The simple bell-boosts-treble explanation of trumpet tone is incomplete.

Evidence

The bell → treble boost → louder upper partials → brilliance of trumpet explanation sounds right and is easy to remember. The problem is more subtle than this, however. Sound from a trumpet is not like beer coming from a spigot, where opening the spigot wider gets you more beer. — Eric J. Heller, Why You Hear What You Hear- An…, 1512