You don't need a lot of air volume to make the trumpet play - compared to what is needed on trombone or tuba. But what you do need on trumpet is air pressure - more than is needed for playing trombone or tuba. That air pressure has to be provided. Maybe this helps for some (unrelaxed?) people as a visualization for playing trumpet (I don't like it though). Fact is: By just relaxation there is not enough air pressure produced to make your lips buzz, especially in somewhat higher frequencies. Why would you (or anybody) not want to stay as relaxed as possible, no matter what stuff you are going to play? .