  A phononic crystal formed in a suspended membrane provides full confinement
of hypersonic waves and thus realizes a range of chip-scale manipulations. In
this letter, we demonstrate the mode-resolved real-space characterization of
the mechanical vibration properties in cavities and waveguide systems. Multiple
resonant modes are independently characterized in various designed cavities,
and wavelength-scale high-$Q$ resonances up to $Q=$ 4200 under atmospheric
conditions are confirmed. This also reveals that the waveguide allows us to
resolve single-mode wave transmission and thereby drive evanescently-coupled
cavities. The methods offer a significant tool with which to build compact and
low-power microwave phononic circuitry for signal processing and hybrid quantum
system applications.
