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Guido
03-30-2005, 02:16 AM
While changing diaphragms at my 2426 Driver last night I realized for the first time that there are slight milling groves at the top plate exactly where the mounting ring of the diaphragm lays on the top plate!

This means that the mounting ring does NOT seal the rearchamber of the driver from the phase plug!

When I put the D16R2421 dia in I realized that these dias have thin metal plates where the mounting holes are. I suspect that these thin plates provide the same “gap” between dia and top plate in a 2420/2421 driver by “lifting it a bit” as this driver does not have the milling groves.

Mounting the D16R2421 dia in a 2426 will consequently increase the “gap” as we have then milling groves + thin metal plates.

If you look at my measurement in the “Playing with a 2426” thread this does not lead to any anomalies in impedance which make me wonder.

The 2416 for example does NOT have this “gap”

What for the heck is this JBL mystery again? I’m confused as I always thought that the rearchamber of an compression driver is airtight sealed from the phase plug section? Shouldn’t it?

Any explanations would be MUCH appreciated.

Mr. Widget
03-30-2005, 10:08 AM
I am not exactly sure what you are asking. The distance between the inner surface of the diaphragm and the domed surface of the phase plug is critical to the driver's performance. I would assume that JBL diaphragms are standardized but I don't know for sure.

By grooved and not grooved are you talking about a machined step in the top plate? The first drawing has one and the second does not.

Widget

Hofmannhp
03-30-2005, 10:51 AM
....... I realized that these dias have thin metal plates where the mounting holes are. ..... .

Hi Guido,

you mean this:
(I also mentioned about.....maybe for cooling air for the voice coil while the dia is moving.)

HP

top is a 2441, the other is a 2420 dia.
BTW: the thickness of this small metal sheets is 0.4mm or ? inch

Guido
03-30-2005, 02:12 PM
I am not exactly sure what you are asking. The distance between the inner surface of the diaphragm and the domed surface of the phase plug is critical to the driver's performance. I would assume that JBL diaphragms are standardized but I don't know for sure.

By grooved and not grooved are you talking about a machined step in the top plate? The first drawing has one and the second does not.

Widget

Yes Widget!
I mean the first one but where the mounting holes are there is no groove and thats why the gap comes. Where the groove is there is a connection between phase plug area and rear chamber.

Guido
03-30-2005, 02:14 PM
Hi Guido,

you mean this:
(I also mentioned about.....maybe for cooling air for the voice coil while the dia is moving.)

HP

top is a 2441, the other is a 2420 dia.
BTW: the thickness of this small metal sheets is 0.4mm or ? inch

The 2421 dia also have the metal sheets. This sheets even increase the gap.

Guido
03-31-2005, 09:50 PM
Interesting!


Nobody seems to know about this phenomenon. Does this mean only I have it in my Drivers?

Mr. Widget
03-31-2005, 11:19 PM
Sorry Guido,

I don't really understand the question. Maybe you could post a pic with your question.

Widget

Guido
04-01-2005, 02:40 AM
Thanks widget. I'll make pics next week.

Maron Horonzakz
04-01-2005, 06:34 AM
As it was explained to me by a JBL service teh. The spacers are to keep the diaphram & phase plug at a specified distence from each other. On some early drivers numbers written inside machined area were coded to help determin spacing also some early diaphram/voicecoils had coded numbers. I dont know if present day drivers have better tolerence,s

Guido
04-01-2005, 11:01 AM
As it was explained to me by a JBL service teh. The spacers are to keep the diaphram & phase plug at a specified distence from each other.

Ok, but fact is that there is no airtight seal between phase plug and rear chamber. I always thought that the area in front of the diaphragm is a preasure chamber.

I will make pics to explain what I mean when I'm back home on Tuesday.

Earl K
04-01-2005, 12:22 PM
Guido,

- The "scalloped" out areas underneath the diaphragms' surround do serve a purpose.

- I've heard ( not from a designer per se. ) that these are to allow the airload under the surround to compress a bit more linearly . Without a "release valve" in this area, trapped air does funny things leading to an increase in distortion products generated by the surround . Somewhat akin to a car "hydroplaning" on wet pavement.

- Other compression drivers might have little "wells" or drill holes in strategic spots that vent into the top-plate or magnet housing. These are usually located under the diaphragms' surround near the plastic rim.

- Anyways, that's the scuttlebutt .


:)

Steve Schell
04-01-2005, 05:08 PM
I like Earl's description of the spaces allowing an escape of what would be considerable pressures generated in the area of the surround. It is also important to remember that most compression drivers allow the pressure changes in the voice coil gap to escape into the motor structure, and the air flow probably aids in cooling of the voice coil. On the other hand, Wente's patent on the Fletcher System compression driver described a brass ring fitted to the forward end of the voice coil gap that sealed the gap and prevented the escape of these pressures. When the later, very similar Western Electric 594A driver was produced, it did not have this sealing ring.

There is also the matter of the distances involved vs. the transit time, at least at the higher frequencies. The phasing plug slits offer a much closer path for the pressure changes over much of the diaphragm's area compared to the paths offered by the voice coil gap or spaces under the mounting ring.

whgeiger
04-01-2005, 06:12 PM
... in the compression chamber, kills driver (horn) output. It has the same effect as a blown head gasket in your car's engine.

On the other hand, venting the back-chamber has no such consequence. It is like removing the muffler.

Regards,

WHG

Ian Mackenzie
04-01-2005, 07:30 PM
While changing diaphragms at my 2426 Driver last night I realized for the first time that there are slight milling groves at the top plate exactly where the mounting ring of the diaphragm lays on the top plate!

This means that the mounting ring does NOT seal the rearchamber of the driver from the phase plug!

When I put the D16R2421 dia in I realized that these dias have thin metal plates where the mounting holes are. I suspect that these thin plates provide the same “gap” between dia and top plate in a 2420/2421 driver by “lifting it a bit” as this driver does not have the milling groves.

Mounting the D16R2421 dia in a 2426 will consequently increase the “gap” as we have then milling groves + thin metal plates.
.

Good observation.

Its seems reasonable that they were at the time compensating for slight changes to the top plates of the respective drivers.

Although backward compatible, I doubt if JBL ever cover the contingency for using the D16R2121 dia in a 2426.(older dai on new drivers)

I have a 2420 driver myself and I fitted the D8R2425 dia without problem(newer dia on older driver). It was a firm fit and it measures perfectly.

Guido
04-04-2005, 02:13 PM
As I'm a lazy guy I took this pic from MatthiasA's post "Is this a JBL 2426 driver?" Thanks Matthias!

The machined parts can easily be seen.

There is clearly a leak between the front and back of the dia. When using a 2421 dia with those thin metal plates the leak will be even bigger as the distance will grow compared to a 2425 dia.

Mr. Widget
04-04-2005, 02:39 PM
I never opened one of those up. I have no idea about those machined slots. I would guess it has to do with a physical clearance issue.

As for back wave front wave issues I would speculate that at these frequencies the voice coil and gap are essentially the seal that prevents the back wave from entering the phase plug.

Widget

Hofmannhp
04-04-2005, 03:04 PM
......The machined parts can easily be seen.

There is clearly a leak between the front and back of the dia. When using a 2421 dia with those thin metal plates the leak will be even bigger as the distance will grow compared to a 2425 dia.

Hi Guido,

now I think to know what you mean......
the 1/3 circle parts between the screws seem not to be machined in difference to the screw bases. For me this is the only reason...to get a precisely deffined (machined) base for the dia.... like a chair with three legs....it will never be shaky. I also think that the pumped air from the dia is a very little with the known dia movements so that in this frequency regions there will be no shortening effect.

my 1/2 cent

Guido
04-04-2005, 03:41 PM
Thanks for all the answers!

Obviously it works with the gap so I should not care. I just wanted to share my "find" with you and tried to understand why it is that way.

However: The 2421 dia plays well in the 2426 core as can be seen in my thread "playing with a 2426".

Elac310
01-08-2014, 05:47 AM
I'm undigging this old thread as it raised an interesting question/issue.

Guido (if you're still around), or others: am I right in understanding from the above that in the 2426, the little mounting plates on the screw holes of the 2421 dia could be useless (or even counterproductive if they increase unnecessarily the distance between phase plug and diaphragm and thus decrease the HF extension) ?

Have you tried to remove these plates to see whether it makes any audible or measurable difference (HF extension especially)?


In another thread, I'm myself wondering about possibilities of reducing the distance between a diaphragm and the core (provided the drivers are not crossed too low nor used in a PA system).

Your present thread is drawing my attention to the fact that the D16R2441 - with which I'm currently "playing" - also have these small distance plates around the mounting holes (and the pole plate of my 2445 on which they are tested is also milled the way the 2426 is)...