**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

Near Sound Field

● Note 1 : the decineper is one-tenth of a neper

● Note 2 : one neper equals 8,686... decibels

Newton and related SI units

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

**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 effective perceived noise level • noise exposure forecastSee also daily noise exposure • noise dose

**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

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 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 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.

*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 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 sound • signal to noise ratio.

See also hearing loss • threshold shift

Noise Induced Permanent Threshold Shift (NIPTS)

See also the IEC Definition of Level

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.

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.

● Note 1 : 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*

● 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

Home • Glossary Search • Certified Instrumentation for Hire