Well I think he is somewhat right. I took physics class in high school and I did pretty good so here is my explanation. What you are talking about also happens in cars, I notice my car gets louder with the windows rolled down and/or door opened up. The reason this happens is because things absorb waves. So your truck being small in size, is absorbing a lot of sound waves, so I'm sure with the sound waves bouncing back and forth (and into new waves) yes that can cause cancellation. So some sound waves are being absorbed, and some are "cancelling out". It's basically like putting a cloth over your mouth and trying to talk, your sound waves don't go as far, but when you take it off, your voice is louder, kind of like when you open your door, there is more space for air to flow and waves to travel. I'm sure their probably is someway to eliminate some of the cancellation, but not sure how to. You could do research if your really interested, maybe invent something people will buy into. But as long as your airspace is limited, that also limits the range in which your sound waves travel.