Ring Radiator Production

[Predrag Dukic]

Hi Predraze,


I agree with You that if there is some kind of acoustic absorber out of the driver's horn (or any other kind of the obstacle) , that can make an interference with the sound propagation ( all of us are aware of the baffle edges interferences, not to mention any other nearby surfaces or objects), and all of that would make great variations in F/R response ( that is "present" at the microphone point in space).
But here I want to emphasize that I would expect that the driver's and its horn's construction and their imperfection could produce some kind of "signature" in the impedance characteristic that can be compared with expectable (correct) one. My expectation is based on MY belief that any kind of "large imperfections" ( the meaning "LARGE", here is disputable), would produce a different amount of reflection energy (compared to the "good" part), and that would, MAY BE, produce some kind of visible behavior in the impedance curves. All that is just my expectation, but may be You can find the answer.

Regards
Ivica

Ivica, (and others ) ;

Now I have a new 1 " PWT, attached to the 2425H driver. In fact two drivers....
Drivers are with close serials, with two original JBL dias.
Two FRs are so close that I think that everything is exactly as it should be- no damage of any kind,
and also magnet flux is probably unaffected by age. ..

The problem is that it does not look like the one published in JBL documentation...

( http://www.google.com/url?sa=t&rct=j...41018144,d.bGE , Figure Nr. 6)

Response is falling down 6 db/oct from 700 Hz onwards, as if the dia. mass breakpoint is around 500 Hz instead around 3500 Hz...
Has anyone ever seen independently published (not by JBL) PWT graph for 2425H driver?

1. Line (Yel) 2425H #1, 1" PWT
2. Line (Gry) 2425H #2, 1" PWT
3. Line (Magenta) 2404H-1, 1/2" PWT

Third line includes 6 dB correction for difference in PWT diameter ( 1/2" vs. 1" )
Absolute SPL is not calibrated, but relative differences are correct.

[1audiohack]

Here is one of mine, a completely brand new JBL 2425 HS on my 2" PWT with a JBL 2327 1" to 2" adapter. Using a calibrated Earthworks M30 mic. Oops, that's not true, I used a TEF TE04 calibrated mic.

[ivica]

Ivica, (and others ) ;

Now I have a new 1 " PWT, attached to the 2425H driver. ..............................

The problem is that it does not look like the one published in JBL documentation...


Response is falling down 6 db/oct from 700 Hz onwards, as if the dia. mass breakpoint is around 500 Hz instead around 3500 Hz...
Has anyone ever seen independently published (not by JBL) PWT graph for 2425H driver?

1. Line (Yel) 2425H #1, 1" PWT
2. Line (Gry) 2425H #2, 1" PWT
3. Line (Magenta) 2404H-1, 1/2" PWT

Third line includes 6 dB correction for difference in PWT diameter ( 1/2" vs. 1" )
.

Hi Predraze,

It is nice to be seen that You have setup an other measurement test place for 1" drivers.
May be some suggestions from 1audiohack would help.
http://www.audioheritage.org/vbullet...l=1#post315250

As I have remembered, the position of the microphone can influence the measurements results , and sound absorber applied in the PWT too.



Regards
Ivica

[Predrag Dukic]

Here is one of mine, a completely brand new JBL 2425 HS on my 2" PWT with a JBL 2327 1" to 2" adapter. Using a calibrated Earthworks M30 mic. Oops, that's not true, I used a TEF TE04 calibrated mic.

It is usually said that the experiment which shows exactly what is expected, is useless. You can't learn anything from it.

If however, results differ from expectations, it can mean only two things:

1. You did something wrong. After correcting the error, You learned something.

2. If You didn't make any error, then You found something new. So again, You learned something...


My 2425H FR graph was unexpected. Not only that it had this prominent resonance, but also looked rougher than Your graph...


And then I remembered...

When I first received the drivers, I dismantled everything, checked the dia,
and also found that the rear chamber damping has disintegrated.
Well, nothing unexpected. PU foam can't last so long... even more than 25 years in this case.

So I removed it, with intention to find a suitable replacement later.

But then I forgot about it.... Yesterdays graphs were recorded without any damping...

When I realized it, of course I tried to find a replacement, which is not easy, so first I tried without a back cap...

The idea was that absence of the cap is kind of an "ideal case".
Any sound coming from the rear side of the dia is effectively dispersed and so....

But then... I wasn't sure about it... So I made another experiment,
with material that was first at hand: a common medical cotton.
Still I wasn't satisfied, because I didn't get expected results and also it is not the original damping material.

So I invested a great effort, visiting various shops and all other places that came to my mind, and finally found
something similar to the original foam. Of course, the original is probably not possible to obtain,
but this looked really close.
And it seems that rear damping material is not a trivial thing...
I still can't explain fully this strong resonance at 750 Hz, and deviation from Your,
and JBL published response, but I got some improvement.

Here are the results:

Yel : Open cap , "ideal damping?"

Gry : with cap, no damping

Orange: long fibre cotton

Cyan : Foam damping


With foam, not only that resonance is 4-5 db lower than the alternatives,
but also there is an improvement in the upper region ( from cca 2-3 kHz to almost the end of the range).
Not much, maybe only 0.5 dB in this band, but there is a measurable difference.
Also those initial smal resonancies at 3 and 6 kHz ( gray line) can be attributed to air space resonance
of the rear chamber. All other cases do not have those small peaks.

I also kept that 2404H-1 magenta line in the graph.

[Earl K]

Hi Predag





- Predrag, ( as an aside & only because I thoroughly enjoy following your research efforts so much ) would you please consider using a "better" screen-capture utility( like the one posted above ) so that we can see ARTA in it's best resolution . Stay away from .jpg when resizing ( & marking-up pics ) when in Paint . It's best to go into & out of Paint using a .png file format ( if that is the finale format type ) .

- Here's the type of quality ( .png files ) that you could be posting ( instead of the current, washed-out / hard-to-follow, pics ) .






- Mind you, one also needs to link into these pics from another location ( since loading them as attachments puts them through another file conversion back to jpg & another visible down-grade in quality )

Thanks !

[more10]

one also needs to link into these pics from another location

Dropbox is very good and very simple, use my invite to get an extra 500 megs.

[Predrag Dukic]

@ 4313B Thanks for the photo. When I received the drivers I forgot to measure and note the dimensions of the foam inserts.
The type in this case was the one on the left; simple round piece. Could You measure its dimensions, and post results?

I doubt that the right foam alone will solve the problem. 10 or more decibels must have some other reason.

Maybe the properties of the diaphragm are also changed... Cold working of the metal in the suspension rim...
Or resin used to dampen resonancies of the diamond pattern changed its stiffness ( outer halve of the suspension is covered with some
yellowish transparent stuff- a resin or a vernice of some kind)

Who knows... I want to find the reason in principle; I want to know....

@ Ivica The damping of the PWT is irrelevant in this case. I tried open, closed end and damping in various lengths and densities.

As long as I use gating, there is no difference.

Position of the microphone is on the wall of the tube, just a 1/4 in from the throat, because I wanted later
to rotate the PWT on the driver, and check any influence or existence of the circumferential modes
( ie. whether the wave is really planar etc..)

[Predrag Dukic]

Earl,

Thank You for Your comments. I am aware that I need to change the way to present my graphs. I am new to this forum, so....

I will try to solve that problem in the following days. Maybe not immediately, because I have some obligations at the uni these days.
So there will be a couple of days of silence . After that I will try to repeat all the photos, and implement Your advices...

Till now I was using PrintKey screen capture, and gif picture format.

[Predrag Dukic]

Dropbox is very good and very simple, use my invite to get an extra 500 megs.

More10,

Sorry, I can't use Your invite. Currently I am using Dropbox for a commercial project ( unrelated to my work at uni) ,

and last time I tried to connect to somebodies other Dropbox, I had some problems.

I can't afford a loss of time trying to figure what was wrong, or whether that was a Dropbox bug...
I must find some other way...

[4313B]

@ 4313B Thanks for the photo. When I received the drivers I forgot to measure and note the dimensions of the foam inserts.
The type in this case was the one on the left; simple round piece. Could You measure its dimensions, and post results?

5-inch diameter
1/2-inch thick