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Thread: Ring Radiator Production

  1. #31
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    FR of 2404H-1 on a 1/2" 6' PWT.

    No damping, just impulse response gated at 10.5 milliseconds

    three lines:

    1. Open tube end, (Yel)
    2. Closed tube end (Rosa)
    3. Last 8in damped with a loose cotton plug (Green)

    From 200 Hz onwards it is essentially the same line, so I can forget damping...
    For some future project, let's say 2" PWT for 2445, I will extend the tube to 12' and gate the impulse response on 21 ms...

    (Forget absolute pressure on ordinate, it is not calibrated... maybe later)
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  2. #32
    Junior Member Turner's Avatar
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    Quote Originally Posted by ivica View Post
    I have several 2405 and 077, ( with silver, blue and gold rings) but almost none of them has the same frequency response. May be interesting to say that Beyma CP-21F has the same "behavior" ( every piece has different response). Regards Ivica
    Have you tried shimming the diaphragms and/or phase plugs? Phase plugs might need to be deburred, also.

  3. #33
    Senior Member grumpy's Avatar
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    My experience has been that in using cleaned/undamaged (whether used or new) diaphragms,
    and with repeatable monitoring of frequency response while carefully snugging up the phase plug,
    one can get what I would call reasonable agreement between these drivers (sufficient to not muck
    up imaging or balance).

    Otherwise, the phase "dog's breakfast" would apply to the comparison plots I've seen when
    less care has been given to the process.

  4. #34
    Senior Member ivica's Avatar
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    Quote Originally Posted by Turner View Post
    Have you tried shimming the diaphragms and/or phase plugs? Phase plugs might need to be deburred, also.
    Some experiments, YES.
    http://www.audioheritage.org/vbullet...l=1#post319363

    I can agree with grumpy's words:

    ".......My experience has been that in using cleaned/undamaged (whether used or new) diaphragms,
    and with repeatable monitoring of frequency response while carefully snugging up the phase plug,
    one can get what I would call reasonable agreement between these drivers (sufficient to not muck
    up imaging or balance)
    ..........."

    for the single driver, but between different drivers, I haven't been so skilful, to get such results.

    Regards
    Ivica

  5. #35
    Junior Member Turner's Avatar
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    Quote Originally Posted by ivica View Post
    Some experiments, YES.
    hahaha, YES, I had a feeling that response was coming. Thanks for that link, lots of great info! We mainly used shims for aftermarket diaphragms - which should be avoided at all costs (aftermarket diaphragms) - paper, tape, foil, anything that worked, well, kinda. Ah, the good old days!

    I just checked 2 sets of silver 2405 diaphragms. I think I see the crease mentioned in the OP, but it doesn't look like it was made with a ballpoint pen, not at 10x anyways.

    Here's a set of 2405H with new D16R2405 installed last spring, tested this AM. Red might need some attention.

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  6. #36
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    FR variability

    Quote Originally Posted by grumpy View Post
    My experience has been that in using cleaned/undamaged (whether used or new) diaphragms,
    and with repeatable monitoring of frequency response while carefully snugging up the phase plug,
    one can get what I would call reasonable agreement between these drivers (sufficient to not muck
    up imaging or balance).

    Otherwise, the phase "dog's breakfast" would apply to the comparison plots I've seen when
    less care has been given to the process.
    Here is a plot of FRs (2404H or H-1) recorded with the same magnet structure, the same inner parts of the horn,
    and the same PWT (to avoid positioning and angle problems with usual FR recording) in place of the outer horn.
    Two pics, with different zoom factor.

    Yel: 2405 dia, small (1.6") phase plug...
    Vio: 2405 dia, big (1.71") phase plug...
    Grn: 2402 dia, small (1.6") phase plug...
    Gry: 2402 dia, big (1.71") phase plug...

    Both dias original JBL, older, mfrd. before 1995

    Name:  2404_all.jpg
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    the last, grey line, is essentially the same as the one I posted some time ago, and I disassembled the tweeter many times since.
    Therefore I am able to get the similar results after remounting.
    But to be sure, I will repeat this mounting/remounting several times more
    to see the variability resulting from the mounting procedure.
    I will post also all results.
    PS: The vertical axis is still not calibrated, so absolute SPL shown is still not correct.

  7. #37
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    So here are results of the next experiment.
    After trying several times more, I chose next combination
    :2402 dia, big (1.71") phase plug, remounted it several times
    and got the same FR, within 1 db from each other.

    Then I repeated it 4 times more, but with some variations.

    1st variation (blue and gray lines): Outer horn (and PWT, together with microphone)
    is mounted rotated for 0 and 180 deg. lines are similar, except that one line falls smoothly at 20 kHz,
    and the other shows a dip and a peak of cca 5 dB.
    There is also a dip around 15 khz, which exists also with 2405 dia.


    2nd variation (violet and green lines):
    Examining inner and outer horns on 6 of 2404Hs, I found that casting of the inner aluminum horn
    does not fit perfectly to the outer plastic horn. all of them had a small gap between the two parts, of cca 0.5 mm.
    I deliberately made my PWT to replicate that small gap, and got it to be a bit thicker,
    cca 0.7 mm- slightly worse than the worst of my 6 2404s .
    All previous measurements include that gap.
    Violet and green lines were recorded with a gasket filling that gap.
    The dip at 15 kHz allmost dissapeared. I conclude from that that imperfection of the casting
    forms an acoustic resonator or filter responsible for this dip.
    I repeated with the gasket in place also rotation of the inner horn and pwt and got the same dip/peak around 20 kHz.
    I conclude that this peak is result of a circumferential mode (HOM) at this frequency, but this requires more investigation.

    For the moment I am satisfied with the finding that EVERY 2404 could benefit
    from placing a gasket between two parts of the horn.
    Gasket should be 13 mm ID, 22 mm OD, and 0.7 mm thick.

    PS to my post of 18 dec: I chose to have my PWT end tightly plugged.
    It does not change results of measurements,
    but prevents external noise to enter the tube during measurement and give wrong results.
    (Don't ask how I.....)
    Name:  2404_2.jpg
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  8. #38
    Senior Member ivica's Avatar
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    Quote Originally Posted by Predrag Dukic View Post
    So here are results of the next experiment.
    After trying several times more, I chose next combination
    :2402 dia, big (1.71") phase plug, remounted it several times
    and got the same FR, within 1 db from each other.

    Then I repeated it 4 times more, but with some variations.

    1st variation (blue and gray lines): ...............

    2nd variation (violet and green lines):
    ...........For the moment I am satisfied with the finding that EVERY 2404 could benefit
    from placing a gasket between two parts of the horn.
    Gasket should be 13 mm ID, 22 mm OD, and 0.7 mm thick.

    PS to my post of 18 dec: I chose to have my PWT end tightly plugged.
    It does not change results of measurements,
    but prevents external noise to enter the tube during measurement and give wrong results.
    (Don't ask how I.....)
    Name:  2404_2.jpg
Views: 990
Size:  53.1 KB
    Hi Predraze,

    It is nice to see on this form such systematic and informative works, with very useful suggestions. Unfortunately PWT tube is fare to be available for most of us, but I believe that such behavior of mechanical imperfection in the driver production may have an influence to the driver impedance, that is fare much easier to be measured for the many of us. This is only my expectation, unfortunately I can not prove it, but may be, You can, and may be , You can give us us some suggestion on how we can improve our drivers.

    Regards Ivica

  9. #39
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    Ease of measurement

    Quote Originally Posted by ivica View Post
    Hi Predraze, It is nice to see on this form such systematic and informative works, with very useful suggestions. Unfortunately PWT tube is fare to be available for most of us, but I believe that such behavior of mechanical imperfection in the driver production may have an influence to the driver impedance, that is fare much easier to be measured for the many of us. This is only my expectation, unfortunately I can not prove it, but may be, You can, and may be , You can give us us some suggestion on how we can improve our drivers. Regards Ivica
    Ivica,

    It is in fact much easier to make a PWT than to do a proper measurement setup with speaker and microphone on the stand.
    In a "normal" measurement You need a quiet place with some free space, You have to measure angles and distances, etc. etc..
    My "setup" is lying on the floor some feet away of the PC....
    Here is a photo. It took an hour or two to assemble, excluding time to go to the plumbers shop...
    It is true that I work at the University, but this is my hobby project, and I didn't need any "heavy artillery" from there to start measurements...
    Also ARTA program is freely downloadable, in fully operational version. Only saving measurement files does not work with unregistered version...
    The key feature of ARTA is the "gated measurement".
    It allows to get the FR from the mic, before the reflection of the pulse arrives from the acoustically unmatched end of the tube.
    The process is easy to understand from the second pic.
    The green portion of the impulse response is used to calculate FR, the rest (red) is simply cut and unused.

    My next PWT is going to be 1", for my 2425s. I will also use green 40 mm PP plumbing tube, which has just about right internal diameter (1,025 in).


    Cost of 1/2" PWT setup:

    ARTA- free
    Meas. Mic. 3 usd (Panasonic WM61a)
    Green PP plumbing tube 3 usd
    Black PP flange 5 usd
    ----------------------
    Total - 11 usd, excluding cable, drop of epoxy glue, and three cable ties.

    1" version is going to cost something around 20 usd.
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  10. #40
    Senior Member ivica's Avatar
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    Quote Originally Posted by Predrag Dukic View Post
    Ivica,

    It is in fact much easier to make a PWT than to do a proper measurement setup with speaker and microphone on the stand.
    In a "normal" measurement You need a quiet place with some free space, You have to measure angles and distances, etc. etc..
    My "setup" is lying on the floor some feet away of the PC....
    Here is a photo. It took an hour or two to assemble, excluding time to go to the plumbers shop...
    It is true that I work at the University, but this is my hobby project, and I didn't need any "heavy artillery" from there to start measurements...
    Also ARTA program is freely downloadable, in fully operational version. Only saving measurement files does not work with unregistered version...
    The key feature of ARTA is the "gated measurement".
    It allows to get the FR from the mic, before the reflection of the pulse arrives from the acoustically unmatched end of the tube.
    The process is easy to understand from the second pic.
    The green portion of the impulse response is used to calculate FR, the rest (red) is simply cut and unused.

    My next PWT is going to be 1", for my 2425s. I will also use green 40 mm PP plumbing tube, which has just about right internal diameter (1,025 in).


    Cost of 1/2" PWT setup:

    ARTA- free
    Meas. Mic. 3 usd (Panasonic WM61a)
    Green PP plumbing tube 3 usd
    Black PP flange 5 usd
    ----------------------
    Total - 11 usd, excluding cable, drop of epoxy glue, and three cable ties.

    1" version is going to cost something around 20 usd.
    Hi Predraze,

    Many thanks for the explanation of your measurements 'setup'. Such equipment is good if the decision is to measure smaller amount of different drivers, but me , as hobbyist, sometime different models are of interest. I know that ARAT is valuable software tools, where one of the possible solution getting F/R response is calculated based on the "truncation of the impulse response".

    Been aware that the reflections can produce errors in the F/R measurements (in ordinary environments that is for hobby-user applicable) I have made my "own" procedure based on the 'Using Complementary binary sequences' in order to get impulse response of the driver, and then applying 'time domain windowing, or truncation', in order to calculate F/R. Usually I have done my measurements in my living room, with the driver positioned about 75cm from the floor ('looking at the ceiling') and microphone about 100cm (3.3 ft) apart, hanged from the ceiling. While the nice outside weather, sometime, I did my measurements outside the house. All in all, the measurements, can be repeated with 'repeatable accuracy. from the measurements to the measurements, and from day to day' . With such, the influence of the driver's horn can be get too, and try to "predict" what would the listener can expect to get.

    Owing to use the two complementary (Golay

    http://www.isg.rhul.ac.uk/~kp/golaysurvey.pdf ,

    http://ompldr.org/vOHkxNw/fulltext.pdf

    http://jenshee.dk/signalprocessing/mls.pdf -"Impulse response measurements using MLS"

    http://cnx.org/content/m15947/latest/

    )

    sequences of about 256kbits (2^18) length each, I can get quite good resolution, which then can be "averaged" in standard 1/N octave presentation.

    I can agree with You that for the scientific research your procedure is much more applicable, but I would stay in belief that such great variation in F/R You have presented us (around 15kHz), has to make some kind of the "reflection" to the impedance characteristics.

    Regards

    Ivics

  11. #41
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    Horn vs. PWT

    Ivica,

    Yes, my project started as a hobby effort, but now I am getting more and more involved.
    Solving the problem of HOM in tweeters is indeed worth a scientific project,
    and it wouldn't be the first one to be undertaken. People at JBL certainly know more about it,
    but the nature of their work requires to keep hardly acquired knowledge to themselves.

    As for the other remarks, I wanted to separate sources of problems, and PWT is a way to do it.
    I will get back to horn issues later.

    I am not familiar with the algorithm used in ARTA to generate noise signal,
    but I suspected it is some kind of pseudo random sequence.

    I might ask the author of ARTA personally some day.
    It is easy to me because we visit the same coffee shop at FESB faculty almost every day.
    In fact, I am not really interested.... One can not know everything...
    I will limit myself to speaker construction and "hardware". I leave software development to others...
    Thanks for the links anyway.
    As for the impedance changes due to the dip at 15 kHz....
    Hmmmm..... let's suppose that the measuring microphone is mounted farther from the throat, not in the first 1 inch or so...
    And let's suppose also that You have a nice, loosely packed plug of a long hair wool somewhere in between.

    This plug might attenuate the sound wave ...let's say 6 dB, or to say, it could "eat" 75% of the driver's efficiency.
    We might expect that the real impedance of this driver with Re of 6ohm falls from let's say 8 ohms to 6.5--
    a corresponding 75% drop in Rt . But, the impedance will in reality remain 8 ohms (25% efficiency),
    and the effect of a well matched acoustic damping or attenuation will pass unnoticed.
    Microphone is going to read 75% decrease of the SPL, but impedance measurement will stay the same.
    That is because the energy is not REFLECTED back to the driver, but simply DISSIPATED in the form of heat.
    Therefore not every dip in FR is visible in the impedance plot,
    but I will do a thorough check of this question one of the next days,

    and I will keep You informed on the results and the progress.

  12. #42
    Senior Member 1audiohack's Avatar
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    This is very nice work Predrag Dukic. The throat leak is a real find. Thank you for sharing your work!

    All the best,
    Barry.
    If we knew what the hell we were doing, we wouldn't call it research would we.

  13. #43
    Senior Member ivica's Avatar
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    Quote Originally Posted by Predrag Dukic View Post
    Ivica,

    ......
    As for the impedance changes due to the dip at 15 kHz....
    Hmmmm..... let's suppose that the measuring microphone is mounted farther from the throat, not in the first 1 inch or so...
    And let's suppose also that You have a nice, loosely packed plug of a long hair wool somewhere in between.

    This plug might attenuate the sound wave ...let's say 6 dB, or to say, it could "eat" 75% of the driver's efficiency.
    We might expect that the real impedance of this driver with Re of 6ohm falls from let's say 8 ohms to 6.5--
    a corresponding 75% drop in Rt . But, the impedance will in reality remain 8 ohms (25% efficiency),
    and the effect of a well matched acoustic damping or attenuation will pass unnoticed.
    Microphone is going to read 75% decrease of the SPL, but impedance measurement will stay the same.
    That is because the energy is not REFLECTED back to the driver, but simply DISSIPATED in the form of heat.
    Therefore not every dip in FR is visible in the impedance plot,
    but I will do a thorough check of this question one of the next days,

    and I will keep You informed on the results and the progress.
    Hi Predraze,

    I agree with You that if there is some kind of acoustic absorber out of the driver's horn (or any other kind of the obstacle) , that can make an interference with the sound propagation ( all of us are aware of the baffle edges interferences, not to mention any other nearby surfaces or objects), and all of that would make great variations in F/R response ( that is "present" at the microphone point in space).
    But here I want to emphasize that I would expect that the driver's and its horn's construction and their imperfection could produce some kind of "signature" in the impedance characteristic that can be compared with expectable (correct) one. My expectation is based on MY belief that any kind of "large imperfections" ( the meaning "LARGE", here is disputable), would produce a different amount of reflection energy (compared to the "good" part), and that would, MAY BE, produce some kind of visible behavior in the impedance curves. All that is just my expectation, but may be You can find the answer.

    Regards
    Ivica

  14. #44
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    Improvement.

    Quote Originally Posted by 1audiohack View Post
    This is very nice work Predrag Dukic. The throat leak is a real find. Thank you for sharing your work! All the best, Barry.
    Thanks... Just my two cents. A cheap way to get an audible difference. I know that some people give an arm and a leg for much smaller improvement.

    The "leak" is not a really a leak through... The aluminum part has a circular ridge of cca 1" diameter around throat.
    The plastic part, outer horn has a groove of the same diameter.
    They should fit one into another, but the ridge is a bit higher than the groove depth, and a slightly different diameter.
    That misfit leaves a small space, in a form of a thin circular chamber around throat.
    It is possible to see the interface between the two from the outside, looking through horn to the phase plug.
    And with a good illumination it is possible to see this thin gap.
    Thickness of this gap varies, depending on Your luck, from 0.1 mm (one sheet of paper) to 0.5 mm,
    which is the thickiest that I saw in my small statistical sample of 6 tweeters.

  15. #45
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    Generalizations

    Quote Originally Posted by ivica View Post
    Hi Predraze,

    I agree with You that if there is some kind of acoustic absorber out of the driver's horn (or any other kind of the obstacle) , that can make an interference with the sound propagation ( all of us are aware of the baffle edges interferences, not to mention any other nearby surfaces or objects), and all of that would make great variations in F/R response ( that is "present" at the microphone point in space).
    But here I want to emphasize that I would expect that the driver's and its horn's construction and their imperfection could produce some kind of "signature" in the impedance characteristic that can be compared with expectable (correct) one. My expectation is based on MY belief that any kind of "large imperfections" ( the meaning "LARGE", here is disputable), would produce a different amount of reflection energy (compared to the "good" part), and that would, MAY BE, produce some kind of visible behavior in the impedance curves. All that is just my expectation, but may be You can find the answer.

    Regards
    Ivica

    Ivica, I agree, just ..... I do not accept any generalizations withouth checking...

    But this dip really dees reflect to the impedance curve....
    Look up my first post with impedance graph...

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