JBL 4" core drivers (2445 etc.): modifying space between diaphragm and phase plug

[Elac310]

Since I was positively impressed by some of the HF qualities of 4" diaphragms in a 2" exit driver and following my attempts to get the most UHF extension out of a 2445 - see http://www.audioheritage.org/vbulletin/showthread.php?34912-JBL-2445-HF-boost-with-autoformer-in-reverse-mode - I'm pursuing my investigations into this area.


It is well known that one of the decisive factors influencing the UHF extension is the distance between the diaphragm and the phase plug (the lower the air volume between the two i.e shorter the distance, the better the UHF extension - see for instance John Eargle's book on Loudspeakers).

I'm now wondering whether:

1) does anyone know what the current distance is in those 4" core drivers - it being understood that due to diaphragm curvature, it is presumably the least at the tip of the diaphragm dome? Since i've managed to implement various types of diaphs in my 2445 drivers (JBL D16R2445, D16R2441, aftermarket ribbed/non aquaplased model normally meant for other JBL drivers etc.) I would assume the core and diaphragm geometries are basically the same throughtout the product range and years (some isolated exceptions ???).

2) provided the driver is not used at too low frequencies (but in the range 2kHz-16kHz area as in my case) and is not used for extreme levels as in PA equipment, it would theoretically be possible to reduce the distance between diaph and core, and if so by how much ?

3) has someone tried a modification ? one that seems feasible would be to remove the lowest of the two metal rings (provided they are not glued too strongly together and it would not distruct the two signal leads) underneath the plastic diaphragm support ring, and to replace it with one or several layers of bristol disks/spacers down to the desired heigth.

[ivica]

Since I was positively impressed by some of the HF qualities of 4" diaphragms in a 2" exit driver and following my attempts to get the most UHF extension out of a 2445 - see http://www.audioheritage.org/vbulletin/showthread.php?34912-JBL-2445-HF-boost-with-autoformer-in-reverse-mode - I'm pursuing my investigations into this area.


It is well known that one of the decisive factors influencing the UHF extension is the distance between the diaphragm and the phase plug (the lower the air volume between the two i.e shorter the distance, the better the UHF extension - see for instance John Eargle's book on Loudspeakers).

I'm now wondering whether:

1) does anyone know what the current distance is in those 4" core drivers - it being understood that due to diaphragm curvature, it is presumably the least at the tip of the diaphragm dome? Since i've managed to implement various types of diaphs in my 2445 drivers (JBL D16R2445, D16R2441, aftermarket ribbed/non aquaplased model normally meant for other JBL drivers etc.) I would assume the core and diaphragm geometries are basically the same throughout the product range and years (some isolated exceptions ???).

.......

As I have remembered the distance from the phase-plug body is at least 0.5mm

reagrds
ivica

[Guido]

....
It is well known that one of the decisive factors influencing the UHF extension is the distance between the diaphragm and the phase plug (the lower the air volume between the two i.e shorter the distance, the better the UHF extension - see for instance John Eargle's book on Loudspeakers).

.....


A regular (non Be) 4" diaphragm shows breakup modes looong before a reduction of dome to phase plug distance would have any effect.
In other words:
4" Ti or Al diaphragms do not have real UHF extension because they are to heavy.

[Elac310]

A regular (non Be) 4" diaphragm shows breakup modes looong before a reduction of dome to phase plug distance would have any effect.
In other words:
4" Ti or Al diaphragms do not have real UHF extension because they are to heavy.


Hello Guido,
Glad to read you (I posted a question in relation to an older thread of yours, which has in fact some connection with the present one: see http://www.audioheritage.org/vbulletin/showthread.php?5293-Perplexity-with-Compression-Driver-Technology/page2 )

Yes, of course, the benefits of Be and the differences in break-up modes have been mentioned time and again but that was not the point of this thread (you're the Guido from Behringer Electric, distributor of a brand of Be diaphragms, right ? we had a telcon recently). Besides, my questions are not related to a specific material of diaph but they refer to general design factors of which any type of diaph could benefit from (whether Ti, Al, Be or else).

[Elac310]

As I have remembered the distance from the phase-plug body is at least 0.5mm

reagrds
ivica

Thank you Ivica. We're playing with tight tolerances, in principle...

[Guido]

Yes, of course, the benefits of Be and the differences in break-up modes have been mentioned time and again but that was not the point of this thread

It is the point.
The improvements you try to get occur in an area where Ti/AL dias do no longer work (show piston behavior).
That might be the reason why nobody cared much about it till now.



(you're the Guido from Behringer Electric, distributor of a brand of Be diaphragms, right ? we had a telcon recently)

Advertising here makes no sense AND isn't allowed. Members here do not buy my diaphragms...well normally not.




Besides, my questions are not related to a specific material of diaph but they refer to general design factors of which any type of diaph could benefit from (whether Ti, Al, Be or else).

Ti/AL will not benefit (see above). If Be would benefit need to be tested.

[more10]

Guido is right.

[ivica]

It is the point.
The improvements you try to get occur in an area where Ti/AL dias do no longer work (show piston behavior).
That might be the reason why nobody cared much about it till now.
.........


May some interesting info can be get from the US20080192972 patent explanations:
[ http://www.google.com/patents/US20080192972 ]

"...This variation includes a phasing plug having a sound
Wave inlet region formed upon the outer circumferential surface
area of the phasing plug and a sound Wave absorption
region formed upon the remaining central surface area of the
phasing plug,.....
....The insert 500A is preferably fabricated from a material
Well suited to absorb sound Waves examples of such material
are rubber, cork, high-density foam or other similar materials....."

Regards
ivica

[Elac310]

Thank you Ivica for the useful info !

For the record: I am not doing any advertising here...In my initial post, I did not even refer to a certain type of diaphragm material precisely to avoid pointing to a specific manufacturer. A comment not related to my questions was done and I wanted to understand.

It is becoming difficult to speak about diaphragms and neighbouring subjects without getting critical/reserved comments about Al and Ti.

Now for what it's worth: using the passive UHF boost circuit referred to in the link of my first post, ultra-clean 2445J cores (5 slit metal phaseplugs), it is with NOS original Ti diaphragms meant for the JBL 2445J (no advertising intended but we have to refer to JBL products on this website, no ? :crying:) that I have obtained the best UHF extension so far (better than with my 1" drivers or dedicated tweeters in fact). And no, the 2445 has not become a "nail shooter" or a distorsion monster (in my system and a home audio context).

The Al and Be diaph I used are clearly smoother, the Be diaph being also a tad more detailed than the NOS JBL Ti, the Al lacking impact, resolution (to my taste) and sound level. In a 2445 core which was designed to host Ti diaph, that is.

I want to continue studying the adequation between phase plug and dia so as to be able (where possible) to get more out of other excellent (non D16R2445) dia.

[ivica]

....

The Al and Be diaph I used are clearly smoother, the Be diaph being also a tad more detailed than the NOS JBL Ti, the Al lacking impact, resolution (to my taste) and sound level. In a 2445 core which was designed to host Ti diaph, that is.

I want to continue studying the adequation between phase plug and dia so as to be able (where possible) to get more out of other excellent (non D16R2445) dia.

Hi Elac310,

May be some of the following papers can help in Your research.

1. http://www.eighteensound.com/staticContent/technologies/products/ppp.htm

2. Bob Smith: "An Investigation of the Air Chamber of Horn Type Loudspeakers"- AS Vol.25 No.2. Mar.1953.

3. Fancher M. Murray : "An Application of Bob Smith's Phase Plug" -AES 61st Convention, Nov. 1978. [ http://www.docin.com/p-455350008.html ]

4. Clifford A. Henricksen: “Phase Plug Modelling and Analysis: Radial Versus Circumferential Types” presented at 59th AES Convention

5. US Patent NO.: US20060034475 [ http://www.google.com/patents/US20060034475 ]

On the other side You can experiment putting adhesive tape on the phase plug in order to reduce the volume between the plug and the diaphragm.
It would be interesting if You share with us your results.

Regards
Ivica

[more10]

The metal phase plug also rings. If you are going to put stuff on it, use somthing which dampens the ringing. I believe bakelite phase plugs are better behaved in this regard.

[Elac310]

My understanding was that bakelite material was an early feature (inherited from JBL 2440, 2441), together with 3 slit (as opposed to 4 slit) cores, and that metal 4 slit cores were designed as an improvement (just as 2446 came after 2445 with phase coherent slit patterns). I may be wrong, of course but this is what I've read and seen on convincing documented posts elsewhere.

Where to apply dampening bearing in mind the miliimeters (or fractions thereof) with which to operate ? I have no (discernible) ringing issues with the 2445.

I will continue looking for the distance between diaph and phase plug, so as to evaluate the room for manoeuvre with non D16R2445 diaphs.

[ivica]

My understanding was that bakelite material was an early feature (inherited from JBL 2440, 2441), together with 3 slit (as opposed to 4 slit) cores, and that metal 4 slit cores were designed as an improvement (just as 2446 came after 2445 with phase coherent slit patterns). I may be wrong, of course but this is what I've read and seen on convincing documented posts elsewhere.

Where to apply dampening bearing in mind the miliimeters (or fractions thereof) with which to operate ? I have no (discernible) ringing issues with the 2445.

I will continue looking for the distance between diaph and phase plug, so as to evaluate the room for manoeuvre with non D16R2445 diaphs.

Hi Elac310,

As I have seen on either 2441, 2445, 2446, 2450J/SL, 2451 4-slit phase plug is applied....

Regards
Ivica

[Elac310]

OK.
For 2446, 2450 etc., which are later (younger) models, it's consistent. Probably there was also an evolution in respect of 2441 from 3 to 4 slits.

I've come accross this paper from 18sound http://www.eighteensound.com/staticContent/download/pdf/18Sound_diaphragm_procedure.pdf (no publicity intended) which gives an idea of the tolerances in the manufacturing process of their drivers; the end user is meant to use shims, where necessary, in order to reach the optimum distance of 0,8mm +/- 0,05mm (!). Drivers and diaphragms bear indivual marks so as to achieve this optimum value... Impressive.
I've seen on original JBL diaphs also some handwritten marks...

[Elac310]

I've seen this tool61326 used by a manufacturer to measure phase plug heigths (link to page showing the tool in case the image disapears at some stage: http://www.vueaudio.com/products/h-12w-h-12n/).

One could use a similar precision tool to do one's own measurements (could also be used for diaph depth?) at least to obtain appropriate matches with suitable self made washers, if needed (before talking about precision machining rectification of old drivers...:blink:

[ivica]

OK.
For 2446, 2450 etc., which are later (younger) models, it's consistent. Probably there was also an evolution in respect of 2441 from 3 to 4 slits.

I've come accross this paper from 18sound http://www.eighteensound.com/staticContent/download/pdf/18Sound_diaphragm_procedure.pdf (no publicity intended) which gives an idea of the tolerances in the manufacturing process of their drivers; the end user is meant to use shims, where necessary, in order to reach the optimum distance of 0,8mm +/- 0,05mm (!). Drivers and diaphragms bear indivual marks so as to achieve this optimum value... Impressive.
I've seen on original JBL diaphs also some handwritten marks...

Hi Elac310,

Many thanks for the info from the 18-SOUND. It is really very nice explanation from 18-sound corp.
It would be nice if we an get something like that from JBL too, or , may be, from some of the AH-forum members, who are "near" the JBL technical stuff.

It seams that such explanation "uncover" the meaning of the hands written numbers marks on JBL compression drivers top-plates

regards
Ivica

[Mr. Widget]

Advertising here makes no sense AND isn't allowed.
Advertising is allowed in the appropriate Market Place area... it's just best left out of the other forum areas as it tends to derail the topic.

Back on the topic of this thread... I'm not sure how JBL does it, but I was at the Meyer Sound factory and I watched as a technician measured every diaphragm installation by hand and followed up with acoustic measurements. To maintain precise performance, they often place custom shims beneath the diaphragm individually for each driver... they do not consider their drivers field serviceable for this reason. In the case of Meyer, I don't think maximum extension is as important as unit to unit consistency and overall reliability.

I know that JBL also performs acoustic testing on their drivers before they are approved, but I don't think their tolerances are quite as tight as a boutique manufacturer like Meyer.


Widget

[1audiohack]

The metal phase plug also rings.

They do?

[ivica]

..... I'm not sure how JBL does it, but I was at the Meyer Sound factory and I watched as a technician measured every diaphragm installation by hand and followed up with acoustic measurements. To maintain precise performance, they often place custom shims beneath the diaphragm individually for each driver... they do not consider their drivers field serviceable for this reason. In the case of Meyer, I don't think maximum extension is as important as unit to unit consistency and overall reliability.

I know that JBL also performs acoustic testing on their drivers before they are approved, but I don't think their tolerances are quite as tight as a boutique manufacturer like Meyer.


Widget

Hi Mr. Widget,

Do You have any knowledge about JBL "numbering marks" that are visible on the top plates of their (for sure) 4"-diaphragm drivers ( 2441, 2445, 2446, 2450, 2451) ?

Regards
Ivica

[NickH]

Doesn't jbl teach how to adjust diaphragm offset in there tech school? A friend of mine has a close friend who went to jbl for this training. He said that jbl teaches how to set the diaphragms offset from the phase plug by shims on the mounting flange of the diaphragm. Now I didn't hear this first hand. But it make since that they would.

Can any of you trained jbl gurus confirm this?

Some metals do ring, some don't. I though jbl uses zinc for there phase plugs. You would be pretty hard pressed to get any resonance from zinc. Maybe with an induction heater though, lol.

[ivica]

Doesn't jbl teach how to adjust diaphragm offset in there tech school? A friend of mine has a close friend who went to jbl for this training. He said that jbl teaches how to set the diaphragms offset from the phase plug by shims on the mounting flange of the diaphragm. Now I didn't hear this first hand. But it make since that they would.

Can any of you trained jbl gurus confirm this?

Some metals do ring, some don't. I though jbl uses zinc for there phase plugs. You would be pretty hard pressed to get any resonance from zinc. Maybe with an induction heater though, lol.

Yes, that would be nice, but here on the forum, about such "things" SILENCE is USUAL answer.

Only very useful info that I have seen has been written by subwoof http://www.audioheritage.org/vbulletin/images/buttons/viewpost-right.png (http://www.audioheritage.org/vbulletin/showthread.php?p=354792#post354792)

http://www.audioheritage.org/vbulletin/showthread.php?35014-4-quot-coil-Nd-driver-history-revisited&p=354792&viewfull=1#post354792

".........
http://www.audioheritage.org/vbulletin/images/misc/quote_icon.png Originally Posted by subwoof http://www.audioheritage.org/vbulletin/images/buttons/viewpost-right.png (http://www.audioheritage.org/vbulletin/showthread.php?p=354792#post354792)

Well there are a few tricks for each mounting method - in general for the older pin diaphrams drill out the locating holes from 1/8 to 9/64 and that should give you plenty of room to "wiggle" This is a common procedure around here..:)

For the shoulder types, the dia is held in position at each mounting screw position against a small portion of a shoulder that is milled into the top plate.

Take a NEW, decent size tool steel *right angle* edge ( as would be found on a wood chisel SIDE or machinists block ) and shave down the diaphragm where it meets this shoulder. DO NOT USE A FILE, SANDPAPER, KNIFE EDGE, LAWN MOWER or something LAYING AROUND...Buy it NEW if you aren't a machinist!

Because slight offsets and material size differences when making the diaphragm assembly happen ( it is a sandwich if you look close ), there are burrs / waste material / etc etc.

By shaving down these areas ( ALL EIGHT ) the diaphragm can move around. Bingo...be sure to remove the burrs you CREATE !

NOW THAT YOU KNOW THIS - it is IMPORTANT to have a low distortion oscillator to align the diaphragm - AND THE DRIVER must have a nominal air mass load at the exit otherwise you will get false response / harmonics that will FOOL you..

NOTE: methods for aligning the NEWER Nd drivers with the dia glued into the back cap are not yet hacked. Someday I will find a need but HELLO, they never blow up right??

Anyways I made a wood box with various holes with wood dowels for guides and lined it with insulation.. stuck a cheapo microphone in the side and connected to the oscilloscope and compared in/out. Wearing earplugs and headphones you can actually hit a driver with some decent power to see if there's any breakup but it REALLY pissed off the cats.

Worked great until I gave it away to a tech and never bothered to make another.

.................."

Regards
ivica

[NickH]

Maybe jbl has them sign a non disclosure agreement.

[ivica]

Maybe jbl has them sign a non disclosure agreement.

May be You are right, but there are a lot of possibilities that such 'secret' to be uncovered.

regards
ivica

[1audiohack]

I really doubt its anything like that. I think we're attempting to assemble a very old puzzle. How long has it been since JBL consumer has used a four inch driver? Most of the folks that worked there are long gone and pro and consumer seem to separate further and further as the years go by.

Another consideration, machine accuracy and work holding have come a long long way and it is likely the kind of tweaking that used to be required is no longer needed.

I have a pair of tweaked (supposedly belonged to one of the senior engineers at JBL) 2450's with SL diaphragms in them that extend unusually high as verified by PWT measurement some time ago. They sound really good in the house. I have the PWT set up again and am going to explore them to see what's up as they also sound like breaking glass when played super loud (out doors on big horns) and if I was to hazard a guess it would be that the diaphragm is in very close proximity to the phase plug assembly. It will be interesting to find out.

I do know from experience that pushing and pulling and fussing with these usually brings about better performance and I have even had a couple of driver/diaphragm combinations I couldn't get happy at all and had to scatter them around and or not use them.

Barry.

[Elac310]

I have a pair of tweaked (supposedly belonged to one of the senior engineers at JBL) 2450's with SL diaphragms in them that extend unusually high as verified by PWT measurement some time ago. They sound really good in the house. I have the PWT set up again and am going to explore them to see what's up as they also sound like breaking glass when played super loud (out doors on big horns) and if I was to hazard a guess it would be that the diaphragm is in very close proximity to the phase plug assembly. It will be interesting to find out.

Barry.

Very interesting !
Yes, please, do let us know of what tweaking you manage to identify on those drivers.