Augmented Reality (AR) audio refers to techniques where virtual sounds are superimposed with real sounds to produce immersive digital content. Headphones are widely used in consumer devices for playback of virtual sounds. However, for AR audio, an important step is to make sure that headphones allow external sounds to pass through naturally. To achieve this, a technique called Hear-Through (HT) processing is commonly employed to reproduce the incoming real sound by playing back processed version of it. In this context, open-back and headphones and closed in-ear headphones have been employed for HT processing. Closed-back headphones provide strong isolation unlike open-back headphones and do not have fittings issue as well as modified ear canal resonance effect found in closed in-ear headphones. In this paper, an investigation of HT design using closed-back circumaural headphones equipped with two pairs of microphones was conducted. An adaptive filtering algorithm was used to derive the ideal equalization filter. To alleviate the direction dependency of the ideal equalization filter and simplify HT filter design, two simplified equalization filters were also introduced. Experiments with objective evaluation using spectral difference and subjective evaluation on both timbre and spatial performance were conducted. These experimental results indicate a close match of the ideal equalized signal with the reference signal in open ear listening, which is slightly degraded in the simplified equalization filters, but outperforms the default ambient hear-through mode in the commercial headphones.
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