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Old 20th July 2010, 10:43 AM   #6721
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Originally Posted by mige0 View Post
John, *if* you would have accepted meanwhile that CMP happens in "true dipole department" as well, I would have asked you to propose a "caps & coil" filter (one I possibly could build for verification by simu and measurement) to completely correct CMP in this department as well.

Michael
First of all, if I understand what you mean, what you call CMP is just a name for a response that is composed of a summation of impulses which when taken individually represent a MP response. In other words the system frequency response is the sum of a series of minimum phase frequency responses. You should recognize that such a summation need not be MP. That is, a system which is CMP does not necessarily have to be MP. Similarly, a system can be MP even though the individual components in the summation are not.

With a dipole, I have said numerous times that the dipole response is not usable above the dipole peak. Being MP is one thing, but the nulls, while exhibiting MP behavior are not suitable for equalization. The point I have made repeatedly is that when you start with a MP response and equalize it to a specific MP target (like a band pass response) then the impulse of the eq MP response will identical to the impulse of the target.

You know we should really define a dipole a little better. What we really have, in the simplest case of two sources separated by some distance, is not a dipole. It is an acoustic doublet. The polar response of an acoustic double approximates a dipole response as the frequency decreases. The difference between a dipole response and the polar response of an acoustic doublet becomes acceptably insignificant about 1 octave below the "doublet" peak. And above that frequency the doublet response really should not be referred to as a dipole as it has no relation to a dipole at all, though it does remain MP.

Now if you want to know if the response in MP or not, make a measurement and then compare the phase of the measured response to the phase generated form the amplitude response using the Hilbert Bode transformation. If, by elimination of only excess delay the measured phase reduces to the HBT phase, then you can conclude that the measured system behaves as a MP system. If that is true, then any MP equalization applied to the system will also result in a MP response. I never said how this eq should be constructed and I would not limit that discussion to caps and coils.
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Old 20th July 2010, 11:02 AM   #6722
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3) the Gary Pimm open-ended cardioid box
Perhaps credit should be given to:

THOMAS J. HOLMES, The "Acoustic Resistance Box" A Fresh Look at an Old Principle, J. Audio Eng. Soc., Vol. 34, No. 12, 1986.

Juha Backman, Theory of acoustical resistance enclosures, Presented at the 106th Convention, 1999 May 8-11, Munich, Germany.

Gary's system, as well and my NaO woofer system are just applications of past develoments.
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Old 20th July 2010, 11:53 AM   #6723
dlr is offline dlr  United States
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Originally Posted by mige0 View Post
John, *if* you would have accepted meanwhile that CMP happens in "true dipole department" as well, I would have asked you to propose a "caps & coil" filter (one I possibly could build for verification by simu and measurement) to completely correct CMP in this department as well.

Michael
Why can't you do this on your own?

Dave

Last edited by dlr; 20th July 2010 at 11:59 AM.
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Old 20th July 2010, 12:48 PM   #6724
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Originally Posted by Lynn Olson View Post
I've made some crude sketches that show the vector relationships of an open baffle compared to a 6 dB resistive-mesh absorber.
Wow, now that makes sense. Thank you for taking an acoustic phenomenon and explaining it in terms an EE can understand. After thousands of posts and understanding very little of it, posts #6716 and #6721 have really helped me understand the basics of combing and baffle size.
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Old 20th July 2010, 02:52 PM   #6725
soongsc is offline soongsc  Taiwan
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Originally Posted by Lynn Olson View Post
As I suspected, it's a time-domain problem, and not just restricted to open-baffle speakers. Any loudspeaker with diffraction and reflections suffers from baffle or enclosure-induced time-domain artifacts, and frequency-based equalization only increases the time-domain error. As mentioned in the posts above, deep nulls are not correctable by any method, since full EQ would require kilowatts of power and would destroy the driver.
True. Hope more people will realize this.
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...
... I'm still curious about Gary Pimm-style resistive boxes, since a modest 6 dB of acoustic attenuation to the rear wave changes the whole picture, getting rid of the deep nulls and cancellations. I'm also happy that Gary Dahl is going ahead with his build, smoothing the way for my version.
After looking at the illustrations of the resistive box, Those would be nice in a very limited bandwidth, cross them too high, and the sound will be sound as though part of the music is compressed. Tricky enclosure to design.
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Old 20th July 2010, 03:28 PM   #6726
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Originally Posted by john k... View Post
Perhaps credit should be given to:

THOMAS J. HOLMES, The "Acoustic Resistance Box" A Fresh Look at an Old Principle, J. Audio Eng. Soc., Vol. 34, No. 12, 1986.

Juha Backman, Theory of acoustical resistance enclosures, Presented at the 106th Convention, 1999 May 8-11, Munich, Germany.

Gary's system, as well and my NaO woofer system are just applications of past develoments.
I absolutely agree with John. The resistive boxes I've setup are just re-hashes of ideas used in the past using modern drivers and damping materials. Pleasing results, fun to build, measure, and listen to, but nothing revolutionary.

The discussions in this thread are great. Lots of great information and view points to observe and absorb.
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Old 20th July 2010, 03:39 PM   #6727
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Originally Posted by Gary P View Post
I absolutely agree with John. The resistive boxes I've setup are just re-hashes of ideas used in the past using modern drivers and damping materials. Pleasing results, fun to build, measure, and listen to, but nothing revolutionary.

The discussions in this thread are great. Lots of great information and view points to observe and absorb.
Thanks for responding Gary. I think it is important to the general audience here that they realize many of the topics being discussed have been well researched over the years. What was old is new again. As I point out on the web page I referred to, this type of enclosure goes back tot he 50's. Of course, what we have today is better materials with which to damp the back wave and better drivers capable of the necessary excursion with resulting better execution of the concept.
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Old 20th July 2010, 03:54 PM   #6728
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Originally Posted by soongsc View Post
True. Hope more people will realize this.

After looking at the illustrations of the resistive box, Those would be nice in a very limited bandwidth, cross them too high, and the sound will be sound as though part of the music is compressed. Tricky enclosure to design.
From some quick calculations (could be in error) it appears to me that if the rear wave were uniformly attenuated by 6dB the nulls would still be on the order of 12dB deep (as opposed to true nulls), the peak would come out around 7 dB, the roll off below the peak would be initially steeper but then plateau at -12dB. This assumes point source behavior.

But a resistively damped box will never have uniform 6dB attenuation. Resistive damping of the rear wave always results in an LP type response.
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Old 20th July 2010, 08:41 PM   #6729
mige0 is offline mige0  Austria
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John, thats partly an elegant summation - in other words - on the perspective I brought up.
Its obvious that OB is not "true" dipole but just an approximation that works pretty well.
You missed to clearly point out that its even an approximation *below* dipole peak, and you also missed to point out that this is so because OB simply is a CMP system (if you have an other term from "ancient" researchers in that topic - no problem for me ).

Now - again - with the discussion about CMP, I'm neither *specifically* focusing on OB - and even less on OB above dipole peak (I gave the reasons why I'm referring to OB nevertheless in demonstration and description and of course above dipole peak is valid for that as well as below dipole peak).

I'd like to stress though, that CMP systems do *not* depend on frequency at first hand - hence I asked for your proposal to correct CMP (be it in OB or elsewhere on constructive / destructive interference in audio) be it as low in frequency you wish.

On the pure theoretical level, I guess, you hardly will come up with anything different than I've shown - at least *if* you correct as 100% for the CMP behaviour as already shown above.

I would not necessarily have restricted you to caps and coils - though I was under the impression that "min phase" correction *always* could be done by caps and coils as well.


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Old 20th July 2010, 09:30 PM   #6730
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Originally Posted by john k... View Post
You know we should really define a dipole a little better. What we really have, in the simplest case of two sources separated by some distance, is not a dipole. It is an acoustic doublet. The polar response of an acoustic double approximates a dipole response as the frequency decreases. The difference between a dipole response and the polar response of an acoustic doublet becomes acceptably insignificant about 1 octave below the "doublet" peak. And above that frequency the doublet response really should not be referred to as a dipole as it has no relation to a dipole at all, though it does remain MP.
John

I don't follow this distinction. Are you saying that the "doublet" is two out of phase sources seperated by a distance and the "dipole" is two out of phase sources seperated by a baffle? Otherwise I don't see what you are getting at.
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