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Old 21st August 2014, 08:17 AM   #1
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Default 2D dipole array with CD (follow up from the 1-4kHz thread)

Following the discussion in this thread, here's a preliminary results of a 4-driver two-dimensional array + planar waveguided tweeter in a dipole configuration (the four 3" wideband units, Visaton FRS8 are placed in a semi-circle around the Neo 8 PDR tweeter on a customized baffle shape. The "outer" two of them are rolled off earlier such that they only contribute in the lower frequency range)

Measurements show 0-30-45-60 degree horizontal response at 1.5 meters with a ~10ms gate in a room (which is rather large, but still will show some reflections). A smaller gate yields smoother response, but starts to miss the dipole effect.

Some remarks:

1. Design goal is constant directivity over a broad range (ideally 300Hz - 4kHz) with no crossover in this area. Currently this works well up to ~2kHz, where it shortly blooms and then directivity goes back again, although not so smoothly controlled as in the dipole area. Crossover is 2nd order LR at~3.5kHz, which does not quite meet the goal, but worked best so far. I intend to try out a 1st order at ~4kHz, something both the wideband drivers and the tweeter(probably with closed back) can handle, but not sure about the off-axis response. Need to try.

2. The blooming at 2kHz look like the dipole peak of the two units inline with the tweeter, which "see" a narrower baffle (further separate measurements of midrange / tweeter are needed and planned) . I see two ways of further improve it which I will try:

- further pushing up the dipole peak. This might be hard or impossible due to the physical baffle constraints.
- use driver directivity, i.e., have the "outer" 2 units roll off upper in frequency such that the dipole peak does not occur at all (by increasing the apparent source size). This was actually my initial idea of using both dipole and driver directivity

3. The waveguide on the tweeter could do a better job

4. During the work on this, I have become a bit skeptical about the real need to improve directivity further. The system already sounds very good. Of course we all want "by the book" directivity, but, as often pointed out, there is a diminishing return somewhere. I'm not saying I'm there, just that we need to keep it in mind.
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Old 21st August 2014, 12:06 PM   #2
flg is offline flg  United States
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Quote:
Originally Posted by bzfcocon View Post
Following the discussion in this thread, here's a preliminary results of a 4-driver two-dimensional array + planar waveguided tweeter in a dipole configuration (the four 3" wideband units, Visaton FRS8 are placed in a semi-circle around the Neo 8 PDR tweeter on a customized baffle shape. The "outer" two of them are rolled off earlier such that they only contribute in the lower frequency range)

Measurements show 0-30-45-60 degree horizontal response at 1.5 meters with a ~10ms gate in a room (which is rather large, but still will show some reflections). A smaller gate yields smoother response, but starts to miss the dipole effect.

Some remarks:

1. Design goal is constant directivity over a broad range (ideally 300Hz - 4kHz) with no crossover in this area. Currently this works well up to ~2kHz, where it shortly blooms and then directivity goes back again, although not so smoothly controlled as in the dipole area. Crossover is 2nd order LR at~3.5kHz, which does not quite meet the goal, but worked best so far. I intend to try out a 1st order at ~4kHz, something both the wideband drivers and the tweeter(probably with closed back) can handle, but not sure about the off-axis response. Need to try.

2. The blooming at 2kHz look like the dipole peak of the two units inline with the tweeter, which "see" a narrower baffle (further separate measurements of midrange / tweeter are needed and planned) . I see two ways of further improve it which I will try:

- further pushing up the dipole peak. This might be hard or impossible due to the physical baffle constraints.
- use driver directivity, i.e., have the "outer" 2 units roll off upper in frequency such that the dipole peak does not occur at all (by increasing the apparent source size). This was actually my initial idea of using both dipole and driver directivity

3. The waveguide on the tweeter could do a better job

4. During the work on this, I have become a bit skeptical about the real need to improve directivity further. The system already sounds very good. Of course we all want "by the book" directivity, but, as often pointed out, there is a diminishing return somewhere. I'm not saying I'm there, just that we need to keep it in mind.
Could we get a pic, dwg or sketch of this configuration?
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Old 21st August 2014, 02:37 PM   #3
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I was just about to patent it, but since you asked..

It looks like a bull head lying on one side The idea is that the drivers inline with the tweeter should "see" a narrower baffle.
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Old 21st August 2014, 02:54 PM   #4
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What does the waveguide look like? When I used the neo8 unbaffled it was 900-4khz. Directivity was the main concern in that design and the neo 8 begins to beam around 4-5khz IIRC. Lately I've been thinking about digging out my neos and making waveguides for them......
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Old 21st August 2014, 02:59 PM   #5
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Originally Posted by natehansen66 View Post
What does the waveguide look like? When I used the neo8 unbaffled it was 900-4khz. Directivity was the main concern in that design and the neo 8 begins to beam around 4-5khz IIRC. Lately I've been thinking about digging out my neos and making waveguides for them......
Sorry, my bad: I meant Neo 3, not 8, which not exactl a tweeter. The wavegude is a very (probably too) shallow vertical one made out of two halved pipes.
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Old 21st August 2014, 07:12 PM   #6
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Quote:
Originally Posted by bzfcocon View Post
I was just about to patent it, but since you asked..

It looks like a bull head lying on one side The idea is that the drivers inline with the tweeter should "see" a narrower baffle.
Wouldn't a symmetrical array be better?
You'll get a symmetrical polar response.
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Old 21st August 2014, 08:33 PM   #7
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Quote:
Originally Posted by bzfcocon View Post
Sorry, my bad: I meant Neo 3, not 8, which not exactl a tweeter. The wavegude is a very (probably too) shallow vertical one made out of two halved pipes.
I was thinking that's probably what you meant. Cool project nonetheless.
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Old 22nd August 2014, 08:10 AM   #8
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Originally Posted by Patrick Bateman View Post
Wouldn't a symmetrical array be better?
You'll get a symmetrical polar response.
In principle, symmetrical would better and one could do a full circle around the tweeter. At this point, however, I am not convinced that the added complexity is worth it. The experiment needs to be carried on a bit further.

BTW, I've found a very interesting paper on circular arrays, although in a different context (small mobile devices). I can't fully follow the math (it's been a while), but the plots are easier to understand. They basically derive an analytical solution how to drive a set of concentric ring radiators (or arrays) in magnitude and phase such that to emulate an oscillating sphere.

http://mellowacoustics.com/articles/...irectivity.pdf
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Old 22nd August 2014, 01:15 PM   #9
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D.B. Keele has some papers referring to the CBT and circular arrays (spherical caps). Could be worth the read?
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Old 22nd August 2014, 03:25 PM   #10
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I have been using a 2D dipole array without CD for about 6 months. I have documented it on Part Express Tech Talk (Discus-an-open-baffle-shaded-spherical-segment-FAST-array). Spaciousness is really good. It has about the best "phantom center" image of anything I have heard. When you close your eyes, you cannot pinpoint the source, yet the imaging is both precise and has depth. I don't want to hijack the thread, just let you know about possibilities.

Marc

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