2 subwoofers and frequency response

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.

Marty

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?