Like Kelly said, some people like to surround their horns with damping material around the bell. This adds mass and acts like a shock absorber to lower the resonant frequency and attenuate ringing.Metal horns sometimes ring if the flare is highly tensioned. So the best way to damp a horn in this case is to reduce tension. Cut the horn - the section dividers - and this destresses the bell and damps it. This is similar to the way bells that are cracked don't ring anymore, which is exactly what we want in this situation.