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 12th May 2008, 05:41 AM #1 diyAudio Member   Join Date: Dec 2006 isobaric loading for small box vs displacement Would the combined displacement of two drivers offer better results than utilizing them in an isobaric config of the same size (small) box? Box size would be a comprimise however, displacement would be doubled. Is this correct?
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Join Date: Feb 2004
Location: Stockholm

Quote:
 Originally posted by danielm Would the combined displacement of two drivers offer better results than utilizing them in an isobaric config of the same size (small) box? Box size would be a comprimise however, displacement would be doubled. Is this correct?
Well, sort of. For frequencies where the driver limits the output, the maximum output level (MOL) will be 6 dB higher for the side-by-side config. Around the vent tuning, however, the vent might limit the MOL.

Below are three systems, all in a box originally designed for one driver in 23 litres. One can see that neither the side-by-side config nor the isobaric config results in a very good frequency response (thick lines), and that the MOL (thin lines) is ~6 dB higher for the side-by-side config except around fp.

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 25th May 2008, 01:49 PM #3 diyAudio Member   Join Date: Aug 2004 Location: Uppsala,Sweden The original question is an ill-defined problem as the sentence containing "better results" doesnīt specify better in which way. In the diagrams we see two examples of mismatching. The box for isobaric configuration should have a volume half of that of the single driver arrangement. The side-by-side should have two times that of the single one. Conclusion made. Itīs important to match the volume of the box to the eventual driver(s) and the type of loading you intend to use. __________________ Without wonderment,no life.
diyAudio Member

Join Date: Feb 2004
Location: Stockholm

Quote:
 Originally posted by danielm Would the combined displacement of two drivers offer better results than utilizing them in an isobaric config of the same size (small) box? Box size would be a comprimise however, displacement would be doubled. Is this correct?

Quote:
 Originally posted by The golden mean The original question is an ill-defined problem as the sentence containing "better results" doesnīt specify better in which way. In the diagrams we see two examples of mismatching. The box for isobaric configuration should have a volume half of that of the single driver arrangement. The side-by-side should have two times that of the single one. Conclusion made. Itīs important to match the volume of the box to the eventual driver(s) and the type of loading you intend to use.

Actually, the question is sort of well-put. It is true that the volume affects the frequency response (which would be part of the degree of "better results"). But with regard to maximum displacement the question is quite ok.

The maximum displacement and so the maximum output level is largely determined by how the drivers are mounted. The exception is the frequency range around the port resonance in a bass-reflex system. Here, the port limits the maximum output level.
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 26th May 2008, 10:05 PM #5 diyAudio Member   Join Date: Aug 2004 Location: Uppsala,Sweden I donīt want to argue. The original question confused me a bit as "better" has to denote something special as lower distortion etc. It may very well be immanent what was meant, so everything is OK. Efficiency is generally defined as the input of a resource or the input of resources into a process and the (wanted) output obtained. A reflex construction has greater efficiency around itīs tuned frequency as the driver sees more loading there. But further down it loses efficiency more rapidly than a sealed box as there is less resistance to the cone. So if we evaluate phase response to a high degree there is a disadvantage to reflex or passive radiator constructions. Of course, the Qtc of the closed box is of importance here. Quote:"The maximum displacement and so the maximum output level is largely determined by how the drivers are mounted. The exception is the frequency range around the port resonance in a bass-reflex system." Well, I say itīs a bit more complicated than that as itīs a system were the properties of the driver/drivers play a vital role. It isnīt fair to reduce their importance..... Naturally; how a given driver is implemented in an enclosure is important. But so is the capability of the driver itself when it comes to linear displacement. I read something about it in "the Cookbook"just minutes ago but sorry , Iīm a bit tired right now. __________________ Without wonderment,no life.
 27th May 2008, 03:20 AM #6 diyAudio Member   Join Date: Dec 2006 I believe Svante's graph above answered my question. By "bettter results" I guess I'm looking for MOL within the intended bandwidth. I have a limitation on box volume which must be very small. I thought that two drivers in an isobaric config 'may' be a better match to the small box and offer more usable output. Then I wondered if the doubled displacement would make up for the lack of box volume. I could see where a side by side driver mounting in a vented box would require a steep hi pass filter to prevent over excusion. Its a question regarding what to do with eight 15" drivers I've had sitting here for yrs. I could use a smallish PA that fits in the back of an estate wagon and thought maybe I could put these to use. I have no TS parameters on the drivers other than an fs of 28hz. I'll try to get some measurements when I find time and go from there. Thanks.

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