I conducted extensive testing on my longbow, with and without silencers, and my arrows, with differing fletching back in the summer of '08. I contacted Truflight reference their testing of fletching noise for advice on equipment and setup. There advice was to use a fixed gain microphone and audio tape recorder with a constant volume gain for the recording. They tested with the recorder 20 yards down range with the microphone pointed towards the bow (and arrow path), 18 inches from the impact point of the arrow. (18 inches was assumed to be an average ear to heart distance for a deer.)
In my testing, my personal Hill hunting bow was virtually dead silent, without the aid of silencing material, at a range beyond 15 yards. Inside this range, the sound was minimal. This mirrored the Empirical data collected from actually arrowing whitetail deer with my combination, as the bow noise proved minimal.
I believe the quietness is all relative to the individual. Even if you measure the sound with a db meter, a bow with a low reading can still be perceived as louder due to the sharpness of the tone. Rick ****** conducted testing the same time I did using a skinny string and he posted a video to demonstrate his perceived quietness of a Black Widow bow using a low stretch string. The bow would probably measure low on a db scale and I take it that he posted the video to demonstrate a quiet BW recurve. But to me the bow sounds loud as it had a sudden "crack" when fired akin to the sudden sound of an axe striking wood or your hands clapping together. The crack of the bow is louder than the arrow striking the target downrange. This is with two puff silencers, limb pads, and a large quiver with a half dozen or so arrows. From my point of view, most every bow I've ever handled or observed with such a string has this same sharp "crack" when fired, some being louder than others. If you stand back, you'll notice that this sharp "crack" carries a good ways.
Flip-side, I generally use 14 strands B-50 which has more of a "whoosh" when loosed that is easily tamed. On all my videos measuring sound like Rick's, the B50 sound has a more gentle "whoosh" followed by a much louder thump of the arrow striking the target downrange. On my longbow, this is with a naked bow without any silencer material whatsoever. The sound is very subjective as I'm sure many of you would think that Rick's bow was quiet whereas I take it as loud. I shy away from any sharp sound from the bow no matter how quiet.
Maybe there would be a way to chart the soundwave on a graph to show more than just peak db. Regardless, I can see where someone would be very happy with any of the combinations as the perception of noise is very subjective.
Best:)