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Old 31st January 2003, 09:42 PM   #1
Doug is offline Doug  Canada
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Default Measuring Xmax

Is there anyone out there with any experience in measuring the Xmax of loudspeakers. Is there any websites that know of with information on measuring Xmax.

Thanks, Doug
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Old 31st January 2003, 10:34 PM   #2
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I think there is no way to measure the Xmas with accuracy unless you destroy the voice coil
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Old 31st January 2003, 11:31 PM   #3
JBL is offline JBL  Canada
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Eminence tell how they rate xmax for their speaker, and I think if
I recall correctly that they don't measure it but calculate it instead.
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Old 31st January 2003, 11:34 PM   #4
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You need a ruler which has a precision of 1/10th of millimeter, as well as a very good eye.

EDIT: That was my 200th post... Too bad it was stupid...
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Old 31st January 2003, 11:46 PM   #5
SY is offline SY  United States
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I'd go with the manufacturer's spec- Xmax is very tricky to directly measure without endangering the driver and with any degree of confidence.
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Old 1st February 2003, 12:17 AM   #6
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Quote:
Originally posted by seangoesbonk
You need a ruler which has a precision of 1/10th of millimeter, as well as a very good eye.

EDIT: That was my 200th post... Too bad it was stupid...

Sean, that was unforgiveable. You are hereby sentenced to spend the next two weeks listening to nothing but an old $19.95 cassette boombox without a CD player!

Doug:
There is expensive measuring equipment out there, but it is generally not necessary for the home hobbyist.

There are two types of Xmax-linear and mechanical maximum. The linear type is the most quoted kind, and is smaller than the mechanical maximum. Within the linear range, the speaker operates relatively distortion free. Outside it's linear range, distortion rises sharply until the speaker hits it's mechanical limit-Xmech. Still, the speaker is able to operate and produce sound outside it's linear range.

Unless specified as Xmech or something similar, Xmax generally means linear Xmax.

If given the Thiele-Small parameters, you usually can calculate the linear Xmax for a speaker. You need the Voice Coil Length and the Height of the Magnetic Gap, (sometimes also called Depth of the Gap, or Air Gap). Just make sure the word "gap" is in there, LOL.

Then:
Xmax = (Voice Coil Length - Height of the Magnetic Gap) / 2.

If Voice Coil length is .75" and the height of the magnetic gap is .25", then Xmax = .25".

For Voice Coil Length, most hi fi speakers run from 3/4" up to 2" for some super-pumpers. The magnetic gap is generally about 1/4"-or 6 mm to 8mm.

If anything, the calculated response gives a stricter figure than the measured response. A calculated Xmax of .25" will often show up as .29" in a DuMax test, for instance. These differences are not important.

For an 8" or larger speaker that gives no specs, assume the Xmax to be .2 inches. If it gives the Voice Coil Length, assume the Height of the Magnetic Gap to be either 1/4" or 6 mm to 8mm. So a speaker with a .75" Voice Coil, you can assume it will have an Xmax of .25". If it has a one inch voice coil, you can assume it will have an Xmax of .36"-which is very good.

In hifi speakers, the Xmech is usually 1.5 to 2 times the linear Xmax. In PA speakers, the mechanical Xmax is frequently four times the linear Xmax-a PA speaker with a linear Xmax of .2 will have a mechanical Xmax of .8, usually.
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Old 1st February 2003, 12:33 AM   #7
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Quote:
Originally posted by kelticwizard

Sean, that was unforgiveable. You are hereby sentenced to spend the next two weeks listening to nothing but an old $19.95 cassette boombox without a CD player!
NOOOOOOOOOOOOOOOOOOOOOOOO!!!!!!!!!!!!!!!!
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Old 1st February 2003, 12:35 AM   #8
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I should add that it is a good idea to check the voice coil length and magnetic gap height for another reason.

Most manufacturers give the Xmax as meaning the distance the cone moves from the center position all the way forward. Or, from the center position to all the way back.

However, some manufacturers, chiefly car manufacturers, give the disatnce the cone moves from all the way back to all the way front-double the distance from center to front.

That is why speaker manufacturers frequently give their Xmax figure as (+ or -).25 in, or something similar. A plus or minus figure means they are measuring from center to front, or center to back, the way they should. A plus or minus figure they are not measuring front to back, which gives an inflated figure.

However, when you know the voice coil length and the gap height, you can double check the Xmax figures, so you can know for sure.

Thought I would just give a graphic of the formula for Xmax. Voice Coil Length is sometimes called Voice Coil Height.
Attached Images
File Type: gif formaula for xmax and vd.gif (3.3 KB, 765 views)
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Old 1st February 2003, 01:09 AM   #9
Mark Kravchenko
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Doug:
The wiz has it down on the money.
For a chiefly by the pants method you can try to shine a brite light onto the spider assembly and see if you can guess the length of the voice coil as you physically move the cone fully up then fully down. To put a real life touch on the magnetic gap height you should look at the thickness of the the top plate or the big washer on the top of the motor assembly that is closest to the cone of your woofer. Have fun
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Old 1st February 2003, 03:54 AM   #10
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This may help:

http://www.linkwitzlab.com/faq.htm#Q29
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