Originally Posted by
RMC
In my post # 5 here I forgot to mention that I also did model response of the 3156 in Altec's 8156 box (Vb and Fb) to see the result. It fits like a glove on the 3156! Not really surprising. This one has the flattest LF response of the three boxes I modeled (3, 4, 5 cu. ft.). It confirms the folks at Altec had it right in the data sheet stating a precise recommended volume of 3 cu. ft., instead of a range of recommended volumes as JBL usually does (e.g. 3-8 cu. ft.). However, even with that 3 cu. ft. flat LF response box, there remains the issue of mid-bass/midrange rising response of the driver (+2.5 db from 500 hz and up) and its possible effect on perceived driver sound if used in a two-way system crossed over at 800 hz for example...
RE "I know a thing or two about this since the 3156 driver is somewhat similar to my JBL 2205H." in my post # 5 here, the 2205 has its own issue too, but less critical than Altec's 3156: box volume being much more "forgiving" for the 2205, but not box tuning frequency in my view. Some relevant T/S of the latter are: Fs 30 hz, Qts 0.21, Vas 10.5 cu. ft. and Xmax 0.14 in. (3156: 38 hz, 0.24, 8.7 cu. ft., 0.15 in., in the same order). I think what saves the 2205 for use in larger boxes compared to Altec's 3156 is the higher Vas number (used in calculating box volume). The lower Fs on the JBL can't hurt also. In case you wonder how things turned-out for me with the 2205H in a vented box, here's an overview...
When I purchased the 2205s in 1981, I called JBL Pro Tech. Services and discussed my box options with Applications Engineer Nat Hecht. I was offered three box choices: 7.1, 4.2 and 3.0 cu. ft. The first one seemed way too large for available space, more so considering the gross volume required to get to 7.1 net, and the third one appeared small to me for a 15" driver. So, at the time I settled for the middle of the road 4.2 cu. ft./Fb 50 hz cabinet. In these days, speaker design software was not as easily available to amateurs as today, but manufacturers such as JBL and E-V had their own in-house box design programs based on T/S parameters. E-V also modeled a box for me many years ago, and I still have those too.
I can report that my choice of box among those offered by JBL, looks like a good compromise for the 2205, after having done my own simulations with a few speaker design computer programs years ago. Even if a 3 cu. ft. box, Fb 50 hz, wasn't bad, response being reasonably flat (though a bit on the negative side in LF) and F3 @52 hz, similar response shape to 3156/8156 combo, with JBL F3 result better than Altec's. The 4.2 cu. ft./Fb 50 hz boxes, show a little edge for me on response since the lows are not in negative territory where it counts (e.g. + 0.5 db @ 60 hz) and F3 @ 47 hz. The slight 2205 driver rising response of 1-1.5 db above 250 hz or so is not an issue in my application considering these bass cabinets are one-way only, and always used in a bi-amp mode (active X-over around 200 hz) with a pair or two of smaller speakers from my lineup.
BTW, the JBL Pro Enclosure Guide, providing GENERAL GUIDELINES for enclosures, does suggest a 4 cu. ft. box with Fb 40 hz for that 15" driver, and many others. However, modeling this 4/40 combo in Winspeakerz design software shows a progressively falling LF response from 200 hz and lower, by almost - 3 db @ 50 hz, with a curve of similar shape to that of Altec's 3156 driver in a 5 cu. ft. box mentioned in my post # 5. Not my cup of tea. Here, that lowering response is purely the result of too low tuning frequency (40 hz) for such a driver/box duo. Based on my simulations, response would be much flatter at Fb 50 hz in that 4 cu. ft. box.
As a Sound Reinforcement driver, the 2205 was primarily designed with rear loaded folded horns in mind (4520, 4530), but it may well be used with good results for general purpose low frequency reproduction in vented enclosures of 3, 4, 5 or even 7.1 cu. ft., as long as box tuning frequency remains around 50 hz for each (about 45 hz for 7.1 enclosure) according to my simulations. An Fb 40 hz is just too low for this woofer and puts more strain on it, unless one uses floor/wall junction placement to boost the deeper low-end by 3 db. That driver, at 97 db sen., as for the 2225, is a middle of the road between max efficiency/lower fidelity (e.g. E-140, 2220 at 100-101 db) and max fidelity/lower efficiency (e.g. 2235 at 95 db).
JBL's 2/80 data sheet recommended enclosure volume of 6-8 cu. ft. appears somewhat overly optimistic or exagerated, in real life, for the 2205. It sure can be used in a large box size, but the useable spl output below 50-60 hz drops rapidly, since it was not engineered to be a subwoofer type driver (i.e. limited Xmax). I wouldn't try to mount this driver in a larger box than I have, since that would mean trying to extract from it lower frequencies than what the woofer was made for. It can only provide some VLF at a reduced output. It can still "rattle the silver" with 120 db @ 60 hz because of amplitude, not LF bandwidth. In fact, at high LF levels, the bandwidth should be high-pass filter limited around 45hz to minimize the risk of over excursion.
Bottom line, some drivers, even if large in size, are not necessarily designed for very low bass reproduction, but may rather be optimized for higher level sound reproduction, excluding deep bass...
Richard