**N·s : newton second** the SI unit of impulse and momentum.

Other noise descriptors ambient noise • background noise • broadband noise • gaussian noise • periodic • pink noise • pseudo random noise • random noise • residual sound • specific sound • white noise • wideband noise

See also constant bandwidth • constant percentage bandwidths • continuous spectrum • fast fourier transform • line spectrum • octave bands.

**Natural Frequency Definition** (IEC 801-24-08) frequency of free oscillation of a system. For a multiple-degree-of freedom system, the natural frequencies are the frequencies of the normal modes of oscillation

See also undamped natural frequency

Natural Logarithm

NC : noise criteria curves

Near Sound Field

NEF : noise exposure forecast

**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

NIC : noise isolation class

See also day noise level • day evening night noise level • evening noise level

NIHL : noise induced hearing loss

NIPTS : noise induced permanent threshold shift

NIST : National Institute of Standards and Technology (USA)

NNI : noise and number index

Nodes

**Noise Definition** (IEC 801-21-08)

a) erratic or statistically random oscillation

b) disagreeable or undesired sound or other disturbance.

Other noise descriptors, ambient noise • background noise • broadband noise • gaussian noise • narrowband noise • periodic • pink noise • pseudo random noise • random noise • residual sound • specific sound • white noise • wideband noise

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.

See also aircraft noise • effective perceived noise level • judged perceived noise level • maximum perceived noise level • noise exposure forecast • perceived noise level • T10 • tone-corrected perceived noise level

See also daily noise exposure • noise dose

**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 *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 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

**Preferred Noise Criterion (PNC)** a noise measurement system for continuous or ambient noise in indoor environments proposed by Leo Beranek in 1971.

See also noise rating (NR) curves

Noise Damage Risk Criteria.

Noise Dose

Noise Dose Badge

Noise Dose Criterion

Noise Dose Criterion Time

Noise Dose Meter

Noise Dose per Hour (DOHR)

Noise Dose Threshold Level

Noise Dosimeter and Sound Exposure

Noise Dose Badge

Noise Dose Criterion

Noise Dose Criterion Time

Noise Dose Meter

Noise Dose per Hour (DOHR)

Noise Dose Threshold Level

Noise Dosimeter and Sound Exposure

**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.

See also daily noise exposure • noise exposure action value • noise exposure limit value • sound exposure

*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.

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 aircraft noise • effective perceived noise level • judged perceived noise level • maximum perceived noise level • noise and number index • perceived noise level • T10 • tone-corrected perceived noise level

**Superseded by the community noise equivalent level which is based on Leq measurements rather than complex calculations.**

See also ambient noise • background noise • residual sound • signal to noise ratio.

See also hearing loss • threshold shift

Noise Induced Permanent Threshold Shift (NIPTS)

See also the IEC Definition of Level

**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.

In America it is common practice to use the NC noise criterion curves and room criteria (RC).

See also sound insulation

Noise Reduction Coefficient

See also hearing protection.

**Noisiness Definition** (IEC 801-29-12) a prescribed function of sound pressure levels in the 24 one-third octave bands centred on 50 Hz to 10 kHz that is used in the calculation of perceived noise level

Note : the prescribed function is given in ISO 3891-1978.

**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

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

Normalised Impact Sound Pressure Level

Normalised Level Difference

**Noy Definition** (IEC 801-29-13) unit of noisiness, equal to the noisiness of a one-third-octave band of noise centred on 1 kHz and having a sound pressure level of 40 dB

See also perceived noise level • PNdB

NPL : National Physical Laboratory (UK)

NR : noise rating and noise rating curves

NR : noise reduction

NRC : noise reduction coefficient

NRDB : noise rating decisive band

NRR : noise reduction rating

**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.