I have found that it is extremely effective to avoid any loudness in communicating. People associate loud voices with anger and aggressiveness. Since I've got a penetrating voice, I've learned to speak only as loudly as necessary for people to hear me.
As some of you know, I do living history. Folks seem to be annoyed by those doing Civil War officer impressions if that person speaks more loudly than necessary to be heard, even in a formation where there are 200 men in line of battle. I figured out it is the difference between being loud enough to be heard, to get everyone moving or whatever, and being so loud that the only reason you're making all that noise is to broadcast that you're in charge.
Ditto firefighting. While one sometimes must shout to be heard in person over the noise from a fire, that's never the case with radio communications, and it's important to keep the excitement level down by speaking into the radio like a BBC news anchor, calm and quiet even if you're reporting the end of the world. Otherwise you spread excitement, not the best thing for firefighting where it's already exciting enough without any help. You don't want adrenaline pumping, you want blood going to the brain for thinking fast.
So yes, people make conclusions based on displays of noise and whatnot. The diffidence that an INTP can bring to a situation is sometimes useful, and can be described with an old-fashioned word: Aplomb. On the other hand, if you are calm or diffident when someone is urgently demanding your attention, it can have odd results (see Banana Mango's WTF? thread).
On the third hand, inappropriate diffidence could be seen as a variation on Kipling's "If": "If you can keep your head when all those about you are losing theirs, then ... you don't understand the problem."
