Uneven response on 2440 with new diaphragm

[Mr. Widget]

Here is a link to a thread I posted long, long ago. This was my first pass at measuring speakers. I was using a top flight measuring system with calibrated mic, but I was just beginning to learn how to use it. I have subsequently learned a lot and have been able to make much more meaningful measurements.

That said, the 2441s were essentially brand new. I had sent them back to JBL who at that time would still strip down a driver and rebuild it at their factory.

Take a look at these posts and view them realizing that the person running the measurements was really pretty clueless at that time.

http://www.audioheritage.org/vbullet...BL-2397-Curves



Widget

[Pellets]

Thank you Widget for passing on this information! I have read it with great interest and will try some of the tricks used for measuring the frequency response. I'm not too concerned by the ragged graphics in the measurement but more the shouty nature of the driver/horn combo, a bit like what you could expect from older Lowther drivers. I will renew my efforts to find the original 2311 horns just to have something to compare with.

I have probably seen pictures of the 2397 horn or similar mounted on top of Olympus cabs, they seem too deep to fit inside the grille.

I´ll come back with a better curve if it also sounds better... it takes some time and experience to make the best of REW (or for that matter the Sound Tools app).

Best regards

[Earl K]

@Pellets

You should be able to achieve smoother response than what you are currently showing.

( Using REW ) run a Full-Range Sweep measurement ( using the normal REW sine stimulus ) that starts at 200hz and then goes out to ( at least 22,000hz ).

Then open & look at your Distortion window , it should look something like this ( this is my 2482 with a Radian R1282 diaphragm mounted on the Selenium HL14-50 conical ).




Pay attention to the profile/tracking of the Harmonic Distortion ( HD ) traces > 3HD should always stay below 2HD ( from 1K to 10K ).

Your diaphragm might not be properly loaded/seated ( that'll certainly be the case if you see a spike in 3HD distortion anywhere from1K to 10K that juts//rises above the 2HD trace ).

FYI, the curve that I show is very sweet sounding with Bach Violin Concerto's playing ( even without a woofer working below it ) // so I think you have a couple of problems to sort out ( due to your stated discontent of your setup).

Also, I very much doubt that 2311's will solve your present discontent ( I sold off my 4 2311's ) since owning the Selenium made them acoustically redundant ( my Seleniums were purchased at least 20 years ago ).

- Personally, I would run your 2440/R1245's full-out, past 10K, and then simply cross-in a tweeter above that point.

[Riley Casey]

If you have significant differences in response when the horn / driver combo is measured stand alone versus mounted in the box behind a grille I suspect it's time to start looking at mounting problems creating resonances or bad grille design.

... This time though the driver/horn was mounted in the cabinet and not free standing as in the previous pictures.

The curve looks quite similar to the previous one apart from the very deep notch at 2.2KHz or so - this could be seen on the previous curves but not that deep. Maybe it's exaggerated by the baffle. Anyhow it seems that the response is quite ragged with at least 5dB variation in the passband. ...

Best regards

[Pellets]

Thanks Earl K, that is potentially very interesting. I´ll try to emulate your measurements - were they done at 1 metre distance? I had a look at the test measurement I did when trying to understand REW and it doesn't look like yours at all... OK, I will take the driver and horn out again and do a new more controlled measurement. This weekend hopefully if my back ache disappears. I´ll come back as soon as I have something meaningful to show.

Riley: I measured with the Olympus grilles off. But the horn is mounted at the top of the baffle with many bits and pieces around it possibly disturbing the frequency curve. We´ll see if this will be bettered when the unit is out of the box again.

Best regards,

[Earl K]

Thanks Earl K, that is potentially very interesting. I´ll try to emulate your measurements - were they done at 1 metre distance? I had a look at the test measurement I did when trying to understand REW and it doesn't look like yours at all... OK, I will take the driver and horn out again and do a new more controlled measurement. This weekend hopefully if my back ache disappears. I´ll come back as soon as I have something meaningful to show.

Riley: I measured with the Olympus grilles off. But the horn is mounted at the top of the baffle with many bits and pieces around it possibly disturbing the frequency curve. We´ll see if this will be bettered when the unit is out of the box again.

Best regards,

My measurement was done at 27" ( not 1 Meter // 40 inches ) .

Distance won't matter that much when checking for 3HD distortion ( assuming 1 meter or less ) .

From a subjective ( quality ) listening perspective > you ought to try a 10db ( resistive pad on those drivers ) to see if they mesh better with the woofer below them .
- This ( pad usage ) assumes you can rebalance the system parts after knocking down the 2440 driver level by that much.
- A 16R resistor paralleled across the driver terminals and then another 16R resistor in series to the Hot terminal of the driver gives this approximate 10db pad.

The idea behind the pad is to better align the dynamic qualities of the disparate parts .
- IOW; knock a 110db CD horn driver combo down to 100db ( dynamic potential ) to better track ( dynamically ) with a 95db woofer.

IanMac has reported ( quite recently ) that this is something that Greg T. does ( and from my own experience is entirely correct ) .
- This treatment makes multiple drivers behave just a bit more like a full-range speaker ( so, the common resistor can be your best buddy here > 5 or 10 watt wirewounds would do for now ) .


Here's a look-see ( of my 2482/Radian on the Selenium HL14-50 conical ) with the 10db pad in place ( it's the green trace with 1/24 smoothing done in XSim // while the red trace is the raw response that I captured before manipulation ).
-

[Earl K]

One other thing to try Pellets ( in the search for better SQ > sound-quality ) is to do a passive/active hybrid when dealing with the 2440 Horn combo.

- While still Bi-Amping ( "Y-Split" the drive signal going to the Horn amp before your electronic crossover ) and then use a passive filter on the Horn/Driver combo by implementing something like in the following pic.
- ( balance levels by using the volume knobs on your horn amp )

With the following values you'll get a response curve very much like this ( this is highly listenable IME ).



This offers you an approximate 650hz crossover point ( low I know > but these big 4" diaphragms can handle this "abuse" at home listening levels ) .
- Just adjust your existing electronic crossover ( for the woofer's low-pass ) for this new point.



PS; "R4" + "L2" are out-of-circuit > IOW > they are just unused placeholders equating to straight pieces of wire .

[Riley Casey]

One big short coming of REW ( or perhaps a get what you pay for element ) is the lack of any coherence data. A narrow notch like the one one in your curve at around 2.3 kHz is often a result of cancellations from a variety of possible sources like reflections or vibrations. A coherence curve like that in Smaart will alert you to the reliability ( or lack there of ) of the measurement. I would be suspicious of validity of that notch particularly if it's not duplicated in measurements taken with the horn outside the box.

Hi again. It took a while to sort out the REW software but now it seems I have a grasp of the functionality. After some calibration I ran a curve of the 2440 through my electronic crossover (800Hz-8k@24 dB/oct).

....
Best regards

[Pellets]

Hi all helpful people,
I just want to share some graphs from the latest measurement on my 2440+HL14-50 combo. The unit is now out of the cabinet and resting on a table. Measuring distance 27". Sorry about the levels, I didn't have the possibility to calibrate levels but they are reasonable (not 150 dB...). Frequency sweep 200-22 000 Hz.

First I measured the unit as is. See the graph, the bad notch at 2.3 kHz is gone but there is quite a huge variation across the passband. Also the 3d harmonic distortion is quite high.

I then took out the diaphragm, cleaned both the gap and the coil carefully, inspected everything and put the diaphragm back on its pins. I tried to move it sideways but there is no play. Tightened down the screws gently and put back the lid. Now look at the second graph. Different isn't it. I have no explanation other than sheer luck. So maybe this is as far as I can get with this particular unit or what do you think?

I´ll come back on the other advices you kindly sent me.

Best regards

[Pellets]

Hi. I admit the graphs from yesterday didn't look too good. Here is one with calibrated levels and correct frequency span and also with 5th harmonic distortion. /P

[Tim Rinkerman]

Have you tried changing the tightness of the screws while passing sine waves through the driver? Just because the driver has alignment pins and they are snug in the holes, doesn't necessarily mean the coil is centered in YOUR driver. Electro-Voice used to be notorious for having to ream the locating holes to get the diaphragm to seat properly, no reason not to suspect Radian of being off a few thousandth's here or there.
People tend to take replacing a diaphragm as a "remove and replace" operation, but there is far more to it to do it correctly. You want clean, smooth response at every frequency..but sometimes it takes hours of tightening, retightening, reaming the alignment holes and "tuning" the diaphragm until it's really right. There is definately an audible difference between one that has been done right and one that was just put in and not clearanced and torqued down correctly.

[Mr. Widget]

Are we sure there is a problem here?

I am not familiar with this horn, but these curves don't scream there is a problem to me just looking at them. This horn and driver will never give you a beautiful textbook case curve.


Widget

[Pellets]

Hi again.
Thank you Widget, maybe it doesn't get better than this with a passband variation of around 5 dB. I compared my latest curve to the one Earl K showed in one of his answers and it seems we are pretty close. His driver is about the same as mine but I guess it has a phenolic diaphragm.

The worry I have is that I don't really know why the response improved so much just by me taking out the diaphragm, cleaning gap and coil and putting it back again. I tighten the screws as you would on a cylinder head, crosswise, and I try to tighten evenly. A more scientific approach would perhaps be to borrow a torque screwdriver and see if the trick is to get the screws down as evenly as possible with a reasonable end force.
To randomly adjust the screws and measure the result would probably take a long time and one problem would be how to go back to a previous better setting.

I read in the brochures (or somewhere) on the 2440 that the diaphragms have the advantage of being field replaceable and if this is so I think there has to be a more definitive way to go when doing the job.

I´ll pull out the second 2440 soon to see if it behaves in the same way as the first one. Next step is to try to implement some of Earl K:s suggestions. And finding a torque screwdriver...

Best regards

[Mr. Widget]

His driver is about the same as mine but I guess it has a phenolic diaphragm.

His 2482 driver and aftermarket diaphragm are different from your driver/diaphragm, but also according to the name of the png file the plot is of a driver with HF compensation... it would take quite a lot of compensation to achieve that HF performance from a phenolic driver.

The worry I have is that I don't really know why the response improved so much just by me taking out the diaphragm, cleaning gap and coil and putting it back again. I tighten the screws as you would on a cylinder head, crosswise, and I try to tighten evenly. A more scientific approach would perhaps be to borrow a torque screwdriver and see if the trick is to get the screws down as evenly as possible with a reasonable end force.
To randomly adjust the screws and measure the result would probably take a long time and one problem would be how to go back to a previous better setting.

If you open up a bunch of these, you will see notations with + and - numbers that had to do with shimming and adjusting the diaphragms. Also, sometimes you need to ream out the guide holes to let you wiggle the diaphragm side to side just a bit to get the best alignment... then there are the inherent inaccuracies of the diaphragms themselves. Not all brand new factory built diaphragms are perfect.

After measuring a few drivers and realizing how difficult it is to get them really well matched, I lost interest in repairs and shipped mine back to the factory to rebuild (no longer a possibility) or sought new drivers. Of course that is no longer possible either for so many of these drivers.

At the factory they had a test jig set up to verify performance as they installed the diaphragms... few of us can match that.


Widget

[Earl K]

Yes, my 2482 is loaded with an aluminum domed, Radian diaphragm ( R1282 ). That's why it has the HF response that you see.
- The 2482 with the original phenolic diaphragm drops like a stone above 4.5K ( and no HF compensation can alter that reality ).



Your curve ( seen immediately above ) shows a natural setup where one could run the driver/horn combo full out ( HF wise ) > and then cross in a tweeter just above 10K-12K ( should sound nice ).