## 1 Introduction  
Active noise control occurs via destructive interference in a given location between a sound field generated by a disturbance source and a controlled secondary source [1]. This technology has proven to be highly effective when dealing with low-frequency noise, and offers multiple benefits in environments where passive treatments are bulky and impractical due to weight penalties [2]. In turboprop aircraft, cabin noise is widely recognized as a major cause of a passenger's discomfort [3]. Low-frequency discrete tones occurring at integer multiples of the blade passing frequency (BPF) dominate the cabin interior noise by affecting passengers' comfort perception during aircraft flight. Practical feedforward controllers, using a multichannel generalization of the well-known LMS adaptive algorithm, have been developed in the literature to reduce the propeller-induced noise in turboprop aircraft cabins [4, 5]. After its first application by [PERSON] [6] and its independent use by [PERSON] [7], the filtered-X LMS algorithm has been widely used for active noise control applications due to its robustness and simplicity of implementation. Successful experiments were conducted on a Saab 2000 turboprop aircraft by using a number of secondary loudspeakers mounted on the trim panels and at foot level [8]. Nowadays, similar solutions are in service within the Bombardier Q-Series Dash 8 and the Raytheon Beech King Air 350 and are optional for other turboprop and business jet cabins. Similarly, [PERSON] et al. demonstrated active engine noise control in 1988 [9]. A few years later, the authors extended automotive active control to the road noise control problem [10]. Today, the car industry offers ANC solutions to provide an improved level of interior comfort and driving experience in luxury cars [11]. The effectiveness of active noise cancelling of the primary noise depends upon three physical concepts: spatial extent, frequency bandwidth, and coherence of signal [12]. The majority of ANC applications rely on feedforward control strategies (either pure feedforward or combined with feedback). The feedforward signal is either taken from a specific sensor (usually an accelerometer in active vibration control [13; 14] or a microphone in active noise control [15; 16]) or extracted from the fundamental disturbance frequency. Additionally, adaptive control strategies are ideal when the primary disturbance acting upon the system becomes time-varying in terms of frequency content, amplitude, and phase. In this case, adaptive filters are used to track these variations and the resulting unknown plants. However, although adaptive control algorithms work well in laboratory conditions, critical issues such as convergence speed, step size tuning, and stability may affect overall system performance and prevent their wide use in practical applications. Modern active noise control systems take advantage of digital signal processing [17]. The control algorithms are implemented on real-time platforms that support a broad range of controller specifications, data acquisition, and plotting features. The control performance is then assessed by comparing acoustic metrics, mainly sound pressure level (SPL), insertion loss, and sound transmission loss, achieved by comparing the active control versus the original uncontrolled counterpart. The performance of local active headrest systems has been thoroughly investigated by a number of authors in both tonal and broadband sound fields [18; 19; 20; 21; 22]. Additionally, a number of virtual sensing algorithms have been developed to shift the zone of quiet to a virtual sensor located at the observer's ear, as first proposed by [PERSON] and [PERSON] [23] and more recently implemented by [PERSON] et al. [24] by using a laser Doppler vibrometer. Within the framework of the Clean Sky 2 Airframe ITD, the CASTLE project is aimed at delivering low-noise technologies to improve the cabin acoustic comfort of regional turbo-prop aircraft. ANC is deemed as one of the main KPIs to advance passengers' well-being.