The transmissibility curves provided by Minus K Technology are generated using a Stanford Research Instruments SR785 Dynamic Signal Analyzer. The output source of the SR785 is set to generate a swept sine signal. This signal is fed into a Labworks PA-138 power amplifier. The amplifier drives a Labworks ET-126 electrodynamic shaker. The shaker is mounted in a heavily reinforced frame that also supports a 650BM-1 isolator. This support isolator has been adjusted to about 2.75 Hz. It has also been modified to work only in the vertical direction. This isolator supports a heavy top plate and whatever ballast weight is needed to bring the total payload including the test isolator up to around 650 pounds. Transmissibility as it applies to our isolators is a ratio of the output signal on the top plate divided by the input signal that the base of the isolator sees. One can also think of it as the ratio of what gets through the isolator divided by what is present on the isolator support. Two similar accelerometers are used to acquire the input and output signals. The input accelerometer is attached to the heavy top plate on the 650BM-1 support isolator. The test isolator rests on the heavy top plate as well. This way the accelerometer measures the vibrations that are fed to the isolator. The output accelerometer is placed on top of the properly loaded test isolator top plate. Both accelerometers are held in place with a thin layer of seismic wax. This works quite well, providing a secure, yet easy to undo bond for measurement. The SR785 acquires both sets of data, calculates their ratio and displays the ratio as transmissibility.. The horizontal transmissibility was acquired in much the same way. The differences were that the support isolator was allowed to move horizontally. The electrodynamic shaker was mounted horizontally. The accelerometers were mounted on their sides, which allowed the ratio of horizontal data to be calculated and displayed as horizontal transmissibility.
