Cannons and the resulting "boom"?

Okay, so I have one of those really small "black powder field canons", with a 3/4" barrel. It's about 4-5" long. When fired, the resulting "boom" is relatively "puny", sounding more like a loud bang than an actual boom. The only time i went to a firing of a real canon was quite an experience, as the boom actually was a boom, and the blast made me stumble backwards...(not from being pushed, but from being scared =P). What I want to know is simple: does the resulting concussion vary because of the diameter of the barrel, or is it some other factor, like length...I really want to purchase a salute canon, but im worried that the sound will only be a "bang". Does anyone know???