  Optomechanical resonators suffer from the dissipation of mechanical energy
through the necessary anchors enabling the suspension of the structure. Here we
show that such structural loss in an optomechnaical oscillator can be almost
completely eliminated through the destructive interference of elastic waves
using dual-disk resonators. We also present both analytical and numerical model
that predicts the observed interference of elastic waves. Our experimental data
reveal unstressed SiN devices with mechanical Q-factors up to $10^4$ at
mechanical frequencies of $f=102$ MHz ($fQ = 10^{12}$) at room temperature.
