The nominal phase and amplitude response of a seismometer at different frequencies is described by a set of numbers known as the "poles and zeroes" of the transfer functionSeismometers operated within their limits can be modelled as linear, time-invariant (LTI) systems where the transfer function is the ratio of the Laplace transform of the output to that of the input. The transfer function is thus the ratio of two complex polynomials, the roots of which provide complete information about the response of the instrument. The poles are the roots of the denominator polynomial and the zeroes are the roots of the numberator polynomial..