Resonant Cavities in the History of Architectural Acoustics

Abstract

Resonant Canities in the History of Architectural Acoustics ROBERT G. ARNS AND BRET E. CRAWFORD The way in which a particular technology is practiced is intrinsically linked to the social setting in which it is found. Technologies, along with other phenomena such as social structure, values and beliefs, and language and symbolism, are part of the cultural matrix. Culture, in turn, both reflects and shapes technology. Considerable insight has been gained from the study of technologies in context and from the study of the changing manifestations of various technologies over time and in different cultures. Studies of this sort provide an opportunity to compare the extent to which a technology is shaped by the culture in which it is observed and the extent to which technology is driven by internal factors, such as fundamental laws of physics or biology, the nature of the environmental problem, or the characteristics of the materials employed. Acoustics is the science of sound, including its production, transmis­ sion, and effects. In architectural acoustics, these scientific principles are applied to improve the hearing of sound in a built space. The resonant cavity, a rigid enclosure connected to the external environment by a small hole—an ordinary vase provides a familiar example—has some­ times been used in architectural acoustics. The air in such a cavity can be set into oscillation by certain sounds in the environment, thus affecting what is heard in the vicinity of the cavity. In the resonant cavity as applied to architectural acoustics we have an opportunity to consider a technological artifact in three very different periods and to interpret these manifestations in terms of the settings and of current understand­ ing of the technology. The first record ofthe use ofthe resonant cavity in architectural acoustics is described in De architectura libri decern, the treatise on architecture of Dr. Arns is professor of physics at the University of Vermont; Mr. Crawford is a Ph.D. candidate in physics at Duke University. This work would not have been possible without the persistence of Professor Patricia E. Mardeusz, University of Vermont Libraries, in locating and obtaining critical materials. The authors also wish to thank ProfessorJean M. Davison, Department of Classics; Professor Jun-Ru Wu, Department of Physics; and Professor HenryJ. Steffens, Department of History, all of the University of Vermont, and the Technology and Culture referees for their helpful observations and suggestions.© 1995 by the Society for the History of Technology. All rights reserved. 0040-165X/95/3601-0005$01.00 104 Resonant Cavities in the History ofArchitectural Acoustics 105 Marcus Vitruvius Pollio, a Roman of the Augustan era.1 The fifth of his ten books is devoted to the architecture of public buildings, including the design characteristics of Greek and Roman theaters. He describes the acoustical problems faced by the builders of open-air structures in which a large crowd is gathered for the purpose of seeing and listening to a public performance. He also provides various prescriptions to guide in the design and construction of such theaters. One of these prescriptions calls for the use of resonant cavities, in the form of empty bronze or pottery vases, to improve the acoustics by serving as “sounding vessels,” separate sources of sound resonating with the sound from the stage. The second manifestation of the resonant cavity as a sounding vessel involves the interior acoustics of medieval churches: empty pots, with open mouths facing inward, embedded in the walls of European churches, a practice that continued into the 17th century. Sounding vessels were espoused again in the 19th century in attempts to improve the acoustics of large concert and assembly halls. Finally, in contrast to the applications in these three historic periods, it was realized, beginning about 1930, that carefully designed arrays of resonant cavities could be applied to advantage in interior acoustics, not as sounding vessels but as sound absorbers. This article addresses several questions. First, in each of these three periods, what was the extent and nature of the use of resonant cavities? Second, what did architects and builders think they were achieving? Third, how was the application of resonant cavities related to the social setting in which we observe that application? Fourth, how...