The preparation of the
synthetic random tape is the most demanding task associated with the tape
technique and, as pointed out previously, requires the more sophisticated
equipment. For this reason, all the tasks which require this level of equipment
and which can be performed by a remotely located agency have been included under
There are several important preliminary steps leading to the actual
synthesizing of the random voltage and recording it on tape:
a. Playback of the 1.0g (peak) sine frequency sweep tape through an
analyzer to determine the sine transfer functions.
b. Playback of the tape deck reference tape to determine tape deck
c. Determination of compensation factors to be applied to the sine
transfer functions. These compensation factors are used to correct the final
random equalization for faults in the sine equalization, or anticipated
variations in the test setup. Some examples are:
- Non-linear tape deck characteristics.
- Non-linear damping characteristics of the test
setup (between 1.0g (peak) sine equalization and 6.0 Grms random
- Variations in resonant frequencies from test to
test of ± 3% (determined
d. Calculation of the random voltage spectrum based on the random
equalization calculated above and the required test spectrum.
The final step is to synthesize the required random voltage spectrum using a
random equalization/analysis system. (It should be noted that the term synthetic
random refers to the method used for equalization rather than the quality of the
random signal. The voltage recorded is a true non-periodic random noise signal
with a Gaussian distribution of amplitudes with peaks up to 3 times the RMS
value). This random voltage is recorded on the sine sweep tape immediately after
the sine frequency sweep. This facilitates the playback of the sine sweep before
running the random portion of the test as setup verification.
The complete tape contains all the data necessary to
perform the sine and random vibration test on a specific vibration