Volume - SPL vs. Freq tradeoff?

[Bill 3]

I have been studying graphs and playing with numbers until I can't see straight. I can go a max of 740 liters and I am using the Fi Q18 Driver. I am trying to keep from having a peak then dip at the tuning frequency which means that I have to lower the volume as I lower the tuning frequency (according to the graphs). Steve Callas has suggested 625l and 13.6Hz but I wonder if I wouldn't be better off going up to 700 with tuning at 15Hz?

What would you do? I know we are only talking a few dB or Hz, but why not aim in the right direction.

Thanks,
Bill

[justin.kerr]

what about 700l at 17hz?

[Bill 3]

justin.kerr wrote:

what about 700l at 17hz?

Well, it does raise the low end quite a bit, but my attempt is to make a true LLT (not sure why) which needs to be tuned at 15hz or below. Also, since I am making these monstrous enclosures, I want to dig as low as I can. Maybe I could have chosen a different driver... I figure I can knock down the higher frequencies (relatively speaking) with a BFD if necessary. There is always a trade off and the deeper you go the bigger the trade off.

All of these are so close, I guess I should just throw a dart and start cutting.

Thanks for the response,
Bill

[Geoff St. Germain]

Part of your answer will come from how large your room is. Larger rooms have less room gain. The idea of the LLT is to attempt to approximately match the anechoic roll off to the room gain to maintain a flat FR into the mid to low teens.

[SteveCallas]

Your parameters must be different from mine, as my model doesn't look like that. Are you using the dual 2ohm coil version?

[Bill 3]

Geoff St. Germain wrote:

Part of your answer will come from how large your room is. Larger rooms have less room gain. The idea of the LLT is to attempt to approximately match the anechoic roll off to the room gain to maintain a flat FR into the mid to low teens.

I am guessing that my room doesn't have a lot of gain (this is the whole reason I got into this sickness - or what you guys call a hobby ). I could play with my current sub to find out how much there is, but it really doesn't go below 25Hz and that is where I would like to test. I am sure it will all be good in the end, just trying to put my best foot forward.

Thanks.

[Bill 3]

SteveCallas wrote:

Your parameters must be different from mine, as my model doesn't look like that. Are you using the dual 2ohm coil version?

I think we went down this road one other time when I was comparing different brands and I was incorrect on my parameters. I corrected them then and then just double checked and I beleive them to be correct. Yes I am using the Dual 2 Ohm numbers. I put in 2.8Ohm for the Re in order for the rest of my numbers to line up. Is that not right? When I input data into WinISD I only input the following: Qms (5.73),Re(2.8Ohm), Mms(342g), Cms(0.13mm/N), BL(15.1N/A), Sd(1210 cm^2), Xmax(27mm).

Here is the graph showing the parameters. I would be happy to be wrong and find out now. Let me know what you think.

Bill

[SteveCallas]

It's the Le. You left it as 0, I assumed at least 4mH. Not a huge difference, but it results in a peakier bottom end with your 700 liter model.

[Bill 3]

Okay, I changed to 4mH for the Le and I like my graph even less. It boosts the output at the tuning point, but then there is a dip between there and about 20-25Hz that I have been trying so hard to avoid.

I guess with this speaker choice, it is something I will have to live with if I want tuning that low. It appears that as long as I don't upgrade my amp anytime soon (ep2500) that my cone excursion and port velocity will be kept in check.

Thanks,
Bill

[SteveCallas]

Don't lose any sleep over it, it's a combination of how the modeling programs work and the small db scale we tend to use for our simulations. For example, in your graph above, your db range is 100-120db, only 20db range. When people use RoomEQ Wizard to measure in room response, they typically use a 50db window. So some small rise within a 20db window is nothing to worry about. In the real world, the in room output will be nice and flat to the low teens.

625-700 liters won't make too much difference as long as you adjust the port length appropriately. Personally, I just don't see the need to go with the extra volume and potentially needing a 30" tube - I'd go 625 liters.

[Bill 3]

Steve, thanks for the words of confidence and the perfactly flat reminder. I already aquired a 30" tube and cut it in half (6'). I have all the endcaps made and I am in the process of painting. It is just a matter of how much to cut off the 30" tube and 8.5" port. No doubt I am making a big deal over nothing in regards to sizing. I am only looking at a 6" height difference between 625 and 700l. I will just make sure I get my port the right length