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Author Topic: Xmax Xmech Clarified.  (Read 6409 times)

Antone Atmarama Bajor

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Xmax Xmech Clarified.
« on: March 01, 2007, 10:31:55 pm »

     I was wondering, most things I've seen about Xmax state  Maximum Linear Travel of Voice Coil.

    Does this really mean that once Xmax is exceeded that

A) The Coil is now traveling through a less linear and stable part of the magnetic Field?

B) The Suspesion Resistance Becomes Less Linear or compliant?

C) Both A and B?

D) The Coil has left the Magnetic Gap?

And as far as Xmech, Xmech far as I can understand its a (calculated) rating for Dammage mechanical can occour once reached or exeeded.

    I guess it depends on if the driver has an over or under hung coil where the coil reaches the  end of the gap.

    I have dislodged the Coil form on a 2245 before, I'm assuming that I successfully exceeded xmech.

    My only thoughts on exceeding Xmech is you probably need a pulse of energy that will provide the speaker cone with sufficient escape velocity, to keep it moving well past the magnetic fields and mechanical resistances that influence it.

    I guess that all depends on what Xmax really means.

    Just little fish swiming in my head.

    Time for Buffy goodness.

Antone-
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Peter Wing

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Re: Xmax Xmech Clarified.
« Reply #1 on: March 02, 2007, 01:02:56 am »

I belive that gap height, and magnetic feild are the factors that play a part part in xmax. When the optimal range of either is exceeded, the driver has exceeded xmax.
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Les Webb

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Re: Xmax Xmech Clarified.
« Reply #2 on: March 02, 2007, 12:30:07 pm »

I've often wondered what happens (if anything) to driver response between Xmax and Xmech.
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Bill Fitzmaurice Authorized Builder

Gareth James

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Re: Xmax Xmech Clarified.
« Reply #3 on: March 02, 2007, 01:06:33 pm »

Anton, xmax as defined by thiele small parameters is the travel available to the coil within the magnetic gap (or the other way around with underhung(overhung?) voice-coils).

As far as real linearity it will be dependent on a lot of other factors including suspension and magnetic field (in and out of the gap).

Some manufacturers rate xmax in terms of measured linear movement (often at resonance - although I've no idea how this affects results its presumably in their favour).

B&C for example quote Xvar as their recommended +/- 10% distortion figures.

Xmech is the rated point at which damage occurs and having an xmech significantly larger than xmax CAN help save a driver in SOME circumstances.

In general I think its best to ignore this figure however and concentrate on maintaining the driver within a linear displacement all the time.

Different drivers will do different things outside of it, all nasty though. It definitely shouldn't be considered "useable displacement" though IMO.
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Tamas Tako

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Re: Xmax Xmech Clarified.
« Reply #4 on: March 03, 2007, 03:54:02 pm »

Hi Antone,


Well I can't answer in a short way.
The Xmax is where the excursion is linear as a function of the input voltage. (not including thermal compression!)
This means, that while cone movement is Force x Springfactor and Force is B*LxI where B is the air gap magnetic field, I is the current in the Voice coil, L is the coil's lenght in the magnetic gap
This is why we use the B*L as force factor.
All this is true with constant B and constant Spring factor.
You can guess, neither of these is really constant in the reality.
However the mechanical parts (spider, suspension) are normaly designed that they are nearly constant in the region where the B is linear in the gap.

The B distribution can be modelled easily with some FEM softwares in these days, so we can guess it is optimized to be nearly constant in modern speakers. The only thing which changes now is the lenght of the VC inside the constant B magnetic gap, which varies with the amount of the excursion!

That means, Xmax one way(linear) is (should be in a good designed speaker):
(Hvc-Hgap)/2

Many manufacturers gives you the Xmax in the +0/-3dB B*L range, which means:one way Xmax:
(Hvc-Hgap)/2+Hgap*0,7

In this case the voice coil at his max excursion point is not 100% in the magnetic gap.

The Xmach comes from that where the mechanical parts limits the Xcursion for example bottoming the VC to the T-yoke or bottoming the cone/VC sealing to the top of the magnet assembly or the limit of the expansion of the spider/s or the surround.

The other big problematic of the speakers is the thermal coused resistance change, which completly changes the performance.

I hope this clears things

cheers,

Tamas
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Ryan Garnett

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Re: Xmax Xmech Clarified.
« Reply #5 on: March 24, 2007, 10:06:45 pm »

Xmech - Maximum mechanical limits of the subwoofer. ie when the physical suspension stops the driver from moving (surround, spyder) or when the coil slams into the back plate.

Xmag - When the BL produced by the coil has reached 70% of it's rest value.

Xsus - Excusion limit due to the mechanical suspension (similar to xmech but doesn't include the coil landing on anything)

Xmax - The lesser of Xsus or Xmag. Or a measurement based upon the length of coil in the gap.
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