In a copy of the "American Rifleman" dated Dec. 1949, there is an article entitled "Food For Thought" about the effects of engine noise on aircrew during long flights. It was thought to have an effect on the ability to think and act quickly and impair concentration to the job to hand.
They enlisted the support of Naval ROTC cadets enrolled at Harvard and active target shooters from the Harvard Employees Rifle Team. Amplifiers that put out the noise level of radial engines were installed and shooters were required to shoot groups in six strings (100-120 shots) from the off hand position. A total of 85 scores were fired during the test.
To everyone's surprise, the average of scores increased during the test! Noise levels were 100-110 decibels, enough to cause shooters to feel their diaphragms vibrate in their chests! All this without ear protection of any kind.
Immediately following the tests, scores returned to their previous levels. There was no comment about damage to the hearing of the shooters.