Aguirre, Sergio Luiz
(2023)
Iceberg: a loudspeaker-based room auralization method for auditory research.
PhD thesis, University of Nottingham.
Abstract
Depending on the acoustic scenario, people with hearing loss are challenged on a different scale than normal hearing people to comprehend sound, especially speech. That happen especially during social interactions within a group, which often occurs in environments with low signal-to-noise ratios. This communication disruption can create a barrier for people to acquire and develop communication skills as a child or to interact with society as an adult. Hearing loss compensation aims to provide an opportunity to restore the auditory part of socialization. Technology and academic efforts progressed to a better understanding of the human hearing system. Through constant efforts to present new algorithms, miniaturization, and new materials, constantly-improving hardware with high-end software is being developed with new features and solutions to broad and specific auditory challenges. The effort to deliver innovative solutions to the complex phenomena of hearing loss encompasses tests, verifications, and validation in various forms. As the newer devices achieve their purpose, the tests need to increase the sensitivity, requiring conditions that effectively assess their improvements.
Regarding realism, many levels are required in hearing research, from pure tone assessment in small soundproof booths to hundreds of loudspeakers combined with visual stimuli through projectors or head-mounted displays, light, and movement control. Hearing aids research commonly relies on loudspeaker setups to reproduce sound sources. In addition, auditory research can use well-known auralization techniques to generate sound signals. These signals can be encoded to carry more than sound pressure level information, adding spatial information about the environment where that sound event happened or was simulated. This work reviews physical acoustics, virtualization, and auralization concepts and their uses in listening effort research. This knowledge, combined with the experiments executed during the studies, aimed to provide a hybrid auralization method to be virtualized in four-loudspeaker setups. Auralization methods are techniques used to encode spatial information into sounds. The main methods were discussed and derived, observing their spatial sound characteristics and trade-offs to be used in auditory tests with one or two participants.
Two well-known auralization techniques (Ambisonics and Vector-Based Amplitude Panning) were selected and compared through a calibrated virtualization setup regarding spatial distortions in the binaural cues. The choice of techniques was based on the need for loudspeakers, although a small number of them. Furthermore, the spatial cues were examined by adding a second listener to the virtualized sound field. The outcome reinforced the literature around spatial localization and these techniques driving Ambisonics to be less spatially accurate but with greater immersion than Vector-Based Amplitude Panning.
A combination study to observe changes in listening effort due to different signal-to-noise ratios and reverberation in a virtualized setup was defined. This experiment aimed to produce the correct sound field via a virtualized setup and assess listening effort via subjective impression with a questionnaire, an objective physiological outcome from EEG, and behavioral performance on word recognition. Nine levels of degradation were imposed on speech signals over speech maskers separated in the virtualized space through Ambisonics' first-order technique in a setup with 24 loudspeakers. A high correlation between participants' performance and their responses on the questionnaire was observed. The results showed that the increased virtualized reverberation time negatively impacts speech intelligibility and listening effort.
A new hybrid auralization method was proposed merging the investigated techniques that presented complementary spatial sound features. The method was derived through room acoustics concepts and a specific objective parameter derived from the room impulse response called Center Time. The verification around the binaural cues was driven with three different rooms (simulated). As the validation with test subjects was not possible due to the COVID-19 pandemic situation, a psychoacoustic model was implemented to estimate the spatial accuracy of the method within a four-loudspeaker setup. Also, an investigation ran the same verification, and the model estimation was performed with the introduction of hearing aids. The results showed that it is possible to consider the hybrid method with four loudspeakers for audiological tests while considering some limitations. The setup can provide binaural cues to a maximum ambiguity angle of 30 degrees in the horizontal plane for a centered listener.
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