JBL Master Reference Monitor

[srm51555]

Here we go, given this is the first time ever taking measurements they are probably not the most professional or even correct but got to start somewhere. Microphone was in the center of the woofer and 10cm away from the dust cap.



and



Let me know if they are what you are looking for or they are correct and you feel something is off in the speaker itself.

Thanks,
Scott

[pos]

Thank you Scott!

I should have been more precise on the type of measurement required here.
For a pressure measurement the mic needs to be like 2 or 3cm from the cone, and preferably not on the dustcap but either on the dustcap/cone junction or on the cone itself.
Regarding the measurement signal, 256k is more than enough, but if you use a sweep you have to let it start lower in frequency, like 10Hz.
I use HOLM so I cannot give you an exact set of parameters for REW. I use a 48kHz sampling rate, a 128k length, and a 10Hz-24kHz sweep signal.
No need for high SPL. You can also disconnect the horn to avoid any problem.

Then what I would need would be an export of the measurement impulse response, either in wav, txt, or the mdat file.

[srm51555]

Thank you Scott!
Regarding the measurement signal, 256k is more than enough, but if you use a sweep you have to let it start lower in frequency, like 10Hz.
I use HOLM so I cannot give you an exact set of parameters for REW. I use a 48kHz sampling rate, a 128k length, and a 10Hz-24kHz sweep signal.
No need for high SPL. You can also disconnect the horn to avoid any problem.

Using the parameters called out above and with the microphone 2CM from the cone (location=third rib counting from the bottom of the dustcap) here is what I received:



I ran out of time to try and figure out how to change the file name to load the actual file. REW wasn't letting me just add a ".txt" to the end and file name change through windows wasn't bringing up anything else either. I will try to figure it out later today. Also the foam thing was on the mic when taking measurements.

Thanks,
Scott

[johanwholst]

Scott:
can you change the scale on the Y axis to show 5dB between lines? Use the looking glass in top left corner or in settings on right.
20dB is too coarse to show anything meaningful unfortunately

[bubbleboy76]

Using the parameters called out above and with the microphone 2CM from the cone (location=third rib counting from the bottom of the dustcap) here is what I received:



I ran out of time to try and figure out how to change the file name to load the actual file. REW wasn't letting me just add a ".txt" to the end and file name change through windows wasn't bringing up anything else either. I will try to figure it out later today. Also the foam thing was on the mic when taking measurements.

Thanks,
Scott

You have to select the measurement, and then "export as impulse response" (or something like that) in one of the top menues.

[pos]

Scott, there are several strange things going on with your measurement.

With a close mic measurement the response should go downward above say 100Hz.
Furthermore, with the 36dB/oct crossover the response should be going downward even more above 800Hz.
And then with the bass reflex loading and the mic away from the port the reponse should also go downward under ~40Hz.
The variations also look strange.

Here is an example of the measurement I took, 48kHz, 128k taps, 1/48oct smoothing.

First with the same type of scale you used :


And then with a scale more suited as Johan suggests:


Here the issues around 200Hz/300Hz can clearly be seen, and the factory EQ is in fact making things worse.

[srm51555]

I was receiving some low SPL level errors with REW so I decided to download HOLM and give it a try. The results are somewhat similar but not sure if totally correct.



And with the following parameters. Note: I was not able to get the 128k length for some reason.



Link to File on dropbox was PM'd

Note: the measurement options and info are from the red curve found above. The second blue measurement is on this post by mistake and couldn't remove it.

[pos]

Thank you Scott, this is great!

Here is a close mic measurement of my "clones" (~50cm internal depth, 5cm cotton wool on the back) with factory EQ enabled:


Here is a close mic measurement of Scott's clones (~30cm internal depth, 2.5cm fiberglass on the back) with factory EQ enabled:


Here are the 3 factory EQ points around 200Hz (197.5Hz, 236Hz, 254Hz):


Here is my measurement with these 3 EQ points removed:


Here is Scott's measurement with these 3 EQ points removed:


So it looks like even with a dimensionally accurate clone like Scott's one it is still better to leave these 3 EQ points out...
I would not try to correct the remaining problems in this area with only a single close mic measurement, but it is clear that these factory EQ points are making things worse for our clones (maybe not the 197.5Hz point, but it does not look ideally placed either).
At that point I would love to see a close mic measurement of an original M2.

[bubbleboy76]

Close mic in room vs. mic more far away in anechoic chamber, does it give the same kind of result, I wonder?

[johanwholst]

Close mic in room vs. mic more far away in anechoic chamber, does it give the same kind of result, I wonder?

No. You will not get the port output in a close mic measurement, but you will see the behavior of the driver in the box. It is useful for correction of the driver itself

[pos]

Some of the resonances also show when measuring the output of the port.
I will do some additional measurements at different SPL levels and mic positions to get a better understanding of what is going on here...

When looking at the correction I first thought these EQ were here to address the comb filtering effect of the back wave, but I am not so sure anymore.

[johanwholst]

Some of the resonances also show when measuring the output of the port.
I will do some additional measurements at different SPL levels and mic positions to get a better understanding of what is going on here...

When looking at the correction I first thought these EQ were here to address the comb filtering effect of the back wave, but I am not so sure anymore.

It must be that lower baffle plate

Mid next week I will be able to measure mine as well. Intresting to compare.
But it would be absolutely brilliant if somebody with a factory made M2 could do the same

[srm51555]

Thank you Thomas for your work on this! Out of curiosity, is your baffle like the actual M2, with the woofer and ports offset from the waveguide or like mine. The reason I ask is your measurement in the 500HZ region is a bit smoother than the one I took and was wondering if that is what member hsosdrum was talking about here, or it could be just coincidence. I wouldn't think it would matter at the distance we set the microphone at but who knows.

Thanks,
Scott

[pos]

Those wiggles at 500Hz are just the result of different smoothing settings.

[Synthetic]

New user, but I thought I could help out.

I'm using a factory M2 with pos's rephase generated openDRC presets.