NEBB S&V CP key terms

A logarithmic measure, in decibels, of the root-mean-square sound pressure relative to the reference pressure of 20 micropascals. It describes how loud a sound is at a specific location and depends on distance and environment.

A dimensionless logarithmic unit expressing the ratio between a measured quantity and a fixed reference. In acoustics it compresses a very wide range of pressures or intensities into a convenient scale, so equal dB increments represent equal ratios, not equal absolute amounts.

The fixed denominator used when computing sound pressure level, standardized at 20 micropascals in air, which approximates the threshold of normal human hearing at 1 kHz. Changing the reference changes the numerical dB value, so it must always be stated.

A decibel value describing the total acoustic energy radiated by a source itself, independent of location or room. Because it is a property of the equipment, it is the preferred descriptor for specifying machinery noise in procurement and design calculations.

A decibel measure of acoustic power passing through a unit area, referenced to one picowatt per square meter. Intensity is a vector quantity, so it can be used to locate and rank sources and to compute sound power without a special room.

A frequency weighting that attenuates low and very high frequencies to approximate the loudness perceived by the human ear at moderate levels. It is the default weighting for most environmental, occupational, and building-services noise limits.

A frequency weighting that is nearly flat across the audible band, with modest roll-off at the extremes. It is used for high-level sound assessment, peak measurements, and as the companion weighting when calculating A-weighted environmental corrections.

Standardized exponential averaging applied to a fluctuating sound signal so the displayed level is readable. Fast uses a 125-millisecond time constant and slow uses one second; the choice must match the measurement standard or limit being applied.

A time constant with a rapid rise and slower decay, designed to capture short transient sounds such as impacts or hammer blows. It is specified in some occupational and community standards and can read higher than fast weighting on impulsive events.

The steady sound level that, over a stated period, would deliver the same total acoustic energy as the actual fluctuating sound. It is the fundamental metric for environmental and occupational noise limits and is always tied to a measurement duration.

The A-weighted level that a single event would produce if all its energy were compressed into one second. It lets events of different duration be compared directly and is widely used for aircraft, rail, and transient equipment noise.

The sound level exceeded for a stated percentage of a measurement period, such as L10 or L90. Percentiles describe the distribution of fluctuating noise, with low percentiles often used to characterize residual or background conditions.

A 24-hour energy-averaged A-weighted level in which nighttime sounds receive a 10 dB penalty to reflect increased annoyance. It is a common community noise descriptor for transportation and industrial sources.

The highest instantaneous sound pressure reached during an event, expressed in decibels without time averaging. It is used for hearing-conservation limits and for assessing impulsive sources such as blasting, impacts, or gunfire.

A frequency range whose upper limit is twice its lower limit, usually identified by its center frequency such as 500 Hz or 1 kHz. Octave-band data reveal how sound energy is distributed across the spectrum and guide treatment selection.

A narrower frequency band with a bandwidth of about 23 percent of its center frequency, giving finer spectral resolution than octave bands. It is standard for detailed acoustical analysis, rating procedures, and tonal-noise assessment.

The process of separating a complex sound into its component frequency bands or lines to identify dominant tones and their levels. It is done with filters or fast Fourier transforms and is essential for diagnosing noise sources and choosing controls.

A graphical or tabular presentation of sound level versus frequency for a given source or location. Comparing spectra before and after treatment shows which frequency ranges were reduced and whether the remaining noise is tonal or broadband.

The sound level present in a space when the source of interest is not operating, including all other equipment and activity. It sets the floor for measurement and strongly affects how much a new source raises the total level.

The difference in decibels between the sound of interest and the background level at the same location. A small difference biases the measured source level upward, so corrections or quieter measurement conditions are needed.

Regions around a source where measurement conditions differ. The near field is close enough that pressure and particle velocity are not simply related and levels vary sharply; the far field is where the level falls by about 6 dB per doubling of distance.

An acoustic environment with no reflecting surfaces, so sound travels outward from a source without returning. Measurements in a free field or an anechoic room can be predicted from sound power using distance and directivity.

A region where sound energy arrives equally from all directions and the level is essentially uniform throughout. A properly designed reverberation room approximates this condition, allowing sound power to be determined from average pressure.

The rule that sound pressure level from a point source in a free field decreases about 6 dB each time the distance from the source doubles. It underpins distance corrections, and it does not hold in enclosed or highly reverberant spaces.

The way a source radiates more sound in some directions than others, described by a directivity factor or index. It explains why a machine can be louder to one side and why measurement positions must be specified relative to the source.

The time required for the sound level in a room to decay by 60 dB after the source stops. It quantifies how live or absorbent a space is and affects both measured levels and speech privacy and clarity.

The fraction of incident sound energy absorbed or transmitted at a surface rather than reflected, ranging from zero to one and varying with frequency. It is the key input to reverberation calculations and the selection of acoustical treatments.

A single-number average of a material's absorption coefficients at 250, 500, 1000, and 2000 Hz, rounded to the nearest 0.05. It is a convenient product comparison but hides performance at low and high frequencies.

A single-number rating of how well a partition blocks airborne sound, derived from laboratory transmission-loss data over a standard frequency range. Higher values mean more isolation, but field performance is usually lower because of flanking and workmanship.

A field rating of airborne sound isolation between two rooms, calculated from measured level differences with a correction for the receiving-room absorption. It reflects the installed construction rather than the laboratory performance of a single element.

The arithmetic difference in sound pressure level between two locations, commonly a source room and a receiving room, without any correction for absorption. It is easy to measure but depends heavily on the receiving room's furnishings.

The quantified range around a reported result within which the true value is expected to lie, combining instrument tolerance, calibration, positioning, and environmental effects. Reporting it honestly is essential when comparing results against a limit.

The documented comparison of an instrument's response against a known reference, typically a portable acoustic calibrator producing a stated level at a stated frequency. It is performed before and after measurements to demonstrate the data are traceable and valid.

An instrument that converts sound pressure into an electrical signal, applies frequency and time weightings, and displays a decibel level. Its class, settings, and calibration status must be recorded so results can be defended.

The rate of change of velocity of a vibrating surface, usually expressed in meters per second squared or in g. It is the quantity most vibration transducers measure directly and is preferred for high-frequency and shock work.

The speed of a vibrating surface as it oscillates, usually given as root-mean-square millimeters per second. Velocity is widely used for machinery condition assessment because it correlates well with human perception and damage potential in the mid-frequency range.

The peak-to-peak or zero-to-peak distance a vibrating surface moves from its rest position, usually in micrometers or mils. It is the most useful descriptor at low frequencies, where clearances and stresses rather than acceleration dominate concern.

Frequency is the number of complete vibration cycles per second in hertz; period is the time for one cycle and is the reciprocal of frequency. Together they identify resonances, forcing frequencies, and machine running speeds.

A condition in which a structure responds with unusually large amplitude when excited near one of its natural frequencies. It can amplify small forces into damaging or annoying vibration, so measurements check whether forcing frequencies coincide with natural frequencies.

The dissipation of vibration energy that limits amplitude growth near resonance and controls how quickly motion decays. It is described by a damping ratio or loss factor and can come from material hysteresis, joints, or added dampers.

The ratio of the vibration amplitude transmitted through an isolator or structure to the amplitude at its input. Values below one mean the isolator is working; above one, the mount may be amplifying vibration.

The use of resilient mounts, pads, or inertia bases to reduce the transmission of vibration from a machine into a structure, or of structure-borne vibration into sensitive equipment. Effective isolation requires the mount's natural frequency to be well below the forcing frequency.

A transducer, usually piezoelectric, that produces a charge or voltage proportional to acceleration. Its mounting method, mass loading, and cable routing strongly affect data quality, especially at high frequencies.

A statistical measure of the effective magnitude of an oscillating signal, equal to the square root of the mean of the squared values over time. RMS is the usual basis for reported velocity and acceleration levels because it reflects energy content.

Peak is the maximum excursion from the mean in one direction; peak-to-peak is the total swing between extremes. These measures matter for clearances, impacts, and shock, and they relate to RMS by a factor that depends on waveform shape.

The ratio of a signal's peak value to its RMS value, indicating how impulsive or spiky the waveform is. A sine wave has about 1.41; higher values signal impacts or bearing defects that RMS alone may understate.

A logarithmic ratio of a measured vibration quantity to a stated reference, such as 10 to the minus 6 meters per second squared for acceleration or 10 to the minus 9 meters per second for velocity. It compresses wide ranges and allows convenient comparison with criteria.

A rating of how strong a machine's vibration is, compared against standardized zones or charts for machines of that type and power. It guides acceptance testing, maintenance decisions, and assessment of whether operation is safe and reliable.

Presenting vibration data against multiples of a machine's running speed rather than against absolute frequency. Orders stay fixed as speed changes, which makes it easier to identify imbalance, misalignment, and other speed-related faults.

Sound radiated after vibration travels through building structure rather than through the air. It can bypass airborne paths and travel long distances, so it is controlled at the source with isolation rather than by adding absorptive treatment.

Sound radiated into a room when vibration or airborne noise inside a duct or enclosure excites its walls. It is a common cause of unexpected noise in spaces adjacent to mechanical equipment and is reduced by stiffer, damped, or isolated panels.

Sound or vibration reaching a receiving space by paths other than the intended separating element, such as ducts, ceilings, or structural joints. It limits the isolation actually achieved and must be found and corrected before blaming the partition.

The span between the lowest and highest frequencies over which an instrument, transducer, or measurement is valid within stated tolerances. Data outside the range may be attenuated or distorted, so results must be interpreted only within it.

Assessment of whole-body or hand-arm vibration transmitted to people, described by frequency-weighted acceleration and daily exposure values. It is compared against action and limit values to protect against discomfort, injury, and vascular damage.