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Does 1/2" make a difference at 375 Hz ?

I own a pair of IMF Monitor Mk III's with KEF B139 oval woofers. The woofers are damaged and I was fortunate enough to be able to purchase a new (unused but 10 yrs old) pair of B139 woofers on ebay. Lo and behold, they are not identical to the originals I have. The diaphragm appears to be the same dimensions, but the frame is a different size and the mounting hole pattern is different. In order to mount these on my IMFs, I need to make an adapter plate. I can make this out of 1/2 inch MDF. The problem is that the front surface of the diaphragm will then be 1/2 inch further forward than the IMF design. By my math, at a crossover of 375 Hz, 1/2 inch is about a 5 degree phase shift. Is this 5 degree phase difference critical for the "time alignment" of the midrange and woofer ??
mbonn
gregm
3,404 posts
 
Hmmm 5" shift is supposedly imperceptible, esp at the low end of the ear's critical range. Maybe it's "imperceptibly better" as duke notes above. You've checked the new drivers' parametres of course (or are they, frame excepted, otherwise identical to the old ones?)
kr4's avatar
kr4
3,672 posts
 
There are some differences between the two versions of the B139 other than the frame. Consequently, the 1/2" shift is not a real issue. BTW, I have also made a B139 swap, although it was to use the new(er) version with the ears in place of the older (raceway) version and found no perceptible performance difference in a Cambridge 500 LS.
mbonn OP
43 posts
 
Thanks for your responses. I have not been able to find spec sheets for the 2 versions of the B139 (6171 vs SP1044), so I am assuming that they are probably similar (maybe that's a bad assumption). In any event, I'll proceed with making the adapter plate and mounting the new ones. By the way, the new ones measure 7.0 ohms DC resistance, and the old ones measure 6.5 ohms DC resistance. Is this significant, or could this simply mean that some of the turns of the voice coil on the old ones got shorted out due to over-driving?
gregm
3,404 posts
 
7 ohms dc indicates a nominal impedance of +8ohms, i.e. a somewhat easier load than the 6,5 drivers.
If the coil were shorted, you'd have a much lower resistance reading
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