You are in for quite a chase. Douglas was one of the first aircraft manufacturers to do extensive acoustic testing of cabins in the late 1930's and early 1940's. Douglas lead the way by stuffing sound deadening material into the hollow aluminum tubing of passenger seats. Subjective noise levels will vary, of course. Vibration accompanying a loud sound will always make the subject think the sound is louder than it actually is. In the late 1940's and early 1950's, several Trans-Atlantic low-cost operators routinely removed the sound deadening materials from Constellations and DC-4's to increase payload at the price of their passengers damaged ear drums. The good ole days weren't always that good. Nostalgia ain't what it used to be.
Last edited by evansb; 1st Apr 2018 at 23:33.