One step at a time towards omnipolars: a bipolar project I'll be working on

[fjhuerta]

Hi all,

I'm starting to believe I should definitely try omnipolars soon. The closest I've ever built was a speaker with OB mids (http://www.htguide.com/forum/showthread.php4?t=33573) - I loved their midrange, although the room I placed them in was less than ideal.

Anyway, I told my wife I wanted to build the Plutos and made a big mistake. I showed her the pics of how they looked like. Quite obviously, she isn't excited about their design (which is to say, she told me NOT to even think about placing them in our living room!).

So it's back to square one. S

I use an old pair of Definitive Technology BP-8 bipolars in our listening room. They sound pretty fine to me, considering their price and age. So I've decided to build a bipolar speaker and see if it helps with my really problematic room.

Since this is more or less a proof of concept, I'll start small. The speakers will use a pair of Zaph's ZA14's each, and a pair of Vifa DQ25s. I'll aim for a 1.8-2.0 Khz L-R 4th order crossover. Theory says I could get away with no baffle step at all, and thus, something close to 87 dB/W.

The thing is, I'll also be trying my balanced MiniDSP. Maybe I'll do a passive crossover later on, but for now, everything will be crossed over using this unit, and powered by two 200W stereo amps (yeah, I know. Overkill. But those are the only units I currently have that are identical!).

If anyone has any pointers regarding bipolar speaker design I'll be more than glad to read about them. I haven't really found much information online. I suppose the first thing is to try to build a column speaker that has as little depth as possible, but I don't know...

[fjhuerta]

bump?

[Mayhem13]

Why not a dipole column?...........BG received excellent feedback from listeners at RMAF with their Neo 10-3-10 dipole MTM and aesthetically it's a great looking speaker. It's certainly a bit more costly than the design you propose, but done well with the right bottom end should be a system you could live with for years to come.

[jonasz]

Or just build the Plutos with an acoustically transparent hood over it? I'm sure it could be done without any performance degradation to speak of. :)

[oldloder]

I was thinking the same thing. I don't think I've seen any pluto pics with anything other than the tubular bases, but couldn't it be wrapped in some sort of cabinet with the hood above?

[Deward Hastings]

One step . . . towards omnipolars.

That’s sort of like “one step towards pregnant”, or, as Yoda said “there is no try”. It helps to know why you are doing it.

“Omnipolar” is not a goal in itself, it is a means to an end. Define, and understand, the end, and how (to what extent) an omnipolar radiation pattern might help accomplish it, and forget about “one step” . . . just do it.

There is general agreement that the direct (directly perceived) radiation from a loudspeaker is best flat over the entire audio range (about 30Hz. to 15kHz.). While probably correct for most recordings this is probably not correct for most classical recordings, but it’s pretty easy to correct with equalization in the electronics. The speaker’s horizontal polar response determines the size of the “listening area” in which this response is accomplished.

There is general agreement that “first reflection” sound should have the same frequency response as direct sound . . . there is considerably less agreement regarding acceptable delay and acceptable amplitude relative to direct, but a minimum of 3 feet from any reflecting surface (some would say more) is a generally accepted minimum. Any discussion of first reflection that doesn’t include floor and ceiling is immediately suspect . . . in this regard a speaker’s vertical polar response is as important as horizontal.

Where it is discussed (which is far from enough) there is general agreement about overall “room response” (which constitutes most of the sound we hear in the typical listening environment) . . . a discussion obviously complicated by the participation of the room in determining the response curve. The general agreement is that this response should be “flat” up to somewhere between 1 and 4 kHz and then begin a linear drop of between 1 and 2 dB per octave. Already rather vague, that is complicated by different recording techniques and differences in desired “audience perspective” . . . classical music in general favoring a greater drop than the “pop” styles. The loudspeaker’s contribution to this curve is determined by its “power response”, the room’s contribution by its absorption curve. To some extent these can be made to complement or balance one another to get the desired overall room response.

It should at once be obvious where “omnipolar” fits into this . . . both its strengths and its weaknesses . . . and why the narrowing polar pattern of the tweeter in typical box speakers works as well as it does. An omni speaker will give the same direct and first-reflection response (if the first reflection surface has a flat absorption profile). An omni speaker will also have the same power response, and that, depending on the room absorption profile, may (or may not) give the desired “room response”. To get the desired room response one has to “tune” the room’s absorption and the speaker’s polar response . . . which makes speaker optimization room dependent (and obviously DIY), and “omni” desirable in many, but not all, cases and at most, but not all, frequencies. Most successful omni designs become directional at the high end, with the same desirable result as with “conventional” speakers.

All the above applies, of course, only above the room’s Schroeder frequency . . . below that different considerations prevail (and almost all speakers are omni anyway).

[LouC]

Doesn't posting on this board suggest many of us are already bipolar? Or at least manic, or OCD?:D

Sorry, I just couldn't resist....

[ernperkins]

Be the man and take the plunge: build a Pluto with a wood frame on the bottom 2/3 and a frame covered with grill cloth on the upper 1/3. Then with a picture of your beautiful implementation in hand I''ll have some ammo to convince my wife!

In case you missed it - the mini-DSP implementation has already been done: http://www.diyaudio.com/forums/multi-way/179407-pluto-dsp-clones-finished.html

Cheers....