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 19th July 2007, 11:09 PM #1 Lindell   diyAudio Member   Join Date: Dec 2005 Location: Planet Earth Compound , Iso-Barik Just to continue the other "Push-Pull" thread. I've read up on the "compound & Iso Barik" theory's. And I understand the deal. What I'm uncertain of is the calculation of the total compound box. Do you calculate the combined volume of the two boxes? Or just the one behind the second woofer? See my drawing below: Thanks!
 20th July 2007, 04:42 AM #2 Circlomanen   diyAudio Member   Join Date: Sep 2004 Location: Göteborg Fig 2 is the right one. The chamber between the drivers should be as small as possible. __________________ We live in a horizontal world, why use vertical topologies???
BHTX
diyAudio Member

Join Date: Feb 2006
Quote:
 Originally posted by Circlomanen The chamber between the drivers should be as small as possible.
Yep, and the face-to-face 'clamshell' method is usually ideal, rather than back to back.

Brisso57
diyAudio Member

Join Date: Sep 2005
Location: the leafy west of Brisbane
Quote:
 Originally posted by BHTX Yep, and the face-to-face 'clamshell' method is usually ideal, rather than back to back.
??? I don't understand. Why wouldn't you fit them in-line, and facing the same way?
That way the volume could be minimised, with the "tail" of the first in the second's "face"? ... taking care that the excursion never exceeded the gap.

Perhaps the cabinetry is trickier, but you would also have the front side of a driver exposed externally. Surely that's better than a rear? (The sound waves not striking the frame.)

Doug
__________________
A speaker-builder's parable: "That burned down, fell over, then sank into the swamp. But the fourth one stayed up. And that's what you're going to get, Lad, the strongest castle of all."

 21st July 2007, 01:58 PM #5 tade   diyAudio Member   Join Date: Feb 2005 Location: Ralieigh nc At those frequencies, there is not "striking" of the soundwaves, they just kind of come out. so definitely cone to cone it a good way. Plus, its seldom you see a subwoofer with the motor on the outside. I really would have liked to have done that with my dayton RS 12" woofers. They have nice motors!
Brisso57
diyAudio Member

Join Date: Sep 2005
Location: the leafy west of Brisbane
Quote:
 Originally posted by tade Plus, its seldom you see a subwoofer with the motor on the outside.
That's because of the dreaded WAF, mostly! javascript:smilie('')

Doug
__________________
A speaker-builder's parable: "That burned down, fell over, then sank into the swamp. But the fourth one stayed up. And that's what you're going to get, Lad, the strongest castle of all."

 21st July 2007, 09:11 PM #7 richie00boy   Did it Himself diyAudio Member     Join Date: Nov 2003 Location: Gloucestershire, England, UK Putting them both the same way loses the benefits of push-pull. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, modules and more.
BHTX
diyAudio Member

Join Date: Feb 2006
Well first of all, even though the performance of both of the drivers motors will likely stick closer together, (since they're in the same exact cooling environments), you must also realize that you now have two large sources of heat dissipation stuck right up against each other, and in the same ridiculously small enclosure. Of course, this has the potential to reduce the thermal handling of the two woofers. And on top of that, remember that heat is not only detrimental to the motor, but also to the entire woofer, including the cone! Heat expands. Why would we want warm air trapped between the backs of both drivers? Besides the whole thing regarding both magnets being together in such a small box, I just don't think it's a good idea.

Second, and perhaps even more importantly, you generally want the space for coupling between the two drivers to be as small as possible, since this air between the two woofers adds to the moving mass of the system, and thus results in less than optimal coupling between them. Since your goal is for the two subwoofers to act as one driver, adding a springy mass of air in between the two of them compromises that goal. It'll also have the tendancy to make predictions of the response and modeling of the system more difficult and inaccurate. Using the back-to-back method of isobaric loading shown in the above diagrams, this volume of air between the two drivers will be a bit larger, versus the face-to-face clamshell method. With that said, this method shown above is even worse in that respect, as far as the amount of air between the two drivers, than the piggy-back tunnel loading method, although this method of loading (piggy-back) also won't provide the benefits of cancelling out driver non-linearities.

As a side note, an easy way to think of it is.. as long as both drivers are face-to-face or back-to-back, you'll receive the major benefits of greatly reduced enclosure size from isobaric loading.

Third...with the volume of the coupling chamber being increased even more than the other inferior methods, the total volume of the isobaric enclosure is obviously approching that of a single conventional subwoofer with one of the same drivers. With that said, you're now getting close to losing a major advantage of isobaric loading.

Quote:
 Originally posted by richie00boy Putting them both the same way loses the benefits of push-pull.
Both the same way? I assume you're referring to the instance of a 'regular' push-pull configuration (not isobaric), where both drivers are mounted conventionally, but with one driver facing the opposite direction, wired out of phase. However, both face-to-face and back-to-back loading arragements causes cancellation of driver non-linearities, cancellation of odd order harmonics, and requires half the enclosure volume than a single driver. Any other push-pull method may give you the first two benefits, but the reduction of required box volume won't exist, as the two woofers aren't physically coupled together.

The main disadvantage to isobaric loading is a fairly significant reduction of 3dB in efficiency per pair of woofers, versus a single one of those woofers. For example, say you have 4 subwoofers, each rated at 87dB/watt. You take those 4 woofers, and put them in two isobaric pairs. Theoretically, efficiency for the combined 4 woofers will still be 87dB with 1 watt applied to each woofer. However, since power handling has also quadrupled, due to using 4 woofers rather than 1, the situation regarding the loss of efficiency can also be seen as being 'evened out' by this large increase of overall power handling of the system.

So, basically, when deciding to go with an isobaric configuration, you're accepting to pay for double the cost of woofers, amplifier power, and possibly having to deal with the disadvantages of having to have at least double the power supply on hand for the massive increase in required amplifier power... all for the main advantage of reducing Vas, and hopefully receiving the other side benefits previously mentioned above.

For most, it's probably not worth it in a home environment. I'd usually prefer a conventional push-pull design, versus isobaric push-pull...unless my enclosure is just insanely huge, or if I want the smallest enclosure possible, or want to attempt impressing less knowledgable and informed individuals with the best low freq extension to enclosure size ratio.

Honestly, I'm still not too clear on the whole reduction of distortion thing with push-pull configurations. I hear and read different claims from different companies and manufacturers, and different opinions and statements from different people. With all that, I just can't get a good grasp on it. I'm currently interested in trying it out myself though.

By the way, I didn't type all of the above solely because I felt someone here needed the information, but so that someone in the future who's attempting to obtain information on the subject might come across it. I didn't search the forum though for info on it yet. I'm guessing there's probably not THAT much out there though. Facts seem to be too cluttered with personal opinions, as well as false statements.

If I myself am misunderstanding anything regarding this subject and have provided false information as a result, please let me know, as I'd very much like to be corrected.

~ Brandin

 22nd July 2007, 11:15 AM #9 Lindell   diyAudio Member   Join Date: Dec 2005 Location: Planet Earth Thanks Brandin, I guess it all depends on the driver and what you want to achieve with it. And the physical size of it. Like for an example getting the maximum bass out of a small woofer like 5" or 6" in the smallest possible enclosure. Normally takes a large vented box, but with iso-barik with a "small" woofer makes for half the size. Since I'm in the recording business we like to play loud. So more bass woofers the better......in a small size..... What about putting a tube between the woofers to reduce air?Same size as the baffel cut out? /L
 22nd July 2007, 01:33 PM #10 richie00boy   Did it Himself diyAudio Member     Join Date: Nov 2003 Location: Gloucestershire, England, UK No I was referring to Brisso57's comment about putting them in 'train carriage' formation. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, modules and more.

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