isobaric 15" subwoofer

[markh]

I wish to build an isobaric design for a pair of subwoofers, using two each of the Dayton 295-130 speakers. Cabinet size is NOT an issue, but deep tight bass is a must. Thinking that the Dayton suggestions might be conservative in terms of size, I was wondering if anyone knows what size I could/should make the sealed front cabinet and how large the rear vented camber should be, to achieve my acoustic nirvana!!

[ccochran]

> I wish to build an isobaric design for a
> pair of subwoofers, . Cabinet size is NOT
> an issue,

From my limited understanding, I'm not sure these two statements fit together. The biggest advantage of an isobaric design is you get to cut the box volume in half, at the expense of an additional woofer.

Why not put them side by side in a normal sealed enclosure?

[Andy_G]

The ONLY reason to go isobaric is to achieve a lower box volume. Other wise its just a waste of a driver.

I would build 2 separate boxes so they can be positioned at different points in the room.

Can't help with the box dimensions, venting etc because I've never used that driver, but I'm sure someone else will.

[billfitzmaurice]

> The ONLY reason to go isobaric is to achieve
> a lower box volume. Other wise its just a
> waste of a driver.

+1. Isobarics made sense 20 and more years ago when the limitations imposed by drivers of the day made a huge box necessary for deep bass. Modern subwoofer drivers have rendered the isobaric concept for the most part obsolete.

[preiter]

> From my limited understanding, I'm not sure
> these two statements fit together. The
> biggest advantage of an isobaric design is
> you get to cut the box volume in half, at
> the expense of an additional woofer.

> Why not put them side by side in a normal
> sealed enclosure?

Isobaric also cuts your efficiency in half.

[donradick]

> +1. Isobarics made sense 20 and more years
> ago when the limitations imposed by drivers
> of the day made a huge box necessary for
> deep bass. Modern subwoofer drivers have
> rendered the isobaric concept for the most
> part obsolete.

bill,
you're expert on subs around here, so please educate me. I'm thinking hard about isobaric subs because my living room space is limited, and amplifier power is cheap and clean these days.

It seems like it makes a lot of sense - use amplifiers to get you to a small box.

Why not?

-don

[Andy_G]

thing is, there are many sub drivers around now that are specifically designed for smaller boxes, so why double your cost ?

[Andy_G]

Also, this "half the volume" thing is a myth.

Yes, there is half the volume behind the second driver, but there is also the necessity , purely due to construction and driver size and shape, to have a small chamber between the 2 drivers. With the extra cabinet timber required, you very rarely end up with a cabinet less than about 2/3 the size of what might have been used with one driver.

[Andy_G]

> rarely end up with a cabinet less than about
> 2/3 the size of what might have been used
> with one driver.

.... unless it was going to be a very big cabinet to start with, and half of very big is still big !!!

[donradick]

> thing is, there are many sub drivers around
> now that are specifically designed for
> smaller boxes, so why double your cost ?

AG

Well, your cost doesn't double, IFAICS.
If I was to use the RS390HF in a ported configuration, PE suggests 8.9 cubes. (uh, Wowza!)
Way too big for my living room.

1/2 the volume = 4.5 cubes, which is doable.
Cost is about 1/3 more, figuring $240 for the amp,
and the 15" at $150 each.

I know my SAF would be way off the scale for a 9 cubes box. $150 extra is a small price to pay (one time) for quality bass in a reasonable size box.

-Don

[markh]

OK - box size is not a problem - I always thought that one of the advantages of the isobaric design (as in a Linn speaker I used to listen to a lot) was that because two speakers were moving in unison, there was no air compression involved and therefore tighter bass without the flabbiness associated with some port and reflex designs. If someone can tell me the equations used to work out optimal sealed and ported box sizes, that would be terrific!!

[donradick]

> OK - box size is not a problem - I always
> thought that one of the advantages of the
> isobaric design (as in a Linn speaker I used
> to listen to a lot) was that because two
> speakers were moving in unison, there was no
> air compression involved and therefore
> tighter bass without the flabbiness
> associated with some port and reflex
> designs. If someone can tell me the
> equations used to work out optimal sealed
> and ported box sizes, that would be
> terrific!!

As far as any reference I've seen on isobarik woofer design, you use the standard box size you get from Unibox or other box design sw, and cut the result in 1/2. That's how I was looking at a very large ported design, and it looks like I can make it much smaller.

-don

[blakeasaurus]

> I wish to build an isobaric design for a
> pair of subwoofers, using two each of the
> Dayton 295-130 speakers. Cabinet size is NOT
> an issue, but deep tight bass is a must.
> Thinking that the Dayton suggestions might
> be conservative in terms of size, I was
> wondering if anyone knows what size I
> could/should make the sealed front cabinet
> and how large the rear vented camber should
> be, to achieve my acoustic nirvana!!

Another BIG advantage of a "push-pull" Isobarik (or Isobaric, I've seen it spelled both ways), is that since any driver produces non-linear behavior in the "push" vs. "pull" directions; the push pull configuration effectively cancels odd-order non-linearities. Look at any Klippel plot of compliance in the push vs. pull directions (of even the most expensive high-end drivers) and you'll see the advantage immediately.

As Vance Dikason puts it (in Loudpeaker Design Cookbook, fifth edition): "This [push-pull] type of setup will cause odd-order non-linearities to cancel and result in substantially reduced driver distortion."

Odd-order non-linear behavior will only "cancel" assuming performance of both drivers is identical (not likely with mass produced drivers). However, distortion will be still be substantially reduced over that of a single driver.

As for calculations, use Vas divided by 2. Efficiency will suffer, but this can be offset if you wire in parralel (effectively increasing efficiency, asuming your amp is OK at 4 ohms nominal impedance).

IMHO, if you're going to do Isobarik, you may as well do it push-pull to reap the above benefits in addition to smaller box size.

Just don't forget to wire the speakers OUT-OF-PHASE and cone-to-cone or magnet-to-magnet with one another or you'll get 100% cancellation of ALL SOUND as well as distortion.

Cone-to-cone is appealing in that your box volume is exactly half of what is would be using a single driver (assuming you can live with the "ugly" basket, magnet, and wiring hanging out of your box). Magnet-to-magnet is probably subjectively better looking, but then you need the sub-enclosure for the inner driver, and your box size increases (not to mention overcoming the challenge of fitting a 15 1/8" OD driver through a 13 7/8" hole).

E-mail me if you'd like some models run with this combination.

Blake

[donradick]

[cole]

> As far as any reference I've seen on
> isobarik woofer design, you use the standard
> box size you get from Unibox or other box
> design sw, and cut the result in 1/2. That's
> how I was looking at a very large ported
> design, and it looks like I can make it much
> smaller.

> -don
mm

[cole]

> mm
mm