N. Ohishi, S. Otsuka, K. Kawabe and K. Tsubono

We describe a new method for the estimation of the thermal noise at off-resonant frequencies and report its demonstration.

In this method, the mechanical conductance is measured at anti-resonant frequencies, where the imaginary part of the mechanical admittance vanishes while the real part, namely conductance, remains. The fluctuation dissipation theorem is, then, applied to the measured conductance in order to evaluate the thermal noise. The merits of this method are that; (1)we need no assumption on the frequency dependence of the loss, and (2)the method can be applied to the mechanical system in which the loss distributes inhomogeneously.

To confirm the validity of our method, we made a mechanical oscillator which comprises of two blade springs and two masses, and measured the mechanical conductance at an anti-resonant frequency. The measured conductance was in good agreement with the one calculated by normal mode expansion.

In addition, we would like to talk about the application of our method to the mirrors in interferometric gravitational wave detectors.