Overview

Vibration is the oscillating motion of an object about an equilibrium position. The transducers that are used to measure this vibration include dynamic displacement transducers, velocity transducers, and accelerometers. The most commonly measured oscillation is sinusoidal, and calibrations are typically made with a sinusoidal source and reference. The measurement system characteristics that are determined during a calibration include low-and high-frequency limits, amplitude versus frequency, phase shifts, and identification of resonances.

Figure 16-1 shows the results of a calibration of an AC responding or dynamic transducer such as a piezoelectric accelerometer. The transducer output is compared to the output of the reference standard, which would have a perfectly flat output amplitude over the frequency range being calibrated (10 to 580 Hz). The amplitude output of the transducer is divided by the output amplitude of the reference standard at each frequency, and the result is expressed in decibels.

Result (decibels) = 20 log ₁₀ (transducer output/reference output)

For the graph in Figure 16-1, the output units and the calibration slope of the two instruments are the same. When the output of the transducer is the same as the reference standard, the ratio of the outputs is equal to one. Since log ₁₀ (1) = 0, then the result in decibels is 0. In the graph in Figure 16-1 this occurs for frequencies from 60 Hz to 450 Hz. For frequencies less than 60 Hz, the transducer's output amplitude is less than that of the reference standard. This results in...