David Nutter and Dr. Timothy Leishman, Physics and Astronomy
Architectural Acoustics is a field of study with many applications in building design, including office spaces, concert halls, and recording studios. Theaters must be carefully designed so that the speech intelligibility of actors is adequate for the audience. Since theater design is varied and unique to each venue, careful planning in theater design is necessary to achieve a listening environment acceptable to proscribed standards. Although tools exist to simulate a room and predict its acoustical properties, testing must be performed in a completed structure to determine if any changes need to be made.
This project dealt with collecting and interpreting data for a theater in the round, and comparing the data to a computer scale-model of the same theater. The Hale Center Theater in Orem, Utah was used for this research. The EASE modeling package was used for the scale model. The theater is unique because the building was not originally designed to be a theater. It was remodeled and fashioned in a round setting, meaning that the audience seats surround the stage, like a smaller version of a sports stadium.
Most acoustical properties of a room can be found by measuring its impulse response. An impulse response is a measurement of the sound pressure level at a given point in the room, over a period of time. It includes information about the direct and reflected sound in the room. Traditionally, impulse responses were measured by using starter pistols or popping a balloon. Now broadband noise signals are used for a more accurate means of acquiring the data (Fig.1).
Using an omnidirectional microphone, impulse response measurements were taken at 64 different audience seat locations. A dodecahedron was used to simulate the head of an actor, and was placed in two separate locations. Only one speaker was used, to control directivity of the sound produced. The sound source was positioned at different degree angles, in steps of 45 degrees, to account for situations where the performer faces a certain way. Impulse responses were taken using an advanced software program that produced a noise pattern and received it back through the microphone, revealing information about the room.
These impulse response measurements were then used to determine a few objective parameters, known as Sound Transmission Index (STI), Clarity (C30), and Reverberation Time (RT60). Though acoustics heavily involves subjective interpretation, these parameters have been created as guidelines for developing ideal listening environments. Initial measurements showed a general uniformity in STI and RT60 values in one of the sound source positions, while C30 has yet to be calculated. More source positions with varying degree values are still planned, in order to provide a more accurate idea of how the audience perceives the performer’s sound. These values will then be compared with acoustical standards to determine whether this listening environment is acceptable, or whether improvements can or ought to be implemented. The expected completion is August 2004.
Noticable differences in impulse response measurements occurred at seat locations that were close to walls, especially protruding walls hanging from the ceiling. The room itself is not symmetric, as the elevations and locations of seats and exits are somewhat different, particularly in the southeast and southwest corners.
One of the major concerns with performing tests in an environment is that the conditions are the same as close as possible throughout the experiment. Unfortunately, the measurements were not able to be taken in one sitting. The main issue was that the theater had been altered somewhat since the first measurements had taken place, because a new production had begun. The stage was covered with carpet, props and furniture introduced, and a seating area on the north side of the building was placed where only a stage existed before. Considerable time was spent in preparing the room so that it matched conditions similar to the first measurement environment, and no new seat locations were considered in the study.
Another challenge was learning to use the modeling software effectively. Using floor plans of the theater, a simple model was created to simulate the same measurements. Part of this project included becoming familiar with this software, and it took considerable time to try to accurately portray the room. However, a drawback with these software packages is that they tend to use formulas for certain conditions that may not be accurate in all situations. In hindsight, less time could have been devoted to the modeling portion of the project, bearing in mind that it is a useful tool for estimating characteristics in structure design.
The value of this research extends beyond becoming familiar with measurement tools and calculation methods. It has provided experience that is comparable to career work in Architectural Acoustics, specifically in consulting work. More importantly, it allows for further research to continue in the optimum design of new theaters in the round, as well as methods for improving existing theaters.