Gracey & Associates
Noise and Vibration
Glossary - N
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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.
Narrow Band Spectra : spectra that have been measured using a narrow frequency bandwidth.
National Physical Laboratory : NPL : 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.
Natural Frequency : 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 were present.
NC : Noise Criteria Curves .
Near Field : that part of a sound field, usually within about two wavelengths of a noise source, where there is no simple relationship between sound level and distance, where the sound pressure does not obey the inverse square law and the particle velocity is not in phase with the sound pressure. See also diffraction : far field : reverberant field .
NEC : Noise Exposure Category.
NEF : Noise Exposure Forecast .
Newton : N : : the derived SI unit of force required to accelerate a mass of 1 kg mass at a rate of 1 m/s2 - approximately equal to the gravitational force on a 100 g mass.
1 N = 1 kg·m/s2 = 105 dynes
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 s-1
Newton second : N·s : the SI unit of impulse and momentum. Impulse and momentum have the same dimensions, but momentum is measured in kilogram meters per second.
1 N·s = 1 kg·m/s
NIC : Noise Isolation Class .
Night Noise Indicator: Lnight : EU defined for describing the annoyance due to environmental noise, based on the LAeq measured over the night period 23.00 to 07.00.
NIHL : Noise Induced Hearing Loss .
NIPTS : : Noise Induced Permanent Threshold Shift .
Nitrile rubber or Buna-N : : is a synthetic rubber copolymer of acrylonitrile (ACN) and butadiene. Some trade names are: Nipol, Krynac and Europrene.
NNI : Noise and Number Index .
Node : a point or line on a vibrating structure that remains stationary.
Noise : 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.
Wideband noise consists of a wide range of frequencies. White noise has equal power per unit bandwidth over a specified frequency range. Pink noise has equal power in constant percentage bandwidths (for example, octave bands) over a specified frequency range.
Random noise is a desirable signal used in acoustical measurements. Random noise is a signal whose instantaneous value varies randomly with time. In the context of assessing hearing damage, any audible sound should be regarded as noise. Pink noise is random noise whose spectrum falls at 3 dB per octave: it is useful for use with sound analyzers with constant percentage bandwidths. Also, periodic or statistically random noise.
Noise and Number Index : NNI : noise unit developed in the 1960s for rating aircraft annoyance. NNI = L + 15 x Log N - 80 where L is the average 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 Control of Noise at Work Regulations 2005 replace the Noise at Work Regulations 1989.
see also sound exposure and noise dose
Noise Criteria : NC : a single number for rating the sound quality of a room, was used extensively by the air conditioning industry to test the background levels on offices etc., widely used in America - see also NR : Noise Rating Curves.
Noise Dose : Dose% : the amount of noise expressed as a percentage of the normal allowable daily collected during a measurement on a worker with a noise dosimeter.
Noise Dose Meter : Sound Exposure Meter also known as dosemeter and dosimeter.
see also sound exposure and noise dose
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.
Noise Exposure : Lex and Lep,d : see sound exposure.
Noise Exposure Category : NEC : used by local planning authorities to determine the suitability of a proposal for residential development. The 4 NECs described in PPG24 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.
Noise Exposure Forecast : NEF : a complex criterion for predicting future noise impact of airports. The computation considers effective perceived noise level of each type of aircraft, flight profile, number of flights, time of day, etc. Generally used in plots of equal NEF contours for zoning control around airports.
Noise Floor : 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.
Noise Induced Permanent Threshold Shift : NIPTS : the component of permanent threshold shift associated with a noise exposure.
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.
Noise Level : For airborne sound, unless specified to the contrary, it is the A-weighted sound level.
Noise Pollution Level : LNP : - a variation of Leq - equivalent continuous noise level - which accounts for short-term variability in noise level. For a Gaussian distribution of dBA level it is defined as: LNP = Leq + (L10 - L90)
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. In America it is common to use the Noise Criterion Curves - NC.
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 Regulations : Noise at Work Regulations.
Nominal Frequency : mid-band frequency of a particular fractional-octave filter. Term is used by the IEC standards, identical to the preferred frequencies defined in the ANSI standards.
Non-stationary Signal : a signal whose frequency content changes within a captured time frame.
Normalized Level Difference : Dn : for airborne sound transmission. A sound insulation index measured under field conditions, between 'real' rooms and deliberately includes effects due to flanking routes and differences in the relative size of the rooms. It attempts however to normalize the measured difference level to the level which would be present when the rooms are furnished by measuring the quantity of acoustic absorption in the receiving room and correcting the difference level to the level which would be expected if there was 10 m² sabin absorption in the receiving room. Accurate knowledge of the dimensions of the receiving room are required.
Noy : 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.
NR : Noise Reduction .
NRC : Noise Reduction Coefficient .
NRR : Noise Reduction Rating .
Null : a low or minimum point on a graph. A minimum pressure region in a room.
Nyquist : a type of plot that displays the imaginary part versus the real part.
GLOSSARY INDEX:
A B C
D E F
G H I
J K L
M N O
P Q R
S T U
V W X
Y Z
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