Using a shake table is the most direct way of obtaining an absolute
calibration. In practice, however, precision is usually poor outside a
frequency band roughly from 0.5 to 5 Hz. At higher frequencies, a shake
table loaded with a broadband seismometer may develop parasitic
resonances, and inertial forces may cause undesired motions of the
table. At low frequencies, the maximum displacement and thus the
signal-to-noise ratio may be insufficient, and the motion may be
nonuniform due to friction or roughness in the bearings. Still worse,
most shake tables do not produce a purely translational motion but also
some tilt. This has two undesired side-effects: the angular
acceleration may be sensed by the seismometer, and gravity may be
coupled into the seismic signal (see 3.3).
Tilt can be catastrophic for the horizontal components at long periods
since the error increases with the square of the signal period. One
might think that a tilt of 10 
rad
per mm of linear motion should not matter; however such a tilt will, at
a period of 30 s, induce seismic signals twice as large as those
originating from the linear motion. At a period of 1 s, the effect of
the same tilt would be negligible. Long-period measurements on a shake
table, if possible at all, require extreme care.
Although all calibration methods mentioned in the previous section are applicable on a shake table, the preferred method would be to record both the motion of the table (as measured with a displacement transducer) and the output signal of the seismometer, and to analyze these signals with a routine like CALEX. Depending on the definition of active and passive parameters, one might determine only the absolute gain (responsivity, generator constant) or any number of additional parameters of the frequency response.