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.
Node : the point of minimum displacement in a Sound Wave.
Note 1 : in practice, this amplitude is generally not zero but simply a minimum. The node is then said to be partial. Note 2 : the appropriate modifier should be used to signify the type that is intended; e.g. Particle Velocity Node, Sound Pressure Node.
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 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.
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.
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.
Category D relates to the situation in which development should normally be refused.
Categories B & C deal with situations where noise mitigation measures may make the development acceptable.
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.
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.
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 (AC) rating method.
For a Gaussian distribution of dBA level it is defined as: LNP = Leq + (L10 - L90)
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.
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 Acoustic Absorption present in the receiving room.
Noise Reduction Coefficient : NRC : a single-number rating system used to compare the sound-absorbing characteristics of building materials. A measurement of the acoustical absorption performance of a material, calculated by averaging its Sound Absorption coefficients at 250, 500, 1000 and 2000 Hz, expressed to the nearest multiple of 0.05.
Noise Reduction Rating : NRR : a single-number rating system used to compare and label hearing protectors.
For example the protected dBA = unprotected dBC - NRR.
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.
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. A 'false' analogue signal at frequencies greater than the Nyquist Frequency appears after sampling at a frequency less than the Nyquist frequency.