Md-90 cabin noise diagnostics ground and flight tests

Measurement of Insertion Loss of an Acoustic Treatment in the Presence of Additional Uncorrelated Sound Sources

“…To distinguish between the different intensity calculations, intensity calculated using equation ( 16) is termed conditioned intensity in the remaining sections of the paper. With q=1, this formulation for multiple reference transducers exactly reduces to the formulation for a single reference transducer given by equations (3) through (7).…”

“…The uncorrelated intensity was calculated using equation (1) from the cross spectrum of the intensity probe microphones. The correlated intensity was calculated using equation (7) from the cross spectra of the intensity probe microphones with the excitation signal to speaker 1 as a reference. These two methods of calculating the intensity at the probe were compared for various excitation levels of speaker 2.…”

“…• Case 3: Only bays 1 and 2 treated (Figure 9c) The intensity at each of the 32 measurement points was calculated using the uncorrelated intensity method equation ( 1), the correlated intensity method equation (7), and the conditioned intensity method equation (16). In the case of the correlated intensity calculation, accelerometer 9 was used as a reference.…”

“…In the case of turbulent boundary layer induced noise inside aircraft, the correlation lengths of the excitation pressure field are small relative to the size of the fuselage and the excitation spectrum is broadband in nature. 7 Thus, distant points on the fuselage can be approximated as independent, incoherent vibratory sources that radiate into the aircraft interior. Therefore, as long as the dimensions of the acoustic treatment are large in comparison to the turbulent boundary layer correlation length, the noise resulting from an untreated and a treated portion of the fuselage can be considered incoherent.…”