# Acoustic Glossary

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

N (newton) the unit of force.

N·m (newton metre) the SI unit of torque, also used to measure energy or work in which case the SI unit is the Joule (J)

N·m·s (newton metre second) the unit of angular momentum

N/m2 (newton per metre-squared) = pressure equivalent to the pascal the SI derived unit for sound pressure.

N·s (newton second) the SI unit of impulse and momentum.

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.

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

National Institute of Standards and Technology (NIST) promotes U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology

National Physical Laboratory (NPL) 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.

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

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,

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 and related SI units

Night Noise Level) (Lnight), is the A-weighted, Leq (equivalent noise level) over the 8 hour night period of 23:00 to 07:00 hours, also known as the night noise indicator.

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

Nodes 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.

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.

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 (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 the 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 (PNC) 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

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

Noise Damage Risk Criteria.

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 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 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.

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

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.

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) is caused by continuous exposure to noise levels greater than 80 dBA over an extended period of time. It may also result from a one-time exposure to an intense impulse sound, such as an explosion.

Noise Induced Permanent Threshold Shift (NIPTS)

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 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 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.

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).

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.

Noise Reduction Coefficient

Noise Reduction Rating (NRR) is a single number rating system, used in the USA, to compare and label hearing protectors.

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

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.

Noy 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