Squeaky Wheel . . . and Other Serious Things, The

Sound and Vibration, Jan 2007 by Miller, Laymon N

An Aircraft Quieting Job

I received a phone call from the Chief Engineer of Cessna Aircraft Company in Wichita, KS. He had learned of BBN's work on aircraft quieting and he said they had a job for us: their Model 310 Twinengine Business Plane. The objective was to reduce the internal cabin noise as much as possible. There were five seats in the plane, two in front for pilot and assistant or passenger and three in back for passengers. There was also a large luggage space behind the rear seats. I went to Wichita and designed a measurement program with their Chief Engineer and Chief Test Pilot. Cessna built some heavy double-wall blankets that would fit over each wall panel and window in the plane, with provision for attaching them and removing and replacing them one at a time in flight. My intention was to build a high TL construction all around the cabin and then to open one section at a time and see how much noise entered the cabin through that opened section. With such a methodical approach, we could determine exactly how much noise was entering through each of the various surfaces and could then treat those surfaces accordingly. I even wanted us to run on one engine at a time during portions of these tests. The Pilot agreed to all this. He was even willing for us to fly 'blind' on an instrument flight plan with all surfaces completely covered for short periods at a time. The Wichita area was full of aircraft manufacturers and their planes were in the air all around. 1 figured if the Pilot was willing, I was willing. Of course, the FAA Air Traffic Control provided safe separation during these tests. With one particular test, we were also able to separate propeller noise and engine exhaust noise.

I was in Wichita for a full two weeks and I received all the cooperation that I wanted. From the test results, I could determine just what kind of lining treatment should be applied to all the wall panels, and I recommended improvements in gaskets for the doors and increased thickness for the side windows. An easy finding was that the small door in the fuselage that opened into the baggage compartment (behind the back seats) was a serious noise leak. I told them about that one evening, and the next day a corrected design was in production. My thought at the time, after working for 13 years on acoustic torpedoes, was: in research work, we measured progress in years; in consulting, we sometimes measure progress in days, or even hours!

When we finished the complete program, they told me that they couldn't do all of this in one year. Instead, they would incorporate a few changes each year over the next four or five years. Sure enough, as I followed their advertisements in one of the aviation magazines, I saw evidences of added improvements in sound reduction for each new model year.

That Crazy Guy-Talking to the Floor

You must realize that in "days of yore" in acoustics we did not always have some of the compact, portable, multi-channel, multifunctional, self-contained equipment that are available these days. In 1959, I was making some vibration measurements that were involved in the design of a new building to be located over an array of underground railroad tracks coming into the central station for that city. While I was making the railroad vibration measurements, I noticed that as people were walking along the walkway, coming from the railroad platforms, I could distinguish vibration signals caused by their walking. With still more careful listening to the signals, I realized that I could detect some muffled conversations of the people talking. That gave me an idea: I talked fairly loudly to the concrete floor where I was measuring, and to and behold, I could recognize my own voice quite clearly. I was actually setting the floor into minute amounts of vibration just by talking to it, and I could then hear the signal that was obtained by my vibration pickup. I did have to ignore the people who would pass by and wonder about that crazy follow, just sitting there, talking to the concrete floor.