Role of diffusers in the acoustic design of the stage enclosure in concert halls
Previous studies on acoustic requirements for performers of classical music proved that early reflections were of critical importance for good ensemble conditions. However, strong early reflections may produce an unpleasant tone quality. Diffusers are sometimes used to reduce the negative effects caused by large specularly reflecting surfaces. The aim of the research project is to better understand what happens when diffusers are applied to the enclosure walls of a stage in a concert hall. Two aspects are examined: the changes produced to the sound field measured on the orchestra platform, and the manner in which musicians perceive diffuse early reflections in comparison with specular early reflections. Results of objective measurements show that the most noticeable property of diffusers on the early sound field is the smoothing of the impulse response. Diffusers attenuate strong early reflections. Besides, diffusers give a better sound energy distribution by providing reflections outside the specular sector: the standard deviation of several acoustic parameters is somewhat smaller with diffusers. It was found that diffusers do not eliminate comb filter effects and that they do not have a great influence on EEL, ST and clarity. The subjective aspect resulting in the greatest consensus among the subjects is the following. The overall tone quality is improved by diffusers. The overall sound quality is influenced by the amplitude of early reflections and by the reception angle of the reflections. Diffusers may also affect articulation, clarity, self and mutual hearing, but to a minor extent. Musicians' requirements regarding the fine structure of early reflections differ according to the size of the ensemble who is performing, the room volume, the power and directivity of the instruments played, and the type of music (or desired tone quality).