N : Sound and Vibration Terms, Definitions, Units, Measurements ...
the unit of force
N·m (newton metre)
the SI unit of torque
, also used to measure energy
in which case the SI unit is the Joule (J)
N·m·s (newton metre second)
the unit of angular momentum
N/m2 (newton per metre-squared)
equivalent to the pascal
the SI derived unit for sound pressure
N·s (newton second) the SI unit of impulse and momentum.
a SI prefix = 10-9
see other SI units
National Institute of Standards and Technology (NIST) promotes U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology
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.
, 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
of a number is its logarithm
to the base of the mathematical constant e
, where e is approximately equal to 2.718281828459. The natural logarithm
of x is generally written as ln x, loge x,
Near Sound Field
Neper Definition IEC 801-22-04,
unit of level of a field quantity
when the base of the logarithm is e = 2,718... . Also, unit of level of a power-like quantity when the logarithm is on the base the square of e, which is equal to 7,389...
Note 1 : the decineper is one-tenth of a neper
Note 2 : one neper equals 8,686... decibels
Newton and related SI units
day noise level •
day evening night noise level •
evening noise level
Nitrile rubber or Buna-N
, is a synthetic rubber copolymer of acrylonitrile (ACN) and butadiene. Some trade names are: Nipol, Krynac and Europrene.
under sound waves
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. More recent ISO standards recommend A-weighting for commercial aircraft noise.
effective perceived noise level
noise exposure forecast
Noise Criteria (NC)
, a single number for rating the sound quality of a room, developed in the USA in the 1950s and 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. Some years later the noise criteria curves were updated by the preferred noise criteria (PNC) curves
Noise Criteria Decisive Band is the frequency band touching the NC Curve.
In 1995 the ANSI Standard S12.2 recommended the balanced noise criteria (NCB) and the room-criteria
The B&K 2250 sound analyser
measures the NC curves directly.
noise rating (NR) curves
Noise Damage Risk Criteria
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 •
noise exposure action value •
noise 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 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.
Category A represents the circumstances in which noise is unlikely to be a determining factor.
Categories B and C deal with situations where noise mitigation measures may make the development acceptable.
Category D relates to the situation in which development should normally be refused.
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 (ND + 16.7 NN ) - 88 dB :: where ND = number of flights during the day and NN = number of flights at night. More recent ISO standards recommend A-weighting for commercial aircraft noise
See also noise and number index
Superseded by the community noise equivalent level which is based on Leq measurements rather than complex calculations
Noise Immission Level is 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)
is caused by continuous exposure to noise levels greater than 80 dBA over an extended period of time. It may also result from a one-time exposure to an intense impulse sound, such as an explosion.
hearing loss •
Noise Induced Permanent Threshold Shift (NIPTS)
Noise Isolation Class (NIC)
is 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
the Lden or LDEN
also known as the day-evening-night level are used. Noise maps published on the internet are widely used in Europe and are becoming used in the UK.
Noise Rating and Noise Rating Curves (NR)
is 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 frequencies (where the ear is more sensitive) are given heavier noise ratings than lower ones, information not taken into consideration in usual measurements.
NR rating is the highest noise rating curve touched by the measured octave band spectrum.
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 practice to use the NC noise criterion curves and room criteria (RC).
Noise Reduction Coefficient
Noise Reduction Rating (NRR)
is a single number rating system, used in the USA, to compare and label hearing protectors.
See also hearing protection.
under the noise at work regulations
or preferred frequency
is a specified octave or fractional-octave filter, as defined by the ISO and ANSI Standards.
Nonlinear System in mathematics and physical sciences, a nonlinear system is a system in which the change of the output is not proportional to the change of the input
See also • linear system
, a signal whose frequency content changes within a captured time frame.
, to a surface is a vector
perpendicular to the surface at a given point.
Also known as normal vector and surface normal
Normal Mode of Oscillation Definition IEC 801-24-14,
mode of free oscillation
of an undamped system
● Note : In general, any composite motion of the system is analysable into a summation of its normal modes, each of which may oscillate entirely independently of the others.
See also other oscillation terms
Normalized Impact Sound Pressure Level
Normalized Level Difference
Null a low or minimum point on a graph. A minimum pressure region in a room.
Nyquist Frequency, twice the maximum frequency of the signal being analysed. It is the minimum rate at which a signal can be sampled without introducing errors.
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