TFNG with DSL work together on the Active Noise Control topic during the PIANO project. TFNG manages all the ANC activities in the project and will integrate the ANC prototypes inside the cabin of the tiltrotor aircraft. DSL will develop and test new ANC algorithm that will be used in the TFNG ANC controller.
Interior aircraft cabin noise in tiltrotor (and more general propeller aircrafts and helicopters) is typically expected by two dominant sources (Figure 1):
- The aircraft engine noise (turbojet or turboprop), transferred via the engine mounts (structure-borne noise) and the propeller noise transmitted directly through the fuselage (airborne noise). Such noise is typically highly tonal at the engine rotation speed and its harmonics.
- The aerodynamic loads as for e.g. the turbulent boundary layer around the fuselage. Such noise is typically broadband.
Figure 1: Noise sources in tiltrotor aircraft
TFNG and DSL analyzed the measurements acquired by LH in the prototype aircraft and conclude to use complementary approaches of Local and Global ANC treatment in order to allow the control of both tonal and broadband noise in the aircraft (see figure 2). Additional measurement done by ICR will also allow to find out if later Active Vibration Control (AVC) system can be used in order to enhance the performance reached by the ANC prototypes developed during this project.
Figure 2: ANC performances aimed in PIANO project
This complementary approaches is necessary for the lowest tones frequencies [50-150] Hz in the tiltrotor aircraft as the speakers integrated in the seat will not be able to generate such deep low frequencies regarding the integration constraints (headrest and cabin size) with the broadband noise performances aimed (local bandwidth [150-350] Hz). As usual, the passive material is required in the aircraft cabin and allows to treat the high frequencies noise that ANC cannot manage (above 350 Hz) but can be significantly reduced as the lowest frequencies are managed by ANC.
Figure 3: Local + Global approaches
Experimental setup and results:
DSL developed a multi-channel virtual microphone detection ANC system (Local ANC, see figure below), which uses three parallel adaptive notch filters and a linear extrapolation technique. The results are encouraging but efforts should be done in order to reduce the speaker size (24 cm diameter at the moment), the number of sensor and their integration in the seat headrest will be complex. DSL submitted a paper entitled “A narrowband virtual sensing Active Noise Control system using ESPRIT for an aircraft interior” to the international Euronoise 2021 conference, presenting the results obtained.
Figure 4: DSL setup for narrowband virtual sensing ANC system
TFNG and DSL decided to build dedicated testbench in order to reproduce the tiltrotor noise cabin environment (see figure 5) for ANC performances evaluation of the prototypes. Subwoofer and speaker outside the “cabins” will reproduce the airborne noise inside the tiltrotor cabin and allow to evaluate the performances of the mock-up and prototypes.
Figure 5: Testbench for ANC prototypes, DSL on the left and TFNG on the right
TFNG is currently finalizing the choice of speakers and subwoofer regarding the seat and cabin size. It is also important to take attention about the weight of the material. In order to do the best compromise an important work is done about the design of the speaker enclosure in order to maximize their low frequency response (level and bandwidth).