Just to add something to Rowuk's great answer to your question: as trumpet players we generally refer to the amount of "work" we have to do to maintain a good vibration in terms of "overcoming resistance". In fact, we are really overcoming impedance.
In electrical engineering terms, resistance is that which resists the FLOW of an electrical current (DC); impedance is that which resists the oscillation of an electrical current (AC). Virtually all of the elements of a trumpet design have something to do with the IMPEDANCE of that instrument: in other words, it's "willingess" or "resistance" to maintain a good, strong vibration within the air column. Poorly placed braces, different metals, design and condition of valves, even the amount (and location) of annealing of the bell can affect the impedance. Because we have to work harder to maintain the vibration in a high impedance instrument we tend to think we are overcoming "resistance" so that's the term we tend to use. (and let's face it, "resistance" is a more commonly known term than "impedance"!)
We've frequently heard how a "large bore" trumpet really doesn't have that much different a "resistance" than an ML or M bore; the reason is that it doesn't take much more energy to push air through a .464" dia tube than it does through a .460" dia tube. The real question should be "how easy does it support the vibration set up by the player?"
A heavier bell horn will tend to resist allowing the metal to vibrate, thus tending to keep the energy of vibration within the air column (pushing the sound "out front") but also reducing the amount of vibration in the metal of the bell and thus, the sound that the player's ears hear coming off the back side of the bell. This is also why we tend to think of a yellow brass instrument as being "brighter" than one of red brass...it is harder and "rings" more. Consider what's been said about Monette: that they really project to the audience easily.... AND that they are so softly annealed that it isn't recommended that you put them on a trumpet stand because they'll bend easily! IE... they are really efficient and they really project "out front" because they resist the vibration of the metal of the bell.
Check out the studies that Renold Schilke did where he experimented with different bells including lead! (horribly impractical but demonstrating the point).
http://www.dallasmusic.org/schilke/B...html#Materials .