Passive sub for Magnepan 3.6r?

Well Marty, the resolution of Greene's posted measurement is pretty low, showing 1/3 octave bands in one dB averaged increments. So I doubt that the actual curve stayed within +/- 4 dB... but that's still impressive in-room performance (which isn't recognizable as such unless you're all-too-familiar with how truly horrible most in-room bass response really is!).

The discrepancy you heard between what the Maggies are doing in the bass region, and what the subs is doing, is primarily an acoustic room-interaction issue.

A single monopole subwoofer will produce a peak-and-dip pattern at the listening position, and these peaks and dips will be far enough apart that they will be audible even if you don't consciously identify them. Now you can move the sub or move the listening position and that will shift and re-arrange the peak-and-dip pattern, but will not eliminate it. You can equalize it and improve the smoothness over a small listening area, but that will likely make it worse elsewhere because the inherent peak-and-dip pattern will be very different in other locations, so instead of cutting peaks and boosting dips you'll be boosting peaks and cutting dips.

Instead, suppose we have two monopole subs, preferably places fairly far apart, and non-symmetrically. Each will produce a unique peak-and-dip pattern at the listening position. The sum of these two dissimilar peak-and-dip patterns will be smoother than either one alone, and because of the time it takes for the ear to register low frequency waveforms the two will be heard as one even if the path lengths to the listening position are somewhat different.

Now the output of a dipole is smoother than that of a monopole because a single dipole can be thought of as two monopoles with one of them displaced 180 degrees in phase rather than physically displaced in space.

My suggestion for achieving similar in-room bass smoothness to a pair of dipoles is to use four small monopole subs, scattered asymmetrically around the room. The result is significantly smoother bass not only in the sweet spot, but throughout the room. This will have in-room smoothness comparable to the bass you get from the dipoles, but with the ability to pressurize the room ("slam") that dipoles inherently don't have. Credit to Earl Geddes for the asymmetrical multisub concept.

Duke
dealer/manufacturer

Hi Martykl,

Thanks for your comments.

A dipole can be modelled as two monopoles separated by a path length (the wrap-around distance) with the polarity reversed on one of them. In fact, that's what a dipole is! Take a Maggie and build it into a wall, and on either side of that wall you'll have a monopole source.

It's true that a dipole does not have the same in-room behavior as two asymmetrically-placed monopoles, but both of them more closely approach the same desirable end result: Greater in-room bass smoothness. This is backed up by several AES papers, and I can dig up citations if you really want but frankly would rather not go to the trouble. My point is, their effect is similar enough to make them easily compatible in the crossover region.

Regarding equalization, if it's addressing a global problem then it will be an improvement throughout the room. If it's addressing a local problem, then it will improve the response in one location but may well make it worse in another. How big that "location" is depends on the specifics, but I agree it's not a head-in-a-vice thing.

Suppose in one location you have a +3 dB peak at 50 Hz, and elsewhere you have a -6 dB dip at the same frequency (this is not at all far-fetched with a single-sub system). That's a 9 dB difference. Equalization cannot fix the 50 Hz region at both locations simulaneously; it can fix one, but at the expense of making the other even worse.

Note that one worthwhile advantage of a distrubuted multisub system is that the variation in bass response from one location to another throughout the room is greatly reduced. So any remaining significant problems are more likely to be global, and therefore EQing them is more likely to be beneficial throughout the room.

What I've been talking about in my posts here is only one aspect of getting good in-room bass from a subwoofer system, but it's the one most relevant to integrating well with dipole main speakers.

Duke

Hi Marty,

Well I can't discount your experience, especially since you were taking actual measurements, nor can I explain it.

In an AES paper entitled "Dipole Loudspeaker Response in Listening Rooms", researcher James M. Kates compares the response of monopole and a dipole at various listening positions and toe-in angles, and he finds the dipole to be usually (but not always) smoother in the bass region.

In an unprinted supplement to his review of the dipole-bass Gradient Revolution, Robert E. Greene records its in-room frequency response, which he finds to be remarkably smooth in the bass region. You can see this supplement at his website:

http://www.regonaudio.com/Gradient%20Revolution%20Loudspeaker.html

Again, I cannot explain your experience. Perhaps the Maggies do not start out "flat" in the bass region; I recall seeing a Stereophile measurement that did not look flat, but it may have been their measurement technique.

Best wishes,

Duke