2 subwoofers and frequency response

Hello all !
What changes does adding a 2nd subwoofer have on frequency response?
Depends on their placement in the room. See the multisub paper here for details: http://www.harman.com/EN-US/OurCompany/Technologyleadership/Pages/WhitePapers.aspx?CategoryID=White papers
Hi Bob,
Thanks for the white paper link but nothing came up just the title white papers with nothing under it
I found it nice article
The more sepakers you throw intoo a space, the more you need to spend time making sure the speakers sound good from the listening positions. If not, you'll run into all sorts of sonic compromises.
If you place the subs in corners, you'll DEFINILTEY need to EQ them very heavily to smooth out the sound. Also if you place half way between wall boundaries, for smoother response, you'll have to deal with phase issues if you have a large seating area.
Probably the most easy way of getting two subs (more dynamic headroom potential, lack of distortion, and solidity of frequencies covered) integrated easily into a room (forget decor considerations) is to place another sub right were tbe original sub -that's properly located for good sound already - is located! (i.e, stack on top or right next to each other (likely straddling a middle wall boundary).
My personal experience is that placing two subs in between (bellow a center channel) a front wall, where the main speakers is, is usually a fool proof way to integrate well, get phase right, smooth frequency response quite a bit. And the main L/C/R setup IS the most critical to get blended correctly with the subs/seating positions, as that's the main soundstage focus area, usually carying the most weight.
If you are the only one or two seating options in a room, you'll probably get the smothest fundamental response (adjust phase control accordingly) if you, yes, place one sub in middle of front wall, and one in middle of side wall. (I doubt most oculd put a sub half way up a wall to further smooth out bass modes in a room).
Bottom line you'll have to balance things well, consider strongly phase response between both subs ( you can easliy cancel out the bass between both subs if not careful, considering diffent seating positions), and integrate them with care to get the best out of things.
I still recommend placing two subs on front wall in center for easiest. But, yes, two can offer more better than one. But it all depends on execution and overall setup
Thanks Queefee,
I will try placing them as close to each other on the front wall. I guess my question would be if my f110 fq response is 22 hz then how low would 2 take me? I understand that placement has a lot to do with it but have you measured how low 2 will go compared to one?
I will try placing them as close to each other on the front wall. I guess my question would be if my f110 fq response is 22 hz then how low would 2 take me?

Actually it doesn't change the F3 at all. It just allows them to play louder (together) which could allow you equalize the the subs to play a lower frequency louder than a single sub could.

But 2 subs will have the same F3 they had individually (ignoring the potential of a room caused reinforcement)
Thanks Summitav
so if it is +/- 3db @ 22hz how many DB would it change? I had a fellow audiophile say that each sub would allow you to hear the FR at 3db louder what do you think ?
Multiple subs in a room can serve two different purposes:

1) help to smooth out the response over a wider seating area given freedom of placement

2) provide cleaner, higher SPL

Stacking the subs won't impact the response, but will give you 2). Decide what you're trying to accomplish and then place accordingly.
The second sub should add about 5db to the sound pressure. If you experiment with placement, you can use the second one to even out the response in the room. I think it's best to place a sub about a foot from the nearest wall, and several feet from the nearest wall. If you are splitting the sub output of a receiver that has auto setup, I'd run the sub setups first then receiver. Also try to keep the subs a similar distance from the seating location so that the delay in the receiver will work for both subs, otherwise you will have to work harder to manually integrate them.

I'm using a pair of matching subs between my towers and Television stand to good effect. The are about 5ft from center to center. It evened out the response and added some detail to the bass. I'm only using a single sub out on my receiver split with a Y so Audyssey can correct them together.
Thanks Summitav
so if it is +/- 3db @ 22hz how many DB would it change? I had a fellow audiophile say that each sub would allow you to hear the FR at 3db louder what do you think ?

What it will do is make the "complete" FR louder due to the fact that "all" of the frequencies produced by each sub will be increased. NOT just the -3 db frequncies.

To take advantage of this you would need to "equalize" some frequncies by "reducing" them, allowing you to produce a flatter and lower residual response.

That is in your example if the "old" +-3 was 22Hz, you could now produce 22Hz at a 'higher" SPL. As long as you reduced the frequencies above 22Hz, you would have a net increase at 22Hz, as well as below that since the roll-off would also be increased.

Point being, bigger subs and multiple subs have higher outputs, and in the end the NET GAIN in Low Frequencies is a function of maintaining a flat response aand taking advantage of the higher output. And just adding subs without equalization will not net you much at all except the same response graph with more subs.

Hope that makes sense.
I'd characterize the answer a bit differently than those above and note that there is no specific "right" answer to the question that is universal.

First, note that the 3db down point is meaningless - unless it's stated against a reference SPL at a given level of distortion. Example: F3 from 90db @ 10% THD is XYZ hz.

If you hold the room's overall SPL constant (in this case 90db) and add a second sub you will typically see F3 down significantly - AT THE STATED DISTORTION LEVEL (i.e. 10% THD). Just how significantly will depend on the particular room, reference level you've chosen for the test and the specific subwoofer(s) in question.

The key point is that adding the second sub allows each individual sub to play at a (3 to 5 db) lower SPL while maintaining the same overall SPL (90db, in our case) in the room. Thus, each sub exhibits less distortion at every frequency. The question is: How much less distortion and - more specifically - how much less distortion right at F3 at the test's given SPL?

If you examine the performance of most (though not ALL) home subwoofers below 50ish hz, you will see that they pretty quickly reach a point at which THD begins to increase almost exponentially with decreasing frequency - the distortion graph goes almost vertical as frequency continues to fall - provided a reasonably high test SPL. Let's call that point where performance goes to hell in a hurry the "critical" frequency. Since adding the second subwoofer effectively reduces the spl at which each sub is playing by 3- 5db, this may reduce distortion a TON, depending on where F3 sits vis a vis the test sub's critical frequency. OTOH, it may reduce it just a tiny bit if the single test sub is behaving well (10% THD) at F3 at the stated SPL.

IME, this happy result is pretty unlikely, unless you have a very small room and/or a monster sub and/or a very high stated THD and/or a very low reference level for testing F3. For context, high power, small cabinet subs (1 CuFT) like the Velo SLP or Sunfire often show 30+% THD at 90db at 50hz.

Hope that makes sense.

Sorry - forgot to add the point that closes the loop. Since distortion is reduced (possibly a LOT) at each frequency, we can state a lower F3 for the stated THD and stated reference SPL. Example: where F3 (down from 90db) was 52hz at 10% THD with a single sub, 52 hz might now show only 4% THD with the addition of the second sub. Therefore, we can "slide down" the response curve 'till we hit 10% THD and maybe we'll get there at 45hz. F3 would drop by 7 hz in this example.
Thanks Bob,Shiitaki, Martykl and Summitav.
You all have been very informative!!!
Ok so I have one JL audio Fathom F110. My room is 14X16 Iam using the audessy system on my Marantz av8003.

the owners manual states that one F110 would be :

27-111 Hz(1.5db)
-3db @ 25hz/120hz
-10 db @ 19 hz/155hz

so adding a second sub would change this how?
I realize that there are a lot of variables but
what would be your best educated guess?

First caveat, I couldn't find output data for the JL110, only the JL113 (available at HTshack.com subwoofer test archives).

The 113 will reproduce 20hz at 95db (quasi-anechoic) with 10% THD, which means that it probably provides more than enough clean output for a room your size for music or movies (unless you require lower distortion for your tastes or you listen/watch very loud). The Audyssey will provide smooth response, meaning that you don't need a second sub to achieve that. In short, a single 113 in your room with Audyssey will probably be very, very good. The only reason you might want a second sub would be if you cross over high enough to require stereo subwoofers to maintain imaging and directionality.

The question is: How far short of the 113 does the 110 fall? Don't know the answer to that one. Maybe someone else has seen data for the 110.

Adding more woofers in a given acoustic space accomplishes many things - and introduces a challenge or two. Dual woofers allows for more dynamic range and efficiency in the bass - more power and surface area equals all the above, plus can help eliminate distoration, provide higher accuracy potential, and more. Getting another woofer in the system, allows you to lower the output of each - again, more efficiency.
Another common concept is dual woofers smoothing the bass response, by locating them in different possitions in relation to the bass modes and the seating possitions/speaker locations. This can be tricky to pull off with maximum effectiveness, however, in a multiple seating arrangment, where you sit proximally closer to one woofer vs the other, cause phase issues, and so forth.
Dual woofers can also be out of phase with each other, and relative to the rest of the speakers in the system.
Yes, Stereo Subwoofers also increases efficiency, and can help locate the bass and proper phase with each speaker with some added ease and effectiveness. (an EQ often helps here, regardless).
Whether to add another sub depends on lots of variables. Sure, in smaller/medium sized spaces, and in less ambitious systems, multiple subs might not makes sense. Still, to be certain, more can indeed be better in an "all out" system, where maximum impact, and system efficiency is demanded. I think the benefits outweighs the negatives - but ONLY as long as the rest of the system in wisely constructed and dialed in, and a there's a little knowledge involved with what's going on with the rest of the set up. Othewise, default to "keep it simple, stupid" is probably the wiser route - stay with the single
...in this case throw a smaller sub in a corner, and a bigger one right up in the middle of front wall between main speakers, and let today's advanced digital EQ's help you out with the rest.
Oh, didn't see your post on room size. I'd probably stick with one well placed sub in that smallish space. Two will get croweded aweful fast, unless you get real creative. Also, you have enough output from that single sub in that space. Also, the very lowest feq's won't be properly represented in that space anyway (could have gone with high output 10" woofer in that space, also). Bass will be on the thick and slow side on the bottom, as room won't absorb enough bass for a proper RT60. In rooms like this, I like to stay with woofer(s) that don't play down as low as larger, because natural boundary reinforcement and RT60 balance better this way. Otherwise you spend your time and technology tying to TAKE AWAY bass energy, and eqing out the large hump at the bottom from boundary re-inforcement, bass nodes, and lack of absorption. Better to have too little that's properly blended than too much that's compromised.
Yeah, stay with the single and EQ it out. My advice, anyway.