Matt Errend wrote on Tue, 06 April 2010 20:46 |
Adam Schaible wrote on Tue, 06 April 2010 21:01 | Hi Art,
Do you know if that is +/- 23mm, or +/- 11.5mm for the JBL driver? Either way, that's a lot.... but if the box is tuned correctly for what you're trying to do, I don't really see 8mm of xmax as an issue.
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That has to be the P-P spec, otherwise the surround would be much much beefier.
I would assume that the "real" xmax is 11.5mm, in relation to the other drivers listed.
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That would be a wrong assumption, and if you look at JBL’s double voice coil differential drive magnet structure you can see they came up with a rather different (and patented) approach to making a woofer than the traditional method.
You can also see how the other JBL woofers have Xmax similar to other traditional drivers.
Xmax was traditionally defined by the overhang method. Overhang is the length of coil in mm that extends past either side of the top plate and is determined by the formula:
Xmax = gap height – [voice coil length/2]
The Audio Engineering Society (AES2-1984) defines Xmax as:
“The voice-coil peak displacement at which the “linearity” of the motor deviates by 10%. Linearity may be measured by percent distortion of the input current or by percent deviation of displacement versus input current. Manufacturer shal state method used. The measurement shall be made in free air at Fs.”
Wolfgang Klippel has a much more specific idea of what Xmax should be, but that is another kettle of fish...
In the 2008 JBL Theile Small parameters and definitions PDF that I quoted the Xmax figures from, their definition for Xmax is:
Peak linear displacement of driver diaphragm (0 to peak).
So the figures are one way.
The 2268 (23 mm Xmax) should be capable of 46 mm “linear displacement” peak to peak, about 1.75 inch of movement.
Art Welter