Some useful terminology that commonly appears in smoothness specifications:

A sensor that measures acceleration. An inertial profiler uses the signal from the on-board accelerometer(s) to compute the inertial reference value (i.e., relative height) that can be combined with the height sensor data to produce a complete profile. See also Height Sensor.
Lack of error. The root-mean-square value of the error when comparing measured values with “deemed correct” or reference values. See also Repeatability.
The maximum value of a periodic curve measured along its vertical axis with reference to the horizontal axis.
Bias Error
Error stemming from systematic problems, including inaccurate calibration, physical damage, or a defect in the profiler’s design.
Blanking Band
A band that is placed over a profilograph trace to “mask” the portion of the trace that is not counted as roughness.
Bode Plot
A frequency response plot made on log-log axes.
Bounce Test
A test performed on an inertial profiler when stationary in order to check the measurement system.
The process of correcting the scale of a transducer.
Correlation Analysis
Analysis that describes how one variable relates to another.
Digital Filter
A calculation procedure that transforms one signal into another to eliminate irrelevant data.
A static, inclinometer-based proprietary device used to measure elevations at individual points (originally used for measuring floor flatness).
Distance Measurement Instrument (DMI)
A transducer used to determine the longitudinal distance that the measurement vehicle has traveled.
Frequency Response
The ratio of output amplitude to input amplitude for a sinusoid.
The ratio that a filter amplifies or attenuates an input signal component at a given wavelength or frequency.
Golden Car
The quarter-car model used in IRI computation. See also IRI.
Half-car Roughness Index (HRI)
A number calculated by applying the IRI algorithm to the average of two profiles.
Height Sensor
A sensor used in an inertial profiler to measure the vertical distance between the sensor and the pavement surface.
High-Pass Filter
A digital filter that produces an anti-smoothing effect that reduces the effect of long wavelengths. See also Low-Pass Filter.
High-Speed Profiler
An inertial profiler (vans, trucks, cars, etc.) that is operated at prevailing highway speed. See also Inertial Profiler and Light-Weight Profiler.
Inertial Profiler
A profile measurement device that measures the pavement profile using an accelerometer to form an inertial reference and a height sensor to measure the pavement surface height relative to that reference.
International Roughness Index (IRI)
A statistic that summarizes the roughness qualities impacting vehicle response based on the Golden-Car vehicle model at a standard simulation speed of 49.7 mph (80 km/h).
The distance required for an inertial profiler to reach an acceptable speed and for the data collection filters used in the profile computation to stabilize. See also Lead-out.
A safe distance for an inertial profiler to operate to a stop position or until the data collection system is turned off. See also Lead-In.
Light-Weight Profiler
An inertial profiler that is relatively light-weight (golf cart, ATV, etc.) compared with high-speed profilers. It is often operated much slower than prevailing traffic speed. See also Inertial Profiler and High-Speed Profiler.
Linear System
A system in which the output is directly proportional to the input.
Localized Roughness
Short sections of roadway that contribute disproportionately to the overall roughness index value. Also referenced as “hot spots”
Longitudinal Profile
A longitudinal two-dimensional slice of a road surface taken along an imaginary line that consists of elevation values and a distance reference for each elevation.
Low-Pass Filter
A smoothing filter, such as moving-average, that reduces the effect of short wavelengths. See also High-pass Filter.
Mean Panel Ratings
A subjective rating system whereby automobile passengers rate the smoothness of a given road after driving on it.
Mean Roughness Index (MRI)
A number calculated by averaging the IRI values from the two wheelpath profiles.
Moving Average Filter
A digital filter that replaces each profile elevation point with the average of several adjacent points.
The ability to obtain consistent results when using different types of measurement devices.
Power Spectral Density (PSD)
A method that describes how the power of a signal (i.e., a time average of signal energy) or time series is distributed with frequency.
The measure of variation between multiple measurements, expressed in standard deviation.
Profile Index
A generic summary number calculated from a profile. This term should be avoided since it may be confused with PrI or RN. See also PrI and RN.
An instrument used to measure road profiles.
Profiler Certification
A procedure used to determine whether a profiler satisfies a specific set of performance criteria.
Profiler Operator Certification
A procedure used to determine whether a profiler operator passes a specific set of written and field operational tests in order to be certified for operating profilers.
A device used to measure smoothness by measuring the deviations of a pavement surface using a mid-point measuring wheel from the reference established by a set of wheels (6 for a California-type) at either end of the device.
Profilograph Index (PrI)
A smoothness index that is computed from a profilograph trace. This is sometimes called Profile Index (PI), but is more specifically called PrI.
Reference Device
A device used to obtain the true profile of a pavement. Devices such as rod and level, Dipstick(TM) and walking profiler are considered reference devices.
The expected standard deviation of measurements obtained in repeated tests using the same device on a single, randomly-selected pavement surface.
The standard deviation of the error included in a single measurement, relative to a reference measurement. The reproducibility of a device includes errors that are systematic with respect to that device, but random with respect to a particular test.
Ride Number (RN)
A calculated roughness index, between 0 and 5, that approximates the Mean Panel Rating for a pavement surface. See also Mean Panel Ratings, IRI, and PrI.
Also spelled out as “Ride Quality”. Measured as accelerations in the vehicle body.
Road Roughness
The deviations of a pavement surface from a true plane surface with characteristic dimensions that affect vehicle dynamics and ride quality.
Rod and Level
Static equipment used to measure elevations at individual points, commonly used for land surveying.
Rolling Straightedge
Profiling equipment generally consisting of a rigid beam (or frame) with support wheels on either end and a measuring wheel at the middle, which is rolled over the surface to be profiled.
Roughness Profile
A plot that shows the variation of roughness over a section of pavement. This is also referred to as a “continuous roughness report”.
Response-type Road Roughness Measuring System (RSRRMS)
A system that measures suspension deflection of either one or two wheels of either a passenger car or a towed trailer and records these deflections as "counts" or as actual measured deflection.
Sample Interval
The longitudinal distance between captured data points.
Segment Length
The length of section of pavement where a smoothness index is recorded.
Signal Processing
The mathematical transformation and analysis of signals.
A series of numbers.
A periodic curve defined by wavelength, amplitude and phase.
Lack of roughness or lack of significant bumps and dips from the pavement surface that cause discomfort to motorists.
Standard Error
The portion of the total error due to random effects.
True Profile
The undistorted shape of a pavement surface.
Valid Profiler
A profiler that provides the same statistical values that would be obtained from the true profile.
Verification Site
A pavement section used to periodically check if an inertial profiler is functioning properly.
Wave Band
A range of frequencies. In profile analysis, wave band often refers to spatial frequencies. See also Wave Number.
Wave Number
The number of wave cycles per unit length.
The distance between peaks or crests of a wave or sinusoid.