Gordon Dix and Dr. Tim Leishman, Physics and Astronomy

Designing and constructing a world-class concert hall is the culmination of many different, yet interrelated, disciplines. The architects must make the building structurally sound, while preserving its aesthetic beauty. Electricity, water, air conditioning, heating, and many other aspects of the inner workings of the hall must be designed and installed properly, while keeping in mind the intent of the structure. Acoustic consultants will often work closely with the architects to ensure that the hall has no acoustic flaws. An acoustical plan for the building must take into account many of the design features of the hall and can become quite time intensive. Once the hall has been completed, further acoustic evaluations of the hall are commonly done to verify the design.

The Church of Jesus Christ of Latter Day Saints funded the construction for a large conference center in Salt Lake City. Once the hall was completed, there were a number of noticeable acoustic problems throughout the hall. The church hired a few individuals to do separate acoustical evaluates of the hall. Although this improved the understanding of the hall, it failed to explain many of the problems. BYU was then asked to do a more thorough evaluation and acoustic investigation into possible causes of the trouble.

A large team, consisting of both students and faculty of BYU, was put together and were all involved in this project and the workload was divided up between us all. My involvement in this project was two-fold: the collection of acoustic data from the hall and the processing of that data into numbers and graphs that could tell us what was happening to the acoustics in the building. The collection of data from the hall proved to be very time consuming but valuable. We got a large drawing of the layout of the hall and selected a couple of seats in each section to be used for every measurement. The provided us with consistency and good coverage of the seating areas.

The first set of data we took was the ambient, or background, noise levels. We used sound level meters and measured the background levels with all machinery, such as the air conditioning, and most of the lights turned off and then on. This allowed us to examine how much noise was being produced exclusively by the ventilation system and the lighting fixtures, particularly up near the stage area where most of it was located. These measurements were important because as the ambient levels increase and begin to interact with the sound from the speakers, the intelligibility of speech is decreased and music can be corrupted. We found this to be an important issue up on the rostrum area where the ambient noise was the highest.

After the noise tests were completed, we needed to acoustically characterize the hall through a variety of tests. The most important measurements we took were impulse responses. An impulse response can be thought of as the acoustic signature of the room. This measurement is unique in the fact that it shows the acoustician how sound acts in a given environment. Phenomenon such as the direct sound, reverberation, echoes, intelligibility, and clarity can all be evaluated through the impulse response, as well.

In order to take an impulse response measurement, you need a source, a receiver, and an analyzer capable of performing such operations. We used a variety of sources for our measurements; each with a different purpose and result. Our omnidirectional source was used most often. The device shown to the left is similar to the omnidirectional source we used. We designed and constructed our own for the experience of building one and also to save quite a bit of money. As you can see from the figure, the entire outer shell is covered with loudspeakers. When a signal is fed to the source, the resulting sound wave propagates in all directions to uniformly excite the room being investigated. Highly directional sources were also used in taking further impulse response measurements to examine singular surfaces and treatments. The in-house sound systems were also used to evaluate the existing audio configuration.

The different types of receivers used in this research project varied as much as the sources, as well. A microphone with an omnidirectional pick-up pattern was used most often. This device receives sound from all angles. On the other hand, microphones that only received sound from in front, behind, or even to the sides were also used to isolate incoming waves of interest. One of the most unique measurements was a polar test which used a microphone with a cardioid pick-up pattern, or a microphone that only picks up sound in front of the diaphragm. This measurement calculates which direction echoes are coming from which then allowed us to pin-point problem surfaces. Another receiver of interest is Kemar. Kemar is a manikin with microphones inserted into its ear where the ear drum would be. Recordings and measurements can be taken with Kemar to get an idea of what the average listener experiences in a given seat location.

Once most of the data had been gathered and brought back to BYU, the second part of my involvement began. Code had to be written to evaluate these impulse responses to extract the useful information such as clarity, intelligibility, and reverberation times. Matlab was used as the software platform because of its complex computational abilities. Large sets of data could be analyzed at once and organized into tables, wave files, and other useful formats. Some computations were so complex and involved that computers were left running overnight to complete their operations.

There are still many measurements that can be taken to further investigate the behavior of sound in the conference center. After all the measurements were taken and the computations were complete, we were able to see more of the big picture. Many of the problems that the hall faces are interrelated and have no quick fixes. I feel it was still a success, though, because I increased my understanding of architectural acoustics and we were able to help the church better understand the needs of the hall for future work.