Definitions, Terms, Units and Parameters
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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.
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.
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.
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.
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
Newton metre second : N·m·s: the unit of Angular Momentum.
Newton second : N·s: the SI unit of impulse and Momentum. Impulse and momentum have the same dimensions, but momentum is measured in kilogram metres per second.
Nitrile rubber or Buna-N: is a synthetic rubber copolymer of acrylonitrile (ACN) and butadiene. Some trade names are: Nipol, Krynac and Europrene.
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. Other noise descriptors :
Noise and Number Index : NNI: noise unit developed in the 1960s for rating aircraft annoyance.
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).
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.
Noise Dose Meter
Noise Dose per Hour
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: see also Daily Noise Exposure l Exposure Action Level l Exposure Limit Value l Sound Exposure
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 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.
Noise Exposure Forecast : NEF: a complex criterion for predicting future noise impact of 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 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 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 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.
Noise Rating Decisive Bandis the frequency band touching the NR Curve.
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: or Preferred Frequency - a specified octave or fractional-octave filter as defined by the ISO and ANSI Standards.
Non-stationary Signal: a signal whose frequency content changes within a captured time frame.
Normalised: measurement in accordance with a Standard or 'Norm'.
Normalised Level Difference : Dn
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.
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. 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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