Other problems are with the thermal sensitivity of the mass position and suspension resonances within the high-frequency portion of the VBB passband. The equation simply describes the ideal thermal noise imparted to an isolated mass levitated in a standard atmosphere by a suspension that controls the period and where motion is limited by a dissipative damping system. Note that it says nothing about the noise caused by the physical components of the geophone. The mass of a seismometer is always in motion, even under the quietest conditions. This causes the suspension members to flex, and when non-linearities of the suspension forces are of the order of the forces of earth accelerations applied to the mass, noise results.
Newtons is applied to the suspension.
But simple suspensions, such as the etched leaf springs of a geophone, are never intended to be linear at these force levels, so micro-bending and warping, along with hysteresis and edge contact effects, result in fairly broadband noise. Obvously, a larger mass will overcome these effects proportionately. This is why very compact but very high quality flexures are used for low noise sensors. A straight flat flexure clearly has a simple and well defined bending pattern compared to the etched spiral leaf spring of a geophone. Its support or contact points are very well constrained, compared to the circularly clamping rings of a geophone.