➀ Researchers from MIT and other institutions have developed a design framework for controlling ultrasound wave propagation in microscale acoustic metamaterials; ➁ The framework involves precisely positioning microscale spheres to tune how ultrasound waves travel through 3D microscale metamaterials; ➂ The work enables tunable elastic-wave velocities within microscale materials and demonstrates an acoustic demultiplexer, paving the way for microscale devices useful for ultrasound imaging or information transmission.