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 22nd February 2004, 06:32 PM #1 diyAudio Member   Join Date: Feb 2004 Location: Ulm Open baffles and directivity To model an open baffel that is not circular, I integrated the function g:= (f,d,a) -> abs(1+exp(I*phi(f,d,a))); (phi is the pase shift) that gives the gain for the two-point-source model at frequency f, distance d and angle a to average from 0.4 m to 1 m: b:= (f,a) -> evalf( Int(g(f,d,a), d=0.4..1)/0.6); This should correspond to a baffle that is shaped like obfp1.gif: This is the frequency response on-axis from 50 to 3000 Hz: There are a lot of peaks and holes due to interference which must be equalized. Still, this response is smoother than that of a circular baffle which has interference holes at all multiples of c/d. Up to 230 Hz, the radiation pattern is a figure-of-eight. This shape however becomes increasingly distorted at higher freqencies. The following graphs show the radiation pattern at 400, 800, and 1600 Hz: At 400 Hz, there's a response of almost 2 dB at +/- 45 degrees. At 1600 Hz at lot of energy is radiated almost orthogonal to the main axis which will cause standing waves at least between floor and ceiling while other room modes are likely to be excited as well because of the lack of directivity. My conclusion is that this baffle is useful in a range from 50 to 230 Hz only. Since it doesn't make sense to construct a different baffle for each interval, I question that open baffles are better than conventional designs. It seems that there are as many advantages as disadvantages. Boris (yuihb01 at compuserve dot de)
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Re: Open baffles and directivity

Quote:
 Originally posted by Yuihb To model an open baffel that is not circular, I integrated the function g:= (f,d,a) -> abs(1+exp(I*phi(f,d,a))); (phi is the pase shift) that gives the gain for the two-point-source model at frequency f, distance d and angle a to average from 0.4 m to 1 m: b:= (f,a) -> evalf( Int(g(f,d,a), d=0.4..1)/0.6); This should correspond to a baffle that is shaped like obfp1.gif: This is the frequency response on-axis from 50 to 3000 Hz: There are a lot of peaks and holes due to interference which must be equalized. Still, this response is smoother than that of a circular baffle which has interference holes at all multiples of c/d. Up to 230 Hz, the radiation pattern is a figure-of-eight. This shape however becomes increasingly distorted at higher freqencies. The following graphs show the radiation pattern at 400, 800, and 1600 Hz: At 400 Hz, there's a response of almost 2 dB at +/- 45 degrees. At 1600 Hz at lot of energy is radiated almost orthogonal to the main axis which will cause standing waves at least between floor and ceiling while other room modes are likely to be excited as well because of the lack of directivity. My conclusion is that this baffle is useful in a range from 50 to 230 Hz only. Since it doesn't make sense to construct a different baffle for each interval, I question that open baffles are better than conventional designs. It seems that there are as many advantages as disadvantages. Boris (yuihb01 at compuserve dot de)
This is interesting, but I cannot see the images, so I'm having a hard time understanding your math. Could you post the images again?
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I'm sorry, something went wrong with the file upload. This is the shape of the baffle:
Attached Images
 ob1sp.gif (7.3 KB, 532 views)

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This is the frequency response:
Attached Images
 ob1fp.gif (6.4 KB, 524 views)

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Join Date: Feb 2004
Location: Ulm
This is the radiation pattern at 400 Hz:
Attached Images
 ob1pp400.gif (8.6 KB, 520 views)

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Join Date: Feb 2004
Location: Ulm
This is the radiation pattern at 800 Hz:
Attached Images
 ob1pp800.gif (8.0 KB, 515 views)

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This is the radiation pattern at 1600 Hz:
Attached Images
 ob1pp1600.gif (7.9 KB, 520 views)

 23rd February 2004, 04:27 PM #8 diyAudio Moderator Emeritus     Join Date: Apr 2002 Location: The Netherlands Blog Entries: 18 "Every advantage has it's disadvantages" - Johan Cruiff
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Re: Open baffles and directivity

Quote:
 Originally posted by Yuihb To model an open baffel that is not circular, I integrated the function g:= (f,d,a) -> abs(1+exp(I*phi(f,d,a))); (phi is the pase shift) that gives the gain for the two-point-source model at frequency f, distance d and angle a to average from 0.4 m to 1 m: b:= (f,a) -> evalf( Int(g(f,d,a), d=0.4..1)/0.6); This should correspond to a baffle that is shaped like obfp1.gif: This is the frequency response on-axis from 50 to 3000 Hz: There are a lot of peaks and holes due to interference which must be equalized. Still, this response is smoother than that of a circular baffle which has interference holes at all multiples of c/d. Up to 230 Hz, the radiation pattern is a figure-of-eight. This shape however becomes increasingly distorted at higher freqencies. The following graphs show the radiation pattern at 400, 800, and 1600 Hz: At 400 Hz, there's a response of almost 2 dB at +/- 45 degrees. At 1600 Hz at lot of energy is radiated almost orthogonal to the main axis which will cause standing waves at least between floor and ceiling while other room modes are likely to be excited as well because of the lack of directivity. My conclusion is that this baffle is useful in a range from 50 to 230 Hz only. Since it doesn't make sense to construct a different baffle for each interval, I question that open baffles are better than conventional designs. It seems that there are as many advantages as disadvantages. Boris (yuihb01 at compuserve dot de)
OK, before we go to the conclusions of this (which I sense that I will to some extent agree with you) I would like to get the model straight. From
g:= (f,d,a) -> abs(1+exp(I*phi(f,d,a))); (phi is the pase shift)
it seems as if you add two sources of equal magnitude. I would add four sources, but the result would be the same; on the front side we would see the point source and its mirror, and an edge reflection from the front source and also an edge reflection from the back source. Maybe you "think" four sources as well?
The back edge reflection will have the same sign as the front edge reflection, but the opposite to that of the front source and its mirror. There is a "+" in your equation above,but maybe you fix this in the phi equation?
So, how do you calculate the phase? I suppose you would calculate the pathway for each angle, first from the source to the edge, and then add the distance difference between the edge and the source, seen from the listener? Then integrate over all angles in the baffle plane to sum up all edge sources?
Do you use any directivity of the edge source? I have found this directivity kind of hard to understand from the articles I have read.

My second question is where you got the baffle shape from? Does it represent a rectangular edge distance distribution between 0.4 and 1 m or have you used some other trick? The fluctuations you see in the frequency response will vary *a lot* with the placement on the baffle, and it is likely that you could find a better baffle shape and/or placement.

A third question would be what you mean with "useful"? If you define the usefulness by that the directivity of the speaker should be a dipole, the you are right. It stops being a proper dipole at about 200Hz (for these dimensions). On the other hand I have understood that people like the dipole because it does not excite the room resonances as much as a point source. Room resonances mainly occur at low frequencies (<200Hz). Above this frequency, the baffled speaker will behave in about the same way as a boxed speaker.

I have posted this link here before, but I'll do it again in case you have not seen it. It is a little diffraction hack of mine, that allows you to experiment with different baffle shapes. It uses the model I described above, with the exception that the response is only plotted straight in front of the baffle.
http://www.tolvan.com/diffract.exe

I don't know if I answered your question, or if there was one, but anyway...

Opinions?

Baffle diffraction is fun!
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Re: Re: Open baffles and directivity

Quote:
 Originally posted by Svante On the other hand I have understood that people like the dipole because it does not excite the room resonances as much as a point source. Room resonances mainly occur at low frequencies (<200Hz). Above this frequency, the baffled speaker will behave in about the same way as a boxed speaker.
My shriveled little gray cells seem to recall at least one person here who suggested that another major advantage of dipoles is the production of pseudo ambience. That would be in the range > 200Hz Yes, no?
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