811 Fins

[GM]

So, I presume the sand and latex paint will work equally as well on the 511 horns. I understand the 511's ring worst than the 811's. I have not had the ringing condition on my 511's but, I would like to know of a remedy should ringing occur.
Thanks...

Yes, though being a larger horn its physical Fs is lower, ergo has a wider BW and higher amplitude, so requires more damping mass, so keep layering it on until a 250-300 Hz sine-wave is < ~-24 dB (and ideally > -35 dB) referenced to whatever your lowest average listening SPL is.

Mounting a horn to a rigid/damped baffle with an area at least = to its mouth area, effectively doubling the mouth's half space radiating area, goes a long ways towards damping it. Ditto using some form of vibration absorbing mounting at the driver end same as cutting out the vane welds and isolation damping them.

GM