Do like I'm doing. Take your horns, with the driver attatched, to a local university engineering department and talk to the NVH prof (noise, vibration, and harshness) and have do an "experimential modal analysis" on the horn and driver. That with tell you the resonent frequency of the horn/driver and from there they can tell you what freq your dampening stuff should be and were to get it. With this method you can make a horn/driver combo as dead as concrete (which by the way has a resonent freq but is to low to notice)