All other things being equal, the closer the speaker is to your ears, the better you'll hear it over the ambient noise. Also reduces the audio wattage you have to feed into it.
Unless you're using loud talkback, and have feedback-howl issues. A noise-cancelling mike is the way to reduce this. A noise-cancelling mike can't tell "noise" from your voice. It can only tell near audio (your mouth up against it) from far-away audio, like 3 or 4 feet. It's really a distance-cancelling mike. And if you hold one a full armlength away from your face, it becomes a voice-cancelling mike.
Putting a speaker on the other side of the truck cab allows a noise-cancelling mike to reduce feedback howl more effectively. But you'll have to turn the receiver volume higher than when it's a half a foot from your ear.
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