To know the acoustic properties of an interior space in each point has been always a difficult task. The solution to this problem has been given during the last years by simulation programs. Nowadays the Acoustic Camera offers a real solution to know the different parameters that describe the acoustic properties of an interior space.
This application note describes the process to measure the acoustic properties of an airplane cabin. For this purpose a one-propeller Cessna C-172 has been measured during the flight and landing processes. The results have shown and localized several excitations inside the cabin.
Acoustic measurement, analysis, and noise source location in cabins of cars, airplanes, trains, trucks… etc. besides rooms, offices, etc.
Cessna C 172 (4-seat, high-wing, one-propeller airplane)
The Acoustic Camera was set up inside the cabin, between the seats, together with the data recorder and the computer (Fig.2).
The system offers the possibility to be used with batteries. Therefore, the use of the Acoustic Camera and its software are appropriate in a circumstance like this one where the measurement has to be done on vehicles in motion. After the setting, the array position has been fitted into the 3D-CAD model of the airplane in order to get the results in 3D.
Despite how loud it was in the interior cabin during the flight, the results show where the squeaks, rattles, and knocking noises (the knocking noises were introduced manually on the fuselage) were localized with accuracy.
During the analysis of the data, certain frequency bands were distinctive:
the frequency band placed between 750 Hz and 1300 Hz shows a leakage from the front part of the roof into the cabin. In this region the connections to the wings are located. However, the results show a stronger emission on the right side.
The frequency band delimited by 2 KHz and 6 Khz showed a leakage at the rear sealing of the left B-pillar and an emission through the right side window.
The higher frequency range shows a distinct emission at the right A-pillar which indicates a defect door fastening. This squeaking noise could be further analyzed with the new psychoacoustic module to collect more information in order to improve the sound quality of the airplane.
The noise generated during the landing came from the right back cabin floor. The analysis of the frequency distance between 660 Hz and 1400 Hz shows the spot of emission in a chassis juncture very accurately.
The 3-dimensional acoustic analysis, in connection with the utilization of portable hardware, permits to realize an analysis of complex noise structures on motion. Both noise types, stationary and impulse noise, can be analyzed and located accurately even in low frequency ranges and in loud environments.