Pendaphonics is a tangible physical-digital-sonic environment and interactive system that engages users in individual, collaborative, group, and distributed interactive experiences. The development of this system, as an element of urban revitalization and as a trans-disciplinary research endeavor, presents strategies for the design and evaluation of low-cost, flexible, and distributed tangible interaction architectures for public engagement, expression, and performance. Pendaphonics is installed in a public media arts space, where over 200 people experienced it during the environment's opening event. Internationally, interaction laboratories at five research universities are advancing explorations of Pendaphonics. This paper presents the development process and findings from observation and evaluation of people using Pendaphonics; diverse social interaction patterns among performers and the general public are discussed. In particular, we identify the repeated and sustained invitation to interact - created by the cyclic motion of a pendulum's simple harmonic oscillation - as a new tangible interaction modality for human computer interaction, in 3D physical-digital-sonic environments. An investigation of this and related elements of Pendaphonics' large-scale tangible interaction scenarios are articulated, along with descriptions of the system's broad potential as a compositional and choreographic tool; an educational exhibit and classroom manipulative; and as an interface that facilitates playful interaction, exploration, discovery and creativity.