Hello Michael,
Arrival time of an impulsion doesn't show the whole picture.
The rise of such an impulsion is mostly made of the highest frequency content.
To have a view on how the lowest frequencies are delayed from those high frequencies you must use the group delay curve (as you use Arta, this should not be difficult).
What we can see on you pulse responses from the one at the top to the one at the bottom is a kind of rounding on the negative arch following the main pulse. This is partly due to the change in frequency response curve but this also surely due to a modification of the group delay curves when increasing the angle from the axis.
Best regards from Paris, France
Jean-Michel Le Cléac'h
Arrival time of an impulsion doesn't show the whole picture.
The rise of such an impulsion is mostly made of the highest frequency content.
To have a view on how the lowest frequencies are delayed from those high frequencies you must use the group delay curve (as you use Arta, this should not be difficult).
What we can see on you pulse responses from the one at the top to the one at the bottom is a kind of rounding on the negative arch following the main pulse. This is partly due to the change in frequency response curve but this also surely due to a modification of the group delay curves when increasing the angle from the axis.
Best regards from Paris, France
Jean-Michel Le Cléac'h
directivity control *is* the same as controlling impulse response (coherence in time of arrival) over radiation angle
So - you have to decide
Yeah, I was under the same impression..
Isn't vertical scale the equivalent in SPL ? So - I don't see the problem ?
Remember its linear scale in IR plots !
Frequency response certainly looks good enough to be called "min phase" - no?
Thanks for pointing there Jean-Michel
I seldom look at GD as I haven't found the "key" to interpret any usefully nor do I have expertise in how it *should* perform in context of other designs.
....except maybe one or two basics I picked up here:
http://www.diyaudio.com/forums/multi-way/123426-horn-vs-waveguide-3.html#post1518072
Can you tell me what actually could be said from above plots?
(same 0 10 20 30 deg measurements as before and same gating at ~4ms, lower plot zoomed in and with "no" smoothing)
Michael
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Does the phase plot show the same thing? For example in software like HOLMImpulse or ARTA. I'll have to compare phase vs GD to see.
Hello Michael,
What I can say looking at you last screencopy is that there is 2 different phenomenons mixed.
One is the normal rise of the group delay when the frequency decreases toward the "cut-off" frequency (whatever it means regarding waveguides).
The other one is shwon through large positive or negative anomalous values of the group delay (normally negative values of the delay should not exist). This is the result of a width of the signal window embedding peaks of reflected energy or delayed energy (HOMs?). You should look at magnification of the pulse graph in order to locate the corresponding delayed peaks (of small amplitude). When there is interference between the direct and the delayed signal then such group delay anomalies appear.
You can also send me the .pir files so I can study them.
Best regards from Paris, France
Jean-Michel Le Cléac'h
What I can say looking at you last screencopy is that there is 2 different phenomenons mixed.
One is the normal rise of the group delay when the frequency decreases toward the "cut-off" frequency (whatever it means regarding waveguides).
The other one is shwon through large positive or negative anomalous values of the group delay (normally negative values of the delay should not exist). This is the result of a width of the signal window embedding peaks of reflected energy or delayed energy (HOMs?). You should look at magnification of the pulse graph in order to locate the corresponding delayed peaks (of small amplitude). When there is interference between the direct and the delayed signal then such group delay anomalies appear.
You can also send me the .pir files so I can study them.
Best regards from Paris, France
Jean-Michel Le Cléac'h
Yes – the referenced plots for
- frequency response
- group delay and
- impulse response
are all one and the same measurement
Thanks for you kind offer – you got mail...
Michael
Hello Michael,
Here are the group delay curves after correct windowing.
You'll notice the evolution of the GD in the interval of frequency between 900 and 1500Hz (GD decreasing with angle from the axis) and then between 2200 and 3000Hz (GD increasing with angle from the axis).
This surely means something...
Best regards from Paris, France
Jean-Michel Le Cléac'h
Here are the group delay curves after correct windowing.
You'll notice the evolution of the GD in the interval of frequency between 900 and 1500Hz (GD decreasing with angle from the axis) and then between 2200 and 3000Hz (GD increasing with angle from the axis).
This surely means something...
Best regards from Paris, France
Jean-Michel Le Cléac'h
Attachments
Again, thanks a million Jean-Michel !
The region around 2500 Hz you point at clearly is the frequency where polar wiggles are.
You better see them in a normalized FR plot:
If we let this polar wiggles aside for a moment - the overall directivity tracking of that horn is pretty good - some 1-2 dB from 1kHz up to 10kHz.
Even the top end from 10kHz until 20kHz is easily within the (nonsense) 10dB limit of CD horns – sure – its a smoothed FR – but even so – I'm pretty happy...
Interestingly this polar wiggle does not show up in the simulation of the min phase horn.
When I saw it slightly smoothing out with the different NEO3-W version I first thought that its been a mis-match of the wave front injected into the horn.
Have tried to close some of the holes the membrane of NEO3 is "breathing" through today – but it does not make that much of a difference – so seems there is more research needed on that effect.
Might be its simply the parallel top and bottom boundary, dunno, will see
Michael
Michael
The region around 2500 Hz you point at clearly is the frequency where polar wiggles are.
You better see them in a normalized FR plot:
If we let this polar wiggles aside for a moment - the overall directivity tracking of that horn is pretty good - some 1-2 dB from 1kHz up to 10kHz.
Even the top end from 10kHz until 20kHz is easily within the (nonsense) 10dB limit of CD horns – sure – its a smoothed FR – but even so – I'm pretty happy...
Interestingly this polar wiggle does not show up in the simulation of the min phase horn.
When I saw it slightly smoothing out with the different NEO3-W version I first thought that its been a mis-match of the wave front injected into the horn.
Have tried to close some of the holes the membrane of NEO3 is "breathing" through today – but it does not make that much of a difference – so seems there is more research needed on that effect.
Might be its simply the parallel top and bottom boundary, dunno, will see
Michael
Michael
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Well, it basically means the FR is lower, thus the phase is also changed when you go off angle. If the phase varies in this beam range, then it cannot be minimum phase can it? Or what is you minimum phase definition? Maybe I'm misunderstanding something....
Isn't vertical scale the equivalent in SPL ? So - I don't see the problem ?
Remember its linear scale in IR plots !
Frequency response certainly looks good enough to be called "min phase" - no?
...
Michael
Hi George,
All minimum phase means is that for any measured response the phase can be reduced to the minimum phase (as obtained from an HBT) plus excess phase resulting from a delay (a linear phase component). Minimum phase for a 3-dimensional sound field only means that as the amplitude changes with position, so does the phase in accordance with the HBT of the amplitude.
Hi John,
I was just questioning the fact why a horn would be called a minimum phase horn taken the fact it's performance and design seems very normal. Use of such technical term should have something unique to the actual design or performance.
Yes, me too. Measure the response. It either reduces to MP or it doesn't. Just because there are HOMs or diffraction effects that may have time domain artifacts doesn't mean it won't remain MP.
But one other thing I have not seen discussed, and I would like to see some horn experts clear up for me, is that, according to the simple horn threory presented by Kinsler and Frey, horns are dispersive; difference frequencies propagate at different speeds. I.e. the speed of sound can not be considered constant. This would, by nesessity, make a horn non MP.
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Hi John - my opinion is the same here. I never did see anything in the MP discussion of waveguides that made any sense. But HOM will not be MP - they have a "trace" velocity along the axis that is different from the main wavefront.
The answer is simple - it doesn't happen, "Horn" theory is wrong.
But, even in waveguide theory, at LFs the modes become complex and do appear to have dispersive effects, but these are not propagating modes, they are evanescent - they disipate exponentially as they travel. So any real propagating wavefront will have a standard wave speed along its waveFRONT. But the wavefront may not be propagating along the axis, hence the speed down the axis of the waveguide will not be the same as the speed of the wavefront.
In Horn theory a real propagating wave at cutoff IS certainly dispersive, but thats precisely where the theory fails. If you take the exact theory of an OS waveguide - which has an analytic solution correct in all aspects - and you apply the Horn equation to its contour you will see that the two solutions differ most precisely at "cutoff". The horn equation predicting dispersion and "cutoff" and the waveguides equations predicting no such thing. At higher frequencies they agree in impedance, but not in wavefront shape, and they disagree completely at LFs, horn theory predicting no sound transmission and the waveguide predicting evanescent wave propagation. Clearly the waveguide equations are the correct ones since this is what happens in a real device.
Another example of where I just don't see how the concept of MP is useful in acoustics.
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I know that we disagree on the MP argument, but I still hold that if we stick out a mic and measures the response at that point, it either reduces to MP plus linear excess phase or it doesn't. HOMs, diffraction, etc may have delays associates with them but that doesn't mean that the net measured response at the observation point can not be reduced to MP pluse a linear excess phase. I haven't measured horns, but the few wave guides I have looked at on tweeters have all shown to have the MP pluse linear phase bahavior. And I certainly haven't seen a measurement of a tweeter on a baffle where the net response, direct plus diffraction, isn't the same, MP plus linear excess phase.
Anyway, things like MP and acoustic centers are pretty useless. I just measure the drivers and removed the excess delay to the mounting flange or fact plate. In that manor I have a good reference point for on axis design.
Thanks for the comments re horn theory.
What actually *is* a horn ? – we possibly must ask first !
To me – and certainly 99% of people out there - it’s a shape .
A horn is a simple shape or structure of quite some stiffness or rigidity starting out big in diameter and ending in a tip after some distance.
When ancient audio freaks discovered that this very shape has special properties when used upside down and replacing the tip by a driver, trumping through the hollow structure – the run after the *magic* contour of horns in speaker design begun.
(literally spoken, the story might have begun even earlier…
)In the years to follow that great discovery - a lot of “special” horns were named :
Conical horns
Exponential .horns
Kugelwellen horns
and so on and so on...
A first mayor turn in looking at horns I see with the development of constant directivity horns.
The ingenuous lies in that these guys were solely after a well defined effect in the resulting sound field - rather than in a hunt to invent new formulas of horn boundary curves.
With Earls
Oblade sheroide horns
I see the next logical step of evolution, in that Earl aimed after – what he thinks is - low diffraction effects *AND* not giving up the CD concept - which in *his* context still is defined as to provide "good audience coverage" first.
With Jean-Michel's LeCleach horn contours – or better put "contour corrections" based on a water right concept of wave front propagation - I see that horns mainly intended for *high quality audio* greatly developed towards smooth and consistent sound filed outcome.
The main drawback I see with Jean-Michel's designs is that advanced directivity control wasn't at top priority – until now at least.
One further step could be considered to look at horn's as to be a diffraction alignment device.
Seeing diffraction as the necessary and actually only (practical) mechanism to smoothly bend the vector of wave fronts "around the corner".
In that context my naming of Min Phase horn is narrowed to smooth and consistent sound fields to result from such designs *PLUS* to stay as close as possible to min phase behaviour to allow to benefit from advanced equalising over an as wide as possible room angle.
This – to my knowledge – *is* quite new and unique and certainly not a small change of the audio paradigm in general – both from the underlying philosophy involved and from the outcome to await.
It's not that I claim to have the "best ever" solution of [/b]Min Phase [/b]horns discovered or developed yet (nor even that I restrict it to *my* effort at all) – its about setting the general *aim* apart form what's been on the list until now.
A new classification if you will.
So - if you watch the stars - by a telescope you have not built - and discover a new structure there - that neither belongs you nor ever will – nevertheless you are tempted to give it a name.
the naming of "min phase" - a simple act of personal pride
Bottom line – its just a looooot of name dropping involved in that horny issue...
Michael
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Actually John I don't disagree with you at all becasue to me the next line is the point
Agreed - to me the concepts of MP don't come in all that handy in the acoustics domain that's all. I don't disagree with anything that you said at all.
the naming of "min phase" - a simple act of personal pride
Bottom line – its just a looooot of name dropping involved in that horny issue...
Michael
Yes, but I haven't seen ant measurements presented that indicate a typical horn any more or less MP that a driver on a baffle is by the definition I put fourth. I looked a coupe of compression driver with different off the shelf, cheap horns or wave guides (to me the difference is semantics) and they all seem to be MP. Where is the departure from MP behavior?
Also, for what its worth, I see a horn and wave guide as the same thing with the primary difference being that a horn's primary function is gain where as a wave guide's primary function is directivity control.
Oh, and by the way, looking at the an impulse generally can not tell you if a device is MP or not.
Yes this is the paramount point you were teaching us.
The pointe behind min phase that I see is that the way you looked at this in audio was the starting point to look at advanced equalization as a possibility to come to close to ideal behavior over a wide room angle (response shaping at will).
Hence I have choosen "Min Phase" horn rather than "Michael Gerstgrasser" horn - together with what I see unique as my "diffraction alignment" concept for horn contours in general I think its a justification to establish a "new" name.
Michael
The pointe behind min phase that I see is that the way you looked at this in audio was the starting point to look at advanced equalization as a possibility to come to close to ideal behavior over a wide room angle (response shaping at will).
Hence I have choosen "Min Phase" horn rather than "Michael Gerstgrasser" horn
- together with what I see unique as my "diffraction alignment" concept for horn contours in general I think its a justification to establish a "new" name.Michael
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