Hello Timo,
As I can see you are using Limp (from the Arta suite). The impedance curve you obtain is very smooth (I guess at lest a 1/24 octave smoothing is used...).
While I am an Arta user and promoter, I don't use it for impedance mesurement in the case I want to locate minute artifacts on them.
As an example I could do a study of the effect of diffraction and/or refllected waves inside a JBL 2405 tweeter to the slit of which I add different mouth prolongations. The different types of horns they figured gave different artifacts on the impedance curve.
Here attached a magnification of an impedance curve showing minutes peaks, probably due to membrane break up.
Best regards from Paris, France
Jean-Michel Le Cléac'h
As I can see you are using Limp (from the Arta suite). The impedance curve you obtain is very smooth (I guess at lest a 1/24 octave smoothing is used...).
While I am an Arta user and promoter, I don't use it for impedance mesurement in the case I want to locate minute artifacts on them.
As an example I could do a study of the effect of diffraction and/or refllected waves inside a JBL 2405 tweeter to the slit of which I add different mouth prolongations. The different types of horns they figured gave different artifacts on the impedance curve.
Here attached a magnification of an impedance curve showing minutes peaks, probably due to membrane break up.
Best regards from Paris, France
Jean-Michel Le Cléac'h
tiki said:
Attachments
Hello Earl,
Simulation of the axial response (using BEM, FEM, Hornresp...) based on constant velocity, predict, for many horns including Le Cléac'h horns (but not only), a rising response with a slope near of 10dB/octave.
Those simulatons are not consistent with the majority of the measurements which one show falt (or nearly) axial response.
That's probably an idiot question from my part , but could it be that we should use a constant acceleration diaphragm model at throat in place of a constant velocity diaphragm model.
Any other idea is welcome!
Best regrads from Paris, France
Jean-Michel Le Cléac'h
Simulation of the axial response (using BEM, FEM, Hornresp...) based on constant velocity, predict, for many horns including Le Cléac'h horns (but not only), a rising response with a slope near of 10dB/octave.
Those simulatons are not consistent with the majority of the measurements which one show falt (or nearly) axial response.
That's probably an idiot question from my part , but could it be that we should use a constant acceleration diaphragm model at throat in place of a constant velocity diaphragm model.
Any other idea is welcome!
Best regrads from Paris, France
Jean-Michel Le Cléac'h
gedlee said:
Hello Wxa666,
What we try to understand is, why the simulation using Hornresp (constant velocity throat model) of the response curve on axis of a Le Cléac'h horn (Fc = 320Hz, T = 0.8):
http://www.diyaudio.com/forums/attachment.php?s=&postid=1857164&stamp=1245230704
shows a nearly +10dB/decade slope above 400Hz, when my measurements at more than 2 meters on axis (current drive and voltage drive) are flat (nearly flat):
http://www.diyaudio.com/forums/attachment.php?s=&postid=1531292&stamp=1212610725
I was hypothesing that the diaphragm was operating in the mass controlled mode and thus, a constant acceleration model should be more convenient than a constant velocity model.
Best regards from Paris, France
Jean-Michel Le Cléac'h
What we try to understand is, why the simulation using Hornresp (constant velocity throat model) of the response curve on axis of a Le Cléac'h horn (Fc = 320Hz, T = 0.8):
http://www.diyaudio.com/forums/attachment.php?s=&postid=1857164&stamp=1245230704
shows a nearly +10dB/decade slope above 400Hz, when my measurements at more than 2 meters on axis (current drive and voltage drive) are flat (nearly flat):
http://www.diyaudio.com/forums/attachment.php?s=&postid=1531292&stamp=1212610725
I was hypothesing that the diaphragm was operating in the mass controlled mode and thus, a constant acceleration model should be more convenient than a constant velocity model.
Best regards from Paris, France
Jean-Michel Le Cléac'h
Jmmlc said:
Given a certain frequency the velocity and the accleration are proportional:
a ~ f * v
hence if a == const:
v ~ 1/f which will give -10dB/decade
From what I read from hornresp You are asked to give the electro acoustic parameters of the driver, the back/front cavities too, and the compression ratio.
If You don't do that the simulation will not provide the actual response, but the part of the horn only. That horn part will alter the diaphragm movement to some extent. You woud have to "multiply" the drivers response, its "force" with the reactance of the horn and further multiply that with the horns radiation that You calculated in the first place. In any case to switch from velocity to accleration will not help a bit.
btw: if the response it raising as shown, what about directivity? I have one further caveat to ask about. What supports in depth Your assumtions of (a) paralleld wavefronts and (b) pependicular pressure gradient onto the horns walls? Isn't it so that with these the parallel to microwave antennas is lost - I really dont know?
by
Jmmlc said:
Hi ...
Constant velocity is clearly not right as that would ignore all impedance effect such as inductance. A driver driven by a current source might be close to constant velocity, but not when driven by a voltage source. A simple comparison of the two measurements would determine if it is an electrical effect or an acoustic one.
hello Earl,
Thanks for the reply.
In the graph
http://www.diyaudio.com/forums/attachment.php?s=&postid=1531292&stamp=1212610725
red curve (top right ) is for current source drive and blue is for voltage source drive.
Using current drive results in a very flat response curve and noticeably the famous hole around 1600Hz-1900Hz of the TAD TD2001 driver, which one is the perfect inverse of a bump in the impedance curve is totally suppressed.
Over 7000Hz the response curve is similar for current source drive and voltage source drive.
Best regards from Paris, France
Jean-Michel Le Cléac'h
Thanks for the reply.
In the graph
http://www.diyaudio.com/forums/attachment.php?s=&postid=1531292&stamp=1212610725
red curve (top right ) is for current source drive and blue is for voltage source drive.
Using current drive results in a very flat response curve and noticeably the famous hole around 1600Hz-1900Hz of the TAD TD2001 driver, which one is the perfect inverse of a bump in the impedance curve is totally suppressed.
Over 7000Hz the response curve is similar for current source drive and voltage source drive.
Best regards from Paris, France
Jean-Michel Le Cléac'h
gedlee said:
Jmmlc said:
So, the driver sports a voice coil of common inductance as I myself expected before earlier
The simulation has to be thought over in consequence. The driver and the horn may be looked at seperately in a freqency regime where the acoustic impedance / reactance (wording?) of the horn is more or less constant. In that regime the hornresp output has to be multiplied with the drivers "plain wave tube" response to achieve a more qualitative amplitude over frequency plot. As far as I know JBL puplished the PWT response for drivers as 2445J etc.
btw: would You mind to give me a hint where to find something to support Your assumptions "parallel wavefronts" and "perpendicular pressure gradient"?
cheers
hello wxa666,
Before to ask such question, you should read IMHO, the threads: "Hornresp"; "horn versus waveguide"; "Geddes on waveguides", "reviving the Onken", "Beyond the Ariel"...
You'll find here several BEM simualtions done by Bjørn Kolbrek:
http://www.diyaudio.com/forums/showthread.php?postid=1765863
Best regards from Paris, France
Jean-Michel Le Cléac'h
Before to ask such question, you should read IMHO, the threads: "Hornresp"; "horn versus waveguide"; "Geddes on waveguides", "reviving the Onken", "Beyond the Ariel"...
You'll find here several BEM simualtions done by Bjørn Kolbrek:
http://www.diyaudio.com/forums/showthread.php?postid=1765863
Best regards from Paris, France
Jean-Michel Le Cléac'h
wxa666 said:
Jmmlc said:
Jean-Michel,
I have to read many 10s of thousends of words more or less off topic to get the clue of Your initiating assumptions? I have to have gotten that wrong. I'm curious whether You are right in saying that You don't postulate a special shape of wavefront, instead of giving it implicitly.
And in the end You think that hornresp has to include the driver in the simulation without knowing it but supposing constant velocity? Why do You use TAD drivers at all from which You don't know PWT response and/or other constructive parameters? With issues due to long neck AlNiCo magnets, but "oxygen free copper shorting rings". Instead of drivers at a 10th of cost with near to perfect resonance free composite diaphragm and flat phase plug design, the latter new patented and by a great margin better than the older ones?
A lotta questions to be answerd. But I better resign.
Thank You!
Hello wxa666
If you have read the threads I encouraged you to read, you should have seen that I am the author of an Hornresp model for the TAD TD2001 most of the parameters of which derive from my own measurements.
http://www.audioasylum.com/forums/hug/messages/12/128658.html
The TAD TD2001 is since long time my favorite compression driver. I guess I was here in France one of its first user in the 80's. I have 6 of them, but I have also many other drivers including Yamaha JA6681, Western Electric WE555...
Modern drivers (JBL, B&C, BMS....) I used to listen don't sound better to my ears...
About the way my horns are conceived, this is explained in Kolbrek's paper:
http://www.audioxpress.com/magsdirx/ax/addenda/media/kolbrek2885.pdf
Best regards from Paris, France
Jean-Michel Le Cléac'h
If you have read the threads I encouraged you to read, you should have seen that I am the author of an Hornresp model for the TAD TD2001 most of the parameters of which derive from my own measurements.
http://www.audioasylum.com/forums/hug/messages/12/128658.html
The TAD TD2001 is since long time my favorite compression driver. I guess I was here in France one of its first user in the 80's. I have 6 of them, but I have also many other drivers including Yamaha JA6681, Western Electric WE555...
Modern drivers (JBL, B&C, BMS....) I used to listen don't sound better to my ears...
About the way my horns are conceived, this is explained in Kolbrek's paper:
http://www.audioxpress.com/magsdirx/ax/addenda/media/kolbrek2885.pdf
Best regards from Paris, France
Jean-Michel Le Cléac'h
wxa666 said:
Jmmlc said:
I don't get how You could ask to take a "constant accleration model" instead of one with "constand velocity" then. That's not a to smart question as You Yourself assumed.
Jmmlc said:
Aha. They have been to expensive to sell them now when smarter drivers are available for a tenth of the cost.
Jmmlc said:
"Jean-Michel Le Cléac’h presented a horn
that does not rely on an assumed wavefront
shape. Rather, it follows a “natural
expansion.”"
I didn't get from what that special shape should be natural in any way. That's why I dared to ask the inventor. Especially this caught my attention:
"... but without making any assumptions regarding the
shape prior to the calculations."
Is it true or is the provision to a special waveform hidden in the initial assumptions?
But as said before, my interest has vanished. I don't think to be cappable to gain further insight, maybe I'm to dump.
Best regards
Hello wxa666,
Fig 24 in Kolbrek's paper is easy to understand and I don't doubt any second the explanation he give are out of your understanding.
The 2 and only inital assumptions done are:
1) we know the expansion law of the area of the wavefront along the curvilinear distance to the throat
2) wavefronts defined as isophase are parallel (= equidistant one from the other)
But if you doubt the explanations given by Kolbrek, then why not to ask him?
Best regards from Paris, France
Jean-Michel Le Cléac'h
Fig 24 in Kolbrek's paper is easy to understand and I don't doubt any second the explanation he give are out of your understanding.
The 2 and only inital assumptions done are:
1) we know the expansion law of the area of the wavefront along the curvilinear distance to the throat
2) wavefronts defined as isophase are parallel (= equidistant one from the other)
But if you doubt the explanations given by Kolbrek, then why not to ask him?
Best regards from Paris, France
Jean-Michel Le Cléac'h
wxa666 said:
Jmmlc said:
Hi ...
Thanks for that data.
Then the cause of difference between the sims and reality at HF has to be mechano-acoustic - either diaphragm breakup (without impedance effects) or acoustic gain in the horn (directivity, modes, that sort of thing). It cannot be electrical related as the cones motion is independent of any electrical characteristics.
Jean-Michel regarding wave front shape.
May I ask you how you look at fig 14-16 from Bjorns paper.
http://www.audioxpress.com/magsdirx/ax/addenda/media/kolbrek2884.pdf
Woldn't you agree that W.M. Hall has gotten confused between the wave *front* versus the sound field / pressure distribution inside the horn ?
Michael
May I ask you how you look at fig 14-16 from Bjorns paper.
http://www.audioxpress.com/magsdirx/ax/addenda/media/kolbrek2884.pdf
Woldn't you agree that W.M. Hall has gotten confused between the wave *front* versus the sound field / pressure distribution inside the horn ?
Michael
Hello Michael,
With a horn the response of which doesn't replicate the power response of the driver, equiphase wavefronts tend to be parallel to pressure wavefronts only at frequency near of the cut-off frequency.
When frequency rises it is obvious that high order modes are progressively more efficient in enhancing the directivity of the horn. This is not questionable.
Constant directivity is not something the Le Cléac'h horn has been conceive for.
The characteristics who defines a Le Cléac'h horn are:
1) quasi absence of reflected waves inside the horn,
2) pure resistive loading on a very wide interval of frequency allowing:
.......a) very small diaphragm displacement and lower distortion,
.......b) constant group delay,
.......c) extended on axis response.
3) Smoothness of the pressure field distribution inside and outside the horn (no nodes of pressure such as in Kolbrek's paper fig 15).
Best regards from Paris, France
Jean-Michel Le Cléac'h
With a horn the response of which doesn't replicate the power response of the driver, equiphase wavefronts tend to be parallel to pressure wavefronts only at frequency near of the cut-off frequency.
When frequency rises it is obvious that high order modes are progressively more efficient in enhancing the directivity of the horn. This is not questionable.
Constant directivity is not something the Le Cléac'h horn has been conceive for.
The characteristics who defines a Le Cléac'h horn are:
1) quasi absence of reflected waves inside the horn,
2) pure resistive loading on a very wide interval of frequency allowing:
.......a) very small diaphragm displacement and lower distortion,
.......b) constant group delay,
.......c) extended on axis response.
3) Smoothness of the pressure field distribution inside and outside the horn (no nodes of pressure such as in Kolbrek's paper fig 15).
Best regards from Paris, France
Jean-Michel Le Cléac'h
mige0 said:
Jmmlc said:
Hi ...
I would say that only equiphase contours can define a "wavefront". Equal pressure amplitude contours do not have to correspond to the wavefront, there is no requirement for that. The phase contours are the "wavefronts". At LF the wavelengths are too long for the pressure variations to be far off from the phase contours. At HF basically anything can occur.