This woofer may fit SOME Acoustic Research speaker enclosures, but it is NOT drop in ready for the classic and BEST speaker system that I own. The AR-3A woofer is slightly less than 12" in diameter and the basket has opposing beveled edges. You would have to use a router and basically ruin the enclosure to make it fit. I had to re-foam the surrounds on the original speakers, since ONLY exact replacement woofers will fit. I also upgraded the ancient and enormous crossover network, replaced the ancient mid range and tweeter. It sounds better now than it did 50 years ago.