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Post by shomrighausen on Nov 23, 2005 12:54:51 GMT -7
Measured fs of 29
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Post by ThomasW on Nov 28, 2005 17:55:27 GMT -7
I'm updating this thread with posts from the AE forum. This is a post from Deon B. He currently works in subwoofer development for Klipsch. In a previous life he was responsible for the R&D of the original Stryke HE-15, and his multiple PR based sub called "The Beast" is legendary in DIY audio circles. ------------------------------------------------------------------------------ This post isn’t to tell you your measurements are so far off that there is nothing wrong with the drivers you received, just to point out a couple things you should keep in mind in the future when measuring drivers.
While working with Klipsch one of the most difficult things to do was to get new incoming engineers and techs up to speed on accurately measuring and documenting drivers. I probably spent more time on this than actually teaching driver and system design.
There are SO many opportunities to mess up a small signal measurement – and then inaccurately interpret the results, it isn’t even funny.
Trying to get Vendors to supply accurate and repeatable measurements was a NIGHTMARE, and required my literally traveling to them, designing and building jigs, fixtures, and test enclosures, then training them on how to correctly use the equipment, and how to look for errors in and interpret the measurements.
1: If you are using added mass, the mass must be securely affixed to the cone to procure repeatable measurements. ANY movement of the mass will affect Mms, Cms and the Q of the impedance peak. And no - resting on the cone is not securely fastened - you would be surprised at how little (even millivolt levels) movement can disturb unsecured mass.
2: Added mass method does not work accurately with low Fs drivers, to provide accurate small signal parameters with ANY method, you need approximately 50% shift in Fo. With added mass on a 19hz driver, you’d need to lower Fs to ~~ 8.5hz, which is below the accurate resolution of most available measurement mechanisms. That aside mass is still the least accurate and least preferred measurement method – Delta compliance is the most reliable. And even so, using Delta compliance, error's are easy to introduce, the system MUST be exceptionally well sealed, AND you MUST have an accurate air volume - including the cutout of the wood baffel, and the area inside the cone.
3: You cannot lay a driver on the ground, or affix it in a jig with the cone face up and get accurate results. You are biasing the drivers suspension, and including this ‘increased spring tension’ in your measurement – which will distort the Cms and VAS measurements, consequently giving a lower Bl reading. (it's working against a pre-biased restistance, thinking the cone is working against only the mass and air load). Adding mass exacerbates this. Measure the driver in a horizontal orientation.
4: It is critical that the impedance peak of the post mass or compliance curve, have a very clear and sharp peak just as in the free air measurement. If the peak is ‘soft’ - again suspect the sollidity of the drivers jig or mounting mechanism. Drivers, especially high MMS drivers must be very solidly affixed during both measurements – or once again – small signal parameters will be affected.
I’ll write up a quickie measurement method post and log it under the knowledge section sometime. _________________ dB
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Post by rbuszka on Dec 1, 2005 13:22:13 GMT -7
So, then, it seems that driver measurement should be left to the professionals, and people should not blow a blood vessel if their own added-mass measurements of high-Mms, low-Fs drivers don't look perfectly right, or else invest in the specialized setup it would take to do measurements the way the pros do.
EDIT by moderator
ok, fine - at least I wasn't being mean, just saying a bunch of unnecessary extra stuff. So don't go away with the impression that I ws being mean.
But I'd like to point out that you reported the name of Deon's subwoofer as the "Breast" instead of the "Beast". Just a humorous observation. You wipe my nose, I'll wipe yours, I suppose. But in a good way. _______________________________________________
The "breast" is what happens when I post after taking migraine meds. The typo is fixed. Thanks...
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Post by shomrighausen on Dec 2, 2005 21:07:11 GMT -7
The contents of this post have been removed by the moderator. And the author may no longer post on this forum.
A copy of the deleted post has been archived.
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