This research focuses on two of the main challenges for headrest loudspeakers in cars – their directivity and their low-frequency performance. In part one, we discuss a method to characterize the directivity in meaningful ways despite the complex acoustic target environment (acoustic near field conditions, presence of head and torso, car interior). Headrest speakers of different classes (open back, closed back, panel) are investigated with a nearfield scanning technique, and best practices for measurements are derived. In part two, we study how nonlinear adaptive control of transducers can improve the bass response and the overall quality of sound reproduction of headrest speakers, especially in typical applications relying on linear, time-invariant characteristics. The theoretical advantages of this approach have been discussed in earlier papers. Here, we will provide measurements to quantify the effect for different driver concepts (see part 1), discuss challenges and implications for concerned active sound algorithms such as individual sound zone control, hands-free communication, and active noise control. The results show that nonlinear adaptive loudspeaker control can considerably expand the usable frequency range while retaining robustness, reducing distortion, and providing a stable response even under varying environmental conditions.
