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?
Anyone knows how to do it?
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....
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. 
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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.
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.
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.
( 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 )
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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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If you could get a woofer to do that you'd be a rich man. My point was that you're worst measurement with any amount of concern or double checking is likely to be much less than 1dB off, and in the end if you hear distortion all you have to do is turn it down a little. If everyone turned it down a little when they heard Xmax, stories of blown woofers would be pretty rare.
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Correct those were related articles about displacement limits and Xmax - NOT specifically on how to use his micrometer.
That's why I suggested that perhaps Mr Linkwitz could be approached directly for details.
I presumed ( perhaps falsely ) that the OP didn't not have access to a scope/analyzer etc to look for non-linearity, and was looking for a simple ( albeit crude ) method of determination.
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Just about everyone has a sound card that can make a tone and listen at the same time these days. Spectrum analyser software is free. On Linux you can use baudline.
But, in the time it takes to set all that up you could get a measurement of similar accuracy very quickly just by listening to the 20Hz sine distortion. Like I said, if you're off by a whole 1mm on a 5mm stroke, it's not going to ruin your design. You can get that close by hanging a ruler over the top of a small piece of tape stuck to the outer edge of the cone. Micrometers don't figure in.
It's best to use the very LF tone with the driver in free air. You don't want acoustic power efficiency for the fundamental, the lesser of that makes the harmonics easier to hear. 20Hz just happens to be a real good frequency too. Lower than that and the harmonics get too low to hear in small amounts, and higher than that as well because they become masked by the fundamental. You need to try to minimize pole vent noise and spider noise pickup (listen to the front of the cone).
But, in the time it takes to set all that up you could get a measurement of similar accuracy very quickly just by listening to the 20Hz sine distortion. Like I said, if you're off by a whole 1mm on a 5mm stroke, it's not going to ruin your design. You can get that close by hanging a ruler over the top of a small piece of tape stuck to the outer edge of the cone. Micrometers don't figure in.
It's best to use the very LF tone with the driver in free air. You don't want acoustic power efficiency for the fundamental, the lesser of that makes the harmonics easier to hear. 20Hz just happens to be a real good frequency too. Lower than that and the harmonics get too low to hear in small amounts, and higher than that as well because they become masked by the fundamental. You need to try to minimize pole vent noise and spider noise pickup (listen to the front of the cone).
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