Large midrange for OB??? Scott G ?

[mige0]

Originally Posted by a1greatdane

...and there's 3 in series of them in a 'traditional' design..

? ?

[a1greatdane]

Dear Michael,

there's 3 cavity resonances in series around the voice coil IE: the air film on the inside of the coil, the bottom of the coil and the air film on the outside of the coil. A 3 of series of potential detrimental resonances directly from the air film signal side of the dome diaphragm against the phase plug and straight up under the surround of the diaphragm, where You can even experience a 4th one!
Cdriver design is not for the faint hearted, which is probably why most designs seems to only get incrementally better (and to many sometimes even worse) copies of each other?

With the best regards

'Dr' O

[mige0]

thanks - that makes sense to me
- but why *traditional* design - without pushing you too hard into revealing your business secrets

I mean - you always have those three cavities mentioned (unless maybe "sealed" VC by ferro fluid) and even if the volume below the surround is vented by holes towards the back volume...

Only way out might be - to some extent - inverse the VC towards the back chamber. Is it this you refer to as a non- "traditional" design ?

Michael

[Lynn Olson]

Originally Posted by a1greatdane

The Radian diaphragm I'm on the other hand very familiar with.

The diaphragm material is a proprietary closely guarded secretive aluminium 'composite' and the voice coil former is directly attached to the diaphragm and for reinforcement/stiffness purposes, in order to avoid 'rocking/flexing' modes, the material is extended a bit out together with the start of the surround.

In order to avoid cavity resonance around the voice coil/gap combination af series of holes is likewise applied to the mylar surround.

Best regards
'Dr' O aka a1olsen aka Sven R. Olsen

Very interesting and illuminating - thank you very much! I talked briefly with the engineers at Radian, and they mentioned that the founder/owner was a metallurgist. If I have my chronology right, Renkus was an engineer that worked for Altec in the 1960's and developed the Mylar-surround aluminum diaphragm, then went off on his own and started Emilar, and then after several financial ups and downs Radian spun off from Emilar, taking the Mylar-aluminum diaphragm technology with them. If the chronolgy is wrong, any comments and corrections would be much appreciated.

I'm interested in the tangential-metal vs stretched-Mylar surround issue. JBL switched to titanium in the late Seventies to eliminate metal-fatigue issues with aluminum, but I found the Murray 1979 AES article on the merits of the diamond-pattern surround a little unconvincing, since it was never made clear if the problem with traditional tangential surrounds was really a quality-control problem or not. They showed data on how tangential surrounds were sensitive to production variations in the thickness of the diaphragm material (no surprise there), but no data was shown if the new diamond-pattern surrounds were any better with respect to process control. (See ZIP file below.)

Of more interest is whether tangential (or diamond-pattern) all-metal surrounds have spurious "noise" associated with the hinge motion of the surround. This would be hard to measure with traditional THD methods, but would probably show up with modern spectral-contamination techniques. There are FR differences above 10 kHz, but operating a large-format diaphragm above the first breakup is problematic - the diaphragm is entering a chaotic region that falls outside the scope of equalization or pre-distortion techniques. The frequency of the first resonance, if I've got it right, is primarily a function of diaphragm material and its diameter. (Titanium, aluminum, and beryllium all have different first resonances for a given diaphragm size.)

If the noise issue is real, that's certainly a major point in favor of flexible surrounds. It's hard to imagine how a folded metal hinge could be mechanically damped in any kind of predictable way, never mind the separate issue of spurious HF radiation from the surround. Rocking motions are very undesirable, so I'd imagine some attention has to paid to that when a soft surround is used. Maybe that's the reason that JBL and Altec stayed with all-metal surrounds - they couldn't reliably solve the problem of rocking motions in their high-power, large-format drivers, while Emilar/Radian were successful in their approach.

It looks like important parameters for a Mylar-surround compression driver are: the method of surround attachment to the diaphragm, correct tensioning of the surround in production, control of surround uniformity and thickness, and appropriate mechanical termination of the surround on to the frame of the magnet assembly. Adding small holes to the surround to minimize or avoid cavity resonances is a new one to me; thanks for passing on the info.

[a1greatdane]

Hi Lynn,
sorry for having been away for a while, but thought I would just mention another aspect besides the electro mechanics IE: production. It's quite diffecult to glue mylar and a metal together... Most probably the likely a course why it's only used by a minority of manufactures?
Best regards
a1greatdane

[Paul W]

For those interested in "large midrange", here is another project I recently completed. "Midrange" drivers are the Seas W26FX crossed at 700Hz to 18Sound ND1460A compression drivers on XT1464 horns.



Here is a link documenting some of the work...HTGuide Forum - The Raptor ... a 10" MTM

Some of the results...

In-room system distortion sweep...



2M vertical polar response, no smoothing, on axis and +/- 15 degrees anechoic (vertically symmetric).



2M horizontal, no smoothing, on axis and +/- 30 degrees (Toole's 60 degree horizontal coverage window).


A few more pics and measurements are posted at the HTGuide link above.

[MBK]

Paul, you must be bored - I thought you already had the system to end the quest for all systems ;-) . Fantastic polars btw. I can only hope I come close to this.

(note to self: must complete the half finished system rebuild that I've been contemplating for two years!).

[Paul W]

These were a stand-alone project, so they don't replace anything. I needed something to take to a local DIY event and already owned the drivers. By adding 4 sheets of plywood and passive crossovers, the "Raptors" came out of the pipe.