2245 in Isobaric configuration???

[Champster]

Ok, so as some of you have followed my latest build, I have opted for a pair of 2245's. Somehow I've wound up with 4 of them all with recent JBL recones.

I was wondering if anyone has any experience using an isobaric configuration with these, or other, large JBL drivers? I have to admit, I don't mind cutting the Vas in half. I have done this in the past as I cloned a Vandersteen Model 4 with 4 Dynaudio 30W100's. I think it really helped lower the distortion of the 12" Dynaudio's but, JBL woofers have such low distortion anyway, I'm not sure there is that big of an advantage other than a reduced cabinet size.

Thoughts/comments?

[NickH]

isobaric loading is a compromise. Just stick 2 drivers in a reflex cabinet and have a stereo pair.


if you have a driver with a weaker motor then isobaric loading has some pros. But you only get half the output I think. The drivers run out of excursion twice as fast.


Hope this helps some,

Nick

[Champster]

isobaric loading is a compromise. Just stick 2 drivers in a reflex cabinet and have a stereo pair.


if you have a driver with a weaker motor then isobaric loading has some pros. But you only get half the output I think. The drivers run out of excursion twice as fast.


Hope this helps some,

Nick

Thanks Nick but could you explain your comment about it being a compromise? I don't see that at all. The cost is 2x, so its no savings. The output is the same as having one driver). The Vas is cut in half, so you save a lot of cabinet space and the excursions doesn't get changed at all. It retains the same excursion as either driver has individually. Distortion is measurably lower due to having 2 motors controlling the same cone area as one driver. I don't see this as a compromise at all, but maybe I'm not understanding how you're using the word???

I'm not confused or uneducated in the implementation of isobaric configurations. I was just wondering if anyone on the forum had tried the alignment with large (15" or 18" drivers) and what they learned from it.
Thanks,
Paul

[NickH]

Hi Paul,

that's exactly it, 2 drivers half the output. Isobaric loading is for drivers with weak or poorly designed motor. The 2245 doesn't fit into that category.

I've thought about trying it before because of the smaller box volume. But I never have. If I were in your shoes I'd put 2 drivers in 2 reflex boxes. Granted they need to be big boxes. But you be getting close to the brown sound with subs like that, lol.

By all means though you use them in what ever way makes you happy. If you do try isobaric make sure you mount the drivers to symmetric sides otherwise you will create some distortion.

there is something to be said for a 2245 sub in a small box, especially if your realestate is limited. You'll need twice the power too, won't ya?

[Lee in Montreal]

Hi Champster

As was mentionned, you will gain absolutely nothing heading the isobaric way, except for reducing your cabinet volume by half, yet needing two drivers per cabinet and one amp per side. But now knowing you have four 2245, you can build four bass bins. Use two in you front set-up and position the other two at the back of your listening room to spread the bass. You will literally have a carpet of bass wherever you are in the room. And there will be a sweetspot where you will literally be floating on bass ;-)

[Mostlydiy]

Two closed cabs in some kind of push pull combination would be awesome. The bass quality would be exelent and with 4 drivers you could play decently loud. Add some eq(Linkwitz transform) if you want to configure the q and lower cutoff.

/Mostly

[more10]

No-Frills in Sweden use a push-pull design in their Barrabas (http://www.no-frills-audio.se/barabbas-2-x-21/). The advantage of mounting one driver with the magnet out is that the nonlinear behaviour of the drivers will be balanced, lower distortion that is.

In order to minimize the baffle area you can also mount the drivers in a PPSL (http://www.diyaudio.com/forums/subwoofers/177905-thread-those-interested-ppsl-enclosures.html) (Push Pull Slot Loading).

Yet another idea is to build 4 boxes (2 with magnet on the outside) and place them in the four corners of the wall where you have your system. This way you will have less problems with cancellations.

[Champster]

Check this out. I'm beginning to believe there is something to this configuration...
http://www.vueaudio.com/Downloads/Datasheets/as-418_datasheet.pdf

[Ian Mackenzie]

What is the crossover point for the 2245H?

Work the design, for the QB5 8 cu ft 3 box /2 = 4 cu ft cu3

If the driver is required up to 300 hz it will need to be front facing.

For that option if the drivers are face to face and inset into the baffle the space used is about 1.5 - 1.8 cu ft 3

The space occupied by the longer ports required for the lower volume VB require more volume approx. 1.5 cu ft 3

So you have to add back the volume for the compound enclosure and the large ports

4 cu ft plus 3-4 ft 3 = 7-8 ft cu, that is the internal volume

For a compound sub like that you mention the enclosed space for the to face to face drivers is significant, then you have the large ports.

The an unassisted enclosure of 10-12 cu ft 3 / 2 = 5-6 cu ft 3, add back 3 cu ft = 8-9 cu ft 3

You also have an 4 ohm load and the in efficiency of the load and only the same output as a single woofer.


I would call compound implementation(duplex) a poor design choice

The moral for the story is the 2245 is meant for large rooms

[Champster]

Ok I'm going to go with the consensus here and use one 2245 in an 8.5cf box just like the 1983 article from Greg Timbers suggests and see how it goes. One interesting thing about the JBL data sheets is that some of their drivers disclose 2nd and 3rd harmonic distortion (2206) whereas others don't (2235 & 2245). The suspicious side of me says that maybe those stats aren't very impressive when omitted. Nevertheless, I'm going to try one 2245 and see how it works.

Thanks for all of your thoughts!

Paul

[Lee in Montreal]

A transmission line would be great too. ;-)

http://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDQQFjAA&url=http%3A%2F%2Fwww.coolcat.dk%2Fbjoern%2FJBL_224 5_H_TL_Designs.pdf&ei=jxtsU5moMsGgyAT6tIHwDA&usg=AFQjCNHYXFner3lJYYexTXWqifo3EQqzzQ&sig2=lG1O-E-eDgg_2hxJg07tZQ&bvm=bv.66330100,d.aWw

[Allanvh5150]

As for 2nd harmonic distortion on a 2235 or 2245, most of the time this would occur above the crossover point so would not be of and real benefit to include the data.

Allan.

[Ian Mackenzie]

Designed by Greg Timbers to partner the L250

I think you will be quite impressed with the 2245H.

Measurements and here say do not translate the subjective performance.

Often regarded as the best sub woofer in the world for music reproduction for the past 3 decades.

Nothing even comes close in terms of linearity and bandwidth.

The 2245H will discern the low bass reproduction accuracy of the finest equipment in the audio chain.

When you get around to setting them up try and get the woofers up off the floor to avoid too much boundary reinforcement.

Use the largest most powerful amplifier you can afford, I currently use an 800 watt per channel amplifier.

[ivica]

......

I would call compound implementation(duplex) a poor design choice

......

Hi Ian Mackenzie,

I have to agree with You, especially if hi-quality drivers (such as JBL bass units) are to be used.

Regards
Ivica

[Champster]

Will Do Ian and thanks for the information/perspective.

By the way, the picture you have at the bottom of your posts. Is that your listening room?

Paul

[Ian Mackenzie]

Unfortunately no...LOL

[4343]

If I had four, I'd be thinking Triple Chamber Bandpass. http://www.jblpro.com/pub/obsolete/4688.pdf

Basically two boxes , tuned to 24Hz, joined to a third box with the drivers tuned to 68Hz.

Build specs in this thread, although not complete measurements.:

http://www.audioheritage.org/vbulletin/showthread.php?6782-4688-TCB-Monster-Subwoofer

[Mostlydiy]

Unfortunately no...LOL

:)

[Champster]

If I had four, I'd be thinking Triple Chamber Bandpass. http://www.jblpro.com/pub/obsolete/4688.pdf

Basically two boxes , tuned to 24Hz, joined to a third box with the drivers tuned to 68Hz.

Build specs in this thread, although not complete measurements.:

http://www.audioheritage.org/vbulletin/showthread.php?6782-4688-TCB-Monster-Subwoofer


Very interesting Mike. I'd never seen that model. So the JBL Data Sheet validates the claim I proposed earlier using phrases like "exceptionally low distortion levels" and "complement the low frequency performance of all JBL systems".

So does the 4688 operate in Stereo or Mono mode?

I wish there were detailed drawings of the cabinets however. Does anyone have one of these to take extensive pictures of the internal chambers?
Thanks, Paul

[rdgrimes]

I'd be looking at 4 boxes with symmetrical placement in the room, and a good EQ system. Most of these cabinet theories are made moot by the application of EQ. Plus the room gain from 4 separate subs is significant. KISS

[macaroonie]

^ this +1 .. Corner placement is highly relevant as there is significant acoustic gain , hence lower distortion. Also less cancellation / augmentation modes to mess with.
Allison Acoustics many years ago showed the pluses of boundary placement for low frequency..

[Champster]

I'd be looking at 4 boxes with symmetrical placement in the room, and a good EQ system. Most of these cabinet theories are made moot by the application of EQ. Plus the room gain from 4 separate subs is significant. KISS


^ this +1 .. Corner placement is highly relevant as there is significant acoustic gain , hence lower distortion. Also less cancellation / augmentation modes to mess with.
Allison Acoustics many years ago showed the pluses of boundary placement for low frequency..


So, if your point is that by adding additional drivers, thus using each driver at much lower wattages to achieve the same SPL, that makes sense. Even for these professional drivers, the harder you push them, the more they will distort. That is simple physics. In my case (a home stereo situation), I'm definitely not pushing any of them to their limits. This is one of my hopes in building my system, that the professional drivers will produce much lower distortion than typical home stereo speakers from Seas, Dynaudio, etc. So to your points, it makes sense that additional drivers would lower their output demands and thus operate in their lowest possible distortion range.

[4343]

So, if your point is that by adding additional drivers, thus using each driver at much lower wattages to achieve the same SPL, that makes sense. Even for these professional drivers, the harder you push them, the more they will distort. That is simple physics. In my case (a home stereo situation), I'm definitely not pushing any of them to their limits. This is one of my hopes in building my system, that the professional drivers will produce much lower distortion than typical home stereo speakers from Seas, Dynaudio, etc. So to your points, it makes sense that additional drivers would lower their output demands and thus operate in their lowest possible distortion range.

Yes, 4 separate locations will be better, making room modes less of an issue, and easier to minimize.

Lower distortion from lower individual output levels helps too.

The TCB also allows you to cancel some distortions caused by non-linearities in the cone suspensions, by way of it's "Push-Pull arrangement where one cone is moving in, and one is moving out. You'd you need 4 more drivers though...

[Ian Mackenzie]

I can run simulations in Bass Box if you like?

Be warned there is no free lunch and the trade offs are response ripple, larger group delay and bandwidth versus sensitivity and Box vol.15

Triple chamber designs are generally not used for HI Fi applications due to the above.

Depending on how much space you have I would consider two 2245 for the mains L & R and centre 2245 for ELF.

That way you get a spread of LF sources and increased Max output below 50 hertz where it matters.

It would be like the equivalent of 6 single 15 inch drivers in terms of displacement limited output.

How are your neighbours!

With room gain you will have sensitivity of 100 db for 1 watt below 100 hertz with 2 x 2245 anyway.

[Champster]

I can run simulations in Bass Box if you like?

Be warned there is no free lunch and the trade offs are response ripple, larger group delay and bandwidth versus sensitivity and Box vol.15

Triple chamber designs are generally not used for HI Fi applications due to the above.

Depending on how much space you have I would consider two 2245 for the mains L & R and centre 2245 for ELF.

That way you get a spread of LF sources and increased Max output below 50 hertz where it matters.

It would be like the equivalent of 6 single 15 inch drivers in terms of displacement limited output.

How are your neighbours!

With room gain you will have sensitivity of 100 db for 1 watt below 100 hertz with 2 x 2245 anyway.


Thanks Ian, I appreciate the offer, but I think I'll just get started by incorporating the 8.5cf cabinet Timbers alignment with one 2245 per side.
Paul