'Very' OB Midrange Baffle Width Study - Page 2 - diyAudio
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Old 26th November 2009, 08:04 AM   #11
Nuuk is offline Nuuk  United Kingdom
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Lynn, not quite the same as you suggest but similar.

Hi-fi DIY - Building full-range open baffle loudspeakers.
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Last edited by Nuuk; 26th November 2009 at 08:05 AM. Reason: Spelling
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Old 26th November 2009, 10:48 AM   #12
Telstar is offline Telstar  Italy
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Quote:
Originally Posted by cuibono View Post
A round 'thong' baffle would be equivalent to no baffle - its width is the width of the driver basket. This was done in the first measurement for each driver.
No, i meant a larger one. Lets say 3-5cm at least larger than the basket flange.
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Old 26th November 2009, 02:12 PM   #13
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Quote:
Originally Posted by Lynn Olson View Post
Good work!!! So much more enlightening than endless computer models. The real world is always so much more interesting than running another iteration of the software.

Still curious what an open mesh (chickenwire or similar) filled with loose filling (felt or cotton) would do for the open-baffle setup. I'm visualizing a pseudo-box made out of an open mesh with layers of felt on the inner surface - a semi-transparent box, in other words.

Even a moderate attenuation of the rear wave would make a significant dent on the depth of the cancellation nulls we're seeing here.
Yes, it i always good to see data, but the simulations tell us what to expect. The measuremets, of course, can account for the precise driver directional characteristics, but this isn't a problem, at low frequency.

As for the mesh box, it is really a matter of the details. Such a box would comprise a low pass filter applied to the rear wave. Thus there are two issues, the attenuation vs. frequency, and the delay introduced, again vs. frequency. The effect of attenuation will be as you say, reduction in the depth of the nulls. The effect of delay would be to shift the angular position of the nulls to the rear. Once the frequency is above the corner frequency of the LP filter the response will undergo a transition to that of the front radiation alone with little impact of the rear radiation on the frequency or polar response. Details will, as always, require measurement, but there is nothing new here that we haven't discussed before in the "Beyond ...." thread.

While I do appreciate the efforts made ot take these measurements, and agree that the details will be dependent of the specific drivers used, I fail to see why they are enlightening. I don't see anything here that would not be expected. The use of dual sealed drivers back to back provides the expected improved symmetery between front and rear.


In fact, last year at the New Engliand DIY event I presented such a speaker which I called the Bird House because the small, 4" midrange drivers were mounted back to back on what looked like the roof of a steep sloped roof Bird House. On the peak was a B&G NEO 3 planar tweeter in dipole mode. The woofer was a 5" unit firing out the bottom of the bird house. Radiation above the mid to woofer x-o was dipole, Through the mid/woofer crossover the radiation pattern morphed from dipole through cardioid to monopole. The design concept was to have full range dipole behavior above the modal region of a typical room, a transition to cardioid response in the frequency range where relfection form the wall behind the speaker would color the sound, and monopole response in the modal region. This was just a proof of principle design initiated form computer simulations. Nothing fance.
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Old 26th November 2009, 03:19 PM   #14
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Let me just introduce some thoughts here. I visited Danmark Radio's new Concert Hall this last weekend and had an extraordinary concert experience.
Which might have some bearing on this discussion. However we measure and theorize we will all end up in our home rooms. And hear what there is to hear. This is what the Conert Hall looks like:

Interactive FullSreens 360° © John E. Kroll

Why am I bringing up this. Well The New DR Concert Hall is debated. First thing is : how is the reverberation time. It is short by any stadards and hence the sound is questioned. However what is heard from the podium is what you all expect to hear from a very good stereo record. All instruments placed on their place on the prodium both 2- and 3- dimensional. No blurring of images, no reveberation ratteling noices. There were no singing in this concert so I cannot judge that part. But otherwhise I would say it was perfect. But would we really hear this at home ?

But that doesn't mean we shouldn't go for perfection !

/Erling

Last edited by skorpion; 26th November 2009 at 03:29 PM.
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Old 26th November 2009, 04:16 PM   #15
cuibono is offline cuibono  United States
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Ah, nice to have some discussion! Thanks everyone for posting. Of course, JohnK is right, this is all to be expected from theory, I just hadn't seen it explicitly demonstrated with measurements here. I wanted to be sure that what was in my head was also in reality. I must have just missed the New England DIY last year, wish I had gone!

I actually suspect that using 4th small driver to preserve the off axis response between 1-2kHz won't make that much of a difference sonically - but there is another big reason for going 4-way: in the midrange, one typically has to apply 18dB of EQ to compensate for three octaves of roll off. This equates to 64W in around 200Hz to get the same SPL level at 1W in at 2000Hz - thermal compression anyone? Splitting the midrange into 2-ways reduces the needed compensation wattage a bit.

Who would want a speaker that is 70dBSPL/1W/1m? (which is what OB's are doing around 150-200Hz)
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Old 26th November 2009, 05:13 PM   #16
cuibono is offline cuibono  United States
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There is another observation I'd like to add: when I first got into open baffle speakers, one of the main 'selling points' was the uniformity of their off axis response - unlike box speakers, with their directivity narrowing with increasing frequency, OB speakers were supposed to have a figure of eight pattern through their passband. They do, but not the advertised -3dB at 45deg, -6dB at 60deg, etc.

The two (fairly different) H-frame dipole woofers I've measured were only down -3dB at 60deg. The 8" midrange drivers I've seen are typically -8 to -16dB at 60deg (between 700 and 1600hz), while dipole tweeters, if baffled, will be something like 0dB down at 45deg. So what we can see is that a full dipole speaker has a fairly variable polar and power response! Add the effect of drivers pushed to their max, both in terms of bandwith and power handling/displacement, and it is kind of a wonder they sound so good!

John, did your 'Bird House' have any 'box' sound to it? I'm curious how dipole versus OB speakers sound. What were your XO points? I wish I had heard them!
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Old 26th November 2009, 08:28 PM   #17
Rudolf is offline Rudolf  Germany
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It would be interesting to know, whether Johns bird house had the midrange drivers in a common air volume or separated by a dividing wall.

What I am missing in Cuibonos measurements is exactly that configuration:
Both Eminence Alpha 6a in a common enclosure, which would "seal off'' the rear output of both drivers from the outer world.

Regarding the measurements already shown: It's always comforting to see how real life follows the theoretical predictions - and to see where real life gets too complicated to be simulated by our simple models any more.

Thank you very much, cuibono, for your effort.
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Old 27th November 2009, 09:51 PM   #18
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The Bird House was just a proof of concept. Sound was ok, but it was never worked on as being a finished speaker. Still, those who heard it thought it showed promise. Bot mids were in a single stuffed chamber. Since there was no net change in volume the chamber didn't load the drivers and the dimensions were such that standing wave frequencies were above the x-o point.
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Old 27th November 2009, 11:21 PM   #19
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too

Last edited by skorpion; 27th November 2009 at 11:31 PM.
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Old 8th December 2009, 12:19 AM   #20
cuibono is offline cuibono  United States
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So, here is where we are going next:

4" drivers in dual sealed enclosures:

Click the image to open in full size.

Appearing today, a cheap poly from PE, and a nice Tang-Band. This is the money shot:

Click the image to open in full size.

The first picture is with the 4" poly in back to back, small sealed enclosures, getting measured.
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