A front resonator can significantly amplify a piezo’s sound level.
These resonators are typically high Q and are designed to enhance the highest requency of interest. The important design parameters of a resonator are the volume and the port dimensions. Ole Wolff uses finite element methods software (COMSOL, Figure 1) to optimize the resonator design. As with all sound sources, it is critical to isolate the front and rear sides of the piezo to prevent sound cancellation.
With nodal mounting it is also necessary to isolate the inner and outer regions (inside and outside the nodal diameter). This is easily accomplished with the resonator and an enclosed back cavity created by the PCB.
Figure 1: Comsol simulation of a front resonator with three holes