responds to the full range signal not just what is low passed.Very clean and quick do also to fact that it uses multiple 8" drivers.Call a sub manufacturer and they will tell you that bigger is better but a large cone and magnet assembly does not always move fast enough to reproduce pitch accurately.If it were Home Theatre thast would be a different story where bigger is better for that "boom in the room".I vote for Vandersteen and if you can afford it the REL Strata III (starting with one and getting second).But all mentioned above are good.You didn't say what kind of assciated equipment you had so all of us are being presumptuous and as you will have noticed freaks.But as it was mentioned before a really cheapo sub good for HT will screw up decent sound.If you are looking try to find one with smaller faster cone and get two but also try to look for one with a servo amp which will produce less distortion than a non-servo.
Gunbei: It's a good question, but fortunately you almost never need to compromise. The flatest response is with subs placed on opposite walls at the midpoint. Now if it were for music and it was a high crossover point (say 80Hz or higher) I would move them back closer to the main speakers but symetrical, and relatively close to the wall (within 1/8 of a wavelength of the crossover point). In a non-symetrical room, this may not be the case. Here's an even more interesting phenomenon (purely hypothetical--because I don't know anyone that's actually done a room like this). If you use 4 subwoofers and place them 1/4 of the crossover wavelength from the corners of the room you get a nearly flat response throughout the room--no peaks or nulls (obviously there would be minor ripples due to furniture and other items in the room in practice, but in theory it's a flat response). In this last example it does assume a mono signal. In your case, you would not need a mono signal because most of the lower bass notes are mono. If the crossover point is high--then that might not be true and you could run into some difficulties, but no more than having 2 main channel speakers that carry the bass information independently. So, I guess I don't really see any significant downside to stereo subs--I hope I've explained that adequately.
Thanks Rives, give a guy with two subs an excuse to buy two more...
Thanks for the helpful info, my system can be viewed by clicking system link by my name, my room is 14x22. I have PSB Silver-I and wanted a sub for 2 channel listening only as an alternative to upgrading to PSB Gold-I. As stated by others here I don't want to muck up sound with slow, boomy sounding sub.
I naturaly was looking at PSB powered subs which are quite affordable and discounted heavily on internet.

The bass from the Silvers is almost enough but needs a touch more extension and authority. I am looking seriously at 10 or 12 inch PSB powered sub which on paper gives equal or greater bass vs Gold-I, but was wondering about the difficulty in general of having seamless integration of sub with Silver so sound is not compromised.

If I get two subs then the cost is approaching the Gold-I, plus don't want all the clutter and connection hassle of two subs.......can the Silvers sound good with one PSB powered sub?
Thanks very much for the response Rives. If I were to use two stereo subs set up on opposite SIDE walls would that be an example of what you're suggesting? Does it matter if they are downfiring or front firing aimed at each other? Because of space limitations I currently have a single mono sub on one of the side walls, but I'm finding that my sitting position is too close to the sub and I lose all the lowest octaves. I was thinking of adding a second sub on the opposite side wall, then moving them closer to the corner behind the main speakers until it sounds fuller. Right now for music the sub is tight and musical but lacks bottom end ooomph. However, for HT I'm happy with it.
I had this dilemma once and decided it was far more cost effective, and would also yield overall better sound, to sell my speakers for a fullgrown full range model. With the seperate sub imaging will be lost. As I understand it you should crossover one octave above your speakers bottom extreme for best results, so around 70Hz, which can be done with the Outlaw Audio bass managment system (www.outlawaudio.com). Also worth note, of the thousands of songs I have on disc of all types of mucis, maybe a dozen have anything portrayed below 35hz, of those I have only 2 tracks with any info from 25hz on down, and it is electronica/techno/trip-hop type of music, so unless you listen to such musik, or I suppose organ music, you're not missing much....
Socrates .. I disagree with some of your suggestions:
1) If speakers are rated to 35Hz, and you cross below 30Hz (it is usually recommended to leave the main speakers unfiltered, and cross below their cutoff frequency ala REL, ACI) then this will have no effect at all on the stereo imaging ... such low frequencies are not directional.
2) Of the several hundred CDs I have at least 75% benefit from the subwoofer, whether it be added weight in the bass drum kick, the sense of the scale of the venue on live recordings, timpani on orchestral music ... I could go on. The type of music that DON'T benefit are extremely limited. Of course the effect is most pronounced with electronica and organ music, but it is very present in most other types of music.
3) Separating the source of sub-bass from the main speakers gives flexibility in placement to allow good imaging on the mains, and good room loading on the sub. Full range speakers don't give you this option.
4)I have heard good full range mains ... in a large room they work very well. I have heard a good subwoofer ... in most rooms it can work very well. Both options are possibilities. However you seem to rule out the subwoofer approach without having had any first hand experience with one. When you say you decided .. was this based on listening, or just a preconception ?
How do you know that I have no first hand experience? An unusual assumption, no? I have heard numerous setups with seperate subs, I find them inferior in many ways to similar quality and priced full range speakers, though sub setups often end up more epxensive in the end once you factor in cables and a good crossover, which is why it makes little sense to me to use seperate subs in most cases when so many good full range speakers can be had on the used market. If nothing else, why not move up the line of your speaker brand of choice, it will likely cost less, be more aesthetically appealing and easier to setup the gear to boot. I also have to imagine that if 75% of your music is better with a sub then you must have very limited range monitor speakers, which is not the case with the originator of this topic. I'm just offering my opinion, take it for what it's worth, just as others will take your opinion....
I have Spica angelus speakers with a REL strata. The amp, crossover, and subwoofer is all one box. Total cost $500 for the Spicas and $850 for the REL. I don't think I could beat the sound (imaging and bass depth) for $1300.

I think our differing opinions come from comparing one-box subs, crossed below the mains to multi-box subs with external amps and crossovers. A one-box-does-it-all sub like REL or ACI contains the crossover, amp and sub, for anywhere between $800 and $1500, and gives bass of depth I have not heard on any main speakers under $2500 a pair. Creating a sub with separate crossovers, amps and cabinets I have no experience with, but it sounds like your opinions would be more applicable in this case.

How did I conclude you had no first-hand experience ... I read "and decided it was far more cost effective, and would also yield overall" and the wording suggests a decision based on presupposition, rather than a listening comparison.
Also you wrote "As I understand it you should crossover ..." this gave me the impression that you had never actually setup a sub. I guess you post was open to interpretation and I made the wrong interpretation.
Gunbei: I think you are headed in the right direction. Subs in the corner behind the speakers can and often do work quite well. It's not as flat a response, but if you keep them within 1/8 of a wavelength of the corner it minimizes the peak/node effect to some degree (eliminates the re-enforcement of back waves coupling or decoupling to the source wave. As to downfiring or forward firing--it makes no difference at crossover points below 65 Hz or so. Above that you might get a very slight difference. You may want to go to our website www.rivesaudio.com and go to the listening room (under acoustic issues). If you click on speakers there, there is a link that you can download a white paper on speaker placement. It goes through all channels and subwoofer placement. Hopefully, it's helpful.
Sean my experience coincides with your advice. I am using ProAc Response 1SC monitors which probably extend down to 40Hz at the most. But I find I must set my Paradigm X30 to it's lowest setting of 35Hz or the excessive overlap at the crossover point begins to muddy the midrange and upper bass of the ProAcs. That's where an extra control for shaping the slope of the lowpass filter would be helpful in giving more control over the rolloff.

Rives, thanks so much for giving me the free advice. I did check your website and found lot's of helpful information. I've been reading around to become more aware of some of the often used guidelines and from there I intend to do a lot of experimenting. I just wanted to make sure I started on the right foot.
Your speakers are very good and extend into the bass regions quite a bit. You do not want to lose any of the benefit of your speakers and there is no reason to try to redesign them by using a traditional, conventional subwoofer. Buy a REL (the size of model depends on size of listening room) and run your speakers full range. Then run the REL off your amp (it draws no current and won't affect anything) and then experiment with the lowest x-over setting you that works. I have ProAc 2.5s and have my REL x-overed at 28HZ. This is a major difference btw RELs and most other systems. I tried, at home, three other subs costing btw 1,200.00 and 2,400.00. The 1,300.00 REL Strata III was far and away the most musical and blended seamlessly with my speakers. You are right to want all the low frequencies you can get, makes a big difference but make sure it's done right. REL is the only make I know that does it right. I've heard Vandersteen's sub does same thing but is far more difficult to set up and adjust.
Since you are looking for music rather than Home theater bombast....try the REL....it comes up under your mains and works off the amp speaker connections.
To add to Rives' comments, front loading and downfiring will have DRASTIC effects on bass output and pitch, even if crossed over at or below 65 Hz. I would suggest listening to the two different designs side by side to see which you like best. My experience is that front firing will typically sound more musical with better pitch definition whereas downloading will provide greater "oomph" with less attack and tonality.

One should also take into account that downloading places more stress on the driver itself ( the suspension can sag pretty drastically over time ) and a system of this nature will require a slightly sturdier amp with greater current capacity / higher damping factor. Sean
PSB made a very nice sounding passive subwoofer called the Subwoofer 1 in the late 80's early 90's. I just purchased one used for really cheap and it works fantastic with my tower speakers of 86 db sensitivity. (The sub is 88) The speaker level high pass is 6db per octave (fairly shallow) so your mains will be still producing some lower bass and not cause collapse of all the bass to the sub. The sub also has a set low pass crossover in it as well I think. This unit is very musical and ideal for a music system. I found most powered subs being sold today very boomy, high distorting regardless of the convenience of Level control and Crossover controls etc. You have to spend $700 U.S or more to get anything half decent (ie Paradigm PW 2200). My room is similar dimensions to yours and at 100db or less the system sounds excellent. Make sure your main amplifier has adequate power into 4ohm load. This sub is a stereo unit (2 8's) which are high compliance. It can also be configured for mono. The sub seems to be very accurate. Try to find one of these used! I only paid about $70 U.S for mine. The only negative is that it is large 22" X 24" Good luck with your system.
Remember...when buying a sub...you are also buying an amp...the size of the driver is really irrelevant...although some purist find them too "thumpy"...I do think Sunfire subs for 2 channel sound are amazing(Bob Carver claim to fame have always been his amps...and we all know what happened after he left Carver)...a friend has them with Maggie 1.6s...which are notorious for sub mating issues..and they are tight,fast,and very musical...a more laid back type of musical sub can be found by REL...and to echo Sean's comments...a garbage sub will only call attention to itself in a 2 channel set up...and surprisingly...since I am not a huge fan of their speakers...B&W makes some pretty darn good subs too...
No discredit to your PSB speakers...but i would pass on a PSB sub...a sub is kind of "icing on the cake" for a system...you want it to be just right....
Also...listen to Danner...this is exactly what RIchard Vandersteen has been saying for years...and why Vandy only makes full range speakers...the crossover for sub/sats is just way too high...good advice...
Danner's comments about surface area only cover part of the equation. Not only is surface area important, so is the amount of displacement. In order to achieve high levels of displacement, a "long throw driver" or one that is capable of making quite a bit of excursion is required. This is especially true if one wants to or is stuck using a smaller driver. As such, one can find an 8" driver that actually displace more air than a 12" driver. It can do this even though it actually has a MUCH smaller surface area due to the longer stroke that it can take. The downsides to such an approach are that the driver will be harder to control due to increased amounts of reflected power and may generate higher levels of distortion.

Like anything in life, there are trade-offs involved. You have to weigh the variables and pick the lesser of two evils. It is rare to find any given product to be a "clear cut" winner in every category. Sean
I purchased a Sunfire Super Junior several months ago to augement LFE when watching movies. It's been a bear of trial and error getting it dialed in and mated with my Magnepan MGIIIA's. It is switched out of the system for music listening. While you want the thing to produce low frequency information, you don't want it calling attention to itself. After months of different settings, I think I am getting it "almost there". My experience with it is as follows: The Sunfire subs have adjustment for gain, phase, and crossover. I currently have the sub in the corner, behind and about 4 feet left of the left speaker; crossover set at 100hz, phase set at normal, and, the gain turned all the way off -then bumped up "just a whisker". Again I only use it for movie LFE, but it produces the low frequency info from the soundtrack with ease, but isn't just boom and rumble. With these settings, I can't tell that the lows are coming from the left corner as I first could. YMMV.