1 Hearing Systems, Department of Electrical Engineering, Technical University of Denmark2 Department of Electrical Engineering, Technical University of Denmark3 Centre for Applied Hearing Research, Center, Technical University of Denmark
In complex acoustic environments, such as a train station or a café, hearing-impaired people often experience difficulties to communicate even when wearing hearing instruments, whereas normal-hearing people are typically able to communicate without effort in such conditions. In order to systematically study the signal processing of realistic sounds by normal-hearing and hearing-impaired listeners, a flexible, reproducible and fully controllable auditory environment is needed. A loudspeaker-based room auralization (LoRA) system was developed in this thesis to provide virtual auditory environments (VAEs) with an array of loudspeakers. The LoRA system combines state-of-the-art acoustic room models with sound-field reproduction techniques. Limitations of these two techniques were taken into consideration together with the limitations of the human auditory system to localize sounds in reverberant environments. Each part of the early incoming sound to the listener was auralized with either higher-order Ambisonic (HOA) or using a single loudspeaker. The late incoming sound was auralized with a specific algorithm in order to provide a diffuse reverberation with minimal coloration artifacts. In order to assess the usability of the LoRA system, one objective and two subjective evaluations were carried out. The objective evaluation showed that the physical characteristics of the acoustic scenario were preserved by the involved signal processing of the system. The first subjective evaluation assessed the impact of the auralization technique used for the early incoming sound (HOA or single loudspeaker) on speech intelligibility. A listening test showed that speech intelligibility experiments can be reliably conducted with the LoRA system with both techniques. The second evaluation investigated the perception of distance in VAEs generated by the LoRA system. These results showed that the distance of far field sources are similarly perceived in these VAEs as in real environments. For close sources (<1 m), a comprehensive study about the near field compensated HOA method was presented and an alternative post-processing was proposed that allowed for the perception of very close sound sources, nearly as accurately as real sources. Beside investigating the auditory system, such virtual auditory environments (VAEs) are also relevant for evaluating and optimizing hearing instruments and communication devices.