Warburton Backbore H for Trumpet, Length overall - 2.85", Bore - 27 at interface, 28 at -215", Comparative resistance - Warburton 4. The "H" backbore fits squarely into the commercial category as it seems to perform best with a fast moving airstream associated with shallow cups. Featuring a longer blank with reverse taper in the venturi, the "H" is preferred by strong lead players who are able to convert the high resistance into an efficient and focused buzz. The benefit is that the "H" projects the full spectrum of their sound rather than just making the highs more piercing.