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

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

National Physical Laboratory (NPL)

**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

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 (N)** the derived SI unit of Force required to accelerate a mass of 1 kg at a rate of 1 m/s^{2} - approximately equal to the gravitational force on a 100 g mass.

Units : kilogram metre per second squared, kg·m/s^{2} : 1 N = 1 kg·m/s^{2} = 10^{5} dynes

**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

1 N·m = 1 J = 1 W·s

**Newton metre second (N·m·s)** the unit of angular momentum.

1 N m s = kg m^{2} 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

Night Noise Indicator

**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 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.

See also • effective perceived noise level • noise exposure forecast

**Noise at Work Regulations 2005** came into force to ensure that workers' hearing is protected from excessive noise at their place of work, by the noise exposure, action and limit values

**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.See also • noise rating (NR) curves

Noise Damage Risk Criteria.

Noise Dose

Noise Dose Badge

Noise Dose Criteria

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.

Noise Exposure

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.

*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 noise and number indexSuperseded by the community noise equivalent level which is based on Leq measurements rather than complex calculations

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

**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.

See also • threshold shift

*Noise Induced* permanent threshold shift (PTS)

**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 rating method.

**Noise Level** for airborne sound, unless specified to the contrary, it is the A-weighted sound level.

**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)** 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).

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

**Nominal Frequency** 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

Also known as *normal vector* and *surface normal*

● 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

**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** 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.

*NR* ▷ noise rating and noise rating curves

*NR* ▷ noise reduction

*NRC* ▷ noise reduction coefficient.

*NRDB* ▷ noise rating decisive band.

*NRR* ▷ noise reduction rating

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