Wendell Diller: "You don't wanna put a subwoofer with a Magneplanar; it doesn't work."

I would expect WD’s woofer/subwoofer integration to be much better than the many trials of past integrations.  Looking forward to a demo.

@josh358: Great post at 11:01 am (and 11:12 too)!

You make an important correction to my saying (in my explanation of dipole cancellation) that bass frequencies are omnidirectional. As you state, it is not that bass frequencies are omni, but rather that sealed/ported subs behave as omni sources because of the woofer enclosure dimensions vs. bass frequency "dimensions" (much larger than the enclosure. In fact, much larger than most room dimensions). This topic requires more time and space to fully discuss than most would care for (again, Siegfried Linkwitz’s writings on the subject are readily available for those interested), but there are a couple of points I would like to make:

One of the benefits of dipole loudspeakers is their radiation pattern: the aforementioned figure-of-8. At higher frequencies that side-speaker null (the dipole front and rear waves meeting on the sides, and due to opposing polarities cancelling one another; +1 and -1 equal 0) greatly reduces the problem of side wall reflections. The QUAD 57 (and the Sanders ESL loudspeaker) is well known to be a single-person loudspeaker: the speaker needs to be pointed directly at the listener, as its frequency response takes a dose dive as you move off axis).

At bass frequencies, those nulls almost-entirely eliminate the excitation of room modes in the room’s left-to-right axis. Fewer room modes = less "room boom". On the other hand, as Josh points out, that dipole back wave creates a problem that non-dipoles don’t (at least not to the same degree): the rear wave (half of the loudspeaker’s output) reflecting off the wall behind the loudspeaker and traveling back to the panel, where its phase relationship with the front wave can cause either frequency reinforcement, or instead cancellation. Ay carumba!

For that reason, dipole/front wall distance experimentation is required to optimize sound quality. But you audiophiles are use to and expect that, right? ;-) But here’s a basic rule: if at all possible, a dipole should be no closer to the wall behind it than 5’, minimum. Why? Because 1- sound travels at approximately 1’ per millisecond; 2- for two sounds to be perceived as separate events, they need to be separated in time by at least 10ms. At a dipole/wall distance of 5’, the dipole rear wave travels the 5’ to the wall behind it, then the 5’ back to the dipole, where it is now separated in time from the front wave by the 10ms required for the two waves to be perceived a separate entities, rather than the rear wave being a "smearing" of the front wave. Dipole 101. ;-)

Dipole cancellation occurs on both left and right sides of a dipole, and results in reduced SPL output. Danny Richie (GR Research) and Brian Ding (Rythmik Audio) advise that their OB/Dipole sub produces 1/4 the output (fed the same signal) as the same driver installed in a sealed enclosure. The answer is of course to use more of them. Hey, its only money, right? ;-) Some fanatics use GRR/Rythmik OB subs stacked floor to ceiling.

The dipole cancellation phenomenon can be tailored in a number of ways. In the old days dipole designers/builders used a wide baffle (in the DIY world that remains common), the dimensions of which would determine the frequency at which cancellation would occur: the greater the distance between the front and rear drivers, the lower the frequency at which cancellation occurs. Danny Richie found an unbraced large panel to be sonically unacceptable (he is very intolerant of resonances), so uses the same style H-frame to house his dual 12" woofers as did Siegfried Linkwitz with his dual 10’s (to see an H-frame is to understand why it is named that). Both designers settled on roughly the same H-frame dimensions: deep enough to lower the cancellation frequency to where they wanted it, but shallow enough to make the resulting cavity resonance occur at a frequency above the passband of the sub (in the GRR/Rythmik, 300Hz). Linkwitz’s H-frame is constructed of a single layer of 3/4" MDF, but Danny---the perfectionists that he is---specifies 1-1/2"

Magneplanar Tympani enthusiasts long ago discovered that if you position one side of the Tympani bass panels right up against a side wall, you would prevent dipole cancellation on that side, resulting in increased SPL output. Another means of increasing the panel output is to brace them to the room structure, often to the wall behind them. Yes, hardcore Tympani enthusiasts are a fanatical bunch (Hi Josh ;-) .

When sold for use in the OB/Dipole Sub, Brian Ding installs in his Rythmik A370 plate amp a "Dipole Cancellation Compensation Circuit". It is basically a filter which is a mirror-image of the roll-off endemic in the OB/Dipole’s output characteristics, which allows the sub to reach quite low in frequency, at relatively-high maximum SPL capabilities. Not Rock ’n’ Roll concert levels (I saw NRBQ in a medium size club last night, and it was pretty damn loud. Not Ramones !LOUD!, but loud), but as much output as most of us need (want is a different matter ;-) .

I of course don’t yet know how the upcoming Magnepan dipole woofer sounds, or for that matter the OB from Eminent Technology (Bruce also offers a "true subwoofer", for the frequencies below 20Hz!). But the GR Research/Rythmik Audio Servo-Feedback OB/Dipole Sub is a hard act to follow. I just wish I had a room big enough for the Tympani bass panels, as they require no augmentation (well, unless you need 30Hz and below), and remain my low frequency reproduction yardstick. The GRR/Rythmik OB/Dipole comes closest to matching the Tympani bass panels as any woofers/subs I’ve yet heard, far better than the woofer towers of my now-sold (back to Brooks Berdan, from whom I bought them) Infinity RS-1b’s (also a servo-feedback design).

Not really. The frequency response and phase of the woofer and midrange have to match through the crossover range. That’s the direct benefit of a woofer with extended high frequency response.

No really. After 120Hz you have direct directional audibility. Howver you need to consider distortion products. Either way if there is energy over 120Hz the sub is locatable and that’s bad. Phase and level matching does not fix that.

The audioamp -- the reduced modal excitation of dipoles is easily measured and heard. It is the main reason that dipole woofers sound better. In subjective comparisons,

 

Woofers sound better, not subwoofers.