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-   -   Estimation of Xmax w.o. any VC data? (http://www.diyaudio.com/forums/subwoofers/172106-estimation-xmax-w-o-any-vc-data.html)

buggsson 18th August 2010 08:54 AM

Estimation of Xmax w.o. any VC data?
 
I have an 8" sub, Response RW-8 that I'm planning to use in a ported enclosure. Apparently, I'm in need of the data for Xmax and there is nothing useful in the very sparse data sheet to help me out. I know I've read somewhere of a way to at least approximate the Xmax value, but I cannot recall where it was.

Anyone knows how to do it?

djk 18th August 2010 10:53 AM

Shine a light through the spider, see how much overhang there is on the VC, add about 25% of the top plate.

Top Shelf 18th August 2010 06:35 PM

Like djk said, I use a green pen laser for better illumination thru
the spider. It also helps to have a good eye @ estimating inches or
millimeters.

Ron E 19th August 2010 12:47 AM

Quote:

Originally Posted by buggsson (Post 2276194)
I have an 8" sub, Response RW-8 that I'm planning to use in a ported enclosure. Apparently, I'm in need of the data for Xmax.

What do you want to know? Really, there is nothing wrong with putting in a value of say 5mm just to see what you get. If you can't see the coil, many inexpensive woofers have an xmax of about 1/2 to 1 times the top plate thickness....

Andrew Eckhardt 19th August 2010 12:59 AM

You don't need Xmax data for design except for choosing minimum port diameter, of course. If you are sensitive to harmonic distortion it's pretty easy to tell when a woofer is reaching its excursion limit by driving it in free air with a good amp and a clean 20Hz sine tone until you start hearing something other than 20Hz. With a decent eye, measure the excursion when unacceptable harmonics (hardly any more than you start off with) are detected. That's woofer Xmax. Watch that parallax. :)

buggsson 21st August 2010 08:12 AM

Quote:

Originally Posted by Andrew Eckhardt (Post 2277043)
You don't need Xmax data for design except for choosing minimum port diameter, of course.

That's the very reason! That seemed to be a good parameter to know, if attainable. I'll give the visual inspection a try, and if it doesn't work, a guesstimate will have to do. It's not very important, the result of theese/or that wooferbox (dual), as I'm just gonna have this pair of drivers to train measurements on, so I don't wreck the "real" drivers when they arrive.

HK26147 24th August 2010 12:59 PM

You could attempt to measure it yourself:

http://www.linkwitzlab.com/images/gr...edge-micro.gif

buggsson 26th August 2010 09:43 AM

For getting the dust cap to move, do you play some test tone, music or other means when doing the trick as per the attached image?

On another forum, there was a guy who measured Xmax electrically, I didn't really understand the method, but it takes some gear to be able to do it, and knowledge of course.

I've arbitrarly decided on 6mm as "my" value. I've pressed down gently with my fingers, and I believe 6mm will be a safe figure. However, I've to read more carefully in the future, I missed you are supposed to shine through the spider, will do later this evening. Maybe I get something different after that.

HK26147 26th August 2010 01:05 PM

Quote:

On another forum, there was a guy who measured Xmax electrically,...
( I believe I've seen this and the embedded link about Xmax, requiring the ability to measure T/S with large signals )

Sigfried Linkwitz's article "Excursion-Limited Spl Nomographs ( SpeakerBuilder 4/84 ) was one of the first articles on the subject.
I linked that pic from his site.

Electrical and mechanical limits
Page index

Perhaps he would provide insight into it's use.
( My first inclination would be to use sinewaves not music,
I've had some ideas for bringing this micrometer into the 21st century, but they are untested )

Andrew Eckhardt 26th August 2010 01:20 PM

I didn't see anything about in at your links, but it should be fairly plain to see the coil leaving the gap on an FFT of the current running into it. Just wire your speaker through some resistance and hit it with a test tone. Send the signal across the sense resistor to your PC running a spectrum analyser and when the harmonics start to spike you're leaving linear excusrion. With a good eye you might even be able to see it on a scope, but you could probably hear it earlier. Any way you do this yoiu're still faced with actually measuring the excursion somehow, which is probably the greater error source. Accuracy is not incredibly important, since getting 3 dB higher output requires double the excursion anyway.


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