Going LLT from sealed

[fireanimal]

Currently I have 4 MJ-18 in a sealed box 20cu/ft internal volume. There are nice, but I am losing out on a lot of the low end. I will post a couple of graphs of the simulation for that box and the actual in room response, because I would like to know what kind of room gain I am getting. My room is L11.5’ W10.5’ H7.5’, it is well sealed with double 5/8 drywall and green glue.

I would like to stay with the current box size that I have now, L96”xW16”xH28.5” but I can go as big as 125x20x28.5. I will be powering with a Crown Xti2000 bridged into 4 ohm’s for 2000 watts. I was considering 2 RL-P15 or 2 IXL18.4 in the smaller box, if that can provide me with more bass than I have now, if not I would go with the larger box and use 2 RL-P18’s or Fi-Q18’s.

There are just so many options, but I think first I would need to know my room gain as a start.

Thanks.

[SteveCallas]

From 40h to 20hz you lost ~14db, and that 12-14db rolloff seems to continue down below 20hz. In other words, it kinda looks like little to no room gain

[fireanimal]

That's not good! Maybe I need a new measurement? I should have some serious gain in such a small sealed room.

[fireanimal]

Not sure if this is right but I did a nearfield measurement of the sub in the room, and another measurment at my seating position. There is a difference in the response at the seating position, but I am not sure how high the room gain is.

RED = Nearfield
GREEN = Listening Position

Thanks.

[StephenP]

Wow, looks like you sit right in a huge null. I would move that sub to a few other locations and re-test before building it all over again.

[fireanimal]

I am not sure of the null that you are talking about, plus the box weighs over 300lbs and is in a fixed location below my screen, so it cannot be moved.

My frequency response at the listening position is +-1db from 25hz to crossover, it is the under 25hz that I want to expand on.

[StephenP]

If the red line is the seating position graph, then you are losing like 12db at 20hz. Nearly any sub you put in that same spot will behave similarly without heavy EQ or room treatment.

[fireanimal]

StephenP wrote:

If the red line is the seating position graph, then you are losing like 12db at 20hz. Nearly any sub you put in that same spot will behave similarly without heavy EQ or room treatment.

That I understand, the red is a nearfield measurment and the green is at the seating position, I am trying to figure out how much UN EQ'ed room gain that I have.

Thanks.

[fireanimal]

I think the room gain is starting around 45hz and is around 8-12db per octave....

Not 100% though.

Thanks.

[erh7771]

If you can move a sub into the listening position and go around the room with a meter or your ear doing 60 - 20 hz sweeps in a loop and listen for the bass.

It took me a second to get rid of a null, creativity will do it also mean you might have to move your seating.

[fireanimal]

I don't understand what I said for all the confusion...I don't have a null and my frequency response is excellent in the listening position. I am trying to find out how much room gain I am getting, because I want to build a LLT or SLLT and want to optimize the tuning and response to my room.

Attached is a old response graph of my subs after EQ at the listening position. It is even better now, but I don't have graph handy.

Thanks.

[cixelsid]

What's so confusing? The room gain is the difference between the 2 plots.

These drivers have a high Fs and a relatively short Xmax. Giving them infrasonic tuning isn't the best idea.

[fireanimal]

I know that I am getting room gain, but I am trying to figure out how much like 10db octave starting at 45hz, and how to simulate that in WinISD.

What was confusing me was why everyone was responding about being in a null and subwoofer placement.

I know that the subs I am using are not good on the low end, and that is why I want to change them and install a LLT to gain it back

Thanks

[Mike P.]

If you are considering other subs, I can vouch for the IXL18.4's. 2 of those in 12.5 cubic feet each would be scary. No comparison in low end response against the MJ-18's sealed. You can view what other members have built here:

http://www.hometheatershack.com/foru...-database.html

[fireanimal]

The IXL's are on my short list of drivers that I am considering right now.

[cixelsid]

fireanimal wrote:

I know that I am getting room gain, but I am trying to figure out how much like 10db octave starting at 45hz, and how to simulate that in WinISD.

As you can see from your plots the amount of room gain varies with frequency. This means there's no one number you can enter into a sim.

[SteveCallas]

What's so confusing? The room gain is the difference between the 2 plots.

No. That near field response doesn't apear to be very......good Take a new one, this time with the mic or spl meter about 4" back from the center of the driver with a folded up blanket, some pillows, or anything else thick and absorbent behind the mic/meter. You should end up with a very deliberate, smooth, predicatble FR. See attached for a close mic measurement of my ported sub to get an idea of the type of smoothness you should be getting.

[fireanimal]

SteveCallas wrote:

No. That near field response doesn't apear to be very......good Take a new one, this time with the mic or spl meter about 4" back from the center of the driver with a folded up blanket, some pillows, or anything else thick and absorbent behind the mic/meter. You should end up with a very deliberate, smooth, predicatble FR. See attached for a close mic measurement of my ported sub to get an idea of the type of smoothness you should be getting.

Thanks, I will do that.

What do you think of using 2 of the new FI IB18 subs in a 100cu/ft enclosure?? I may use a room beside my theater and build a large enclosure....I was thinking 95-100cu.ft 12" Port 13" Long for a 12 hz tune. $200 with all that excursion might be hard to pass up.

[fireanimal]

Here is the graph, hopefully it turned out okay.

[a1161979]

Ummmm is it just me or does that look the same as the first graph just with different axis

[Mike P.]

Looks like about 12 db gain at 20hz.

[fireanimal]

thanks.

[SteveCallas]

Looks like about 12 db gain at 20hz.

The relative output levels between the two graphs isn't important, but rather the shape of the FR. So take his at the seat graph and just lower it on the chart some until it more closely matches his near field measurement. Also, on that near field measurement, something is interferring with a proper response between ~14-30hz, but since we know how it SHOULD measure, I would draw a straight line from 14hz to 40hz to get a truer reading and rolloff profile, and sure enough it matches pretty closely to 12db/octave.

Now the rolloffs start similarly and the at the seat one flattens a bit down low. Looks like room gain starts ~25hz and is at its most potent in the mid teens yielding 6db. So in designing your LLT, I would keep in mind that room gain for your room starts ~25hz and yields ~7db/octave. That is a very rough estimate.

[fireanimal]

Thanks for the input on the room gain.

What do you think of 2 Fi IB18's 1850L's and 12" port 16" long?? It seems like it would match my room gain fairly well. The yellow line is my current setup 4xMJ18 sealed.

[SteveCallas]

1850 liters means you would need an adjacent space to use as the enclosure, otherwise they would probably be too big. But the response looks great.