Open Baffle musings

[charlielaub]

Re: Open Baffle musings

After a little more tweaking, I have version 1.3 of the crossover that looks and sounds nice:

NOTE: the measurement is only valid above 200 Hz.

I did a nearfield measurement on the Exodus Anarchy and it seems that it rolls off starting around 80Hz with a Q around 0.7. This means that the speaker is really a "monitor" that should be used with subwoofers. I have some lying around, plus I would like to build a W-frame sub with a couple of 18" drivers that I have. Together with these, it would make a nice OB system.

Again, nice agreement with ACD on this (revised) version of the crossover (1.0k Hz elliptic):


Having fun and listening to one speaker for the time being. Tomorrow I might throw together another, for a stereo pair...

-Charlie

[johnk...]

Re: Open Baffle musings

SL did a very nice job of executing known concepts and we all give him credit for that. But please, the the ideas, the foundation, came well before. SL did not invent dynamic driver dipoles. In the strictest sense, he cut wood, just as I and everyone else building these systems does. (Not trying to minimize his contributions, just emphasize a point.) Aside from Olson, who tested (measured) 1st order bidirectional (dipole) and unidirectional (cardioid) speakers, and also a 2nd order unidirectional speaker, in addition to the theory he presented there was Newman at Electro Voice back in '78 who also built and tested a dipole woofer panel using dynamic drivers. There was early work on acoustic resistance boxes (unidirectional speakers -- cardioids) that dates back the the '50, by Kalusche. And let's not forget Beranek's book in which he discuss doublets and the characteristic of unbaffled circular piston, or Kinsler and Frey who discuss the behavior of direct radiators mounted on circular baffles and pointed out that such a configuration can be considered as a doublet or dipole with separation equal to the baffle radius.

Your not the only one who was doing audio "stuff" back in the 60s', what ever that has to do with it?

[charlielaub]

Re: Open Baffle musings

I'm posting a follow up on my OB speaker:


I continued to play with the crossover yesterday, and continue to listen today. I've been able to obtain flat on axis response and verify with measurements (shown in previous posts above) but have not measured off axis response. I can move around and the image stays stable and the tonal character does not change much. I am very much enjoying the "box-less" sound and I have been listening to various recordings that had revealed energy storage issues with other speakers that I have built (boxed speakers). So at this point, I am pretty happy with things and I will probably make a pair and build a real baffle for them. I might look into some alternate ways to attache the nude tweeter to the baffle (current version is dangling by one screw!).

I still believe that even though the speaker is not operating in the "single lobe" regime of the dipole, that the combination of driver radiating area and position on the baffle (of the midwoofer) largely wash out the lobing pattern that many ascribe to "dipole" operation. But this is not a bad thing, and I believe that the compromises result in good performance and reasonable directivity, since achieving CD for all frequencies requires extreme means. This is just a 2-way speaker, so there is only so much that can be done or expected.


Here's a thought:
There has been much talk about "bloom" with dipoles. If I understand correctly, this is the result of the +6dB interference peak in the pattern... So why no do this: install driver 2: a monopole. Locate it at the rear, very close to driver 1 that is producing the bloom. Filter the signal supplied to driver 2 with a bandpass filter so that it only will be reproducing the frequencies around the "bloom peak" and set the phase so that it's opposite in phase to the rear of driver 1. Wouldn't the monopole be able to "cancel" most of the rear radiation that is causing the bloom while leaving other frequencies unaffected?

-Charlie

[fdieck]

Re: Open Baffle musings

I'm posting a follow up on my OB speaker:
[ATTACH=CONFIG]30961[/ATTACH]

I continued to play with the crossover yesterday, and continue to listen today. I've been able to obtain flat on axis response and verify with measurements (shown in previous posts above) but have not measured off axis response. I can move around and the image stays stable and the tonal character does not change much. I am very much enjoying the "box-less" sound and I have been listening to various recordings that had revealed energy storage issues with other speakers that I have built (boxed speakers). So at this point, I am pretty happy with things and I will probably make a pair and build a real baffle for them. I might look into some alternate ways to attache the nude tweeter to the baffle (current version is dangling by one screw!).

I still believe that even though the speaker is not operating in the "single lobe" regime of the dipole, that the combination of driver radiating area and position on the baffle (of the midwoofer) largely wash out the lobing pattern that many ascribe to "dipole" operation. But this is not a bad thing, and I believe that the compromises result in good performance and reasonable directivity, since achieving CD for all frequencies requires extreme means. This is just a 2-way speaker, so there is only so much that can be done or expected.


Here's a thought:
There has been much talk about "bloom" with dipoles. If I understand correctly, this is the result of the +6dB interference peak in the pattern... So why no do this: install driver 2: a monopole. Locate it at the rear, very close to driver 1 that is producing the bloom. Filter the signal supplied to driver 2 with a bandpass filter so that it only will be reproducing the frequencies around the "bloom peak" and set the phase so that it's opposite in phase to the rear of driver 1. Wouldn't the monopole be able to "cancel" most of the rear radiation that is causing the bloom while leaving other frequencies unaffected?

-Charlie

I was thinking of this very approach a while back but thought everyone would say "why not just use a notch filter to remove the bloom, as some are using notch filters to remove dipole peaks". I like the idea because it could easily be tuned for amplitude and frequency for EQ. Another think that it would be nice is to have the driver in a movable "Watson type enclosure" for omni in stead of 2 pi radiation on a baffle. The same basic approach could be used for a filler driver, ala Bang & Olufson or Duelund.


http://www.diy-audio.narod.ru/litr/duelund-filter.pdf
http://www.musicanddesign.com/Duelun...mld-filter.pdf

After my concerns about complexity and creeping elegance, I can ony say "A foolish consistency is the hobgoblin of little minds".

http://dictionary.reference.com/brow...eping+elegance

[johnk...]

Re: Open Baffle musings

I think you have to consider when the bloom arises form. If you think of the dipole as two source separated by a distance, d, that distance defines the on axis peak frequency. As you move off axis the effective "d" degreases. This moves the dipole peak higher in frequency. As a result, while the on axis response is starting to decrease in amplitude towards the first null, off axis the peak is still getting higher. The more off axis you move, the higher the peak frequency because the effective "d" keeps getting smaller. At 90 degrees, the effective "d" is zero. The peak frequency goes to infinity and the 6dB/octave roll off form f = infinity yeilds a null at all finite frequencies.

The bottom line is that the bloom is not a single frequency problem.

Take a look at ths figure.



Notice how the peak moves higher in frequency as you move off axis in the waterfall plot, untill directionality comes into play.

[Deward Hastings]

Re: Open Baffle musings

"why not just use a notch filter to remove the bloom, as some are using notch filters to remove dipole peaks".

That is, in effect, what SL did with the shelving filter in ORION . . . the problem is that it then becomes a balancing act between flattening power response (good) and putting a dip in on-axis response (bad). And it doesn't address the perceptual effect of the changing direction and apparent source of first reflections. With ORION (my experience anyway) it is better to keep on-axis flat and correct the "bloom" with room absorbers. In any case SL, motived at least in part by this very issue, as solved it with LX521.

[diy speaker guy]

Re: Open Baffle musings

Ah, look what I put together this afternoon:

[ATTACH=CONFIG]30937[/ATTACH]
Why, it's one of those open baffle speakers...
-Charlie

That's what I'd call extremely large, not medium.

Putting aside the technical aspect for a moment (or is it because of the technical aspects?) why do you want such a huge baffle? I realize the concept from the beginning included a large baffle, as did your previous musings over the last few months. But why?

On the completely opposite end of the spectrum I posted a thread a few months ago about a completely baffleless OB mid/tweeter combo that could cover from 200 hz and up and would work with a wide variety of drivers. The mid was only used in the area below the first dipole peak and even so I was able to work out a simple passive crossover for it. It's not high sensitivity or high power handling but functional enough to be more than just a novelty. There was no interest at all in this (somewhat radical) idea but here it is, if it's of any interest. http://techtalk.parts-express.com/sh...-less-2-way-OB (Just more musings, I didn't build anything yet.)

So what I'm saying, if I'm able to to do all this with the challenges of a passive crossover and no baffle at all, why do you want such a huge baffle?

[fdieck]

Re: Open Baffle musings

I think you have to consider when the bloom arises form. If you think of the dipole as two source separated by a distance, d, that distance defines the on axis peak frequency. As you move off axis the effective "d" degreases. This moves the dipole peak higher in frequency. As a result, while the on axis response is starting to decrease in amplitude towards the first null, off axis the peak is still getting higher. The more off axis you move, the higher the peak frequency because the effective "d" keeps getting smaller. At 90 degrees, the effective "d" is zero. The peak frequency goes to infinity and the 6dB/octave roll off form f = infinity yeilds a null at all finite frequencies.

The bottom line is that the bloom is not a single frequency problem.

Take a look at ths figure.



Notice how the peak moves higher in frequency as you move off axis in the waterfall plot, untill directionality comes into play.

I wonder if this might might make a case for an omni or even a dipole firing upwards, which have more constant output for rotation about the Z axis, for the frequency range where the dipole peak would be. Before I am laughed off the thread,consider Gilbret Brigg's Wharfedale SFB/3 from 55(!) years ago. I get this feeling Mr. Briggs is peering down from heaven, saying BTDT, and finding all of these musings very amusing.

http://www.inner-magazines.com/news/...haferdale-SFB3
http://www.hi-fiworld.co.uk/loudspea...responses.html
http://www.troelsgravesen.dk/SFB.htm
http://www.troelsgravesen.dk/super3.htm

[Deward Hastings]

Re: Open Baffle musings

why do you want such a huge baffle?

I beleive the intent is to put a 12" or 15" at the bottom of it for dipole bass . . . a reasonable alternative (if you have the space) to an "H" frame . . .

[charlielaub]

Re: Open Baffle musings

I think you have to consider when the bloom arises form. If you think of the dipole as two source separated by a distance, d, that distance defines the on axis peak frequency. As you move off axis the effective "d" degreases. This moves the dipole peak higher in frequency. As a result, while the on axis response is starting to decrease in amplitude towards the first null, off axis the peak is still getting higher. The more off axis you move, the higher the peak frequency because the effective "d" keeps getting smaller. At 90 degrees, the effective "d" is zero. The peak frequency goes to infinity and the 6dB/octave roll off form f = infinity yeilds a null at all finite frequencies.

The bottom line is that the bloom is not a single frequency problem.

Take a look at ths figure.

Notice how the peak moves higher in frequency as you move off axis in the waterfall plot, untill directionality comes into play.

Ah, there really isn't any getting around the physics of space and time, is there??? OK, well enough of my fantasy of another driver to cancel the rear radiation... back to the current system...

I have to admit I am using some EQ to kill a peak in the tweeter's response between about 2k and 3k Hz. This could be the first +6dB dipole peak, but it seems too narrow for that. It's also similar to a peak around that frequency in an MTM system I recently built with another pair of these drivers:

Compare the plots below:




The top figure is the measured on-axis response of the tweeter in the current MWOB system. The lower plot is from the MT system with the driver rear mounted to the baffle, in a sealed box configuration with a small subchamber, and firing through a small waveguide. When I was working up the MTM system I had thought that the peak around 2k-3k Hz was due to the waveguide, but maybe it is a driver resonance since I am seeing something similar in the MWOB as well. It could be the +6dB peak, or that could be the smaller peak around 4k Hz (which seems to match my models better). The driver is operating nude and has a 2" wide frame, so a +6dB peak seems to be located too low in frequency if it is happening at 2.2k Hz...

-Charlie

[diy speaker guy]

Re: Open Baffle musings

I beleive the intent is to put a 12" or 15" at the bottom of it for dipole bass . . . a reasonable alternative (if you have the space) to an "H" frame . . .

Ok, but it would still only have to be 18 inches wide max. Charlie seems to like really, really big baffles (which he calls medium size :D) and I'm just wondering what that's about.

[charlielaub]

Re: Open Baffle musings

That's what I'd call extremely large, not medium.

Putting aside the technical aspect for a moment (or is it because of the technical aspects?) why do you want such a huge baffle? I realize the concept from the beginning included a large baffle, as did your previous musings over the last few months. But why?

On the completely opposite end of the spectrum I posted a thread a few months ago about a completely baffleless OB mid/tweeter combo that could cover from 200 hz and up and would work with a wide variety of drivers. The mid was only used in the area below the first dipole peak and even so I was able to work out a simple passive crossover for it. It's not high sensitivity or high power handling but functional enough to be more than just a novelty. There was no interest at all in this (somewhat radical) idea but here it is, if it's of any interest. http://techtalk.parts-express.com/sh...-less-2-way-OB (Just more musings, I didn't build anything yet.)

So what I'm saying, if I'm able to to do all this with the challenges of a passive crossover and no baffle at all, why do you want such a huge baffle?

Well, OK, toe-MAY-to ta-MAH-to. I've made larger OB systems in the past, and I was kicking around plans for a larger (like 48"x48") OB at one time. On my scale this is "medium sized". It also happens to be the size of a piece of sheet goods that I can buy locally and which will fit in my car. I did some modeling first to determine the best location and then cut the hole for the midwoofer. The tweeter is just screwed on. I was up and running in about 6 hours start to finish, including measurement and crossover development.

As I am sure you know, the larger/wider the baffle, the lower in frequency that you get reinforcement before you transition to the 6dB/oct rolloff characteristic of OB loading. I was originally planning for extension down to 40-60Hz with this system using a 12" woofer in addition to a midwoofer (a true midrange with limited Xmax). I decided to switch to the current midwoofer at the last minute. They are close in size and I could almost use the same hole for both, but unfortunately not.

With the current driver, I am already getting response down to 80Hz so I might just leave it as is and cross over there to subs. The midwoofer is a 7" size driver with 12mm Xmax, and with the larger baffle I can get quite a lot of output from this driver above 80 Hz.

I looked at your thread. I think your concept is doable. Why not build it and measure the FR and then post in that thread again about it. That might get more interest.

-Charlie

Edit: I should add here that I might do another "less wide" (but probably medium on your scale!) 3-way open baffle system with a nude midrange and tweeter using the drivers I originally proposed but with the tweeter operated as a monopole (see post#1 in this thread for details).

[diy speaker guy]

Re: Open Baffle musings

As I am sure you know, the larger/wider the baffle, the lower in frequency that you get reinforcement before you transition to the 6dB/oct rolloff characteristic of OB loading.

Sure, I'm just wondering why you are trying to use the baffle to eq instead of doing a narrow baffle with dsp. The wide baffle will have more max spl potential though if you plan to use it down to 80 hz at high spl, so if that's the reason I get it.

I looked at your thread. I think your concept is doable. Why no build it and measure and then post in that thread again about it. That might get more interest.

-Charlie

The main reason I haven't done it yet is I don't have drivers that are attractive from all angles. A wishlist would include a truncated frame on the woofer (so it will stand up by itself and stay put) with an attractive cone and motor structure. If I built it with stuff I currently have it wouldn't be attractive, so it would just be a proof of concept. I'm almost certain the concept is sound so I haven't bothered taking it further until I can afford some really fancy looking drivers.

[charlielaub]

Re: Open Baffle musings

Sure, I'm just wondering why you are trying to use the baffle to eq instead of doing a narrow baffle with dsp. The wide baffle will have more max spl potential though if you plan to use it down to 80 hz at high spl, so if that's the reason I get it.

Yes, one reason is the increased capability and/or reduced driver strain at lower frequencies. I am also more or less going in the opposite direction as the narrow baffle or nude driver crowd - they hope to achieve constant directivity by only operating each driver below the +6dB peak in the OB response (at this is what I understand to be their intent). Instead I am using a wide baffle and locating the driver in such a way that I minimize the dipole peaks and nulls that can form when using small drivers and symmetric placement. So all of these things are in play.

The main reason I haven't done it yet is I don't have drivers that are attractive from all angles. A wishlist would include a truncated frame on the woofer (so it will stand up by itself and stay put) with an attractive cone and motor structure. If I built it with stuff I currently have it wouldn't be attractive, so it would just be a proof of concept. I'm almost certain the concept is sound so I haven't bothered taking it further until I can afford some really fancy looking drivers.

I would try a proof of concept first, before investing in expensive drivers...

This is probably another motivation for my OB configuration - I can use relatively inexpensive drivers, and only TWO to cover 80 Hz to 20k Hz. In doing so I am not creating a system that has the same goals as the LX521 or NaO Note, but that is "open baffle" and uses some principles from dipole designs. I have a large listening space in my home, and locations where these speakers would be demoed are also typically "large", so the baffle size is not a problem IMHO.

-Charlie

[diy speaker guy]

Re: Open Baffle musings

I would try a proof of concept first, before investing in expensive drivers...


-Charlie

Yes, I've been meaning to but never got around to it. At the time I wrote that I didn't even have a small robust neo tweeter to play with but I do now. At the time, I was looking at this little guy https://www.solen.ca/pdf/tangband/w2748sg.pdf as a replacement tweeter if a normal tweeter didn't cut it. In a lot of ways that little mid/tweeter might be better than a normal tweeter for this application.

At that time they were $14 each at Solen but since then they have steadily climbed to almost $50 each!!! It looks nice, but not at that price, I thought it was a bit expensive at $14. (But then again I think everything is too expensive.)