Question regarding bandpass enclosure

[wrankin]

Hey gang,

I recently picked up a set of Boston Acoustics SubSat 7's cheap on Ebay. The system consists of a pair of small 2-way satellites and a passive bandpass subwoofer. I had a quick question about the layout of the subwoofer.

Being the eternal engineer, I of course didn't even bother plugging the subwoofer in and immediately took the back of the cabinet off to check out the guts. Internally there are two decent looking 6-7 inch woofers each with their own sealed enclosure. The drivers then radiate into the third enclosure which contains the tuned port.

The one thing I did notice is that one of the woofers is installed "upside down" (ie, with the basket extending into the tuned chamber) while the other driver has its basket in the sealed portion. This was obviously an intentional part of the design, and the two drivers are wired in opposite phase.

So, why did they do this? Is it an effort to reduce distortion?

Thanks,

-bill

[Jonathan Bright]

Yes, I think you are correct in assuming it is distorion that is the factor they are trying to address. As I understand it people think that the characteristics of the cone movement are different going away from the magnet assembly compared with moving towards it and mounting any pair of woofers in the manner described will aid in cancelling some of the anomolies. But others may have a more technical/competent answer. BTW, I know how you feel. Always have a look inside before playing!!!

Jonathan
Edit; Yup, just read your post again. If the enclosures are sealed then there will be a difference on the surounds/suspension etc because the air on the inside will be in compression for movement in one direction and then be expanded in the other. There will be a mild variation in the characteristics.

[technofreak]

I'm using push-pull myself - cheap Visaton BG20 8" fullrangers that's midrange turned out not to be so good, too high harmonic distortion for my liking (though subjectively with music not that bad), now playing up to ~250 Hz in push-pull, for the time being providing bass to complement Eminence Alpha 6 midrange. I can run those Visatons beyond their X-max, no noticeable distortion whatsoever.

IMO push-pull is a very good and easy substitution for horn-loading if you want to reduce distortion at bass frequencies. It don't work for midrange/highs, though, because drivers must be within 1/4th of wavelenght.

As noted in prev. post, push-pull evens out the magnetic field* and suspension differences of the "+" and "-" side of cone excursion, reducing 2. harmonic distortion.

*Shorting rings do the same, but they're more effective at higher frequencies, whereas push-pull is more effective at lower frequencies.