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 1st May 2009, 09:12 PM #1081 diyAudio Member     Join Date: Dec 2004 Location: Novi, Michigan The system is not letting me post the page, but it can be seen here www.gedlee.com/downloads/morse_pg429.pdf __________________ Earl Geddes Gedlee Website Read about the highly acclaimed Geddes loudspeakers
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Join Date: Apr 2009
Quote:
 Originally posted by gedlee Lets forget about the minimum phase comment, thats a whole other can of worms. I stand by my statements as correct as shown by Morse in said text and they are not wrong simply because YOU say that they are wrong. The burdon of proof is on you since I have attached my proof from the work that I described.
Hi,

You cited Morse.

I read it. Nothing on that page supports this:

"I turn off this source, the sound field slowly (relative to the decay rate) changes from 49 Hz to 51 Hz."

The citation lacks all definitions one has to know to understand what it is about. What is it aiming for? Example given, how are thoses series defined? Your proof is none. It is Your turn to show explicitely how Your claims can be derived from Morse (if Morse is an authority, which he is).

I'm quite sure that You didn't mean that decay MAKES an other frequency. Decay changes proportions between frequencies that have ever been there. Or in case of switching off a source, are part of the signal. To switch off a source is a signal.

Morse himself mentions a temporal symmetry between switching on and off. Woud You turn on the source, the sound field slowly changes from 51Hz to 49Hz? I'm quite sure all the argument is about some sloppy wording.

so long

diyAudio Member

Join Date: Mar 2005
Location: Taiwan
Quote:
 Originally posted by gedlee I do. I have. You ignore it. That's the conflict.
I think in areas that I did find myself incorrect, I did openly admit it, not face saving remarks. If I diagnose my own data which show different trends, of course I beleive my data as you would believe yours. We just never got down the the exact conditions we each used. But bear in mind that I am constantly generating new data an looking at much more different test conditions than others have made public. The lack of comparible tests always leaves room for interpretation and opinion.
One thing that I always try to stick to is to focus on the issue, not on the person. Therefore, I accept the fact that disagreements exists, which is very common, and a fact in life.
Having worked with a few large companies in the US, many opinions of mine initially get ignored, but normally it gets respected a year or to down the road. So as you can see, I have no problem with disagreements.
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 2nd May 2009, 10:59 AM #1084 diyAudio Member     Join Date: Aug 2004 Location: US My point is that MP has nothing to do with how the system acts after the signal is removed. MP is just a mathematical relationship with response the the transfer function. I believe form the many discussions with Earl on this subject that Earl disagrees with the MP concept because in acoustics the transfer function is a four-dimensional function of position and frequency, x,z,y,f, whereas for an electrical system the transfer function is a one dimensional function of F at the output. My point has always been that for a given position, x,y,z, the resulting spatial transfer function is either MP or not MP. The TF changes with position, and relative to that position the response again, may or may not be MP. The point is that the position, x,y,z defines the transfer function. At that point the transfer function then can be determined to be MP or not. Does it matter? That depends on your outlook. From the system point of view, the only thing MP does is to allow us to recognize the amplitude correction implies phase /time correction. If the system is non MP that doesn't hold. The other point about MP is that if two tweeters which measure on axis exactly the same and are MP, then substitution of either tweeter in a system which was designed for flat on axis response with one of the tweeters will have flat response when the other is substituted. If the MP relationship continues to hold for off axis positions and both tweeters have the same polar response, then the system will also have the same polar response with either tweeter. If MP doesn't hold, you would have no idea what the system would look like with the second tweeter, on or off axis. Now it is not necessary to for CAD software to require MP or even consider it. But it is inherent in the design process because it is inherrent that for a dirver the response, T(x,y,z,f) for any position x,y,z is typically MP, or MP plus some constant time delay. But when we talk about in room response, low frequency or otherwise MP really isn't the issue in any case. We are stuck with an 4-dimensional transfer function, T(x,y,z,f) and there is no way to make T(x,y,z,f) have some predefined characteristic by applying a single equalization function Eq(f). T(x,y,z,f) * Eq(f) is still a three dimensional function of space. Even with multiple subs and multiple eq, T(x,y,z,f) would equal T1(x,y,z,f) * Eq1(f) +T2(x,y,z,f) * Eq2(f) +...... + Tn(x,y,z,f) * Eqn(f) where there are up to n woofers and equalization functions. There may be ways to optimize the position of each woofer and its corresponding eq to have relatively smooth (low frequency) response over some range of positions, but the only true way to have uniformly smooth response over a wide regions if to eliminate the x,y,z dependences in the room transfer function(s). __________________ John k.... Music and Design NaO dsp Dipole Loudspeakers.
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Join Date: Apr 2009
Quote:
 Originally posted by john k... ... the only true way to have uniformly smooth response over a wide regions if to eliminate the x,y,z dependences in the room transfer function(s). [/B]
Hi John,

You do not aim for that, do You? The ansatz of Earl Geddes seems quite convincing to me. Take what is affordable (3 subs + 2 mains = 5 sources) and follow an algorithm to yield at least a second best solution. To some extent the second best is equivalent to the best, when the limited human hearing skills are taken into consideration.

This is straight forward engineering. Some kind of sound field generation would be more satisfying to the scientist. Alas it is not practical due to limited budget, limited time, limited knowledge.

cheers

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Join Date: Dec 2004
Location: Novi, Michigan
Quote:
 Originally posted by xpert You cited Morse. http://www.gedlee.com/downloads/morse_pg429.pdf I read it. Nothing on that page supports this: "I turn off this source, the sound field slowly (relative to the decay rate) changes from 49 Hz to 51 Hz."
I'll stand by my statement and claim that it is supported by Morse. If you need more information to understand it then LOOK IT UP.

 2nd May 2009, 10:17 PM #1087 diyAudio Member     Join Date: Dec 2004 Location: Novi, Michigan John I don't disagree with a anything that you say, except maybe "MP has nothing to do with how the system acts after the signal is removed". Decay is part of the system response. Since the steady state response defines the impulse response and the impulse response defines the response for all time and all signals, this statement does not seem like it could be correct. At any rate you understand very clearly the limitations of the concepts for acoustics and why I don't bother with it. If it works for you fine. __________________ Earl Geddes Gedlee Website Read about the highly acclaimed Geddes loudspeakers
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Join Date: Apr 2009
Quote:
 Originally posted by gedlee I'll stand by my statement and claim that it is supported by Morse. If you need more information to understand it then LOOK IT UP.

Earl,

sorry. You are the one to understand things. I would like to help You. Please show us how You could come to that forementioned misconceptions.

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Join Date: Apr 2009
Quote:
 Originally posted by gedlee John I don't disagree with a anything that you say, except maybe "MP has nothing to do with how the system acts after the signal is removed". Decay is part of the system response. Since the steady state response defines the impulse response and the impulse response defines the response for all time and all signals, this statement does not seem like it could be correct. At any rate you understand very clearly the limitations of the concepts for acoustics and why I don't bother with it. If it works for you fine.
Quatsch,

The term "the system acts after the signal is removed" is an unlucky wording in this context. This wording is from Morse whom You brought in as an authority. You shouldn't take john_k into account for that.

If You are talking about a transfer function there surely is NO difference betweeen steady state and any other state. This very statement is from me, it was not Yours. Please don't use my argumentation against john_k, whilst You try to make him resonsible for Your own misleading wording.

As far as I have understood Morse from Your very (brief) citation, he does NOT support any of Your statements. First of all, what is decay within a cavity described by eigenmodes (may be) and some sort of damping on the reflective surfaces? Do You try to derive Your claims from an understanding of that situation?

What is different compared to, example given a one dimensional oscillator mass/compliance type, electric circuit bild up with reactive impedances or much simpler an organ pipe? The concept of eigenmodes encourages to look at the whole systems as a sum of parts that superimpose linearly.

Do You try to tell us, that the eigenmode concept is not valid here? To only flutter with some papers/books is not very kind. Do You think john_k does understand Your objections? If not why not try to explain the basic concept behind Your doubt.

Thank You

diyAudio Member

Join Date: Aug 2004
Location: US
Quote:
 Originally posted by gedlee John I don't disagree with a anything that you say, except maybe "MP has nothing to do with how the system acts after the signal is removed". Decay is part of the system response. Since the steady state response defines the impulse response and the impulse response defines the response for all time and all signals, this statement does not seem like it could be correct. At any rate you understand very clearly the limitations of the concepts for acoustics and why I don't bother with it. If it works for you fine.
Earl,

What I was saying about MP having noting to do with decay was in regards to a system undergoing forced oscillation at some frequency and then when the forcing function is removed it will decay with oscillation at its natural frequency if the system is sufficiently underdamped. That characteristic is independent of whether the system is MP or not. That's all. Don't read anyting into it.

xpert:

At this point, what ever satisfies the listener is fine with me. Bass in a small room is never going to be "right" over a wide area. Different methods will achieve more or less satisfying results that other approaches. We can argue the merits of one approach over another, but ever floats your boat is good enough for me.

Frankly, in my experience, I find a deep null some where between 30 to 50 Hz region most dissatisfying. I'm personally more tolerant of a little boom compared to a complete suck out. For example, a recording of a big bass drum without the low frequency part sound a lot worse that the same thing with a little boom. I'd rather hear it wrong than not at all. :0
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