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Jennice 8th May 2005 10:15 AM

4x isobaric
Hi everybody,

I have found info on the use of two drivers in isobaric coupling, but what happens (in terms of sensitivity and box design math) if I use 4 drivers? Two should be facing out into the listening room, and the other two should be hidden inside the box, with isobaric coupling to the outter two.
The drivers are Peerless SLS-8 drivers, nominal 8ohm with about 87dB sensitivity each.
My plan is to put connect two drivers in parallel (giving 4 ohm), and then connecting two of these sets in series, giving 8 ohm again.

If I have understood it correctly, two drivers in parallel (mounted isobaric) end up with the same sensitivity as a single driver. (if this correct?)

If so, I am splitting the power (with two "4-ohm pairs" in series), but getting double the cone surface. Where does this lead me?
Can anybody tell me the overall sensitivity of this system?


Ron E 8th May 2005 08:14 PM

It will be the same as a single driver.

Isobaric is a waste of money and cone area. You'd be better off making a single sub with two woofers (or even all four) of the desired box size - then make a linkwitz transform to get the Q you want.

You are better off getting an amp that can handle 4 ohms than run with a limitation to 8 ohms loads anyway.

Bill Fitzpatrick 9th May 2005 12:41 AM

No, you're better off putting 2 in parallel on one channel and 2 in parallel on the other.

fazman 9th May 2005 12:59 AM


Originally posted by Ron E
It will be the same as a single driver.

Isobaric is a waste of money and cone area. You'd be better off making a single sub with two woofers (or even all four) of the desired box size - then make a linkwitz transform to get the Q you want.

You are better off getting an amp that can handle 4 ohms than run with a limitation to 8 ohms loads anyway.

Ron E
Could you tell us why isobaric is a waste of time?
On another thread I posted some questions regarding help with my JBL 15" subs and it was suggested to me to run them in an isobaric configuration (also no room for two subs it had to be one enclosure)
I made a makeshift isobaric loaded protoype and so far the results are very promising.
You can look under the thread help me make my decision on new subwoofer or not.
As you will read my initial plans were to change my sub for a Parts EXpress Titanic 15 or something similar.

I'm interested to read your views on this if not for anything more but to learn more about this crazy addiction we all have.
Thanks Dave.

BassAwdyO 9th May 2005 03:55 AM

he didnt say it was a waste of time, only cone area and money. Cone area is wasted because you used two drivers and get only the cone area of one, and time is wasted because you spend twice as much on drivers.

But, you get double the themal power handling and half the enclosure size, plus canceling of some distortion and a slight increase in linear Xmax.

Jennice 9th May 2005 06:28 AM

...and the madman returns :)
Hi folks...

Thanks for the replies. I was not planning to make it as a dedicated sub, but as the low-end of a 3-way. (Don't worry, I got someone to help me with the X-over).

Thus, I would use 8 speakers in total, 4 for each side.

If I understand it correctly, using 4 drivers (2 isobaric sets) in each enclosure, it would give me the sensitivity of one driver, but with better power handling?
Am I correct, that this setup would have lower displacement (cone movement) for a given SPL than a single driver (since there are two cone surfaces to move the same air as one would have to do)?

I have penty of these drivers around. I have also considered using the 4 drivers per enclosure, all facing out (2 front, 2 rear). This would give better efficiency - but how much more?
Also, it would need a rather big enclosure - which isn't very compliant with a good WAF ;) .


Jennice 9th May 2005 10:00 AM

Hi again... just a quick update.

I plan to use the bass drivers with a HDS134 or HDS164 for the mid's, and a scanspeak 970k or 930k series for the top.

If I crossed at about 200 - 250, I am concerned about the displacement of the HDS134 (+/- 3.5 mm).

The HDS134 is supposed to sound better, but the HDS134 will certainly be better to handle the power at these frequencies.

Ron E stated that the sensitivity of 2x 2isobaric would be the same as one driver. This would suit the mid-driver well, as they both have about 87dB sens.

The tweeter it at about 89, so it would need a little series resistor, but I wouldn't "waste" a lot of energy this way (only in the tweeter).

What do you guys think.


Edit: I just realized that maybe this shouldn't have been posted in the sub-section (although my primary question was about the bass drivers), but it's too late to worry now I guess...

Ron E 9th May 2005 05:19 PM

As the gentleman said, a waste of money because you pay for two drivers and only get one, and a waste of cone area for the same reason. IT may be a valid way to make some drivers you already have work for some application where you need a smaller box, but IMO if you are starting from scratch your money is better spent looking for a driver that will work in that application in the first place.

You could also call it a waste of power because you need twice the power to get the same SPL. Isobarik is not a free lunch. Depending on how it is done, it doesn't always save as much space as you think bacause if it is not clamshell (which looks ugly) you have a tunnel to build.

Since the OP has a bunch of these lying around and is presumably making a multi-way with passive crossovers, it should work OK, but I think just using 2 active drivers speaking to the outside world in parallel might work better in the end for baffle step compensation purposes..... it is really hard to say as there was not enough detail given.

For a straight powered sub application, it is much better to use a Linkwitz transform and use all the cones for making noise. You can build small box with a Q of 1+ and transform it to the Q and F3 you desire. You will get better max SPL's and less distortion for a given SPL because each cone will only be moving half as far.

You should be able to take the 8"ers up to 500Hz with no problem if you want to. Cuts down on midrange IM distortion.

Jennice 9th May 2005 07:14 PM

Hi all,

What would happen if I placed all 4 drivers facing out of the enclosure (useful driver surface)? I would mount 2 drivers in parallel, and two of these sets in series, getting back to 8 ohm.

I'm fairly new to this, so please comment on my math:
2 drivers in parallel should give +3dB for "doubled surface", and +3dB for half impedance (double power), resulting in a 6 dB gain.

If I take two of these sets in series, I would loose 3dB to reduced power, but gain 3 dB due to doubled (again) surface. If I am correct, this should give 6dB gain over the single driver, while having a better power handling. Is this correct, or am I wrong (if so, where)?


azrix 9th May 2005 11:10 PM

As far as I know, you have that correct. You should gain 3db for doubling cone area once (2 drivers), another 3db for doubling again (4 drivers), and none from impedance since that didn't change. Which gives you a 6db net gain. Depending on frequency and driver layout, though, you can have lobing issues with that many drivers. I'm not sure what exactly happens to the power handling. Anyone know if it double or quadruples over a single driver?

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