Binaural technology and issues related to sound quality analysis and spatial hearing
Sammanfattning: Binaural technology makes it possible to record, store and reproduce sound field in a perceptually authentic way, i.e. recorded sounds are correctly reproduced so that it is as if the listener was present in the recorded environment. This method involves recording and correctly reproducing sound pressures at each of the eardrums of the listener. Recording can be made with small microphones placed in the ears of a listener. However, artificial heads are most often used. During the past decade binaural technology became a useful tool in sound quality assessment. Sound quality refers to the adequacy of a sound in the context of a specific technical goal or task. It is measured during listening tests with human subjects. In this thesis an improvement of sound quality is related to a reduction of noise annoyance. This thesis concerns the use of binaural technology for assessment of noise annoyance in a steel plant and noise annoyance from diesel engines using listening tests and psychoacoustic parameters. Accuracy of sound source localization while listening to binaurally recorded sounds and its influence on noise annoyance judgments is addressed as well. Finally, the method to relate head-related transfer functions and anthropometric measures of the pinna are investigated. The thesis is based on six experiments. All experiments involved listening tests under laboratory conditions. For this purpose, sounds were reproduced through headphones or loudspeakers. Subjects listened to different sounds and responded by evaluating noise annoyance and/or localizing the sound source position. The first experiment assessed noise annoyance in steel plant control rooms, and on the floor while wearing hearing protective devices. Interaction effects between loudness, sharpness, roughness and tonality were found to be significant in the description of noise annoyance in control rooms. The comparison of noise annoyance using four different hearing protectors showed significant interaction between hearing protector type and gender. Ear cups reduced noise annoyance for male subjects more than for female subjects. Ear plugs had a similar effect for both genders. Loudness and roughness were better predictors of annoyance than was A-weighted sound pressure level. The second experiment evaluated speech perception while wearing hearing protectors when exposed to steel plant noise. Results did not show significant differences between different hearing protectors or the condition without hearing protection. The third experiment addressed annoyance judgments of diesel engine sound when running on six different fuels. The investigation made it possible to identify the type of fuel that produces less annoying sounds (than the other fuels). The differences in annoyance judgments for different fuels and the same engine running speed could not be predicted by the use of an existing annoyance index. The fourth experiment analyzed how sound source position influences diesel engine noise annoyance. Results showed that sounds coming from the front of the listener were rated as more annoying than sounds coming from the sides of the listener. Differences can be described as a function of loudness and sharpness combinations in left and right channels. Localization errors appeared to have influence on noise annoyance judgments as well. The fifth experiment investigated the effect of a short training session on the localization performance prior to listening to binaural recordings. Results showed that subjects perceived fewer sounds inside the head after training session. However, these sounds were often incorrectly localized as front-to-back or back-to-front reversals. Finally, the method to relate the features of head-related transfer functions and anthropometric data of the pinna was investigated. The method was found to be subjective in nature and of limited value.
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