OP
A double hose regulator exhausts from the body which is located behind and out of sight of marine life a photographer might approach. The noise and immediate blast of bubbles is diffused further by the outlet holes and blocked from view largely by the divers body as is the sound. It is more than a few inches different, it is much different. Marine life do see and hear the exhaust and it is the first thing they see and hear with a single hose diver and it is the last thing they see with a double hose diver. The difference is considerable and noticeable in the behavior of shy organisms.
As well, a double hose two stage regulator is much quieter, perception, to the diver, there are no bubbles exploding in the ears or slapping the face and neck. That difference is considerable as well and quite noticeable to many divers.
Another not often touted feature or advantage is the buoyant mouthpiece, it does not hang down or pry on the jaw and upper teeth like many (all) single hose second stages do, which for some people can cause headaches. I am particularly sensitive to this as courtesy of a drunk driver, when I was 12 yo, I got to go through a windshield which knocked my front teeth out an broke my jaws. My mother also got her teeth knocked out. I have implanted front teeth but got to go through HS with none. My mom has a bridge. I dive single hose regs, certainly, it is not a huge deal, I am pretty durable,
, but I do find the buoyant mouthpiece more comfortable and a lot of people do, for their own reasons.And JFYI, the double hose, two stage regulator contains both stages and exhaust in the cans/body, there is no stage at the mouth (nor purge valve either). The continuous loop and high exhaust position (for normal swimming position) provides for easy exhaust and in the case of the Argonaut, the large mushroom valve is effortless.
I understand, particularly for divers who have no training or experience with double hose that they may not want to put the effort into adjusting their diving style or the needed equipment selection/choice to fully realize the advantages and negate true disadvantages and peculiarities of a double hose regulator (single hose have their own set of peculiarities, you just never had a choice before). That is fine, but I am not going to limit my arsenal, viva la difference (or something like that).
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