by tomlinson holman
Long-time readers will remember that some time ago I complained editorially about the acoustics of the Staples Center in Los Angeles. One of those complaints was that the low-frequency reverberation time was very long, due to the fact that there is literally no place to put low-frequency absorption. For a concert, the floor and “walls” are covered with seats, and the ceiling provides too little space and not enough weight-carrying capacity to work.
One of our erstwhile contributors to this magazine, Dr. Peter d’Antonio of RPG Diffusor Systems, has patented at least part of a solution: use the concrete riser construction of most of the seating area itself to form a series of tuned overlapping Helmholtz resonators, thus providing low-frequency absorption from the very area occupied by the audience (except on the main floor). U. S. Patent 5,787,656 details this approach. It says in part:
Progress in acoustical design has been hampered by several physical constraints. Since the program occurs on the floor level of the building, and the audience encompasses additional floor area as well as a large portion of the wall area, the locations in the arena where acoustical treatments can be enhanced are limited. In a conventional arena, only the ceiling and a portion of the upper walls are available for acoustical treatment. Where acoustical treatments are proposed within reach of the audience, such acoustical treatments must not only perform their acoustical functions, but must also be damage-proof and protected from abuse. In the typical covered stadium or area, the roof is extremely large and is not designed to support significant additional weight loads over and above those anticipated such as, for example, through rain or snow. Therefore, only limited, lightweight, high-frequency absorptive treatments can be effectively applied there.
One sound pattern that is especially annoying to an audience within an indoor area or stadium is a heavy or “boomy” sound with excess reverberation in the low-frequency region below 500 Hz. Sound problems below 500 Hz are further exacerbated by the extended low-frequency performance of sound reinforcement systems. The extended low-frequency energy delivered by sound reinforcement systems common in today’s arenas and stadiums, combined with the inability to architecturally absorb excess low-frequency energy, cause a sound problem. As such, a need has developed for an approach that can be employed to selectively absorb low-frequency sound in the frequency range below 500 Hz employing the area where the audience sits. It goes on to describe the solution as the series of tuned resonators, including design equations for them, that act as absorbers. Measured data is included. Practical considerations include how this kind of absorption is particularly good compared to thick fuzzy materials that the audience can tear up.
