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Old 27th July 2013, 12:36 AM   #241
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Afaik, the diaphragm has a phenolic ring that the main diaphragm is permanently affixed to. Like the cheap phenolic tweeters or the phenolic diaphragms in those cheap horn loaded tweeters.

I wrote up a comment on the K-55 mod that got lost in internet space on posting... but the gist of it is/was that by drilling the holes in the phase plug the compression ratio is dropped somewhat. The uneven path length between the original path and the path through the holes doubtless has an effect at some high freqs, but that may be above the usable top end of the driver anyhow.

So, putting a space above the phenolic mounting ring, and below it will indeed reduce the compression as well as providing some extra space for diaphragm excursion.

That doesn't explain the apparent improvement in frequency response... unless the extra (stock) compression creates a highish Q condition. Also I would expect a drop in sensitivity with the increased volume due to the spacers?

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Old 27th July 2013, 06:52 AM   #242
Radian is offline Radian  Germany
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Quote:
Originally Posted by bear View Post
......by drilling the holes in the phase plug the compression ratio is dropped somewhat. The uneven path length between the original path and the path through the holes doubtless has an effect at some high freqs, but that may be above the usable top end of the driver anyhow.
The only effect that the holes in the phase plug has is a bit stronger hf.
Important only for those who want to be able to get by with just a bullet
tweeter above the K55. There is no ill effect that we can see. Neither in the
response nor in the distortion department.

Noteworthy here is the fact that the compression ratio has almost no effect on
the frequency response from 100hz to 3khz. With that many holes we are
talking about a compression ratio variation of at least 30% .
In the picture I attached you see two identically modified units
only comparing holes vs. non holes.
Attached Images
File Type: png K55 holes in PhasePlug.png (260.3 KB, 167 views)
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Old 27th July 2013, 07:31 AM   #243
Radian is offline Radian  Germany
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Although we seemingly have found a more suitable driver, I think the lessons we
can glean from the modified K55 are very valuable for us:

1. Compression ratio has very little effect in the frequency
range the K55 should be used in. At least on big horns.

2. Qtc on some of these drivers is way to high, therefore a back chamber
increase exhibits great gain in the lower region

3. Softening the surround (be it by making holes in it or by clamping it more on
the outside) lowers fs, exhibiting, in the right relationship with the back
chamber volume, a further low frequency extension.
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Old 29th July 2013, 03:57 PM   #244
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Quote:
Originally Posted by Radian View Post
Although we seemingly have found a more suitable driver, I think the lessons we
can glean from the modified K55 are very valuable for us:

3. Softening the surround (be it by making holes in it or by clamping it more on
the outside) lowers fs, exhibiting, in the right relationship with the back
chamber volume, a further low frequency extension.

Afaik these compression drivers are operating below Fs. Fs tends to be somewhere in the upper mid frequencies.

By making the surround more compliant Fs might be lowered somewhat, but it's not down at 100Hz I would not expect.

Lowering the Q of the system would indeed tend to flatten the shape of the response. (assuming Fs at the mid point of the peak of the stock curve) But I would expect an attendant reduction in the SPL measured for the highest part of the curve, IF the Q reduction was the cause of the apparent LF extension. But for the LF to lift up implies another mechanism at work.

An impedance curve on the Plane Wave Tube would show the Fs of the driver.

Most compression drivers are limited at LF by the stiffness of the surround, which forms a mechanical limitation, afaik. (at very very low levels, they'll pretty much all move at LF, but as the level goes up they still are excursion limited by the suspension)
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Last edited by bear; 29th July 2013 at 03:59 PM.
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Old 29th July 2013, 05:20 PM   #245
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Bear,
I don't have any real figures to go by but I would suspect that the resonant frequency of these devices is higher than the low frequency range that is being desired here. That being the case you could operate below the resonant frequency as long as you had a reasonable upper frequency cutoff which in my eyes questions the use of a compression driver to produce these low frequencies in the first place. As far as a reasonable output down to 100 hz with a compression driver even on a horn sounds like you are asking for device failure due to suspension failure or coil burnout from over powering the device.

On another subject entirely if these compression drivers are using a composite phenolic surround material drilling round holes in that material is a known failure mode in composite construction. The edge of the hole between holes will propagate cracking over time and this will be a failure mode, a standard miss application of composite construction known very well in the aircraft industry and composites in general.
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Old 29th July 2013, 08:45 PM   #246
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Certainly, they will operate down there, but one would have to restrict the max SPL to modest levels as found in a typical home environment. Also there is no reason not to use multiple drivers on a manifold to get higher SPL since this is for LF applications and the phase/path length issues that might happen if ur interested in full HF response seem to not apply.

I'm still trying to understand how or why the LF response became improved over the stock unit, other than a change to the rear volume (load).

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Old 29th July 2013, 11:16 PM   #247
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Bear,
It would seem to me that you would be changing the compliance by changing the air volume that the diaphragm would have to work against. It would seem that increasing the air volume both in front of and behind the diaphragm would change the loading it would see, not the same as changing the ratio of open to closed area of the phase plug, but the distance between the diaphragm and the phase plug usually is at a minimum causing refraction of the air waves.
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Old 29th July 2013, 11:43 PM   #248
3GGG is offline 3GGG  Australia
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Quote:
Originally Posted by bear View Post
Certainly, they will operate down there, but one would have to restrict the max SPL to modest levels as found in a typical home environment. Also there is no reason not to use multiple drivers on a manifold to get higher SPL since this is for LF applications and the phase/path length issues that might happen if ur interested in full HF response seem to not apply.

I'm still trying to understand how or why the LF response became improved over the stock unit, other than a change to the rear volume (load).

_-_-
This is exactly what I will be doing, multiple drivers on a manifold. At home levels distortion should be low in the 100Hz region where the driver is operating below Fs.
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Old 30th July 2013, 09:10 AM   #249
Rewind is offline Rewind  Sweden
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Okay, speed trials of the un-modded K55-V!

Setup 1:
JBL 12" 2204H 60Hz-200 Hz.
K55-V in a wooden 1.75m deep horn with 75cm diameter, inspired by the Goto S-150, that has 200 Hz as its lowest crossover point, hence the bandpass 200-650 Hz.
JA6681B 650-4000 Hz.
Beyma 1" above and beyond.

Versus

Setup 2:
JBL 12" 2204H 50Hz-650Hz.
JA6681B 650-4000Hz.
Beyma 1" above and beyond.

No ribbon supertweeter today. Too few amps are operational.

K55-V
+ The K55 gives me a feeling that Sinne Eeg, in the song "Brief Hesitation" is in the room , and not just in a speaker.
- The K55 sounds a little strained, compared to the JBL 12". Is this what could be horn honk? I suspect the culpret to be strange aerodynamical anomalies caused by a 1.75 meter cushion of air in front of the diaphragm.
- I was expecting the sound to be clearer, but maybe I am used to metal diaphragms and shorter horns. The deepest horn I have heard before today is the 40cm deep horn I use with the JA6681b. Do I dare to use the JA-6681b in the Goto horn? Naah.

JBL 2204H 12"
+ The 12" sound effortless. It does not have 1.75 meters of air to push against.
- The 12" JBL sound pretty boxy. As if was a speaker, and not a live performance. Go figure!

My room is too small to really assess it properly. I should really wait until after I have cooked lunch, because I get a way better, and very different sound walking into the kitchen, six meters away from my listening room, while still standing in the straight, and rather narrow beam of sound coming from the horns.

I doubt having a doorway between me and the system is the best way judge its performance. Still, it is slightly more enticing than the JBL 12" was before. It is a keeper!

I posted some pictures, as there are only some minor details left to fix. The horn needs some more sandpaper, another paint job, and a new appartment.
Attached Images
File Type: jpg 20130730_082026_1.jpg (569.4 KB, 105 views)

Last edited by Rewind; 30th July 2013 at 09:34 AM.
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Old 30th July 2013, 10:01 AM   #250
Rewind is offline Rewind  Sweden
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I realize that just highpassing the compression drivers works well, except for the 12" 2204H, that is doing a rather poor job as subwoofer. Sound is even more alive/live now. At lest on singer song writer music, like Tony Joe White - Lazy.

Video:
https://www.facebook.com/photo.php?v...85153912794167
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