History lesson: Improving Acoustics With Volume Cavity Resonators
By Kerry VonDross
Providing a good acoustical environment is critical to the well-being of people occupying interior spaces; especially for rooms having hard-surfaced, highly reflective enclosures.
Volume resonators—acoustical devices comprising a hollow cavity connected to the atmosphere via an aperture—are one means of achieving suitable environments for human occupancy and superior audibility for social interaction.
Origins of cavity resonators
The Vitruvian Man by Leonardo da Vinci (c. 1492) is one of the world’s most iconic images. It depicts the human body inscribed within the fundamental geometric patterns of the circle and the square. Da Vinci’s illustration of the ideal human body is based on concepts of geometry and human proportions as developed by the Roman architect and engineer Marcus Vitruvius Pollio (1stcentury BC) in his 10-volume work, De Architectura. Vitruvius’ texts, also known as the Ten Books on Architecture, are the earliest known treatise on architectural theory. He wrote guiding principles about the design of Roman structures, their materials and building methods, as well as precepts concerning the aesthetic and practical effects of architecture on the lives of citizens.
In his Book V, Vitruvius describes the proper arrangement and construction principles of various public places such as Roman forums and basilicas; emphasizing their design should follow the classic principles of symmetry and harmony and adopt proportions that imitate nature. Vitruvius also provides design strategies for building a proper community theater including site and orientation, foundation, enclosures, seating, and acoustics.
Sympathetic to the importance of good sound quality in a theatrical venue, Vitruvius provided design insights toward improving acoustic intelligibility. He wrote on the science of sound transmission through air movement, how stage voices best reach the audience, reflective vocal sounds and echoes, and on “methods increasing the power of the voice in theaters through the application of harmonics.”
Due to the provenance of Vitruvius’ writings, he is often regarded as the father of architectural acoustics. He is most noted for his examination of using echéa (literally: echoers) as instruments for improving the sound in theaters. These “sounding vessels” were bronze urns. Their size and number were proportionate to the volume of the amphitheater, distributed in tiers with equal space between them, placed free-standing in hollowed arched niches, and supported on wedges to minimize damping. Uttered stage voices, upon entering the cavities of these sounding vessels, were to cause it to resonate in unison with the voice frequencies and produce an increased harmonious note, thereby increasing the clearness of voice sounds for the audience.