Question: What is blow resistance?

GR : It is the acoustical resistance created by the mouthpiece when playing the instrument. Such resistance depends on the specific design of the cylindrical bore and the backbore. The blow resistance number that we use is a ratio, which is a multiple of the trumpet bore velocity head. The mouthpiece blow resistance affects the acoustical impedance of the resonating air column. I use this when analyzing mouthpieces and matching the player to the mouthpiece.

The bore size alone doesn't provide me with enough information to accurately make a match. Blow resistance is a function of diameter and length. I also calculate the volume of each mouthpiece. This includes the cup, bore and backbore.