nano : n an SI prefix = 10-9 see other SI units
Other noise descriptors • ambient noise • background noise • broadband noise • periodic • pink noise • pseudo random noise • random noise • residual noise • specific noise • white noise • wideband noise
See also • undamped natural frequency
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 = 105 dynes
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 metre per second.
1 N·s = 1 kg·m/s
Other noise descriptors • ambient noise • background noise • broadband noise • narrowband noise • noisiness • periodic • pink noise • pseudo random noise • random noise • specific noise • white noise • wideband 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. More recent ISO standards recommend A-weighting for commercial aircraft noise.
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.
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.
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 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 & 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.
See the UK Government Web Site for more details.
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 noiseSee also noise and number index
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.
See also • 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 rating method.
Noise Level for airborne sound, unless specified to the contrary, it is the A-weighted sound level.
See also the • IEC Definition of Level
Noise Mapping 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 gaussian noise distribution, the dBA level it is defined as : LNP = Leq + (L10 - L90)
Noise Rating and 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 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 to use the NC Noise Criterion Curves.
See also • sound insulation
Noise Reduction Rating a single-number rating system used to compare and label hearing protectors. For example the protected dBA = unprotected dBC - NRR.
Nominal Frequency or Preferred Frequency : 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
Non-stationary Signal a signal whose frequency content changes within a captured time frame.
Also known as normal vector and surface normal
See also other oscillation terms
Normalized Sound Insulation measurements in accordance with a Standard or 'Norm'.
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.
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.