Acoustic-Vibration Coupling in an Ultra-Quiet Cylindrical Laboratory*

Atomic-scale fabrication and imaging techniques require an ultra-quiet environment to reach optimal resolution. Scanning tunneling microscopy (STM) is among the most demanding, with exponential sensitivity to perturbations in the tip-sample distance. To reduce vibration noise, STM facilities typically employ a massive inertial block floating on soft pneumatic springs; however these blocks are prone to coherent acoustic standing waves coupling to their vibrations because of their flat sides. We describe a new low-vibration facility at Harvard University that employs a cylindrical, rather than prismatic block, designed to minimize the coupling of acoustic standing waves between the straight walls and the curved block face. Here, we use a movable microphone to measure the acoustic response throughout the room, to map out the spatial eigenmodes. We compare the measured acoustic modes to COMSOL Multiphysics simulations of the room. Finally, we use an accelerometer to measure the vibration response of the cylinder and compute the acoustic-vibration transfer function.