2. speakers
  3. 1083195223

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
audiokinesis
2,722 posts
 
In my opinion (as a long-time amateur builder of mediocre speakers), that 5 degree phase shift you calculated is audibly inconsequential. And chances are by moving the voice coil of the woofer forward that 1/2", you just might make your speakers imperceptibly better.
gregm
3,395 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
3,650 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.
More to discover