Question to the electromechanical specialists
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 30th May 2007, 09:12 PM #1 crazyhub   diyAudio Member   Join Date: Nov 2005 Location: alsace Question to the electromechanical specialists Hello all, When we add a supplementary magnet to a driver, the magnetic field is better concentrated in the gap, thus BL and efficiency increase, Qes and Qts decrease, which is logical. However the modified value of the magnetic field also acts on Qms and Rms which are purely mechanical parameters... Why? i.e.: specs from manufacturer: SEAS standard CA12RCY: Qms=2.17 Rms=1.04Ns/m added magnet CA12RCY: Qms=1.87 Rms=1.21Ns/m Thanks. Hubert __________________ crazyhub
 30th May 2007, 09:24 PM #2 joe carrow   diyAudio Member   Join Date: May 2004 Location: Oakland, CA If I had to guess, I would say that the larger magnet structure is restricting the airflow somewhat. This would increase the mechanical resistance, which would in turn decrease the Q of mechanical resonance. I didn't do the math to determine that the change in Rms is sufficient to account for the change in Qms. Where did you find this data? I looked at Seas.no, but didn't see it.
 30th May 2007, 09:52 PM #3 Svante   diyAudio Member     Join Date: Feb 2004 Location: Stockholm It could also be a difference between individual drivers. Numbers in datasheets are presented with three digits, giving a false impression of accuracy. It could well be that the difference has nothing to do with the magnet. It could also be that the different magnetic circuit affects the measurement method somehow. Rms is calculated from the impedance curve, and I would bet on that a change in eg voice coil inductance would be seen in the measurement of Rms too, even if the actual physical Rms is intact. __________________ Simulate loudspeakers: Basta! Simulate the baffle step: The Edge
crazyhub
diyAudio Member

Join Date: Nov 2005
Location: alsace
Joe wrote:
Quote:
 If I had to guess, I would say that the larger magnet structure is restricting the airflow somewhat.
Could be an explanation but...I doubt: the area that slows down the airflow behind the spider doesn't increase with the supplementary magnet and the hole in the back plate remains open. I guess an indirect effect of the magnet strength but in what a way?
Quote:
 Where did you find this data? I looked at Seas.no, but didn't see it.
At the moment you are here:
http://www.seas.no/Prestige_line%20line%20up.htm
scroll down. Between "prestige midranges" and "prestige woofers" you will find the "Seas Woofer Shielding Guide"
__________________
crazyhub

 30th May 2007, 10:19 PM #5 crazyhub   diyAudio Member   Join Date: Nov 2005 Location: alsace Svante, vs standard drivers t/s parameters, see their variations here, once magnet added: http://www.seas.no/seas_woofer_shielding_guide.htm so no "impressions" but real modified values. Your explanation about Rms could be right... What about Qms? I found its calculation: Qms = Fs sqrt(Rc) / f2 - f1 Any idea of what are Rc and f2 - f1 ? __________________ crazyhub
 31st May 2007, 01:39 AM #6 KBK   diyAudio Member   Join Date: Dec 2003 Location: The Wilds Of Canada peak distortion decreases and considering how the ear works..this is why we are far more impressed with drivers like the MDT33 tweeter, as an example. It is the effect of the added magnetic stability under HUGE accelerations..that is largely the key to why they sound better. In the stated case...oddly enough..the shielded driver, if the effcts of magnetic reactive 'speed' (dynamic magnetic modulations become the steel cover's hysteresis curve!!!) could be negated..it might be found to have lower distortion or play to similar percieved distotion levels..at 3 or so DB higher than the unshielded unit. If one were to try both drivers, and be able to pull the steel cup off of the shielded one..and put it back on..and listen to the two 'effects'..you will get exactly what I mean. Thereafter, you will never fail to hear the effect of a shielding cup. In the end, all driver concerns are purely dynamic and (can be)seemingly random in nature, and have little to nothing to do with the static measurments and similar tests. __________________ "Extraordinary claims require extraordinary evidence." - Carl Sagan. Sometimes the claims aren't extraordinary. Sometimes the weakness of men's minds are the culprit. Mass mentality has nothing to do with obviousness or truths.
 31st May 2007, 03:48 PM #7 Michail Bugaria   diyAudio Member   Join Date: May 2006 Very good question, crazyhub! It does not give me rest already enough long after I spent some experiences with additional magnets. In all of a case I am hardware fixed change both Qes, and Qms... The scheme for measurement and some pictures of process http://www.diy-audio.narod.ru/audio/ts.htm Qms=1 / (2*pi*Fs*Cms*Rms)
crazyhub
diyAudio Member

Join Date: Nov 2005
Location: alsace
KBK wrote:
Quote:
 this is why we are far more impressed with drivers like the MDT33 tweeter, as an example. It is the effect of the added magnetic stability under HUGE accelerations..that is largely the key to why they sound better.
Please go further but with a technical explanation...because at same spl and same Sd, two drivers have in fact the same acceleration at a given frequency. Thanks.
Michail wrote:
Quote:
 In all of a case I am hardware fixed change both Qes, and Qms...
Not sure I understand what you wrote...In your measurements with added magnets, did Qms and Rms vary? Thanks.
__________________
crazyhub

Svante
diyAudio Member

Join Date: Feb 2004
Location: Stockholm
Quote:
 Originally posted by crazyhub Svante, vs standard drivers t/s parameters, see their variations here, once magnet added: http://www.seas.no/seas_woofer_shielding_guide.htm so no "impressions" but real modified values. Your explanation about Rms could be right... What about Qms? I found its calculation: Qms = Fs sqrt(Rc) / f2 - f1 Any idea of what are Rc and f2 - f1 ?
I'd put it this way:

Qms=2*pi*fs*Mms/Rms

In that way it is pretty clear what would happen if Rms changes .

fs/(f2-f1) seems to be the inverse relative bandwidth of the fs resonance, which effectively is Qms. I don't know how sqrt(Rc) fits in there, or even what it is.
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 31st May 2007, 07:39 PM #10 AKN   No snake oil diyAudio Member     Join Date: Aug 2005 Location: In the middle of Sweden Hi, Svante, Qms = Fs sqrt(Rc) / f2 - f1 whereas Rc=(Z@fs)/Re Formula useful when measuring TS parameters. http://www.epanorama.net/documents/a...arameters.html __________________ / Anders

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