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I really don't think it's going to be anywhere near as accurate as you think it will be. It'd require you to have the same speed at the same distance and angle to the mic every time. Otherwise, if you swung faster, but the maximum speed was another 6" past the microphone and another 2" away from it, it could read as significantly quieter. I think you're ignoring some really basic physics here, my man. And the fact that microphones in phones are highly directional, too. It's not like an open-air mic in a quiet room. I think this idea lacks almost all merit.

I played a hole that looked just like that at Full Cry at Keswick Hall, down near Charlottesville.  Its a Pete Dye design, but clearly this hole imitates the work of some older architects.  Golf Club Atlas suggests there's something of Langford and Moreau in the wrap-around bunker. After my Ohio trip, and a recent visit to the Gettysburg area, I'm up to 321 courses, with at least one additional planned for October.  A long way behind @jsgolfer, but still a lot of fun places.

I get that if I do a couple swing today at my home course, and later in the week at my house the decibel readings could be totally different. The surrounding environment of both are totally different so the reading are likely to be different. But for one session only, the current one with the same environment from start to finish, if my normal swing produces 90 decibels as a base line, I should be able to test other swings in order to find one that produces more decibels and expect it to be faster than the base line. I look at this the same way as I work with my PRGR. With 7 iron at home, my average ball speed is 115 but at my home course ball speed is around 120 (same ball, same intent. I guess this is because at my home I don't hit that hard down on the ball to prevent injuries). So.. when the other day I tested a new swing and averaged 122 at home I knew that at the course it should be around 127 and It was almost there at 125.   My question is.. if the environment is the same from swing A to swing B, could I compare their speeds by comparing their decibels? Not the exact speeds but witch one is faster?   

I'll report back. 😁

Guess? Make a spreadsheet. 😄