Effects of acoustic treatment on the interior noise of a twin-enginepropeller airplane

A study of the cabin acoustics of a Fairchild Merlin IVC twin-engine propeller airplane is described^ The sound field was measured at six locations inside both an untreated "green" airplane and a completely finished airplane. Several flight conditions were tested, including different altitudes, engine power settings, and cabin pressures. The overall sound pressure level for each test condition and microphone position was computed from a one-third octave band analysis of the data. The blade passage frequency and its integral multiples, which dominate the low-frequency noise spectrum, were examined using a narrowband analysis of the data. The insertion loss due to the added acoustical treatment was determined by comparing the narrowband results from the two airplanes. These insertion loss values varied widely, depending on many factors, such as position in the cabin, multiple of blade passage frequency, cabin pressure, and engine torque. The space-averaged sound pressure levels corresponding to specific tests of the treated airplane were found to be in good agreement with predictions from the Propeller Aircraft Interior Noise (PAIN) computer program.