Monthly News SEMMER 1995

Acoustic Engineering Ensures Optimal Sound for Concert Halls, Sports Facilities and Other Spaces

Located in Sumitomo Marine and Fire Insurance Company's Nagoya building, Shirakawa Hall is a shoebox-style hall reserved exclusively for classical music performance. A special feature of the hall is the installation of seats designed to minimize reverberation time differential between a full hall and an unoccupied one in order to make acoustic conditions during rehearsals as close as possible to those during public performances. (Construction by Kajima in a joint venture, design by Nikken Sekkei and Yamaha Acoustic Laboratory.)

Kajima engineers recreated the acoustic conditions of Yamabiko Dome in a special anechoic simulation chamber (left) and, using the acoustic CAD system, determined optimal speaker placement and stage design.

Acoustic engineering is usually considered to focus exclusively on interior design, above all concert halls, while in fact it is an integrated discipline that encompasses a variety of different technologies and applications. In addition to quality of sound, for example, acoustic engineering is concerned with preventing noise and vibration originating outside and inside a structure, minimizing acoustical impediments, and matching the reverberation, or volume level, to the intended use and size of the space. Kajima's world-class technology and impressive accomplishments, including scores of concert halls, attest to the particular emphasis it has put on acoustic engineering.

Concert Halls

Concert halls make special demands in terms of acoustic engineering. In a "dead" hall - that is, one with little reverberation - even a great musician will sound anemic, while in a space that has excessive reverberation, the nuances of a performance may be lost. When designing a concert hall, Kajima brings a full arsenal of advanced technology and know-how into play, including its acoustic CAD system as well as acoustic modeling and audible reverberation assessment.

Once the rough shape of the hall is decided, it is input into the acoustic CAD system, making it possible to graphically display the paths of sound diffusion from stage to audience. When problems such as long-path or flutter echoes arise, the effectiveness of possible remedies - whether changing wall angles or interior materials - can be easily assessed on the system. After setting the optimal shape and materials for the space, a precise reduced-scale model is made. The reverberation at each seat is then tested by means of dummy heads equipped with two microphone "ears." With an electrical pulse from the stage serving as the sound sources, the sound "heard" by each dummy is recorded and, after processing, can be played back to indicate the sound characteristics at any particular seat in the hall.

Sports Facilities

In large facilities, such as sports domes, the challenge is to make public address systems clearly audible to a great number of people. Because there is typically little sound-absorbing surface in relation to the volume of the space, prolonged reverberations and echoes reflect back from surfaces with a considerable time lag, making it difficult to hear announcements. Kajima's acoustic CAD system calculates the path of sound waves from the public address system to audience seating, taking into account the directionality of individual loudspeakers in multi-speaker installations. Sound waves reaching individual seats, as calculated by the system, are combined with equivalent sounds recorded in an anechoic space. When replayed in a simulation chamber utilizing 16 speakers, the quality of the sound - or comprehensibility of announcements - at various locations can be readily assessed and appropriate design modifications selected.

Television Studios

In television studios, where the primary emphasis is on images, wall units used as backdrops pose special acoustical challenges. In particular, when a studio has multiple wall units, flutter echo can be a serious problem. By utilizing its acoustic CAD system, Kajima engineers can determine the best angle for tilting the wall units to minimize this type of problem, avoiding the need to treat them for sound absorption.