  # Vibration

Vibration is the alternating movement of a body around its equilibrium position. These vibrations are useful during vibration testing.

There are other concepts related to vibration:

• The amplitude of a vibration quantifies its level. The amplitude can be expressed as root mean square (RMS), average, peak, or peak-to-peak values. The crest factor is the ratio between the peak value and the RMS value of a signal. For a sinusoidal signal, the crest factor is equal to √2.
• Power Spectral Density (PSD), expressed in g2/Hz, quantifies the power of a random signal within a frequency band. PSD is obtained by calculating the ratio between the square of the RMS acceleration and the frequency bandwidth. PSD is used in vibration testing according to ASTM, ISTA, and ISO standards.
• Harmonics are multiples of the fundamental frequency of a periodic signal.
• An octave is the interval between two frequencies with a ratio of 2.
• The force required for a vibration test is the product of the total moving mass (sample, fixture, etc.) and the required maximum acceleration.
• RMS (Root Mean Square) is the square root of the average of the squared values. It is used to calculate the effective value of a signal. In vibration testing, we refer to G rms.
• Resonance is the maximum response of a system to a constant excitation. Anti-resonance occurs when the system response is at a minimum with constant excitation.
• Velocity is expressed in meters per second or millimeters per second. Vibrational velocity is the derivative of displacement. Acceleration is the derivative of vibrational velocity. Acceleration units are in m/s2 or g (1 g = 9.81 m/s2).
• A periodic signal repeats itself identically, and its values can be predicted in advance. This is the case, for example, with a sinusoidal signal.
• Decibels (dB) are used to evaluate the increase or decrease in the amplitude of a signal based on a logarithmic ratio.
• A subharmonic is a frequency component that is a submultiple of the fundamental frequency of a periodic signal.

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