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Thread: JBL Master Reference Monitor

  1. #1426
    Senior Member pos's Avatar
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    That port measurement looks very clean!
    There is something happening around 120Hz, that also shows on my measurement (could be a coincidence though).

    Regarding tuning measurement, here are two methods:
    A simple and precise one described by sebackman here: http://www.audioheritage.org/vbullet...l=1#post388697
    A less precise but even simpler one that I described here: http://www.audioheritage.org/vbullet...l=1#post388714

  2. #1427
    Senior Member ivica's Avatar
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    Quote Originally Posted by Synthetic View Post
    I'm using a Crown XLI 2500 per speaker.

    I'll get an impedance measurement, but it'll be a little while. You might have to explain a little more on the second option. How would I do that?

    I did a close mic of the port. Looks like 22Hz?


    Also I took more close mic measurements at 3 different spots on a different m2. They all have that peak at ~282Hz.
    Hi Synty,

    Looking at the port responses it seems to me that a kind of internal (in the box) resonances are established round about 130Hz and 260Hz . It can be seen if compare either responses on the same graph.
    I wonder what would be the port response if the ports are mounted somewhere in the mid (above the bass) positions, such as 4344, or 4345.
    On some other DIY project, almost the same behavior can be seen.
    The internal sound become out of phase relative to bass front sound from 130Hz, while round 260Hz becomes in-phase.
    I believe that POS can give us better explanations.
    I wonder can such resonance or standing waves be reduced by adding more absorbent material or put a kind of angled surface in the box

    Regards
    Ivica
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  3. #1428
    Senior Member ivica's Avatar
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    Quote Originally Posted by pos View Post
    Ivica,
    A close mic woofer measurement should be immune to boundaries effects.
    Albeit not representative its purpose here was to compare the reponse of our clones with an original M2, as a target.

    Regarding port measurement, most of the resonances that I measured under ~150Hz through the woofer also appear there:
    Hi POS,

    Interesting, or better to say strange driver (only) response round 30Hz, I have no idea what can make such notch round 30+ Hz, so sharp....
    Have You ever tried putting port in the upper box place (such as 4345 or 4344...)

    May be each port resonance, as You have two ports.

    Regards
    Ivica

  4. #1429
    Senior Member pos's Avatar
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    Hi Ivica

    Quote Originally Posted by ivica View Post
    I wonder can such resonance or standing waves be reduced by adding more absorbent material or put a kind of angled surface in the box
    I'd love to try this kind of arrangement on the back wall: http://www.thomann.de/gb/the_takusti...set_big_36.htm
    (ideally using basotect, maybe by cutting this kind of corner bass trap in slices: http://www.thomann.de/gb/the_takusti...e_480_gray.htm )

    Quote Originally Posted by ivica View Post
    Interesting, or better to say strange driver (only) response round 30Hz, I have no idea what can make such notch round 30+ Hz, so sharp....
    I believe this is a mode of my room (~29.5Hz, 1-0-0 mode of my 5.8m long room).
    That is why I did not try to correct anything under ~150Hz
    Some of the resonances I measure through the port in this range are indeed in line with the room mode I calculated.
    And yes, my speakers are placed in corners

    Have You ever tried putting port in the upper box place (such as 4345 or 4344...)
    nope, I suck at building/modifying boxes so I cannot experiment much there

  5. #1430
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    Quote Originally Posted by pos View Post
    That port measurement looks very clean!
    There is something happening around 120Hz, that also shows on my measurement (could be a coincidence though).

    Regarding tuning measurement, here are two methods:
    A simple and precise one described by sebackman here: http://www.audioheritage.org/vbullet...l=1#post388697
    A less precise but even simpler one that I described here: http://www.audioheritage.org/vbullet...l=1#post388714
    With method 2, I find cone excursion to be a minimum at about 27Hz. Air movement from ports was a maximum at around 22Hz. Excursion from the drivers went up a lot after 22Hz. I'll get some resistors and try method 1.

  6. #1431
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    Quote Originally Posted by ivica View Post
    Hi Synty,

    Looking at the port responses it seems to me that a kind of internal (in the box) resonances are established round about 130Hz and 260Hz . It can be seen if compare either responses on the same graph.
    I wonder what would be the port response if the ports are mounted somewhere in the mid (above the bass) positions, such as 4344, or 4345.
    On some other DIY project, almost the same behavior can be seen.
    The internal sound become out of phase relative to bass front sound from 130Hz, while round 260Hz becomes in-phase.
    I believe that POS can give us better explanations.
    I wonder can such resonance or standing waves be reduced by adding more absorbent material or put a kind of angled surface in the box

    Regards
    Ivica
    Interesting find. I didn't notice that.
    Name:  PortandWooferCloseMic.jpg
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  7. #1432
    Senior Member ivica's Avatar
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    Quote Originally Posted by pos View Post
    Hi Ivica
    ........
    I believe this is a mode of my room (~29.5Hz, 1-0-0 mode of my 5.8m long room).
    That is why I did not try to correct anything under ~150Hz
    Some of the resonances I measure through the port in this range are indeed in line with the room mode I calculated.
    And yes, my speakers are placed in corners
    ....
    Hi POS,

    If You have applied proper 'windowing' row-data, then no wall reflection would make any interference with the driver signal,
    so room influence can be neglected. I have no idea about measuring procedure You have done.
    As You know, I prefer to use MLS like response (delta impulse - response: "Impulse Resposnse Measurements using GOLAY Complementary Sequences"), so that influence of the wall reflection can be 'controlled'.

    Regards
    ivica

  8. #1433
    Senior Member pos's Avatar
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    I use a sweep that HOLM handles using Farina's method. I believe REW does the same.
    This has some advantages over a MLS signal of equivalent length.

    I also do not use any kind of windowing (outside of the length of the measurement signal itself of course, which leads to 128k taps at 48kHz here) and something like 1/24oct to 1/48oct frequency smoothing.

  9. #1434
    Senior Member ivica's Avatar
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    Quote Originally Posted by pos View Post
    I use a sweep that HOLM handles using Farina's method. I believe REW does the same.
    This has some advantages over a MLS signal of equivalent length.

    I also do not use any kind of windowing (outside of the length of the measurement signal itself of course, which leads to 128k taps at 48kHz here) and something like 1/24oct to 1/48oct frequency smoothing.
    Hi POS,

    I use Farina's method when THD measurements are of interest, but if I prefer to do higher resolution MLS seems to me more applicable, especially if outside measurements can be done....

    One question, have You ever seen some explanation about port position in the box, especially if one dimension of a box is 'too long' relative to the others two.

    Regards
    ivica

  10. #1435
    Senior Member pos's Avatar
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    Sweep measurement seem to give me a lower noise floor and better repeatability than MLS in HOLM, but I must admit I did not dig the subject too much.

    I don't know about any rational regarding port placement.
    Some say to keep them close to the woofer, but given the length of these wave I cannot imagine the summation not being in phase anyway.
    It is also said that having the ports on the back of the enclosure is a good idea to avoid the colored midrange to leak through it, but I have no first hand experience/comparison here either.

  11. #1436
    Senior Member ivica's Avatar
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    Quote Originally Posted by pos View Post
    .....
    I don't know about any rational regarding port placement.
    Some say to keep them close to the woofer, but given the length of these wave I cannot imagine the summation not being in phase anyway.
    It is also said that having the ports on the back of the enclosure is a good idea to avoid the colored midrange to leak through it, but I have no first hand experience/comparison here either.
    Hi POS,

    You are right if talking about the LF resonance, what is the reason port is applied, but at the higher frequency, the best would be that the port would be closed, unfortunately it is not possible to be, so some reflections in the box, may be is the reason of the peaks and deeps in the total FR resp.

    regards
    ivica

  12. #1437
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    Quote Originally Posted by pos View Post
    It is also said that having the ports on the back of the enclosure is a good idea to avoid the colored midrange to leak through it, but I have no first hand experience/comparison here either.
    GT stated this once on this site. Ports on the back are better than ports on the front.

  13. #1438
    Senior Member pos's Avatar
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    So that is the reason why you sold your 4365 and got LSR305 instead?

  14. #1439
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    How much coloration can a port add?

    Are we talking resonances?

    Also couldnt we take precautions in lining the cabinet to avoid any more colorations from reflections and port noises?

  15. #1440
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    Quote Originally Posted by chrapladm View Post
    How much coloration can a port add?
    Enough for JBL to come up with the PR8, PR10, PR300 and PR15x...

    The M2 is quite literally "ducted ports done right".

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