Recently proposed Head-Related Transfer Function acquisition methods include head-tracking to allow unconstrained subject movements during the measurement. This enables fast measurements with less equipment than previous fast measurement approaches. In this paper, we propose a novel acoustic head-tracking concept, which is particularly suited for this application. Only a head-mounted microphone array and additional recording channels are required in addition to a regular measurement setup. Unlike other head-tracking systems, the tracking data is inherently synchronized to the acoustic measurements and the angle of sound incidence can be accurately determined without knowledge of loudspeaker positions. The concept is developed and evaluated in simulations and measurements, which show that the proposed acoustic head tracker outperforms a comparison device.