Open Baffle musings

[charlielaub]

Re: Open Baffle musings

Over at least part of its range anyway . . . it is certainly a plausable candidate. With a bit of baffle you can get decent dipole behavior (and output) down low enough to permit crossover at 2kHz. The cone will be beaming (forward) at a (roughly) dipole-equivalent pattern by around 4kHz., above that it will be progressively narrower (beaming). The rear lobe above 4k will simply be a mess because of diffraction and occlusion by the frame, spider and magnet, but the overall reflected sound may not be all that bad. To avoid vertical lobing it will want to be as close as possible to the mid (without introducing baffle interference) . . . that, combined with the decidedly non-dipole pattern you're going to get from the mid mounting position (as proposed) poses problems. My suggestion would be a stepped sub-baffle (super-baffle?) like you-know-who does above the slab for the combined mid and tweeter that would provide the close spacing required and overall dipole behavior that includes the midrange as well.

The initial fr curves will not be pretty at all, but the necessary dipole equalization and crossover will be easily handled by the miniDSP.

OK, I think I am homing in on the objection... if you want to have a nice "two circles and line" radiation pattern, you pretty much have to stick to that part of the response below the first dipole peak. Otherwise, your pattern starts to have more lobes, and this is I guess undesirable. In order to stick to the part of the response below the first dipole peak, you need to make the baffle as small as possible, to push the first dipole peak as high as possible in frequency. Then you just EQ out the 6dB slope, cross over to the next higher driver around or just below the peak frequency, and you can sit back and enjoy you nice two-lobe dipole pattern... Am I getting this right here?

Of course you run into some problems at higher frequencies where objects like the size of the magnet on the back of the driver are comparably to wavelength, and therefore diffract the rear wave and mess up the pattern. This is a motivation for using a planar open back tweeter like the Neo3...

Deward, JohnK, etc. your thoughts on the above?

-Charlie

[fdieck]

Re: Open Baffle musings

This is really a good text on the topic

How open baffles work - from point sources to small drivers in big baffles to big drivers in small baffles (version 12.08.2010):

http://www.dipolplus.de/frameset.htm

(in the downloads section)

[diy speaker guy]

Re: Open Baffle musings

This is really a good text on the topic

How open baffles work - from point sources to small drivers in big baffles to big drivers in small baffles (version 12.08.2010):

http://www.dipolplus.de/frameset.htm

(in the downloads section)

Thanks for this, the explanation of what we are talking about here is on page 4 on the right hand side.

A point source dipole radiates in the figure 8 pattern only up to
the first on axis SPL peak (D/λ = 0.5).

[charlielaub]

Re: Open Baffle musings

This is really a good text on the topic

How open baffles work - from point sources to small drivers in big baffles to big drivers in small baffles (version 12.08.2010):

http://www.dipolplus.de/frameset.htm

(in the downloads section)

Uh, what is so great about that web site? Lots of Edge simulations and some handwaving generalizations. They don't even mention Jeff Bagby's Diffraction & Boundary Simulator (BDS), which can be found here:
http://audio.claub.net/software/jbabgy/BDBS.html


SL touches on the dipole radiation pattern change in the first couple of plots from this page on his old Phoenix project:
http://www.linkwitzlab.com/models.htm#A1
Note on the left plot he illustrates that the pattern has changed at the first null. This pattern really is more like two donuts back to back in 3D.

For instance, here are a bunch of patterns taken from a web page on dipole antennas:


The patterns are really 2-D slices of half (the forward hemisphere) of a 3-D shape. To get the shape in 3-D you have to "spin" the pattern around the axis running vertically through the image and then mirror that (to represent the rear pattern). That's how SL's four leaf pattern corresponds to the two leaf pattern in the figure above (labeled 1/2 lambda high) which is donut.


These patterns are created for point sources, or more specifically in the case when the size of the radiator is much smaller than the radiating wavelength. As the driver diameter increases, the interference patterns are "smeared out" and peaks and dips smoothed to some degree. This tends to ameliorate the lobing to some extent. You can easily sim this in the Edge or DBS.

Can someone make some specific remarks on the radiation pattern issues that I just raised?

-Charlie

[diy speaker guy]

Re: Open Baffle musings

Can someone make some specific remarks on the radiation pattern issues that I just raised?

-Charlie

Sure, maybe this is why John K (in a post above) and several hundreds of people using MJK style wide baffle OB's don't complain about radiation patterns.

(As for myself, I have no special appreciation for the german site, but the specific paper he linked to contained the missing info from my previous explanations. I have no idea how important the radiation pattern is in real life, I don't have any experience with active OB and only a little with passive OB.

As a side note though, using the drivers in their completely linear range (wrt diffraction) is a compelling and attractive argument in it's own right, without even considering the lobing effects.)

[fdieck]

Re: Open Baffle musings

[QUOTE=charlielaub;1869682]Uh, what is so great about that web site? Lots of Edge simulations and some handwaving generalizations. They don't even mention (BDS), which can be found here:
http://audio.claub.net/software/jbabgy/BDBS.html


You're welcome.

I like both edge (for multiple driver and strange shaped baffle capability) and BDS. I will refrain from any further comments that, at best would be handwaving, and at worst finger waving (I believe you know which one).:rolleyes:

Don't forget http://www.musicanddesign.com/A_B_C_Dipole.html as well.


http://www.dipolplus.de/frameset.htm

[weinstro]

Re: Open Baffle musings

Hi Charlie,

Don't let all the hype about narrow baffle dipole systems deter you. Trust me on this, I know!

33" x 39" baffle, 2 SLS 12" woofer + 2 10" SLS woofer per side + ScanSpeak 21W8554 + SB Acoustics SB29RDC. I would recommend 1/2 carpet felt over the back side of the midrange to limit rear radiation.

John,

Did you publish a write-up on this system? I thought you had at one point, but I'm unable to locate it on your website. I'd be interested in reading that again, if you can indulge my curiosity.

Thanks and regards,

Rob

[Face]

Re: Open Baffle musings

John,

Did you publish a write-up on this system? I thought you had at one point, but I'm unable to locate it on your website. I'd be interested in reading that again, if you can indulge my curiosity.

Thanks and regards,

Rob

http://techtalk.parts-express.com/sh...ew-open-baffle

[weinstro]

Re: Open Baffle musings

OK, I agree, for high frequencies the motor is kind of in the way. Only an open-back planar could work then, I guess.

You could try an inexpensive tweeter on the back side. Perhaps some effort could be made to get a reasonably close harmonic distortion profile, like the front firing tweeter. I'd personally be tempted to use two identical small flange or flangeless tweeters, one on either side of the baffle. It might just be good enough and the rear firing tweeter can be padded to taste.

Regards,

Rob

[johnk...]

Re: Open Baffle musings

OK, I think I am homing in on the objection... if you want to have a nice "two circles and line" radiation pattern, you pretty much have to stick to that part of the response below the first dipole peak. Otherwise, your pattern starts to have more lobes, and this is I guess undesirable. In order to stick to the part of the response below the first dipole peak, you need to make the baffle as small as possible, to push the first dipole peak as high as possible in frequency. Then you just EQ out the 6dB slope, cross over to the next higher driver around or just below the peak frequency, and you can sit back and enjoy you nice two-lobe dipole pattern... Am I getting this right here?

Of course you run into some problems at higher frequencies where objects like the size of the magnet on the back of the driver are comparably to wavelength, and therefore diffract the rear wave and mess up the pattern. This is a motivation for using a planar open back tweeter like the Neo3...

Deward, JohnK, etc. your thoughts on the above?

-Charlie

Yes you are getting it, but the problem is getting the dipole peak higher than about 2k Hz. The peak is at C/(2d). At 2k d has to be 3.3 inches. The reality is that it is necessary to look at driver directionality. Some of the guys at DIY audio played around with wave guides but even that is tough.

The thing is all these designs take advantage of directionality to curtail front to back cancellation above the peak. That generally makes the response different front to back due to all the blockage on the back side of the driver. Certainly the back side of thee cone is radiation the same energy as the front (nominally) but it is scattered. That's not a bad thing since that rear radiation ends up being reflected and diffuse. But if you want symmetric free field response the idea of back to back drivers, while appealing, typically makes the separation great enough that the dipole peak ends up sufficiently below the frequency where directionality would prevent dipole bloom and the multi lobing at slightly higher frequency. To get an idea, here is an image of the axial and polar response of a coupled of 37mm audax domes mounted back to back. You can see the dipole peak and then directionality starts to come in before a good null is formed, but the polars show the bloom as you get above the peak. it would be desirable to have directionality come in a little before the peak, to counter the bloom, which requires moving the drivers closer, but they are as physically close as then can be already. In this example the response would be useful to only about 1k. So you need a bigger diameter driver to lower the frequency where directionality helps, but then back to back mounting ends put pushing them further apart and the response looks the same, but shifted lower in frequency.

[Deward Hastings]

Re: Open Baffle musings

Can someone make some specific remarks on the radiation pattern issues that I just raised?

Not sure what you want to hear . . . on the face of it the multiple lobes are bad. Since their location (angle) varies with frequency the result (apart from power response variations) is frequency response variation (both direct and reflected) at any particular listening position.

As a side note though, using the drivers in their completely linear range (wrt diffraction) is a compelling and attractive argument in it's own right

Yes. Physics makes the single-driver full-range impractical, the added constraint of constant (dipole) directivity makes it impossible. In general the most we can get from a single driver/baffle is three octaves . . . other constraints limit even that. Since we're looking for 9 octaves in a "full range" speaker it's obvious why 4-way is the end point of most evolved dipole designs.

I don't see the point of rehashing why constant directivity is a good idea . . . if that's not understood and "part of the plan" then there's little point in dealing with all the dipole issues in the first place. A loudspeaker does not become "a dipole" just by taking the box away, but at the same time taking the box away does not free dipoles of all other design considerations. If anything it makes them more pronounced, since excursion and equalization demands become greater and some of the "fudge factors" that come with a box (like "make the messy back wave go away") are lost as well.

If you want to make a full range dipole there are just things you have to do (and other things that you can't). If you disregard those things you won't get the constant directivity and radiation pattern (and resultant benefits) that are the reason for undertaking a dipole design in the first place, so why bother.

[Deward Hastings]

Re: Open Baffle musings

if you want symmetric free field response the idea of back to back drivers, while appealing, typically makes the separation great enough that the dipole peak ends up sufficiently below the frequency where directionality would prevent dipole bloom and the multi lobing at slightly higher frequency.

Yes. That was/is the problem with the back-to-back tweeters in ORION . . . although they made the treble "bi-directional" (in some ways an improvement) they are "dipole" over exactly no part of their range. Until they start beaming there is nothing to control the side lobe(s), and that really compromises the "constant directionality" of the design. The tweeters in LX521 (also not aligned to be or constituting a dipole) are larger (diameter) domes than they otherwise need to be presumably to insure that they are already beaming at the crossover frequency chosen.

[diy speaker guy]

Re: Open Baffle musings

I don't see the point of rehashing why constant directivity is a good idea . . . if that's not understood and "part of the plan" then there's little point in dealing with all the dipole issues in the first place. A loudspeaker does not become "a dipole" just by taking the box away, but at the same time taking the box away does not free dipoles of all other design considerations. If anything it makes them more pronounced, since excursion and equalization demands become greater and some of the "fudge factors" that come with a box (like "make the messy back wave go away") are lost as well.

If you want to make a full range dipole there are just things you have to do (and other things that you can't). If you disregard those things you won't get the constant directivity and radiation pattern (and resultant benefits) that are the reason for undertaking a dipole design in the first place, so why bother.

I agree that constant directivity (where practical) is a great goal and the only way to be truly pure dipole.

On the other hand, Charlie might be right and those lobes might not measure as bad as the pics show, if that is the case then obviously the lobing issue above the first dipole peak is not nearly as pressing.

I'm all for keeping a constant radiation pattern but at what cost? Even John's tweeter isn't narrow enough to cover the high frequencies so there's no perfect option. Where do we draw the line? Is it wrong for hobbyists to try a wide MJK style OB with a fullrange driver? It's not pure dipole but I can tell you from experience that it's not that bad, although I didn't do any comprehensive measurements.

[Flint]

Re: Open Baffle musings

Charlie, it's possible you could use a single SB29 as a sort-of dipole tweeter with the back chamber sawed off. Someone posted an image at diyAudio that looks to be the ferrite version, and the motor has a large pole vent. I would guess the neo version has a similar vent but with a smaller overall size. I'd have sawed the chamber off one of these had I thought to try...

[diy speaker guy]

Re: Open Baffle musings

Charlie, it's possible you could use a single SB29 as a sort-of dipole with the back chamber sawed off. Someone posted an image at diyAudio that looks to be the ferrite version, and motor has a large pole vent. I would guess the neo version has a similar vent but with a smaller overall size. I'd have sawed the chamber of one of these had I thought to try...

At tweeter frequencies that's a tuned chamber and it will have associated resonances. It's hard to imagine how it would actually sound on the backside.