Planar Dipole roll-off?

[AJ]

I'm discussing dipole rolloff with a buddy who owns Maggie 1.6s, and I've heard them in his large, well-balanced room. My question is why do Maggies (I own some MMGs and the same goes for them) seem to defy the laws of dipole physics and still have reasonably flat bass output down to ~40hz without the need for EQ?

Considering their baffle size, I don't understand what gives them their low-end lift when everyone else building dynamic dipoles has to EQ? Is there something about line sources that get to cheat in this area, or does Magnepan build in a rising response in their bass panels to compensate for the (apparently) unavoidable loss in the low end?

Enquiring minds want to know.

[Pan]

EQ can be acomplished in the crossover or in a electronic crossover/EQ of some sort.

I think the bass panels have a fairly high Q to begin with and some EQ in the passive crossover would take care of the rest.

It's very possible to design dynamic dipoles as well without active EQ and end up with a tolerable impedance.. all depending on size, requirements and so on.


/Peter

[MJK]

If you look on my site under OB articles, you will find two examples of reasonably compact OB's that easily get to 40 Hz or lower without requiring EQ. It is a function of the driver Qts and the baffle size. If you use a driver intended for a box speaker on an OB the bass will definitely need to be EQ'ed.

[LineSource]

My Apogee Full Range bass planar magnetic panels are tuned to have a distributed top-to-bottom resonance to avoid BOOM-BASS, and have a Q ~2 to create low frequency bass boost. The lowest resonance is at floor level to get the most gain. Apogee used a Kapton film on their panels which has little physical stretch, and this allowed tuned high-longevity multi-spring tensioning clamps along one edge.

[AJ]

Pan & Martin & LineSource - thanks. I don't see anything resembling passive EQ in the crossovers. They're basically 2nd order, or a combination 1st/2nd depending on what year the Maggies were built.

Both your comments on the high Q make sense though; I didn't stop to think about that with planars. I see it easily enough with dynamic drivers, and understand the high Q concept for dipoles, but I didn't translate that to the Maggies. This explains why I'm getting more bass than I "should" from my own dynamic dipoles.

[Pan]

The EQ in a passive crossover could simply be a series coil that is big enough to shape the response into something desirable.

Bass drivers with Q normally associated with boxes can easily be used open baffled as well. There's no need really to go the high Q way (allthough some may argue that it makes sense). Drivers with Q around 0.35 with sufficiently low Fs may be used.


/Peter

[AJ]

Pan,

I've been watching alot of DIY efforts lately with lowish Qts drivers and for the most part, everyone is EQing them. Mine have a Qts of ~.65, which I don't consider high, just better than .35 and I've been surprised at how little EQ I need to get a reasonable 40hz from them. I guess I never really connected what I saw as a not-too-high Qts with a relatively easy 40-45hz and then the conversation about the Maggies came up. I didn't connect the dots because I don't see planars in terms of typical T/S parameters.

It's a lot clearer now and it's steering me to want to try a quad of Alpha 15's per side instead of my sixteen Warrior 12s. After living with them for a coupla months, the higher Qts Alphas are probably more what I'm looking for. Thanks again.

[john k...]

Originally posted by AJ
Pan,

I've been watching alot of DIY efforts lately with lowish Qts drivers and for the most part, everyone is EQing them. Mine have a Qts of ~.65, which I don't consider high, just better than .35 and I've been surprised at how little EQ I need to get a reasonable 40hz from them. I guess I never really connected what I saw as a not-too-high Qts with a relatively easy 40-45hz and then the conversation about the Maggies came up. I didn't connect the dots because I don't see planars in terms of typical T/S parameters.

It's a lot clearer now and it's steering me to want to try a quad of Alpha 15's per side instead of my sixteen Warrior 12s. After living with them for a coupla months, the higher Qts Alphas are probably more what I'm looking for. Thanks again.

There is a reason to use low Qts low Fs drivers. Yes, they require more Eq but but because of the characteristics of the poles in the response (the slow initial roll off) it is possible to retain a 2nd order acoustic response fairly easily w/o applying eq below the targeted system Fs. On the other hand, higher Qts woofer offer other choices for eq. See my article on
gradient woofer equalization.

[AJ]

Hi John

Thanks for all the work you've put up on your website - I've been reading it over the past coupla months and absorbing it slowly. I'm sure I'll end up building both high and low Q bass panels. I'm not opposed to EQ and have an amp that can deal with the extra power needed, but there's always something missing in the back of my mind if I don't build it and listen. I gotta hear both versions.