### Vibration Terms and Definitions

**Strength and Mechanics of Materials**

The following are vibration related terms and definitions.

Acceleration

A vector quantity that specifies rate of change of velocity.

Accelerometer

A sensor whose electrical output is proportional to acceleration.

Charge amplifier

An amplifier whose output voltage is proportional to the output charge from a piezoelectric transducer. Has the advantage
that voltage output is not affected by length of connecting cable from the transducer.

Compliance

The ease with which a system may be displaced or compressed for a given force. The reciprocal of stiffness.

Critical damping

The minimum viscous damping that will allow a displaced system to return to its original position without oscillation

Displacement

A vector quantity that specifies the change of position of a body. Usually measured from the rest position.

Damping

Any means of dissipating vibration energy within a vibrating system.

Damping ratio

Ratio of actual damping to critical damping, at a resonant frequency.

Degree of freedom

In a mechanical system, equals the minimum number of independent co-ordinates required to completely define the position
of all parts of the system at any instant of time.

Dynamic mass

Ratio of applied force to resulting acceleration during simple harmonic motion.

Dynamic modulus

Ratio of stress to strain under vibratory conditions.

Excitation

An external force or motion applied to a system that causes the system to respond in same way.

Force

Retardation Agency which gives a mass an acceleration.

Impact

Excitation of a structure with a force pulse e.g. using an Impact Hammer.

Impulse

Well defined pulse. Periodic impulse is repeated impulses.

Integrator

An electrical frequency filter used to convert a vibratory acceleration signal to one whose amplitude is proportional to
velocity or displacement.

Jerk

A vector quantity that specifies time rate of change of acceleration.

Mechanical impedance

Ratio of applied force to resulting velocity during simple harmonic excitation., Called driving point impedance if
force and velocity are measured at the same point, otherwise called transfer impedance.

Mobility

Mechanical admittance. Inverse of mechanical impedance.

Modal analysis

A process of determining the mode shapes and associated parameters, natural frequency and damping.

Natural frequency

The frequency at which a resiliently mounted mass will vibrate when set into free

vibration.

Periodic vibration

An oscillatory motion whose amplitude pattern repeats after fixed increments of time.

Random vibration

A vibration whose instantaneous amplitude is not specified at any instant of time. Instantaneous amplitude can only be
defined statistically by a probability distribution function which gives the fraction of the total time that the amplitude lies within specified amplitude intervals. Pseudo,
or Periodic and Burst random are special forms.

Resonance

Conditions of peak vibratory response where a small change in excitation frequency causes a decrease in system response.

Response

Motion or other output resulting from an excitation, under specified conditions.

Shock

Rapid transient transmission of mechanical energy.

Simple harmonic motion

A periodic motion whose displacement varies as a sinusoidal function of time.

Stiffness

Ratio of the change in force to the corresponding change in displacement of an elastic element.

Transmissibility

Ratio of the amplitude response of a system in steady state vibration to the excitation amplitude.

Velocity

A vector quantity that specifies time rate of change of displacement.

Vibration isolator

A resilient support that reduces transmissibility.

Vibration meter

An instrument for measuring oscillatory displacement, velocity or acceleration.

Vibration severity

A criteria for predicting the hazard related to specific machine vibration levels.

Voltage preamplifier

A preamplifier which produces an output voltage proportional to the input voltage from a piezoelectric accelerometer.
Input voltage depends upon cable capacitance.