Bǎi is an interactive sound installation that uses a pendulum speaker as an interface for audience participation. We track the movement of the hanging speaker with an HTC Vive tracker which allows to use its motion as input for the interactive dialogue. A 6-speaker setup is surrounding the pendulum and reacting to it. While interacting, the behaviour of the installation changes and goes through different states and levels of excitement. This results in a dynamically changing sound environment for the audience to explore. This paper elucidates the design goals we intended to achieve for the audience experience and the behaviour of the installation. We present a comprehensive description of the development process, including physical, software and sound design. Meanwhile, we discus different forms of interacting with the pendulum speaker and the surrounding speakers.