Acoustic Glossary


N : Sound and Vibration Definitions, Terms, Units, Measurements etc., ..

N • N·m • N·m·s • N·s under newton

nano (n) a SI prefix = 10-9 see other SI units

Narrowband Noise

, a noise which has it's energy distributed over a relatively small section of the audible range.

Other noise descriptors • ambient noisebackground noisebroadband noisegaussian noiseperiodicpink noisepseudo random noiserandom noiseresidual noisespecific noisewhite noisewideband noise

Narrowband Spectra

that have been measured using a narrow frequency bandwidth.

See also • constant bandwidthconstant percentage bandwidthscontinuous spectrumfast fourier transformline spectrumoctave bands.

National Physical Laboratory (NPL)

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.

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 of a number is its logarithm to the base of the mathematical constant e, where e is approximately equal to 2.718281828459. The natural logarithm of x is generally written as ln x, loge x,
NCNoise Criteria

Near Sound Field

NECNoise Exposure Category

NEFNoise 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 (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.
Units : kilogram metre per second squared, kg·m/s2 : 1 N = 1 kg·m/s2 = 105 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 m2 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

NICNoise Isolation Class

Night Noise Indicator

NIHLNoise Induced Hearing Loss

NIPTSNoise Induced Permanent Threshold Shift

Nitrile rubber or Buna-N, is a synthetic rubber copolymer of acrylonitrile (ACN) and butadiene. Some trade names are: Nipol, Krynac and Europrene.

NNINoise and Number Index

Node under Sound Waves


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.

Noise Definition

IEC 801-21-08,
a) erratic or statistically random oscillation
b) disagreeable or undesired sound or other disturbance

Other noise descriptors • ambient noisebackground noisebroadband noisegaussian noisenarrowband noiseperiodicpink noisepseudo random noiserandom noiseresidual noisespecific noisewhite noisewideband 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 LevelNoise Exposure Forecast

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).

The aim of the Noise Regulations is to ensure that workers' hearing is protected from excessive noise at their place of work, which could cause them to lose their hearing and/or to suffer from tinnitus (permanent ringing in the ears).
The regulations set exposure limit values and exposure action levels

The Control of Noise at Work Regulations 2005 replace the Noise at Work Regulations 1989.

See also • sound exposurenoise dose
For others details see the Government and/or HSE websites.

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.

The B&K 2250 sound analyser measures the NC curves directly.

See also • balanced noise criterianoise rating curvesroom-criteria

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 personal noise exposurenoise exposure action valuenoise exposure limit valuesound 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 A represents the circumstances in which noise is unlikely to be a determining factor.
Categories B & 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.

See the UK Government Web Site for more details.

Noise Exposure Forecast (NEF)

a complex criterion for predicting future noise impact of airports. The calculation is based on the effective perceived noise levels (EPNL) and takes into account each type of aircraft, flight profile, number of flights, time of day, etc. Developed by the US Federal Aviation Agency to generate equal NEF contours for zoning control around airports.

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 index

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

Noise Floor

a measure of the extraneous noise sources within a measurement system. Ideally the 'noise floor' should be 10 dB lower than the sound level to be measured

See also • ambient noisebackground noiseresidual noisesignal to noise ratio.

Noise Immission Level

is 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.

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.

See also the • IEC Definition of Level

Noise Mapping

in noise mapping the Lden or LDEN also known as the day-evening-night level are 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 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).

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.

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 to use the NC noise criterion curves.

Noise Reduction

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 sound absorption present in the receiving room.

See also • sound insulation

Noise Reduction Coefficient

Noise Reduction Rating (NNR)

is a single-number rating system used to compare and label hearing protectors. For example the protected dBA = unprotected dBC - NRR.

Noise Regulations under the noise at work regulations.

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

Non-stationary Signal a signal whose frequency content changes within a captured time frame.

Normal, to a surface is a vector perpendicular to the surface at a given point.

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

Normalized Impact Sound Pressure Level
Normalized Level Difference


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.

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 levelPNdB

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.

NRnoise rating and noise rating curves
NRnoise reduction
NRCnoise reduction coefficient.
NRDBnoise rating decisive band.
NRRnoise reduction rating

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. It is the minimum rate at which a signal can be sampled without introducing errors.

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