# Glossary

The vibration monitoring industry has specialized jargon that is commonly used when talking about vibration sensors, accelerometers and associated applications. It's important that you understand common terminology to communicate effectively with vibration monitoring, reliability, condition monitoring and process automation professionals. We hope you find our glossary useful to find specialized terms and definitions used across vibration, condition monitoring and predictive maintenance.

DAMPING

Dissipation of oscillatory or vibratory energy with motion or with time. Critical damping (q.v.) is that value that provides the most rapid response to a step function without overshoot. Damping ratio is then C/Cc.

DAMPING RATIO

For a system with linear viscous damping, the ratio of the actual damping coefficient to the critical damping coefficient.

DAMPING RATIO CHANGE

The change in the damping over a specified temperature range. Usually specified in %/°C. This specification usually found in damped accelerometers such as piezoresistive and variable capacitance accelerometers.

The volume inside the pressure port of a transducer at room temperature and barometric pressure.

DECIBEL

Ratios of identical quantities are expressed in decibel or deciBel or dB units. Magnitude thus refers to some standard value, in terms of the base 10 logarithm of that ratio. In measuring acoustic or vibration power (as in PSD or ASD of random vibration), the number of dB = 10 log10 P/P0. P0, the reference level, equals 0 dB. In measuring the more common voltage-like quantities such as acceleration, the number of dB = 20 log10 E/E0. E0, the reference level, equals 0 dB.

DEFLECTION

The change in length along the primary axis or the distance a diaphragm moves at the center between no-load and rated-load conditions.

DEGREES OF FREEDOM

In mechanics, the total number of directions of motion (of all the points being considered) on a structure being modeled or otherwise evaluated. In statistics, the number of independent variables used in constructing a mathematical model representing some collection of random variables.

DETERMINISTIC VIBRATION

A vibration whose instantaneous value at any future time can be predicted by an exact mathematical expression. Sinusoidal vibration is the classic example. Complex vibration is less simple (two or more sinusoids). See also Periodic Vibration.

DIAPHRAGM

The sensing membrane which is deformed when pressure is applied.

DIFFERENTIAL INPUT (Amplifier)

A symmetrical input circuit configured such that both input lines have equal impedance and transfer characteristics with respect to the transducer grounding structure. The amplifier then amplifies only the difference between the two inputs, rejecting any common signal (Common Mode).

DIFFERENTIAL PRESSURE

The difference in pressure between two measurement points.

DISCHARGE TIME CONSTANT

A term sometimes found on IEPE datasheets. It is the time constant created by the output characteristics of the internal electronics. The time constant determines the low frequency point of the accelerometer e.g., the -5% low frequency point is 0.5/time constant. It is the time required for the sensor’s electronics to discharge to 37%of its original value following a step function input.

DISPLACEMENT

The change in position of a body or point with respect to a reference point. A vector quantity specifying the change of position of a body or particle, usually measured from the mean or rest position.

DISTORTION

In mechanics, any unwanted motion. If sinusoidal motion were desired at a fundamental frequency, any motion at harmonics or subharmonics of that frequency, or any mechanical "hash" (perhaps due to parts colliding). In electronic measurements, any unwanted signal; e.g. amplifiers may generate unwanted signals by clipping, nonlinearity or harmonic distortion.

DOUBLE AMPLITUDE

The total excursion of a simple harmonic quantity; the peak-to-peak value. DURATION: Of a shock pulse, how long it lasts. For "classical" pulses, time is usually measured between instants when the amplitude is greater than 10% of peak value.

DYNAMIC MASS (Apparent Mass, Effective Mass)

The complex ratio of force to acceleration at a point in a mechanical system during simple harmonic motion. Usually expressed as: Za=F/a