First off let me say I have no electrical background and I am not DIY (soldering and the like) kind of guy either so my thoughts are worth what you are paying for them...
I can see how the copper sleeve could stop airborne nasties from entering in at the IEC opening but a noisy cord like the one I have when the copper sleeve is in place isn't necessarily stopping anything from entering. It may only be acting as a shield on outbound noise from being detected. The noise traveling along the cable may be going right into the unit and the noise is less audible at the copper sleeve because its blocking it from coming out. It may still be blocking anything more from going in (which is a good thing) but it isn't necessarily blocking this noise from going into the unit. Something that may block something from entering may be just as effective blocking something from coming out as well. I still think insulated cords may be the best cord to use this tweak on.
Well thats the extent of my technical BS and I defer this to people more knowledgeable in these matters...
I can see how the copper sleeve could stop airborne nasties from entering in at the IEC opening but a noisy cord like the one I have when the copper sleeve is in place isn't necessarily stopping anything from entering. It may only be acting as a shield on outbound noise from being detected. The noise traveling along the cable may be going right into the unit and the noise is less audible at the copper sleeve because its blocking it from coming out. It may still be blocking anything more from going in (which is a good thing) but it isn't necessarily blocking this noise from going into the unit. Something that may block something from entering may be just as effective blocking something from coming out as well. I still think insulated cords may be the best cord to use this tweak on.
Well thats the extent of my technical BS and I defer this to people more knowledgeable in these matters...