ANC performance with the developed head tracking system. (a) Four 15-s samples of the time-domain signal observed by the HATS. The upper 30 s duration shows the sound pressure with ANC off and on for the stationary situation, while the lower 30 s duration shows the sound pressure with ANC on with the tracking system off and on for a moving HATS. (b) The corresponding sound pressure level (dB re. 20 μPa) of the four signals.