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Radial Position
The average location, relative to the radial bearing centreline, of a shaft’s dynamic motion. Applies only to sleeve bearings.Random Excitation
where:
- G = rms level in g units
- B = frequency bandwidth in Hz
- Go = acceleration density in g2/Hz
Crest factor = 
Random Noise
Noise whose instantaneous amplitude is not specified at any instant of time. Instantaneous amplitude can only be defined statistically by an amplitude distribution function.Random Noise Burst
A signal where the amplitude varies at random and the signal is not continuous, but occurs periodically.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 that gives the fraction of the total time that the amplitude lies within specified amplitude intervals. Pseudo, Periodic and Burst random are special forms.Ray
At higher audio frequencies, sound may be considered to travel in straight lines, or rays, in a direction normal to the wave front.RC Circuit
An RC circuit is used for exponential averaging, as the impulse response is a decaying exponential.Reactance
The opposition to the flow of electricity posed by capacitors and inductors.Reactive Absorber
A sound absorber, such as the Helmholtz Resonator, which involves the effects of mass and compliance as well as resistance.Reactive Intensity
The part of the sound field that does not contribute to the net flow of energy.Reactive Silencer
A silencer in air-conditioning systems that uses reflection effects for its action.Reactive Sound Field
A sound field in which the particle velocity is 90° out of phase with the pressure. An ideal standing wave is an example of this type of field, where there is no net flow of energy, and constitutes the imaginary part of a complex sound field.Real Modes
(Sometimes called normal modes). In a real mode, all points on the structure reach a maximum or a minimum value at the same time and all pass through equilibrium at the same time.Real Part
A plot of the real part of the frequency response function versus frequency. For a single degree of freedom, the magnitude is zero at the damped natural frequency.Real-time Signal
A signal that is analysed on-line in real-time (as opposed to post-analysis of a recorded signal). For true real-time analysis, overlap analysis should be possible.Reasonably Steady Sound
A sound whose level fluctuates through a total range of less than 8 dB(A) as measured with a sound level meter set to “S” time weighting.Receiver point
A position (and in some cases, direction) where the response of interest is measured. Example: “Driver’s right ear”.Record Length
The length of time, T, for which each sample of a signal is recorded. With random signals, it is the recorded length that determines the accuracy of the result.Recruitment (of loudness)
A manifestation of auditory dysfunction usually associated with the sensory elements of the cochlea. It is characterised by a raised threshold and by a more rapid rate of increase of loudness with sensation level than a normal ear.Rectangular Time Window
A time window that has a zero value outside the specified time record and unity within the record length. In the FFT analyzer, the rectangular window is actually no window at all. It is also called rectangular weighting, or uniform weighting, and is used when the signal to be analysed is a transient rather than a continuous signal. See also Hanning Window.Reference Sine
A pure sine wave with known characteristics. This provides a base for making comparisons.Reference Transducer
One of the array of transducers necessary to identify the partial fields during STSF measurements. A reference transducer can be a microphone, an accelerometer, a pressure transducer inserted in the cylinder of an engine, etc.Reference Zero (for pure tone audiometers)
For air conduction, an array of sound pressure levels of pure tones at audiometric frequencies, which correspond to the normal threshold of hearing of young persons. For each frequency the value is expressed by the sound pressure level measured in an acoustic coupler or artificial ear when the earphone, driven by a specific electrical signal, is placed on the coupler. This value is known as the Reference Equivalent Threshold Sound Pressure Level (RETSPL) for the frequency in question. The specific electrical signal is such that the sound pressure level it generates under the earphone when placed on the average human ear corresponds to the modal value of the threshold of hearing for a group of otologically normal young persons. For bone conduction, the reference zero is defined analogously by reference equivalent threshold vibration levels when the bone vibrator is loaded by a specified artificial mastoid or mechanical coupler.Reflection (of sound)
The return of a sound wave from a surface. For surfaces large compared to the wavelength of impinging sound, sound is reflected much as light is reflected, with the angle of incidence equalling the angle of reflection.Reflection-phase grating
A diffuser of sound energy using the principle of the diffraction grating.Refraction
The bending of a sound wave from its original path, either because it is passing from one medium to another with different velocities or by changes in the physical properties of the medium, for example, a temperature or wind gradient in the air.Regression (statistical) Analysis
For example, used for the correlation and comparison between objective and subjective sound quality tests.Reliability Testing
An essential part of manufacturers’ quality assurance testing. Vibration tests are used to simulate the dynamic working environment of a structure to test the structure’s response before it is put into operation. The main application areas are: the automotive, aerospace and electronics industries, computers and consumer goods.Repeatability
The consistency (or variation) of readings and results between consecutive sets of measurements. It has nothing to do with accuracy.Repeated Root
When more than one mode shape is found at a particular modal frequency.Residual Intensity, LI,R
The sound intensity level measured when the same signal is fed to both channels of a sound intensity measuring system.Residual Noise
This is Ambient Noise without Specific Noise. It is the noise remaining at a point under certain conditions when the noise from the specific noise is suppressed.Residual p-I Index
The residual pressure-intensity Index describes the phase mismatch between the two channels used for sound intensity measurements. It is equal to the mean pressure minus the intensity with the probe in a sound field where the pressure is uniform over the volume containing the microphones (for example, using Sound Intensity Calibrator Type 3541).Residual Terms
Terms added to a curve fit algorithm to take into account the effects of modes outside the range being fitted. These terms consist of a mass term on the low-frequency end and a stiffness term on the high.Residues
A mathematical term used to express the strength of a particular mode for a particular pair of degrees-of-freedom in the model for measured frequency response functions.Resistance
That quality of electrical or acoustical circuits that results in dissipation of energy through heat.Resolution
The smallest change or amount a measurement system can detect.Resonance
Conditions of peak vibratory response where a small change in excitation frequency causes a decrease in system response.Resonant Frequency
The frequency of maximum amplification for a given damping ratio. See Resonance.Resonant Frequency of First Bending Mode (unloaded beams)
where:
- C = constant, function of method of support
- E = elastic modulus
- I = moment of inertia of cross section
- g = acceleration of gravity
- L = length
- W = weight per unit length
| Support Method |
C |
| Cantilever |
0.56 |
| Point support each end |
1.57 |
| Both ends fixed |
3.56 |
| Totally unsupported |
3.56 |
Resonator
A device that resonates or vibrates in sympathy with a source of sound or vibration.Resonator Silencer
An air-conditioning silencer employing tuned stubs and their resonating effect for its action.Response
Motion or other output resulting from an excitation, under specified conditions. See Frequency Response.Response Spectrum
The frequency response function, also called the response spectrum, is a characteristic of a system that has a measured response resulting from a known applied input. In the case of a mechanical structure, the frequency response is the spectrum of the vibration of the structure divided by the spectrum of the input force to the system. To measure the frequency response of a mechanical system, one must measure the spectra of both the input force to the system and the vibration response, which is most easily done with a dual-channel FFT analyzer. Frequency response measurements are used extensively in modal analysis of mechanical systems. The frequency response function is actually a three-dimensional quantity, consisting of amplitude vs. phase vs. frequency. Therefore, a true plot of it requires three dimensions, and this is difficult to represent on paper. One way to do this is the so-called Bode plot, which consists of two curves, one of amplitude vs. frequency and one of phase vs. frequency. Another way to look at the frequency response function is to resolve the phase portion into two orthogonal components, one in-phase part (called the real part), and one part 90 degrees out of phase (called the “quadrature” or “imaginary” part). Sometimes these two phase parts are plotted against each other, and the result is the so-called Nyquist plot.Reverberant Field
The region in a room where the reflected sound dominates, as opposed to the region close to the noise source where the direct sound dominates. The same as diffuse field.Reverberation
The persistence of sound in an enclosure after a sound source has been stopped.Reverberation Chamber/Room
A room with hard boundaries and long reverberation time, especially designed to make the sound field inside it as diffuse (homogeneous) as possible. Can be used for measuring sound absorption coefficients.Reverberation Time (RT60 or T60)
The tailing off of sound in an enclosure because of multiple reflections from the boundaries. The reverberation time of a room is the time (in seconds) taken for the sound pressure level at a specific frequency to decrease 60 dB from its steady-state value when the source of sound energy is suddenly interrupted. It is a measure of the persistence of an impulsive sound in a room as well as of the amount of acoustical absorption present inside the room. Rooms with long reverberation times are called live rooms.
Acceptable Reverberation Times at 500 Hz
Reverse Indicator Method
Method for taking shaft alignment readings with indicators mounted radially at opposite ends of a spanned section (on each machine).RFZ
Reflection-free zone.Rigid Body Mode
Represents the non-deformed mode shapes of a body suspended on soft supports. Examples are a car body, ship or aircraft. A body has six rigid modes: heave, surge, yaw, pitch, roll and sway.Ringing
High-Q electrical circuits and acoustical devices have a tendency to oscillate (or ring) when excited by a suddenly applied signal.Ripple
Ripple appears in the pass-band of a filter. We measure the height of the ripple within ± ½ line spacing around the centre frequency in dBs. Ripple is also referred to when discussing time window weighting, as a time window is effectively a type of filter.RMS
The root-mean-square value of a time-varying quantity is obtained by squaring the function at each instant, obtaining the average of the squared values over the interval of interest, and then taking the square root of this average. For a sine wave, if you multiply the RMS value by the square root of 2 (1.414), you get the peak value of the wave. The RMS value, also called the effective value of the sound pressure, is the best measure of ordinary continuous sound, but the peak value is necessary for assessment of impulsive noises. Also, used to describe the mathematical process of determining the average value of a complex signal.RMS Sound Pressure
The square root of the time averaged square of the sound pressure.Roll Off
The attenuation of a high-pass or low-pass filter is called roll off. The term is mostly used for high frequency attenuation.Room Constant
where:
- R is the room constant in square metres
- St is the total area of the room in square metres
- a is the average absorption coefficient
where:
- an is the absorption coefficient of component surface, Sn
or
where:
- T is the reverberation time in seconds
- V is the room volume in cubic metres
- St is the total surface area in square metres
Room Criteria (RC)
A single-number noise rating system developed in 1981 to diagnose and rate the HVAC noise exposure in a room. This system is more effective than the noise criteria (NC) System in rating noise with strong low-frequency content.Room Mode
The normal modes of vibration of an enclosed space. See Mode.Root Mean Square Spectral Density (RMSSD)
This is the square root of the power spectral density. Root Mean Square Spectral Density (RMSSD) is a scaling unit.Roots
The roots of the characteristic equation are complex and have a real and imaginary part. The real part describes the damping (decay rate) of the system and the imaginary part describes the oscillations or damped natural frequency of the system.Rotational Play
Looseness, usually in a coupling, where a rotor can rotate a given distance before the rotational play is out and the coupled shaft begins to rotate also.Roughness
A measure of the modulation amplitude and frequency modulation of a sound where the modulation is in the range from 15 to 300 Hz. Maximum roughness occurs around 70 Hz and gives the unpleasant sensation of a stationary but rough sound. Above this frequency, the time constant of human hearing starts to take effect and reduces the perception of modulation. When determining roughness, the loudness data must be sampled in real-time with a high sampling rate.Round Window
The tiny membrane of the cochlea that opens in the middle ear that serves as a “pressure release” for the cochlear fluid.Run Down
Where a motor or other rotating or reciprocating machine is decelerated downwards past an upper known RPM to a lower known RPM and a measurement is made. The tacho frequency is supposed to decrease.Run Up
Where a motor or other rotating or reciprocating machine is accelerated upwards past a lower known RPM to an upper known RPM and a measurement is made. The tacho frequency is supposed to increase.Brüel & Kjær Sound & Vibration Dictionary End User Agreement
