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Vibration Amplitude Measurement


The following definitions apply to the measurement of mechanical vibration amplitude.


Peak Amplitude (Pk) is the maximum excursion of the wave from the zero or equilibrium point.

Peak-to-Peak Amplitude (Pk-Pk) is the distance from a negative peak to a positive peak. In the case of the sine wave, the peak-to-peak value is exactly twice the peak value because the waveform is symmetrical, but this is not necessarily the case with all vibration waveforms, as we will see shortly.

Root Mean Square Amplitude (RMS) is the square root of the averageof the squared values of the waveform. In the case of the sine wave, the RMS value is 0.707 times the peak value, but this is only true in the case of the sine wave. The RMS value is proportional to the area under the curve -- if the negative peaks are rectified, i.e., made positive, and the area under the resulting curve averaged to a constant level, that level would be proportional to the RMS value.


The RMS value of a vibration signal is an important measure of its amplitude. As mentioned before, it is numerically equal to the square root of the average of the squared value of amplitude. To calculate this value, the instantaneous amplitude values of the waveform must be squared and these squared values averaged over a certain length of time. This time interval must be at least one period of the wave in order to arrive at the correct value. The squared values are all positive, and thus so is their average. Then the square root of this average value is extracted to get the RMS value.

Average Amplitude, which is simply the arithmetic average of the signal level over time, is not used in vibration measurements, and we will not consider it further.


The RMS value must be used in all calculations regarding power or energy in a waveform. An example of this is the 117 volt AC line. The 117 volts is the RMS value of the voltage, and it is used in calculations of the wattage (power) drawn by devices connected to it. Remember that the RMS value of a sine wave is 0.707 times the peak value, and this is the only wave form where this is true. We will see shortly that this is important.

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