A prototype real-time analyzer was made available by Spectral
Dynamics Research Corporation in the late ’60s, and this single-
channel spectrum analyzer was used to estimate the response
spectrum from an impact to a machine tool base. This spectrum
measurement had good agreement with the response spectrum
estimated from the measured FRF using sinusoidal testing. Based
on this result, a serious effort was initiated to develop a measure-
ment process that would use the newly developed FFT algorithm to
estimate an FRF from the FFTs of the digitized input and responses
signals. In 1968 and ’69, the large applied dynamics computer in the
Department of Electrical Engineering at the University of Cincinnati
was used to develop a software program that used the analog part
of the hybrid computer to digitize the force and the response (ac-
celerometer) signal measured by testing a machine tool base. The
IBM 1130 computer (digital part of a hybrid computer) was used to
compute FRFs and coherence functions. These measurements were
compared to the FRF function measured with a Spectral Dynamics
transfer analyzer, and the comparisons were good.