THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

270 11. Acoustics of Enclosed Spaces: Architectural AcousticsNesholm) Architects and the acoustical consultant, Cyril M. Harris, combinedthe shoebox design with state-of-the-art materials to achieve maximum warmthand balance.The location of Benaroya Hall in a busy sector of Seattle posed special challengesto the designers. They had to contend with a railroad tunnel running diagonallybeneath the auditorium and a nearby underground bus tunnel. A slab of concretemore than 2m (6 ft) thick, 24 m (80 ft) wide, and 131 m (430 ft) long was pouredunder the hall to swallow the sound from the tunnels. In order to combat otherexterior noises, the designers essentially encased a building within a building. Theauditorium, weighing 12 million kilograms, rests on 310 rubber pads, which absorbvibration from the tunnels. The pads are 38 cm 2 and are composed of four layersof natural rubber sandwiched with 0.32-cm (1/8-in.) steel plates.All electrical, plumbing, and other noise-generating equipment are located outsidethe auditorium box. Any penetration of the box is made with flexible connections.Water is known to transmit sound very well, so the fire sprinkler system isleft dry and it will flood with water only when a fire is detected. The ventilationsystem is connected to the outside by a sound trap, which channels air throughnarrow openings between perforated aluminum boxes of sound insulation. Verylarge vents collect air below the floor and move it slowly behind the auditoriumto another sound trap and from thereon to fans. The basic idea of the ventilationsystem is to move a high volume (2400 m 3 /min) of air at low speed, eliminatingnoise created by fast-moving air in conventional systems.Instead of frame construction, the walls are built of precast concrete panels. Theheavy mass helps to cut down building vibration and provides a stiff, hard surface toreflect concert sound. Side walls, back walls, and ceiling are covered with panelingshaped like truncated pyramids to reflect sound at various angles to aid diffusion.Randomly spaced wood blocking behind the angled paneling creates framed soundboxes that reflect both high and low frequency sounds so that no tone is eliminatedfrom the music. Side walls are covered with particle boards veneered with a dense,fine-grained hardwood from a single makore tree. The ceiling is suspended fromthe roof by hundreds of metal strips. The ceiling is coated with 3.8 cm (1.5 in.)of plaster in irregularly shaped panels to diffuse sound. The plaster is sufficientlydense to prevent the ceiling from vibrating. House lighting is imbedded in theceiling to minimize sound leakage. Access to the light bulbs is achieved above theceiling through heavy, removable plaster caps.11.16 Band Shells and Outdoor AuditoriumsOver the past several decades there has been an increasing trend toward outdoorconcerts, either at band shells or in semi-open structures. These types of structuresare more economical to construct than full-fledged indoor concert halls, andthey also meet the criteria of providing an informal setting for audiences seekingentertainment in a usually rural environment, away from the metropolitanareas.