High spatial density measurements of the vibro-acoustic response of some aircraft structures

Experiments involving spatially dense interior pressure and surface velocity measurements (10–1000 Hz) on aircraft structures both in the laboratory and in-flight are reported. In the laboratory experiments, the structures were predominately excited by point forcing functions applied at a number of locations on the fuselage. The structural-borne energy couples energy into the interior at resonant frequencies of both the structure and the interior cavity. The normal modes of the cavity tend to dominate the problem at frequencies above 100–200 Hz where the density of states is very high. Further, we also report on in-flight measurements involving the use of a scanned microphone array (55 microphones at 32 axial stations) in the passenger cabin of a turbo-prop powered aircraft. The surface velocity was determined through near-field acoustic holography (NAH). The counter-clockwise rotating propellers sweep past the fuselage, resulting in a slap of air pressure on the aircraft sidewalls. This motion creates a dominant antisymmetric (left/right) surface velocity and interior pressure distribution at the blade passage frequency and its harmonics. The results associated with two laboratory experiments and one in-flight experiment, along with some comparisons to complex three-dimensional finite element-infinite element numerical models, will be reported.