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In Reply to: RE: how on earth could that be time aligned? posted by Jmmlc on June 27, 2007 at 08:48:53
hello Jean Michel
>>>(the presence of irregularities on the surface of the diaphragms and the suspensions is often questionnable)may you know something abought the components used in the Goto drivers ? Someone i know in italy is in a project, to produce drivers similar to Gotos ore Ale's. He has almost all element's together, and is working with a acoustic engeneer with good experience to design the driver. However, he does not have still enough informations abought the frp edge ( Fiber Reinforced Polymer ) suspension. May it be Mylar , fibre reinforced ? what kind of fiber?
if you have fotos, ore know more details abought this subject, ore may be able to indicate someone, that would be great. I am helping out with all info's, i can gather on the net.
>>From the absence of large compression ratio a larger dispacement of the diaphragm results and then a quite high level of distortion (compared to a driver like the JBL245x by example) and a bell shaped frequency response curve.So that seems to be negative...
>>IMHO the rear cavity is also too small.i dont understand, the rear cavity of what ?
rich wrote :
>>The very low compression ratio sounds less 'squeezed' than the typical 10:1 ratio unit.
and this would be a positive effect.
so the question is: does not use a phase plug in this frequency range, in the end, bring better, ore worse results ? also, as it is used this way, probably would it not beam ?
Avantgarde uses a different impedance coil so it requires no attenuation in the network.
don't understand this, Rich.
rds Angelo
Edits: 06/27/07Follow Ups:
Hello Angelo,
About what I called "rear cavity":
Behind the diaphragm of the Community M200 there is a (closed?) chamber, you can see it on the drawing in the pdf document. This cavity is quite small and possess a quasi rectangular axial section. I don't know if it is damped.
About suspension material:
Polyester films (under the trademark Mylar by example)is the favored material. They cab be found in different thickness and eventually reinforced by fibers (Dacron...).
The image of the "poor air" squeezed inside the compression volume between the diaphragm and the phase plug is often used by detractors of compression drivers who promote the direct radiating loudspeakers. Bad compression drivers can for true exhibit resonances in relation with the compression chamber, the phase plug slits, the air volume directly in relation with the coil, etc...
When the compression volume and the phase plug are well designed those parasitic resonances are not audible and there is virtually no negative point to the use of compression drivers if they are used on good quasi infinite horns.
Best regards from Paris,
Jean-Michel Le CLéac'h
"From the absence of large compression ratio a larger dispacement of the diaphragm results and then a quite high level of distortion (compared to a driver like the JBL245x by example) and a bell shaped frequency response curve."
Funny the data sheets don't show either. The JBL2445 on the 2380A has a higher peak/trough from 500hz~4Khz (about 8dB) than the Community does (about 6.5dB), and the Community has much less distortion too (JBL over 10% at 10W at 3.5Khz vs the Community hitting about the same 10%, but with 75W in! Even adjusted for dB down from actual output (the JBL has a peak where the Community has a dip) the Community has less distortion.
http://www.jblpro.com/pages/pub/components/2380a.pdf
http://www.community.chester.pa.us/files/specs/new/m200a.pdf
"If you use a bit of equalization between 500Hz and 3000Hz then the efficiency fall to 105 or less. "
The JBL is only 105dB at 500hz, the Community is 110dB. In the 1Khz region the JBL is 108dB, the Community is 112dB.
"IMHO the rear cavity is also too small."
"rear cavity of what"
Under the diaphragm. Make it bigger if you wish, it is designed for 500hz, but you can open it up and do anything you like.
"and this would be a positive effect."
Yes less distortion and more effortless sound at any volume level.
"as it is used this way, probably would it not beam ?"
It acts like a 2" point source, which is capable of over 90* dispersion to beyond the recommended 4Khz crossover point. Being a dedicated midrange driver dictates a different design than a wide-band unit.
"don't understand this"
The Community is nominally 112dB/2.83V in the 2Khz region where it is about 10 ohms. The Avantgard need it to be only about 107dB/2.83V/1M. Rather than add a bunch or resistors (and many audiphiles don't like resistors when used to attenuate horns), they raise the impedance so the driver draws less power. I am told the version they have made is around 30 ohms (if it was about 20 ohms it would be about 109dB, 40 ohms would be about 106dB).
A few years back I sold these with a money back policy. Most of the buyers already had JBL 2" drivers. No one ever returned a set, they sold their JBLs instead. The Community looks OK on paper, and I think it sounds even better than it looks.
Best results with the Community will be had on a horn without a narrow diffraction slot (ie: a JBL 2380 has a narrow slot, a 2350 or 2397 does not). Those $250 a pair black Tractrix horns on eBay look like they might sound good.
Hi djk,
You just seem to be a living audio encyclopaedia:-) Thanks for sharing!
I have some additional questions that I hope you will respond to:
1. Do you happen to know what kind of 1" driver the Avantgarde Trio uses?
2. Which of the 2" Community drivers (M200, M200A, EM282, or EM280) sound best for hifi mids?
3. Have you had a chance to hear the BIG 4" M4 in a home setup, and if "yes" is it worth the prize for an "all-out" midrange driver?
Thanks a lot!
Regards
Peter
hello peter
one question i can answer you : they use a 1" beyma cp380m.
what i still wonder, is witch mid-bass they use. i guess it's a cheap monacor.
angelo
Hi Angelo,
Thanks for sharing!
Regards
Peter
thanks, Jean Michel .Rich
i will try out a Radian 950 pb. ( do u anyway have experience with this driver ? ) I want to use it up to 5khz. May i get better results then, if i would invert, and use the back radiating part, coupling direct to the horn, as avantgarde does, without phase plug, and no compression ?
Angelo
Hello,
Angelo's question was related to hifi use not professional audio.
In hifi use that's very rare to feed a compression driver having an efficiency larger than 110dB/1W/1m with more than 0,1watt RMS.
On such low signals the displacement of the diaphragm of a compression chamber will be very small compared to a direct radiatior (even horn loaded). Distortion at low acoustic level is fully related to the displacement of the diaphragm. Low diaphragm displacement = low distortion.
At large acoustic level, the distorsion is essentially due to the non linear property of the air itself and that's true that at very high SPL all compression drivers will show high distortion level.
Best regards from Paris,
Jean-Michel Le Cléac'h
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