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Old 14th September 2007, 02:21 AM   #2001
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Quote:
Originally posted by salas
Lynn,

1. Are you going to design for a reference axis located between the wide range and the RAAL, or on the RAAL?

2. You did not tell your opinion about the open back compression driver dipole idea asked earlier...
1. I always use a reference axis between the mid and tweeter, and design the crossover so the center of the lobe is aimed right at the listener. I want the nulls as far away as possible.

2. Yes, compression drivers can be used "barefoot" with the rear chamber removed. But that won't fix the inherent diaphragm breakup mode in the 14~21 kHz region, and throws away a tremendous amount of horn-gain/impedance-matching and resultant drop in IM distortion at lower frequencies. So it amounts to a tradeoff - a big, well-made dome tweeter (that needs a lot of EQ) versus the HOM's (or multipath distortion, as I think of it) of a horn or waveguide (and much less EQ).

It's significant that over in the Dr. Geddes thread, at least one person has tried a short foam plug in the throat of the horn and is reporting good results. Since the throat area has been identified as one of the main culprits in generating HOM/multipath distortion, it looks like a little bit of open-cell foam in the right area could do a lot of good. Since pressures are highest in this part of the horn or waveguide, the lossy foam in turn should be most effective in damping unwanted modes.

The old advice of "put a sock in it" might turn out to be true.
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Old 14th September 2007, 02:27 AM   #2002
Salas is offline Salas  Greece
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- Thanks. Classic English school always took midway axis too.

- Barefoot = bare loaded, got it.
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Old 14th September 2007, 02:41 AM   #2003
AJinFLA is offline AJinFLA  United States
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Default Re: Very interesting thread

Quote:
Originally posted by dlr

AJ,

You keep referring to "soft, flexing, lossy paper cones", but this is a bit disingenuous. The truly good driver examples such as the SS units are not soft, they are relatively stiff doped-paper with significant internal damping and don't flex nearly as much as implied. There's also a very simple way to compare any two drivers in this regard. Make a range of distortion tests, such as those that Mark K and zaph make. All implications of "soft", "flexing", "lossy", etc., are really irrelevant without some sort of objective evaluation for comparison.
Hi Dave,

You have to put what I said in context. That was a reply to Tom in reference to pro audio drivers, not the SS, Seas, etc.
I'm well aware that paper can be stiffened significantly, ala the new Seas Nextel cones, to the point where they have almost a metal like upper resonance peak.
I agree with the rest of what you said. There are rigid cones that do not exhibit the the same amplitude peak(s) as the W Seas magnesium. Toole's Ceramic skinned aluminum appear to push the peak further up and lower in amplitude. Even coated aluminum can have relatively benign peaking.
Now lets take a look at the type of driver I was referring to.8CX21
I suspect there may be some bending and flexing there.
My point was (again) there are bad and good examples of both.
I still prefer good metal over good paper or cloth. Others obviously don't.

cheers,

AJ
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Old 14th September 2007, 02:55 AM   #2004
Salas is offline Salas  Greece
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AJ, I think that Ted Jordan knew how to keep Alu working on his side, better than most. I have a JXR6 HD pair that I look to implement in a low cross 2 way in the short future, and I can't hear it ringing through initial evaluation.
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Old 14th September 2007, 06:12 AM   #2005
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Quote:
Originally posted by Lynn Olson
[snip]
This means the symphonic spectra superficially looks as dense as noise, but in reality is highly correlated with itself and the hall reflections. Any perturbation to the fine spectral and time structure does enormous violence to the performance, since so much is going on all at once - indeed, the sheer density, complexity, and fleeting spatial relationships are an integral part of the composer's and conductor's intentions.
[snip]
Lynn,

I believe that time-fidelity is at least as important as frequency-response to maintain the perceptual parsability of complex orchestral and choral recordings. As I continue on my personal 'audio-trek' - which shares a lot in common with your current efforts - I have recently been working on time-fidelity... with very encouraging results.

Here is a recent impulse response measurement for the Gecko system:

Click the image to open in full size.

This measurement has ZERO editorializing.

For me, this accomplishment has been a personally and aurally rewarding step in the right direction.

Edward
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Old 14th September 2007, 06:52 AM   #2006
gedlee is offline gedlee  United States
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Quote:
Originally posted by EdwardWest


This measurement has ZERO editorializing.

Edward

And yet it is physically impossible.
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Old 14th September 2007, 11:32 AM   #2007
dlr is offline dlr  United States
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Default Re: Re: Very interesting thread

Quote:
Originally posted by AJinFLA


Hi Dave,

You have to put what I said in context. That was a reply to Tom in reference to pro audio drivers, not the SS, Seas, etc.
I hadn't see the context on that, my mistake then. There are still a lot of folks around who make the same reference but in the context in which I took it, guess I was reacting to that in seeing your post.

Quote:
I'm well aware that paper can be stiffened significantly, ala the new Seas Nextel cones, to the point where they have almost a metal like upper resonance peak.
I'm not sure what I think of the Nextel. I haven't heard any, so I'd only be conjecturing as to their sound.

Quote:
Toole's Ceramic skinned aluminum appear to push the peak further up and lower in amplitude. Even coated aluminum can have relatively benign peaking.
That's an interesting driver, I've not seen it before. I see that the description is "wide range driver", more apt, but it seems that it's being touted as a full range for usage in the examples.

Though it uses neodymium, the impedance curve would indicate that it may still not be a low distortion design. I'm curious about that aspect of the driver. At that price, I'd expect to see better than standard distortion figures. Too bad they don't show any. I also always question a manufacturer that does not post 30 and 60 degree off-axis FR graphs.

Quote:
Now lets take a look at the type of driver I was referring to.8CX21
I suspect there may be some bending and flexing there.
My point was (again) there are bad and good examples of both.
I still prefer good metal over good paper or cloth. Others obviously don't.

cheers,

AJ
With that driver it's hard to say what is related to cone material, cone geometry (resonance modes) or possible diffraction due to the coincident driver as well as diffraction due to the cone profile. I wouldn't even hazard a guess without knowing more details.

Dave
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Old 14th September 2007, 11:33 AM   #2008
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Quote:
Originally posted by soongsc

What happens first will arrive at the ear first. Since electrical compensation happens first, you hear the wrongly compensated signal first. Unless you can make the compensation after the cone mode is excited and before the wave reaches the ear, you cannot ideally correct for it. I think if someone does a finite element analysis of the acoustic wave then you can clearly see it. I recall that Manger has a patent that does some kind of compensation acoustically, now this is possible, but the compensation will be perfect at specific measurement locations only.
I’ not going to get all upset if I can not convince you but I wish you only to consider this. Forget about nonlinear effects and concentrate only on the direct sound. Now, if I have two perfectly linear sources which have different frequency response, one with a big resonance peak at 4k Hz, and I equalize the resonance so both drivers have identical on axis response, then the impulse response associated with the direct sound will be identical for both systems and the impulse response is the time response so how can the direct sound that reaches my ear be different? Yes, it takes time for the resonance to build, but if the resonance grows as exp(At) and I apply equalization that goes as exp(-At) the system output is exp(0) = 1.0. You are correct in that what happens first reaches your ear first, but the equalization corrects the input time response so that what happens first at the output is controlled in such manor that it is what we want to happen first. I also agree that this eq. may not produce the perfect correction off axis. That was one of the other factors I grouped into “There are numerous other potential sources of problems too” and would possibly account for why two different but perfectly linear sources with difference frequency response may still sound different when equalized to have perfectly identical on axis response. This is part of the what make the problem so complex and another possible reason to stay away from driver with nasty breakup peaks.

This is not a feedback system. This is a linear correction to a linear system. In a linear system if you know before hand that the system produces an error you can subtract the error from the input to eliminate it in the output. This is different than a nonlinear system because you can not predict the error in a nonlinear system. It will vary with the input. In a nonlinear system we look at the output to see what the instantaneous error is and then we subtract the instantaneous error from the input. This in turn changes the input which may change the error created in the output, so we have to do this continuously because the error is constantly changing. I’m not an expert in feedback systems like amplifiers but I do know that in a nonlinear system the best approach is to design the system to be as stable and as close to linear as possible and then apply the least amount of feedback required to clean up the system.
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Old 14th September 2007, 11:39 AM   #2009
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Quote:
Originally posted by gedlee



And yet it is physically impossible.

And is this unedited 3-way system, on axis impulse response impossible too? Sorry that it is inverted. My mic inverts. (~60k/sec sampling rate, first 3 msec).

Click the image to open in full size.
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Old 14th September 2007, 01:39 PM   #2010
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by john k...


I?not going to get all upset if I can not convince you but I wish you only to consider this. Forget about nonlinear effects and concentrate only on the direct sound. Now, if I have two perfectly linear sources which have different frequency response, one with a big resonance peak at 4k Hz, and I equalize the resonance so both drivers have identical on axis response, then the impulse response associated with the direct sound will be identical for both systems and the impulse response is the time response so how can the direct sound that reaches my ear be different? Yes, it takes time for the resonance to build, but if the resonance grows as exp(At) and I apply equalization that goes as exp(-At) the system output is exp(0) = 1.0. You are correct in that what happens first reaches your ear first, but the equalization corrects the input time response so that what happens first at the output is controlled in such manor that it is what we want to happen first. I also agree that this eq. may not produce the perfect correction off axis. That was one of the other factors I grouped into “There are numerous other potential sources of problems too?and would possibly account for why two different but perfectly linear sources with difference frequency response may still sound different when equalized to have perfectly identical on axis response. This is part of the what make the problem so complex and another possible reason to stay away from driver with nasty breakup peaks.

This is not a feedback system. This is a linear correction to a linear system. In a linear system if you know before hand that the system produces an error you can subtract the error from the input to eliminate it in the output. This is different than a nonlinear system because you can not predict the error in a nonlinear system. It will vary with the input. In a nonlinear system we look at the output to see what the instantaneous error is and then we subtract the instantaneous error from the input. This in turn changes the input which may change the error created in the output, so we have to do this continuously because the error is constantly changing. I’m not an expert in feedback systems like amplifiers but I do know that in a nonlinear system the best approach is to design the system to be as stable and as close to linear as possible and then apply the least amount of feedback required to clean up the system.
I understand what you are saying, and I agree it covers most part of the signal regeneration. Preshaping a signal is a very common technique used in control, and I understand that we are not talking about feedback. However, we must realize that presure is generated from different parts of the surface and different time throughout the signal variation.
Well thinking about direct sound, at the first cone mode, I'm visualizing in the VC moving first, part of surface close to where VC and Cone connect move a bit later, then finally the outer edge of the cone follows. Throughout this first 1/4 cycle is the onset, if compensation prevents this part from being faithfully reproduced, then the sound will not be as good as a more well behaved cone. I won't be upset if I cannot let you understand this part.
Now if we wanted to do somthing really exotic using real=time software technology to it's extreme, in might be possible to model the energy storage charateristics of a cone, and depending on what the previous state and new signal state is, more adequately compensate for cone defficiencies. But I don't think anyone is going to spend millions on a project like that.
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