Acoustic Glossary


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

N : N·m : N·m·s : N·s under Newton

nano : n an SI prefix = 10-9 see other SI Units

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

Related Terms : Broadband NoisePink NoiseRandom NoiseWhite Noise
See also : Constant BandwidthConstant Percentage BandwidthsOctave BandsFFT- Fast Fourier Transform.

Narrowband Spectra spectra that have been measured using a narrow frequency bandwidth.

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 : ln

NC : Noise Criteria

NCB : Balanced Noise Criteria.

Near Field
Near Sound Field

NEC : Noise Exposure Category

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 : 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

NIC : Noise Isolation Class

Night Noise Indicator

NIHL : Noise Induced Hearing Loss

NIPTS : Noise 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.

NNI : Noise and Number Index

Node : under Sound Waves

Noise 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 : Broadband NoiseNarrow Band NoisePeriodic NoisePink NoisePseudo-random NoiseRandom 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.

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.

Balanced Noise Criteria is a refinement of NC : Noise Criteria. The rating is determined by the SIL Speech Interference Level and gets the Classification (R) for Rumble if the spectrum is rich in low frequency sound, the Classification H for Hiss if the spectrum is rich in high frequency sound, or Classification (RV) for Rattle and Vibration and Rattle if the low frequency spectra is likely to produce audible rattling in lightweight building elements.

The B&K 2250 Sound Analyser measures the NC curves directly.

See also the Noise Rating Curves : NR.

Noise Dose
Noise Dose Meter
Noise Dose per Hour
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 ExposureNoise Exposure Action LevelNoise 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 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.

Superseded by the Community Noise Equivalent Level which is based on Leq measurements rather than complex calculations

Noise Floor a measure of the signal created from the sum of all noise sources and unwanted signals within a measurement system. Signals beneath the noise floor cannot be measured directly. Ideally the 'noise floor' should be 10 dB lower than the sound level to be measured

Noise Immission Level 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 : NIPTS

Noise Isolation Class : NIC 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 for noise mapping the Lden or LDEN also known as the day-evening-night level is 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 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 : NR 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.

Noise Reduction Coefficient : NRC

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

Noise Regulations Noise at Work Regulations.

Noisiness 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 the perceived noise level.

Nominal Frequency or Preferred Frequency - 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

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

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 Oscillation

Normalized measurement in accordance with a Standard or 'Norm', for examples see Sound Insulation

Normalized Impact Level : Ln
Normalized Level Difference : Dn

Noy a 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. see Perceived Noise Level.

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

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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