Posted 21 July 2012 - 03:53 AM
I discovered why I have the buzz in one of my AR 3as. I replaced the bad foam surround with cloth surround, a technique that has been documented on the internet. Thinking that the bass speaker has to be free to move, I allowed extra cloth so I could form a curl much like the original foam has. With cloth this allows the speaker lots of movement (probably too much). Anyway when feeding a 32 HZ sine wave, I can eliminate the buzz and get a pure sounding bass by tugging the surround towards the outside in an area about 25 degrees wide. since the cloth is glued to the frame and to the speaker cone, I have to figure out how to make this slight outward pull permanent without trying to get the cloth off the frame. Believe it or not, since the surround is cloth, I have thought of using a safety pin to shorten the surround fabric and create the tug. With 32 HZ in it's easy to check the sensitivity of the cone (probably the voice coil) to buzz, by gently tugging on the surround as you move around the cone. It seems centered every where else except this 25 deg area. Glad I had my little B&K tone generator, because it really tests the sensitivity of the cone to buzz. Finally, I don't have enough experience to know if this Ar 3a had a voice coil issue, or whether allowing extra cloth surround caused the problem. In other words I don't know how much support the surround is supposed to give the cone and the voice coil.
As others have pointed out you want to be gentle while feeding a sine wave to a speaker, because you can easily generate more bass energy than you will ever see in music. Feeding a sine wave is an interesting test when done gently. You can determine the point at which the speaker doubles the bass and distorts by producing twice the input frequency. Up till this point with a good speaker you feel the bass at 32HZ as much as hearing it. If the speaker cranks out a high volume before doubling, it's a good one. The bad thing about the test is every speaker will double (distort) at some point so unless you have maybe a Klipsch horn enclosure, you can forget about extremely loud undistorted bass.