So - why DON'T people go with open baffle?

[Sydney]

Re: So - why DON'T people go with open baffle?

There is a misconception that LF is omnidirectional, and LF directivity cannot be affected.
That is incorrect and it is a matter of the size of the device vs the wavelength propagated.
And directivity can be implemented with multiple drivers and delay.

[johnk...]

Re: So - why DON'T people go with open baffle?

There is a misconception that LF is omnidirectional, and LF directivity cannot be affected.
That is incorrect and it is a matter of the size of the device vs the wavelength propagated.
And directivity can be implemented with multiple drivers and delay.

Dipole bass is a little different. If you move a dipole woofer closer to a wall, if the dipole axis is perpendicular to that wall, the low bass will go away. This is just the opposite of a monopole which increase deep bass as the woofer gets closer to the wall. Look here:

http://musicanddesign.com/Boundary_reflections.html

[charlielaub]

Re: So - why DON'T people go with open baffle?

Dipole bass is a little different. If you move a dipole woofer closer to a wall, if the dipole axis is perpendicular to that wall, the low bass will go away. This is just the opposite of a monopole which increase deep bass as the woofer gets closer to the wall. Look here:

http://musicanddesign.com/Boundary_reflections.html

John,

Can you direct me/us to info (on your web site?) about how much delay is required to create a cardiod type directional pattern. Thanks!

-Charlie

[Sydney]

Re: So - why DON'T people go with open baffle?

I noticed this:

It is fairly well known that when a monopole woofer is placed close to a reflecting surface there is
reenforcement of the radiated sound pressure. "Close" is interpreted as meaning a distance significantly
less than a wave length

The distances I quoted were 1/4 wavelength distances for frequencies: 100Hz and below.

I didn't see a mention of distance ( D, d ) in the simulation charts,
A "classic" comb pattern is evident and the distances can be "guesstimated".
The +/- .5 octave wide notch I was describing is also evident in the simulations.
I was describing for a single driver/radiating source, and said: "And directivity can be implemented with multiple drivers and delay."
The link had this "When the source is a dipole, equalized for flat free space response,"
Using 2 pistons separated, changes the situation/options. For one; including a polarity flip in the electrical domain, combined with the spatial offset of the radiators.
That approach is also one implemented in the Pro world.

Thanks for the link

[Deward Hastings]

Re: So - why DON'T people go with open baffle?

Dipole bass is a little different. If you move a dipole woofer closer to a wall

But "closer to a wall" defeats the purpose of dipoles in general. Since the "rest of the speaker" needs to be well away from the wall to function properly what happens to the woofer when close to the wall is not of any significance. It is pertinent to the original question, though . . . another reason people don't go with dipole speakers is the placement requirement. Mine are five feet from the front wall, for example, and I sometimes move them out a foot further. Not all rooms (layout or decor) will support that. If the speakers can't be placed (considerably) more than three feet from the front wall you probably shouldn't be looking at dipoles in the first place.

This all goes back to the question of "constant directivity" and matching speakers to the room they will be used in. For a (somewhat) closer to the wall placement (not too close or it's self-defeating) a cardiod woofer has potential advantage (on the forward axis, anyway), but then where's the cardiod midrange and tweeter that close-to-the-wall placement also favors?

And that goes back to the even bigger question . . . the loudspeaker and the room are part of an integrated system. This is, for the most part, ignored . . . people place their loudspeakers where dictated by other considerations than optimum sound, and take what sound they get as a result, with the occasional "fix-its" of (usually inadequate) "room treatments". Given the high probability of less than ideal speaker placement the relative, or theoretical, advantage of other idealized speaker properties fades, and the possibility that less idealized speakers will work as well, or even better in the particular circumstance, rises. It all works against widespread addoption of dipole speakers. They have profound advantages for some types of music (accurate reproduction of accoustic sources) and in some acoustic environments (the "ideal room" deserves a thread of its own). For a buyer/builder primarily interested in HT or rock-and-roll played loud they are less easy to justify, and the compromises necessary to make them work to those purposes (including compromising on dipole bass) are questionalbe at best, as those compromises substantially reduce the dipole advantage in those areas where dipoles work best.

[johnk...]

Re: So - why DON'T people go with open baffle?

The problem is that acoustically close is a function of wave length. 3 or 4 feet from the wall is not acoustically close at 1k H but at 30 Hz it is. No disputing that dipole woofer can sound fine, but they are the most sensitive to placement.

[johnk...]

Re: So - why DON'T people go with open baffle?

John,

Can you direct me/us to info (on your web site?) about how much delay is required to create a cardiod type directional pattern. Thanks!

-Charlie

The delay is the equivalent of the distance,d, between sources. Td = d/c, c = sound speed. The rear source is delayed by Td.

[Deward Hastings]

Re: So - why DON'T people go with open baffle?

I have an inquisitive question (not a flame, no really): what advantage is it to have sound coming from the back of the speaker that is out of phase with the sound from the front?

Over most of the range of a dipole loudspeaker the "phase" of the back radiation is not an issue . . . by the time it is heard it has traveled several wavelengths and been reflected off of walls not equidistant from the source, so the backwave as heard is fully decorrelated from the forward ("direct") sound. There is extensive discussion of why this reflected sound is desirable (and not enough, in my opinion, of when and why it might not be) at the linkwitzlab site.

At lower frequencies (where speaker-to-wall spacing is a wavelength or less) you get similar cancellation nulls to those that you get with an omni (conventional box) speaker, just at different frequencies. Because the speaker-to-wall distance varies with angle the nulls are never as sharp as simple simulations suggest, but they do appear (and generally can be corrected for) in the listening area.

[Deward Hastings]

Re: So - why DON'T people go with open baffle?

No disputing that dipole woofer can sound fine, but they are the most sensitive to placement.

This is the kind of statement that is rendered moot by reality. Once you draw the "placement box" required by dipole speakers over the rest of their range you find that *within that box* dipole woofers are no more sensitive to placement than woofers of other radiation patterns, while their reduced exication of room modes and easy integration to the (dipole) midrange remain advantages. It should be of no concern to anyone contemplating dipole loudspeakers that dipole woofers don't work well when placed close to the wall . . . the rest of the speaker doesn't either.

[johnk...]

Re: So - why DON'T people go with open baffle?

This is the kind of statement that is rendered moot by reality. Once you draw the "placement box" required by dipole speakers over the rest of their range you find that *within that box* dipole woofers are no more sensitive to placement than woofers of other radiation patterns, while their reduced exication of room modes and easy integration to the (dipole) midrange remain advantages. It should be of no concern to anyone contemplating dipole loudspeakers that dipole woofers don't work well when placed close to the wall . . . the rest of the speaker doesn't either.

The point was what is optimal for upper frequencies may well be acoustically close for low frequencies.

[Pete Schumacher]

Re: So - why DON'T people go with open baffle?

Over most of the range of a dipole loudspeaker the "phase" of the back radiation is not an issue . . . by the time it is heard it has traveled several wavelengths and been reflected off of walls not equidistant from the source, so the backwave as heard is fully decorrelated from the forward ("direct") sound. There is extensive discussion of why this reflected sound is desirable (and not enough, in my opinion, of when and why it might not be) at the linkwitzlab site.

At lower frequencies (where speaker-to-wall spacing is a wavelength or less) you get similar cancellation nulls to those that you get with an omni (conventional box) speaker, just at different frequencies. Because the speaker-to-wall distance varies with angle the nulls are never as sharp as simple simulations suggest, but they do appear (and generally can be corrected for) in the listening area.

So, what is the advantage, if any, of dipole over bipole in the range of the returning uncorrelated sound? I realize that as you move down in frequency, the room excitation changes, but for midrange and up, why would bipole be any different than dipole? Is it purely a radiation pattern issue?

[Deward Hastings]

Re: So - why DON'T people go with open baffle?

So, what is the advantage, if any, of dipole over bipole in the range of the returning uncorrelated sound? . . . Is it purely a radiation pattern issue?

I think that's a fair generalization . . . once you're above any dipole pattern control the rear phase doesn't matter. With spaced (back to back) tweeters the rear radiation slips in and out of phase as frequency rises because of the spacing anyway. The other issue, though, is what happens at crossover . . . transitioning from a dipole mid to a bipole tweeter would produce the expected on-axis null from out-of-phase drivers, and that would probably have negative effect on the reflected frequency response (depending also, of course, on acoustic center considerations and all that other stuff).

[johnk...]

Re: So - why DON'T people go with open baffle?

So, what is the advantage, if any, of dipole over bipole in the range of the returning uncorrelated sound? I realize that as you move down in frequency, the room excitation changes, but for midrange and up, why would bipole be any different than dipole? Is it purely a radiation pattern issue?

If the sources are uncorrelated it make little difference whether it is dipole or bipole other than where the off axis peaks and null occur. The higher the frequency the more peaks and null vs angular position. If you look at the on axis dipole response above the dipole peak the position of the peaks and nulls reverse for a bipole. Above the dipole peak the sources become uncorrelated. When you change the phase by 180 (bipole) it is still uncorrelated but the radiation pattern changes. It is still a daisy pedal affair but the position of the pedals changes.

[spasticteapot]

Re: So - why DON'T people go with open baffle?

A question for the folks who own dipoles: How far away from the wall do you need to put 'em?

(A similar question applies to the NaO-style woofers, though I suspect the cardioid radiation trick might mediate this a bit.)

Also, what's to be done about tweeters? Using a monopole tweeter seems just a bit inelegant for this particular application, but two domes is kludgy and expensive. Neo3PDRs solve the problem nicely, but it does seem to rather limit the output.

What's a "suitable driver"? Some 15" paper cone breakup engine with a Q of over 1?

http://www.aespeakers.com/drivers.php?driver_id=32

And then there's the ego-driven tomfoolery of "can't do this" or "can't do that" or "can't use a W22" because Linkwitz already did that, and you don't want to design JaO (Just another ORION). Lots of bad design decisions spring from "just gotta be different" . . .

I have four SS Revelator 5.25s. Very much not W22s. And some subwoofers capable of a higher xover point than the XLS subs he used.

I'd probably be better off with some Revelator 18Ws, but I don't have any.

which I feel as though I have finally "nailed" witha the new Sunflowers.

New sunflowers?

Here's what I could use from you guys, being that I think I represent the "People who don't go with open baffle".

Interesting. It seems that the big reason for non-dipole speakers is mostly just that dipoles are about as cost-effective as using gold-plated Ferraris as delivery vans.

(But wouldn't you rather get office supplies from a gold-plated ferrari?)

[Saurav]

Re: So - why DON'T people go with open baffle?

A question for the folks who own dipoles: How far away from the wall do you need to put 'em?

About 3' from the wall for me, less from the various cabinets / racks in front of the wall.

Also, what's to be done about tweeters? Using a monopole tweeter seems just a bit inelegant for this particular application, but two domes is kludgy and expensive. Neo3PDRs solve the problem nicely, but it does seem to rather limit the output.

Depending on your budget, RAAL makes a dipole tweeter, and there are one or two other expensive ribbon / AMT type dipole tweeters. Way outside my budget, so I'm using the Neo3. I've considered a CD + WG on the front and a dome / bullet on the back, at that point it wouldn't be dipole in the treble any more. Some people have tried similar things with decent results, but I haven't measured anything like this myself.

I tried dipole midrange with a front-only WG tweeter, and didn't like it, even though the front half had pretty good dispersion match through the crossover. The back wave had no treble, I suspect that was the culprit. To my ears, the soundstage up through the midrange was well behind the speakers, but the cymbals and other treble would always be at the plane of the speakers. It was distracting and bothered me.

P.S. If you get to the point of building and measuring a prototype, and need some help with figuring out a crossover for the DCX, feel free to ping me.