Relationship between sound produced in a space and its listeners, of particular concern in the design of concert halls and auditoriums. Good acoustic design takes into account such issues as reverberation time; sound absorption of the finish materials; echoes; acoustic shadows; sound intimacy, texture, and blend; and external noise. Architectural modifications (e.g., orchestral shells, canopies, and undulating or angled ceilings and walls) may act as focusing elements to improve sound quality.