Le15A...how low does yours go?

[JRL]

I recently played a test cd on my S7's (original surrounds, but treated with brake fluid, so nice and compliant), and was surprised to find no real ourput below 40 cycles. The test disc has bands at 20, 30, 40, 50. Has anyone else checked their LE15A's at low frequencies?

[Bill Roberts]

Egads. Who came up with the idea of treating them with brake fluid? It certainly appears that most adhesives would be broken down using brake fluid..not to mention the paper cone absorbing the fluid and it weaking the structure of the cone rim.

To answer your question, Low FQ response has a ton to do with how the driver is loaded. I suspect given the proper enclosure and load that 30hz should be reached quite powerfully, within a very few dB of 100hz if your goal is to extract maximum lowest frequency from them.

I have been able to reproduce (to its reference of average response) 20hz from 6 inch woofers in properly designed enclosures made around reaching those frequencies.

Enclosures make the difference.

I also want to state that I have not read up on "use of brake fluid" on a loudspeaker but certainly if they need to be loostened up any at all; that a few hours at 16hz in free air at 5 watts would not do, a complete rebuild should be of order.

[JRL]

Bill, thanks for the reply. The brake fluid "fix" is well documented in the archives here, and may very well not apply to other speakers. It restores the compliance of JBL surrounds that have stiffened with age. As to the design of the enclosure, the woofers are in the original JBL Apollo enclosures, so I presume that some basic acoustic engineering was incorporated in the design.

[Bill Roberts]

In those days, I suppose the intended reach to be about mid 40's solid. Many loudspeakers were never meant to do below the 40's else risk upper bass taughtness. Many enclosure tuned to get into the 20's lack the tidyness in the mid bass (90 to 250).

All sound acoustic principals involve trade-offs. To truly have it all, one must sacrifice somewhere..and usually this is sensitivity or at the other end, enclosure size.

[Lawrence HF]

My experience with LE15A Fs 20Hz. Take it out of cab and check,JBL says tune box to 32Hz. Works well 3.5cu/ft, better around 5cu/ft, mine in 8 cu/ft JBL plans, with 3 ports at bottom about 4"round and 5and 1half long. They will produce well to 32 Hz, and lower. Stiff old surround, air leaks etc may be limiting your performance, also they should be within 1 db or less variation between each LE15A over the freq. range . Check voice coils. I am 99% sure your cab is tuned to 32HZ, audio signal generator needed by hobbyist of audio, if U don't have borrow, then U can check many things. You should have very strong output,(most port) at 32Hz. My 2-cents worth of answer.

[GordonW]

Yeah, the resonance of an LE15 is ~20 Hz, but the Q (damping ratio) is VERY low. As a result, it's VERY strong in the midband (relatively high efficiency) but the strong damping means there's not much output near resonance, even with a ported enclosure.

Realistically, the only way to get significant output from an LE15 would be to do ALL of the following:

1) use multiples, since they don't have a big X-max (linear excursion... the Xmax of an LE15 is MUCH lower than a 2235, for example.

2) use a relatively BIG box, even for an LE15 (probably in the order of 8-10 cubic feet per driver, tuned at 20-25 Hz or so.

3) use an active boost filter, such as the type of design used in the BX63 crossover or the type of design in a Linkwitz transform filter. This will provide the "resonance magnification" to give strong output down to driver resonance.

As you can see, it's not that simple. Most people, when faced with this situation, simply use a 2235 or 2245 (or some other driver combo) instead...

Regards,
Gordon.

[Lawrence HF]

I think JRL just wants to get his LE15a to work like it should, without building another system. The 2235 will move more air at low freq, but in his system, I think LE85 LX5 the mid performance of LE15a blends well. With 2235 many people use cone mid, then horn. I think if he put 2235 he would get more bass, but the overall sound would be different, and it would cost $$ to buy em. I find the bass tight and clean, not reproducing movie sound where some people want earth shaking sound at 20Hz. For that, build something. It goes low enough for me, and strong enough, there is always something better. The member above, Gordon is a real expert on these things, and I would look to him for definitive advice and information. I do know one recording studio where I live did replace LE15's with 2235 in all except one room.

[JRL]

Thanks, Lawrence, you've summed up my thoughts very accurately. I did not want to reinvent the wheel, just to determine what the normal bass parameters should be in an Apollo enclosure. I do have a subwoofer, although I don't use it all the time. I wanted the S7 system for nostalgia's sake, as I had one in the late 1960's, and have another system for real accuracy and fine resolution. Frankly, I've been using the JBL's with rock concert DVDs...I click on the loudness contour, turn off the lights, crank up the volume, sit back and enjoy the concert experience.

[paragon]

Hello,

I think that all old big JBL speakers are not going
below 35 or 40 Hz, because they are high efficient
drivers with a low QTS (Flat Butterworth).
I can simulate drivers with an enclosure you
want to build (very, very good to the real).
If you want 20 Hz from your drivers, you had to do
more than youst build an reflex enclosure.

Regards
Eckhard

[Mr. Widget]

As has been mentioned by several others on this thread the LE15A/2215 will not work as a sub without coaxing. I first tried a pair and then two pair and was never able to get the lows I wanted. The 2235H is much better for the lows and quite respectable in the midrange as well. There are those that prefer the LE15A in the mids. I am on the fence on that one.

The LE15A was designed when bass response to 35Hz was all that was expected or required from a speaker and it will do that very well. An LE15A in a 5 to 8 cu ft box tuned to 30-35Hz will give you excellent results.

If you want to get 20Hz out of it you should use an 8 cu ft box and tune the box to 20Hz and add about 10-12dB of boost. Your maximum SPL will be limited to around 100dB below 50-70Hz. With a box that big I would prefer a 2245, but then it will not do the mids.

Widget