Definitions, Terms, Units and Parameters:
N, N·m, N·m·s, N·s,
: under Newton
nano : n
: SI prefix = 10-9
see other SI Units
Narrow Band Noise
: noise which has it's energy distributed over a relatively small section of the audible range.
Related Terms :
Constant Bandwidth, Constant Percentage Bandwidths
, Octave Bands
, FFT- Fast Fourier Transform
Narrow Band Spectra
: spectra that have been measured using a narrow frequency bandwidth.
: the frequency at which a resiliently mounted mass will vibrate when set into free vibration. The frequency of oscillation of the free vibration of a system if no Damping
: Np : is a logarithmic unit for ratios of measurements of physical field and power quantities.
Newton : N
: the derived SI unit of Force
required to accelerate a mass of 1 kg at a rate of 1 m/s2
- approximately equal to the Gravitational
force on a 100 g mass.
Units: kilogram metre per second squared, kg·m/s2
1 N = 1 kg·m/s2
Newton metre : N·m
: the unit of Torque
. Also used to measure energy or work in which case the SI unit is the Joule J
1 N·m = 1 J = 1 W·s
Newton metre second : N·m·s
: the unit of Angular Momentum
1 N m s = kg m2
Newton second : N·s
: the SI unit of impulse and Momentum
. Impulse and momentum have the same dimensions, but momentum is measured in kilogram metre per second.
1 N·s = 1 kg·m/s
Nitrile rubber or Buna-N
: is a synthetic rubber copolymer of acrylonitrile (ACN) and butadiene. Some trade names are: Nipol, Krynac and Europrene.
: the point of minimum displacement in a Sound Wave
: in practice, this amplitude is generally not zero but simply a minimum. The node is then said to be partial.
: the appropriate modifier should be used to signify the type that is intended; e.g. Particle Velocity Node, Sound Pressure Node.
: any sound that is undesired by the recipient. Any sound not occurring in the natural environment, such as sounds emanating from aircraft, highways, industrial, commercial and residential sources. Interference of an electrical or acoustical nature.
Other noise descriptors :
Narrow Band Noise
Noise and Number Index : NNI
: noise unit developed in the 1960s for rating aircraft annoyance.
NNI = L + 15 x Lg N - 80
where L is the log average Peak PNdB noise level and N is the number of aircraft movements.
35 NNI was rated as low annoyance and 55 NNI high annoyance.
NNI contours were 'drawn' around Heathrow at the time.
Noise at Work Regulations
: The Control of Noise at Work Regulations 2005 (the Noise Regulations) came into force for all industry sectors in Great Britain on 6 April 2006 (except for the music and entertainment sectors where they came into force on 6 April 2008).
The aim of the Noise Regulations is to ensure that workers' hearing is protected from excessive noise at their place of work, which could cause them to lose their hearing and/or to suffer from tinnitus (permanent ringing in the ears).
The regulations set Exposure Limit Values and Exposure Action Levels
The Control of Noise at Work Regulations 2005 replace the Noise at Work Regulations 1989.
: Sound Exposure
and Noise Dose
For others details see the Government and/or HSE websites.
Noise Criteria : NC
: a single number for rating the sound quality of a room, used extensively by the air conditioning industry to test the background levels on offices etc., due to the air-conditioning. The measured Octave Bands
are compared with the NC Curves - based on equal loudness curves. The NC rating is the value of the highest NC curve touched by the measured spectrum.
Noise Criteria Decisive Band
: is the frequency band touching the NC Curve.
The B&K 2250 Sound Analyser
measures the NC curves directly.
: NR : Noise Rating Curves
: Sound Exposure
Noise Emission Level
: the dBA level measured at a specified distance and direction from a noise source, in an open environment, above a specified type of surface. Generally follows the recommendation of a national or industry standard.
Daily Noise Exposure
Exposure Action Level
Exposure Limit Value
Noise Exposure Category : NEC
: used by local planning authorities to determine the suitability of a proposal for residential development. The 4 Noise Exposure Categories described in
take account of both day and night time noise levels from road, rail and air transport. The NEC levels should not be used for assessing the impact of industrial noise on proposed residential development although at a mixed noise site where industrial noise is present but not dominant, its contribution should be included in establishing the appropriate NEC.
Category A represents the circumstances in which noise is unlikely to be a determining factor.
Category D relates to the situation in which development should normally be refused.
Categories B & C deal with situations where noise mitigation measures may make the development acceptable.
See the UK Government Web Site
for more details.
Noise Exposure Forecast : NEF
: a complex criterion for predicting future noise impact of airports.
The calculation is based on the Effective Perceived Noise Levels
: EPNL and takes into account each type of aircraft, flight profile, number of flights, time of day, etc.
Developed by the US Federal Aviation Agency to generate equal NEF contours for zoning control around airports.
NEF = EPNL + 10 lg10
+ 16.7 NN
) - 88 dB :: where
= number of flights during the day and NN
= number of flights at night.
Superseded by the Community Noise Equivalent Level
: CNEL which is based on Leq measurements rather than complex calculations
: a measure of the signal created from the sum of all noise sources and unwanted signals within a measurement system. Signals beneath the noise floor cannot be measured.
Noise Immission Level
: the total quantity of sound impinging on the ear over a long period, expressed in decibels. It can be calculated from LEX, ref. duration
and the number of years of exposure.
Noise Induced Hearing Loss : NIHL
: may be caused by a one-time exposure to an intense impulse sound, such as an explosion, or by continuous exposure to noise levels greater than 80 dBA over an extended period of time.
: Threshold Shift
Noise Isolation Class : NIC
: a single number rating of the degree of speech privacy achieved through the use of an acoustical ceiling and sound absorbing screens in an open office. NIC has been replaced by the Articulation Class
(AC) rating method.
: for airborne sound, unless specified to the contrary, it is the
A-weighted Sound Level
: for noise mapping the
Lden or LDEN
also known as the day-evening-night level is used. Noise maps published on the internet are widely used in Europe and are becoming used in the UK.
Noise Pollution Level : LNP
: a variation of
Leq - Equivalent Continuous Noise Level
- which accounts for short-term variability in noise level. For a
distribution of dBA level it is defined as: LNP
Noise Rating Curves : NR
: a method for rating the acceptability of indoor environments for the purposes of hearing preservation, speech communication and annoyance, based on curves developed by Kosten and van Os (1962). Sound Pressure Levels
measured in octave bands are compared with these curves from which a noise rating (NR) is obtained. It will be seen that higher frequencies (where the ear is more sensitive) are given heavier noise ratings than lower ones, information not taken into consideration in usual measurements.
The NR rating is the highest NR Curve touched by the measured Octave Band
Noise Rating Decisive Band
is the frequency band touching the NR Curve.
The B&K 2250 Sound Analyser
measures the NR curves directly.
In America it is common to use the NC Noise Criterion Curves
Noise Reduction : NR
: the numerical difference, in decibels, of the average Sound Pressure Levels
in two areas or rooms. A measurement of noise reduction combines the effect of the Sound Transmission Loss
performance of structures separating the two areas or rooms, plus the effect of Acoustic Absorption
present in the receiving room.
Noise Reduction Coefficient : NRC
: a single-number rating system used to compare the sound-absorbing characteristics of building materials. A measurement of the acoustical absorption performance of a material, calculated by averaging its Sound Absorption
coefficients at 250, 500, 1000 and 2000 Hz, expressed to the nearest multiple of 0.05.
Noise Reduction Rating : NRR
: a single-number rating system used to compare and label hearing protectors. For example the protected dBA = unprotected dBC - NRR.
: Noise at Work Regulations
: a prescribed function of Sound Pressure Levels
in the 24 one-third octave bands centred on 50 Hz to 10 kHz that is used in the calculation of Perceived Noise Level
: or Preferred Frequency
- a specified octave or fractional-octave filter as defined by the ISO and ANSI Standards.
: a signal whose frequency content changes within a captured time frame.
: measurement in accordance with a Standard or 'Norm'.
: a linear unit of noisiness or annoyance. 1 Noy is defined as the noisiness of a 1000 Hz tone at a SPL of 40 dB, 2 Noys is twice as noisy as a 1 Noy and half as noisy as 4 Noys. see Perceived Noise Level
NPL : National Physical Laboratory
: is the UK's National Measurement Institute and is a world-leading centre of excellence in developing and applying the most accurate measurement standards, science and technology.
: a low or minimum point on a graph. A minimum pressure region in a room.
: twice the maximum frequency of the signal being analysed. A 'false' analogue signal at frequencies greater than the Nyquist Frequency appears after sampling at a frequency less than the Nyquist frequency.
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