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Old 30th May 2007, 09:12 PM   #1
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Default 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
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Old 30th May 2007, 09:24 PM   #2
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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.
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Old 30th May 2007, 09:52 PM   #3
Svante is offline Svante  Sweden
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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.
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Old 30th May 2007, 10:01 PM   #4
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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"
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Old 30th May 2007, 10:19 PM   #5
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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 ?
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Old 31st May 2007, 01:39 AM   #6
KBK is offline KBK  Canada
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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.
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Old 31st May 2007, 03:48 PM   #7
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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)
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Old 31st May 2007, 06:16 PM   #8
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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:
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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.
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Old 31st May 2007, 06:55 PM   #9
Svante is offline Svante  Sweden
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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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Old 31st May 2007, 07:39 PM   #10
AKN is offline AKN  Sweden
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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
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