Measuring compression drivers. Questions.

[kelossus]

Hi

This weekend I will be installing a "lightly used" pair of 2441 diaphragms into my 4350 system. Once installed I would like to measure the CD's to ensure the diaphragms and the drivers are good to go.

Am I able to do this with the CD's hooked up to the passive network? I would not connect any of the drivers, just the 2441's. I understand the network may do something to the FR but as long both sweeps are similar can I take that as an indication that all is well? Is REW the right software to use to do the sweeps?

The diaphragms have a few tiny dents and one larger crease that was apparently from the previous install. I am hoping that these imperfections don't cause any anomalies. These diaphragms are hard to get and the drivers are quite difficult to remove from the 4350 cabinet. I only want to do this once.

[Robh3606]

I would run an impedance sweep on them no network and also do a low power sine sweep to listen for any issues. Thy should match up pretty close. Just put a big cap in series to protect against DC. I will try to find the JBL installation instructions and post. It should look something like this. Black curve is a 4" on a waveguide as an example. Figure a couple of peaks on the low end and smooth above.



Rob

[kelossus]

Just a follow up. I ended measuring the drivers still within the passive networks. I just wanted to confirm that both the diaphragms measured fairly closely. Sweeps were taken within the drivers operating range within the network, 1k-10k.

I had never opened up my 2440's so I was not aware what diaphragms were installed. I knew they had been cracked open in the 90's. Much to my pleasure I found a mint pair of 375 diaphragms. These speakers had not been used in nearly 20 years so I can assume the 375 diaphragms have had little to no use

One of the 2441 diaphragms does not look good at all. I switched diaphragms between drivers to rule out the driver itself. The bad diaphragm measured consistent no matter what driver it was in. It appears to have a fairly large dip easily visisble in the sweeps.

Measurement distances were not perfectly controlled but I took multiple sweeps and the results always looked the same. All sweeps were taken with the drivers attached to the 2311 horn outside the box sitting on a chair. 1/48 smoothing applied to sweeps.

Any thoughts on the sweeps? Is one of the diaphragms just worn out? They do have a few marks on them but oddly enough the one with more substantial creasing/marks measures the best.

2440 BOTH

2441 LEFT

2441 RIGHT

2441 BOTH

[kelossus]

Also on the sweeps which show both drivers I used the "Separate" Function in REW. There is not a drop in output between the diaphragms.

[Mr. Widget]

Looking at those curves, since the vertical scale is set at 20dB per each delineation, any noticeable difference is actually quite extreme. If the software allows you to run it with 5dB per delineation you will be able to compare them more accurately. Any difference of 1-2 dB between the two diaphragms will be quite audible and will very likely be out of spec.


Widget

[kelossus]

Could the uneven response of the diaphragm be a result of the coil rubbing somewhere?

[Mr. Widget]

Could the uneven response of the diaphragm be a result of the coil rubbing somewhere?

Could be. It could also be due to improper centering/alignment, or a partially damaged diaphragm due to wear/age.


Widget

[kelossus]

Could be. It could also be due to improper centering/alignment, or a partially damaged diaphragm due to wear/age.


Widget

Is the response of the "good" 2441 diaphragm what you can expect of this driver functioning properly?

Just checked and I can't change the delineation of the sweeps.

[Earl K]

Is the response of the "good" 2441 diaphragm what you can expect of this driver functioning properly?

Just checked and I can't change the delineation of the sweeps.

The response of your "good" 2441 diaphragm looks to be normal.

Its rising response will flatten out somewhat when you put the 2308 ( slant-plate ) in front of the 2311 horn.

You might want to upload your REW file here so others can look at it for you.

Simply "Zip" it first so that the forum software allows it as a proper file type to be uploaded.



PS; There are a couple of places to change the vertical db scaling ( one is within a small icon that has 2 sets of arrows overlaying a rectangular box ).

PS2; The "bad" response is so off it has me thinking its not a bona-fide JBL diaphragm ( you might want to post a few pics so that others can help with that determination ).

[kelossus]

The response of your "good" 2441 diaphragm looks to be normal.

Its rising response will flatten out somewhat when you put the 2308 ( slant-plate ) in front of the 2311 horn.

You might want to upload your REW file here so others can look at it for you.

Simply "Zip" it first so that the forum software allows it as a proper file type to be uploaded.



PS; There are a couple of places to change the vertical db scaling ( one is within a small icon that has 2 sets of arrows overlaying a rectangular box ).

PS2; The "bad" response is so off it has me thinking its not a bona-fide JBL diaphragm ( you might want to post a few pics so that others can help with that determination ).

Thanks for the reply mate.

I noticed on the 375 diaphragms that came out of the drivers they had shims underneath the plastic ring. Could the lack of shims on the "bad" diaphragm cause this kind of uneven response? Also strange that the 2441 diaphragm that measures well did not require any shims to give a decent sweep.

Diaphragms are both JBL's. I will attach pictures of them below as well as the REW files compressed as you suggested.

2441 REW SWEEPS @ https://ufile.io/w1dvztyg

[1audiohack]

I would also say you're done.

What made it happen for you?

Barry.

[kelossus]

Had a bit of a play and this is where I'm at. I did try to keep the microphone to driver distance the same but they may have been out by a few cm here and there. I think this result is good enough? Would you be happy to call it a day with these sweeps?

Strangely enough I could not get a consistent distortion curve. For whatever reason these last sweeps I took today look worse then my initial sweeps. Even on the driver that hasn't been touched.

[Earl K]

Had a bit of a play and this is where I'm at. I did try to keep the microphone to driver distance the same but they may have been out by a few cm here and there. I think this result is good enough? Would you be happy to call it a day with these sweeps?

Strangely enough I could not get a consistent distortion curve. For whatever reason these last sweeps I took today look worse then my initial sweeps. Even on the driver that hasn't been touched.

I'd recommend that you now stop fiddlin with diaphragm fitment ( those 2 are as matched as anyone else might achieve ). So, Congratulations!

Distortion specs won't be reliable until measurements are made above a certain level of background room noise ( iow; the "room noise" makes the results unreliable when the drivers distortion is below that level ).



PS; typically one displays the frequencies in a logarithmic scale ( you've chosen linear ).

[kelossus]

Also how do you determine if you should add/remove a shim?

[Earl K]

Also how do you determine if you should add/remove a shim?

By measuring FR response ( before & after ).

One can make and use paper shims .