Intriguing relief on 2405 diaphragms

[jeanmichelt]

Hi everyone,
to clean the gaps of my 2405 alnico tweeters, I've disassembled them. Strangely, they both present an almost symmetrical, concave relief on more or less 50% of the internal part of the diaphragm. I do not see any external cause for this relief and I have not found any other example of this situation on the net.
I have attached a picture of the diaphragm with arrows. Does anyone have an opinion on the matter.
Thanks a lot in advance for any reply.
Jean-Michel

[JeffW]

There was a story about JBL assembly techs running a ball point pen around the edge of those diaphragms to give better response, but it seems that would have been around the entire circumference. Yours sure looks like a pen has been used on it, maybe they figured out a way of tweaking the alignment with the pen and only used it on part, hard to say.


Hi Dave,

I've never measured any 075s, but by reputation they are not particularly smooth or extended in response. The moving mass is pretty heavy for a tweeter, so the rolloff on top is to be expected I suppose. From your graph it looks like the spec would be plus or minus 3dB from 3700Hz. to 14kHz., not too bad really, and pretty close to JBL's published specs for the similar 2402H:

http://www.lansingheritage.org/html/jbl/specs/pro-comp/2402.htm

A funny story- at some point years ago someone in the factory discovered that the 075 would measure smoother if a dry ball point pen was run around the diaphragm edge to introduce a bit of a compliance. This procedure was then incorporated into the factory assembly process.


Thought you might get a kick out of this, hopefully it's new info for you.
One of the guys I work with (he shall remain nameless) working at JBL Pro for over 20 years and over the years went from the line to engineering. Anyway, he told me about the production of the ring radiators 2405 etc. The production yield was very low. Fall out because of the high standards for frequency response JBL required was killing them. Then a lady on the line figured out a way of fixing the diaphragms that did not pass. The tooling guys tried and tried to implement this fix into the die but they just couldn't get it to work as well as her method, she had the magic touch. And what was this method? She would simple run the tip of a ball point pen around the diaphragm, this added crease made the diaphragms play very smoothly and within spec. I don't know if this is true but he swears it is and says that you can see it on many of the old diaphragms if you look closely. It's a great story, especially how the tool guys couldn't duplicate the results, a personal touch.

From another site, but this guy did work for JBL


Those curves are from a really good one. The ring radiators vary all over the place. The 3 peaks that make up the response can be quite nasty. They did get somewhat better when they trained a lady to go around the perimeter with a ball point pen to crease the edge just inside the suspension point. (I kid you not. You can usually see the blue ink line.)


Anyhow, DSP on the actual measured individual responses should be a significant improvement. I would measure and average across, say +-15 degrees and EQ to that.


David S.

[1audiohack]

I have seen many of the “ball formed” diaphragms but I don’t recall seeing what you have there.

Interesting for sure.

Barry.

[Robh3606]

Strange it's just on half and not all the way round??

Rob :)

[jeanmichelt]

Thank you very much for your explanations; do you think it would be better to complete the circle ?
Regards.
Jean-Michel

[edgewound]

Thank you very much for your explanations; do you think it would be better to complete the circle ?
Regards.
Jean-Michel

If it's working well as is, I wouldn't experiment, since new Northridge factory diaphragms aren't available, unless someone has a stash of NOS.