HAM-SAS "simply" stands for Horizontal Access Module Seismic Attenuation System. In other words, the HAM-SAS is a system expressely designed to fit in the tight space of the LIGO HAM vacuum chamber.
It should provide an attenuation factor of about 70dB above 10Hz for the auxiliary optics carried by the triple pendulum suspension.

Seismic Attenuation Strategy

This Seismic attenuation system is based on the property of the mechanical oscillator which attenuates, as a second order low pass filter, above its resonant frequency; therefore the attenuation is purely passive like a passive RLC resonant circuit. No active control is applied to reduce the seismic noise in the frequency band of interest.

Mechanical resonators have the unpleasant property (for us) of having very high quality factors, which amplify the residual RMS displacement noise. It is necessary to reduce such RMS noise otherwise it becomes impossible to properly control the payload position. To fix such a problem, seismic attenuators designer always (almost) relies on some of kind active damping: inertial damping to improve the overall performance, or relative to ground if the resonances are low enough to not compromise the attenuation performance.

A smarter solution is possible when tunable mechanical oscillators are used.
Tunable mechanical oscillators can become unstable or better, very close to stability. Under this condition, a very weak force is necessary to keep the oscillator steady (stable), and no oscillator resonances will practicaly show up; therefore, there is no need to damp the harmonic resonances, and just a DC active control with a quite easy control law is needed.
The side benefit of such strategy, which is not marginal at all, is that the passive attenuation is maximized thanks to the lowest as practical as possible filter cut off frequency.

Internal modes of the mechanical structures spoil the passive attenuation. In other words, they generate peaks and saturation in the transmissibility ,(the ratio of seismic excitation over the response of the attenuation system), compromising the performance of the system.
It is therefore necessary to make such structures as stiffer as possible to relegate internal resonances and transmissibility saturation at frequencies, where the seismic noise spectra is very small.

Contributed by Virginio Sannibale.