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

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/vbullet...n-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.

Quote:

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Originally Posted by 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/vbullet...n-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 ???).

.......

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

reagrds
ivica

Quote:

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Originally Posted by Elac310

....
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).

.....

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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.

Quote:

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Originally Posted by Guido

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.

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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/vbullet...chnology/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).

Quote:

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Originally Posted by ivica

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

reagrds
ivica

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

Quote:

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Originally Posted by Elac310

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

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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.

Quote:

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Originally Posted by Elac310

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

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

Quote:

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Originally Posted by Elac310

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).

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

Guido is right.

Quote:

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Originally Posted by Guido

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.
.........

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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

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.

Quote:

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Originally Posted by Elac310

....

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.

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Hi Elac310,

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

1. http://www.eighteensound.com/staticC...oducts/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

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.

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.

Quote:

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Originally Posted by 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.

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Hi Elac310,

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

Regards
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/staticC..._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...

I've seen this toolAttachment 61326 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: