## 3 Experimental Set-Up  
An experimental campaign was carried out at CIRA to evaluate the proposed technology in laboratory conditions. A set-up consisting of a dummy seat with a headrest was equipped with two monitoring and error microphones and tested within a semi-anechoic chamber. Similar experimental conditions were also investigated in [25] to assess the acoustic performance of two passive noise control concepts based on either a tailored headrest shape or an innovative nanofibrous textile material. The receiving room was a semi-anechoic chamber with dimensions of \(6.5\times 5.3\times 4.2\) m (volume 145 m\({}^{3}\)) and a cut-off frequency of 70 Hz. A binaural head was also used to replicate passenger volume and acquire reliable acoustic data at the position of the ears. In the laboratory set-up shown in Figure 4, the noise disturbance was generated by a dodecahedral source at 200 Hz. This tonal acoustic field was assumed to replicate the second turboprop harmonics detected by passengers within the cabin as the primary noise field to be cancelled. The free space next to the passenger was an approximation of the cabin environment, whereas the two speakers on the passenger's sides simulated the counteracting noise field. The acoustic acquisitions were made over the entire audible range (20-20,000 Hz). The analysis compared the acoustic field, which was recorded and then elaborated in terms of SPL and sound intensity, with the acoustic control activated and non-activated, at the position of the ears. The multichannel filtered-x version of the adaptive LMS algorithm was developed in MATLAB/Simulink to derive the optimal control signals driving the control loudspeakers. A C code was then generated in Real-Time Workshop and implemented via a DSP control board for real-time experiments. Figure 3: The block diagram of the \(2\times 2\times 2\) FxLMS algorithm for active noise cancellation of the ANC headrest system. A schematic of the experimental set-up is shown in Figure 5. The dodecahedral source emitted the disturbing noise, whereas the counteracting acoustic field was generated by two (left and right) Pioneer TS-G17111 17 cm dual-cone (170 Watt) loudspeakers. During all the tests, the microphone signals were conditioned by a PCB 16 channel ICP conditioner. Three types of acoustic sensors were used: two (left and right) 1/\" control microphones, two embedded microphones in the binaural head, and one probe for sound intensity measurements. The first couple of microphones provided the error signals processed by the active control system. The second one was used to monitor the ANC performance in terms of sound pressure level reduction. Finally, because one of the objectives of the experiments was to assess the dimension of the quiet zones at increasing distances from the passenger, a dedicated probe, shown in Figure 6, was used to map the sound intensity (both magnitude and direction) around the passenger's head. Figure 4: ANC tests under semi-anechoic conditions. Figure 5: Schematic of the ANC experimental set-up. The sound intensity was measured by a 3D Sinnenter Soundbrush system. This acoustic tool combines 3D sound intensity with a visualization of the sound source to provide instant 3D visualization of the sound intensity magnitude and direction during measurements. The acoustic probe contains the data acquisition unit and a color-illuminated sphere so that its position can be determined by a tracking camera, while an inertial platform provides a 3D orientation of the probe. In this way, the system allowed us to measure the sound intensity vectors and map scalar sound pressure amplitudes at multiple circles of varying radii from the same center point. A LMS SCADAS III data acquisition system was also used to measure the acoustic signals and calculate both the primary and secondary FRFs prior to active control system implementation. A digital dSPACE DS1103 PPC controller board with a real-time processor and a comprehensive I/O was used for control prototyping.