NEBB S&V CP flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Reference sound pressure p0

p0 = 20 micropascals (2×10⁻⁵ Pa), the standard reference for airborne sound pressure level, chosen near the threshold of hearing at 1 kHz.

A-weighting purpose

A-weighting approximates human loudness perception at moderate levels by attenuating low and very high frequencies; it is used for environmental and occupational noise limits.

Decibel addition rule

Incoherent sources combine on an energy basis: L_total = 10 log10(10^(L1/10) + 10^(L2/10)). Two equal levels add 3 dB; ten equal levels add 10 dB.

Octave band center frequencies

Standard octave bands run 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000 and 16000 Hz; each upper band edge is twice its lower edge.

Point source distance law

In a free field, sound pressure level from a point source decreases 6 dB per doubling of distance: Lp2 = Lp1 − 20 log10(r2/r1).

Directivity factor Q

Q = 10^(DI/10), relating on-axis intensity to the spherical-average intensity. Free space gives Q = 1, a hard floor half space Q = 2, a corner Q = 8.

Sabine reverberation time

RT60 = 0.161 V/A in metric units, where V is room volume in cubic metres and A is total absorption in square metre sabins.

Room constant R

R = S·ᾱ/(1 − ᾱ), where S is total surface area and ᾱ is average absorption coefficient; larger R means a weaker reverberant field.

Sound power level formula

Lw = 10 log10(W/W0) with W0 = 1 picowatt (10⁻¹² W). Sound power is a property of the source and is independent of the measurement environment.

Sound intensity level

LI = 10 log10(I/I0) with I0 = 1 picowatt per square metre. Intensity is a vector quantity, allowing sound power determination by surface scanning.

Near versus far field

The near field lies within about one wavelength or the largest source dimension, where level varies strongly; the far field follows inverse-square distance behaviour.

C-weighting use

C-weighting is nearly flat across the audio range and is used for high-level noise assessment and for computing A-weighted levels from octave band data.

One-third octave bandwidth

One-third octave bands have upper-to-lower edge ratio 2^(1/3) and centre-to-edge factor 2^(1/6); they give finer spectral detail than octave bands.

Equivalent level Leq

Leq is the energy-averaged sound level over a stated time period, computed as 10 log10 of the mean of 10^(L/10) values.

Background noise correction

When measured level exceeds background by 10 dB or more, no correction is needed; at 3 dB difference subtract 3 dB; below 3 dB the result is unreliable.

Free field definition

A free field is a space with no reflecting boundaries, so sound radiates outward without reflection; an anechoic chamber approximates it above its cutoff frequency.

Accelerometer mounted resonance

The mounted resonance frequency sets the usable range, typically to one-third of it; stiffer mounting such as a stud preserves the highest usable frequency.

IEPE accelerometer

IEPE (integrated electronics piezoelectric) sensors contain built-in charge conversion and need a constant-current source, typically 2 to 4 mA, from the analyser.

Piezoelectric static limitation

Piezoelectric accelerometers cannot measure true static acceleration because the charge decays; they are unsuitable for steady centrifugal or gravitational offsets.

RMS of a sine wave

For sinusoidal vibration, RMS equals peak divided by the square root of two, approximately 0.707 times the peak amplitude.

Crest factor

Crest factor is the ratio of peak to RMS amplitude; it equals 1.414 for a pure sine and rises for impulsive or shock-containing signals.

Velocity reference level

Vibration velocity level uses a reference of 1 nanometre per second: Lv = 20 log10(v_rms/v0); the same 20 log form applies to acceleration and displacement.

Acceleration reference level

Acceleration level commonly uses a0 = 1 micrometre per second squared in ISO usage; some standards cite 10 micrometres per second squared, so confirm the cited reference.

Wk and Wd weighting

ISO 8041 defines Wk weighting for vertical (z-axis) whole-body vibration and Wd weighting for horizontal (x and y axis) exposure assessment.

FFT frequency resolution

Frequency resolution equals sampling rate divided by number of samples, or one over the record duration; longer records give finer resolution.

Overall RMS from spectrum

The overall RMS of a spectrum is the square root of the sum of squared individual band or line RMS amplitudes, not the sum of peaks.

Integration slope

Integrating acceleration to velocity or displacement applies a 1/(2πf) factor per integration, attenuating high frequencies and amplifying low-frequency noise.

Vibration meter calibration

Field calibration uses a handheld shaker at a known frequency and amplitude, commonly 159.15 Hz (1000 rad/s) at 10 m/s², checked before and after measurement.

Transverse sensitivity

Transverse sensitivity is an accelerometer's response to motion perpendicular to its sensitive axis, usually under 5 percent of the main-axis sensitivity.

Sampling and aliasing

Sampling rate must exceed twice the highest frequency of interest; anti-alias filtering before digitising prevents high frequencies from appearing as false low frequencies.

Hand-arm vibration exposure

Hand-arm vibration is assessed with Wh weighting and expressed as A(8), the 8-hour energy-equivalent acceleration, compared against exposure action and limit values.

Velocity as damage indicator

Vibration velocity in the 10 to 1000 Hz range correlates well with structural and machine damage and is widely used for condition monitoring criteria.