Nano-: a standard metric prefix meaning one thousand millionth (i.e. 10‚Äæ9).

Narrowband filter: a band pass filter with a small bandwidth, i.e. less than one thirds octave.

Natural frequency: the frequency of free or natural vibrations of a system.

Near field: of a sound source; the region or space surrounding the source where sound pressure and acoustic particle velocity are not in phase, and the sound pressure varies with position in a complex way.

Neighbour or neighbourhood noise: noise from domestic premises: household appliances, radios, televisions, music systems, noisy pets, DIY activities, intruder alarms, parties or similar events.

Newton (N): the SI unit of force; the force required to produce an acceleration of 1 m/s² in a mass of 1 kg.

Node: a point, line or surface in a standing-wave pattern where some characteristic of the vibration, e.g. the displacement, is zero.

Noise: unwanted sound or unwanted signal (usually electrical) in a measurement or instrumentation system.

Noise and track keeping (NTK) system: a computerised system used at some major airports whereby radar is used to monitor flightpaths (tracks) of arriving and departing aircraft in order to identify aircraft which are not keeping within required limits, and to match noise events from nearby noise monitoring terminals with aircraft.

Noise criteria (NC) curves: a method devised by Beranek in the 1940s for rating or assessing internal (mainly office) nose. It consists of a set of curves relating octave band sound pressure level to octave band centre frequencies; each curve is given an NC number, which is numerically equal to its value at 1000 Hz. The NC value of a noise is obtained by plotting the octave band spectrum against the family of curves. In order to meet a particular NC specification the noise level must be either below or equal to the SPL in each octave band.

Noise dose: an amount of noise energy, usually A-weighted, received by a person, resulting from a combination of sound pressure level and exposure time; see also under personal daily noise exposure level, LEP,d.

Noise exposure category: a term used in Planning Policy Guidance Note 24 Planning and Noise.

Noise exposure forecast (NEF): a noise index used mainly in the United States for aircraft noise.

Noise immission: the amount of noise exposure received at a particular location.

Noise index: a method of evaluating or rating a noise, usually by assigning a single number to it, based on some combination of its physical characteristics (sound pressure level, frequency, duration) and other factors such as time of day, tonal characteristics and impulsive characteristics.

Noise limit: a maximum or minimum value imposed on a noise index, e.g. for some legal purpose or to determine eligibility for some benefit.

Noise mapping: the production of computer software generated maps showing how the predicted levels of outdoor noise levels vary with location, e.g. from street to street in an area. They show ‘sound (or noise) immission contours’ (see also noise immission) and may be used (according to the EU Noise Directive) to help generate noise action plans.

Noise nuisance: has been defined by the World Health Organisation as ‘a feeling displeasure evoked by noise’. Statutory nuisance has a more specific meaning and is subject to legal action under the Environmental Protection Act 1990.

Noise pollution level, LPN: an index devised in the 1960s for assessing environmental noise, based on a combination of its LAeq value and its variability, expressed in terms of standard deviation; it is now rarely used.

Noise rating (NR) curves: a method of rating noise which is similar to the NC system but intended to be applicable to a wider range of situations; the method was defined in ISO R1996, now withdrawn, but the NR system continues to be used, particularly for offices and is used in BS 8233.

Noise reduction coefficient: a single-figure number sometimes used to describe the performance of sound absorbing materials, based on a combination of its absorption coefficient at various frequencies.

Noise zone: region where the long-term average rating level lies between two specified levels such as, for example, between 65 and 70 dB. The noise zone number for this example is 65-70 dB (defined in BS 7445)

Non-linear: in general there is a non-linear relationship between two quantities if they are not directly proportional to each other; if in measurement and instrumentation systems the input exceeds the linear range, then non-linearity results in a distorted output.

Normalised: corrected or standardised in some way, as in normalised level difference, Dn, defined in BS EN ISO 140-4, where the measured level difference is corrected on the basis of the amount of sound absorption in the receiving room.

Normal mode: a natural mode of a vibrating system.

Normal threshold of hearing: the modal value of the thresholds of hearing of a large number of ontologically normal observers between 19 and 25 years of age.

Noy: a unit of noisiness related to the perceived noise level in PNdB by the formula: PNdB = 40 + 10 log2 (Noy).

NPRs (noise preferential routes): specified aircraft departure routes at some airports designed to minimise noise exposure to major centres of population near to the airport. Compliance with NPRs is monitored by noise and track keeping (NTK) systems.

Nyquist frequency: the frequency, which corresponds to half the sampling rate of digitized data, above which aliasing occurs.