## 2 ANC Headrest System  
The zone of quiet in a diffuse sound field has been extensively studied in the literature [28; 29; 30]. [PERSON] et al. [31] investigated the spatial extent of the zone of quiet while controlling pressure with a remote secondary source in a pure tone diffuse sound field. It was defined by a _sinc_ function, with the primary sound pressure level attenuated by 10 dB over a region size of about one-tenth of the acoustic wavelength. [PERSON] and [PERSON] [32] investigated the control of both the pressure and the pressure gradient in a diffuse sound field with two secondary sources. A considerable attenuation of the primary field can be achieved if the primary and secondary sources are positioned within half of a wavelength of each other at the frequency of interest [33].  
There are many challenges in developing an ANC headrest system that meets the practical implementation requirements commonly used in aviation. In practical applications,although the noise levels inside the cabin may be efficiently predicted by simplified vibro-acoustic models, the actual characteristics of noise sources and their vibro-acoustic coupling with the airframe and fuselage lining panels remain uncertain. Computing and hardware costs may be also an issue, especially as a consequence of the limited size and weight margins for instrumentation and control systems onboard. Quiet-Bubble(tm) technology, developed by Silentium Ltd. [34], is an example of one of the most advanced commercial solutions with proven efficiency in the low-medium frequency range. Such a system offers an interesting dB/kg ratio when applied in place of passive NVH systems. This is achieved via a multichannel algorithm that allows control of the sound field produced by an array of counteracting loudspeakers. A detailed description of the control logics can be found in [35]. Other examples include the systems of Ultra Electronics [36], which were successfully tested in a passenger cabin onboard a Saab 2000 in the late 1990s to reduce the fundamental frequency of propeller noise at 97 Hz.  
In this study, a dedicated mock-up of a cabin seat equipped with integrated sensors and actuators was investigated to demonstrate the potential of the proposed active noise control concept under laboratory conditions. The secondary loudspeakers were placed at the inner side of the headrest and the error microphones at the end side. The sound pressure was minimized at the physical error signal to create a localized zone of quiet extended to the passenger's ears when the active noise control system was operating. The system was conceived to actively reduce the narrowband periodic noise locally detected by passengers through an active noise canceling function (commonly delivered by active headsets) without the need to wear a headset.  
The proposed concept is shown in Figure 1. A pair of speakers was integrated into the seat headrest to create a local comfort area on both sides of the passenger's head, thus improving the passenger's comfort perception. The monitoring microphones were located near the aircraft headrest and used as an error signal to drive the secondary loudspeakers through destructive interference.