A company responsible for the interior design of a business jet discovered an annoying sound in their new top-class business airplanes during steady flight. Business jets have a high standard of quality and the sound impacts experienced during a flight should be kept at a minimale, comfortable level. To address the acoustic insulation during the next maintenance cycle of the aircraft, the major sound sources during the flight needed to be identified. For this purpose, an NVH measurement (Noise Vibration Harshness) with the Acoustic Camera was used.
Bombardier BD-700 - 1A10 Global Express
The spherical array was set-up in three different positions, in front, in the middle and in the end of the passenger cabin. The data recorder and the mobile power supply were placed between two groups of seats. The set-up of the system took less than 20 minutes.
As basis of the three-dimensional analysis, a 3D model of the cabin was created using a Faro Scanner (see Fig. 3). Due the lengthened shape of the aircraft, three scanner positions where measured and fitted together into one 3D model. After the measurements the 3D-model was imported to NoiseImage and fitted via a special, patented algorithm. For the analysis the acoustic data was mapped onto the 3D model.
During the flight, only stationary noises occured. The sound pressure levels (SPL) showed a significant increase of 2 dB in the front and the middle sections of the cabin. After applying an A-weighting filter, the difference did not seem to be as significant (less than 1 dBA). The major emissions could be located in the middle of the passenger cabin, above 630 Hz.
The acoustic data was analyzed in the frequency domain and in the time domain.
The origin of the noise was found in different third octave bands of 250 Hz, 315 Hz, 3150 Hz and 6300 Hz. The sound was radiating from the floor in the middle of the cabin and from the side of the fuselage.
These Figures visualize the results from two different array positions. The loudest sound sources were located at the ceiling above the chosen measurement position, 85.22 dB (front) and 83.06 (back) dB. In both scenarios, ground-reflected sources, around 68 dB, can also be seen.
