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If I understand you correctly, the enclosure size/shape should have little to do with the THD
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No. In a ported sub, there will almost always be a temporary decrease in THD near the tuning point, as the driver shifts from having to use a lot of excursion above tuning, to not much at all at tuning, and then it rises exponentially at some point below tuning. Other variables can and will change the specific levels of THD, including enclosure volume with a static tune and amount of power, but excursion plays a large role, and THD measurements of a ported sub typically resemble excursion use closely. The more linear the motor technology being used, the less this holds true, but it holds pretty true for the Plus drivers.
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and the distortion coming from dual cylinders should be essentailly the same as from the Plus/2.
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No. What I'm saying is that Ed's review of the PBPlus (NOT Plus/2) should closely resemble the performance of one 20-39 PCPlus.
Here is his review.
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Could that be the cause of the discrepancy?
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No, tune is determined by enclosure size and porting - the drivers used have nothing to do with it. Looking at Ilkka's measurements of the 25-31 PCPlus and the PB10, using THD as a guide, it would seem they are both tuned a bit higher than what one would expect.
The only way to know for sure is to do a close mic FR measurements of the driver. Do you have an spl meter?