Help Topics - N

Natural frequency

The frequency at which a system tends to vibrate with large amplitude for very small input energy. It falls at the point where a system changes from being stiffness controlled at low frequencies to being mass controlled at higher frequencies.

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Natural wet bulb temperature

The temperature reported by a thermometer that has a wet wick over it supplied continuously with water from a reservoir.

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Near field

The sound field close to a noise source where the sound pressure does not obey the inverse square law and the particle velocity is not in phase with the sound pressure. Measurements made in the near field can vary greatly for small changes of microphone position. This can sometimes account for the differences found in noise measurements obtained with a personal noise dosimeter and those obtained from a hand-held sound level meter when an operator stands very close to a noisy machine.

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Nephelometer

Optical measurement device based upon the scattering effect of light by particulate.

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NIOSH

The National Institute for Occupational Safety and Hygiene in the United States. An organization dedicated to investigating the harmful effects of varied industrial hazards on the working population and promulgating regulations to control their harmful effects.

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Noise

Unwanted sounds at a particular place at a particular time.

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Noise dose (dose%)

The amount of noise received by a worker during the workday expressed as a percentage of a certain reference level for a given duration. The usual allowable noise dose is typically set at 100% dose equivalent to an equivalent continuous noise level of 90 A weighted dB over a standard 8-hour working day. Other noise levels exist that are considered to represent the 100% noise dose but the time interval is almost always the 8 hour day.

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Noise event

The increase in the current sound level that exceeds a selected threshold value and thereby identifies a significant change in the acoustic climate. Noise events are typically considered to be significant when the instantaneous level exceeds the underlying background level by more than about 15 dB. Events may be transient and last less than a second, such as a single blast or may last for 20 to 30 seconds such as a passing train or aircraft taking off at an airport.

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Noise exposure

The absolute measure of the amount of noise experienced by an operator at work. It is proportional to the square of the sound pressure and the duration. It is different from noise dose in that it is an absolute measurement rather than a percentage of some chosen combination of noise level and time. Typically specified in European Noise at Work legislation and expressed in Pa2h or Pa2s. 1 Pa2h = 3600 Pa2s. a continuous noise level of 90 dB lasting for 8 hours is approximately equivalent to a sound exposure of 3.2 Pa2h.

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Noise exposure level (LEP,d)

The expression of noise exposure as a logarithmic term in dB. In European Noise at Work legislation it is expressed as the Daily Personal Noise Exposure level LEP,d.

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Noise floor

The lower limit of accurate measurement capability in a sound level meter where the effect of the inherent electrical noise is superimposed on the actual signal. It is normally quoted as the minimum level in dB that can be measured by the meter on the lowest dynamic range setting. Only results of 10 or more dB than the noise floor should be considered to be accurate in a typical sound level meter specification.

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Noise isolation class

A single number rating derived in a prescribed manner from the measured values of noise reduction between two areas or rooms. It provides an evaluation of the sound isolation between two enclosed spaces that are acoustically connected by one or more paths.

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Noise level

For sound transmitted primarily through the air it is usually taken to be the A weighted sound pressure level in dB.

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Noise ordinance

A document that sets out how noise in a community is to be treated in terms of limits of level, duration, times of occurrence etc during the day. Different limits may apply to day times that to night times and between residential, industrial or commercial zones. Limits may be written in terms of absolute maximum levels not to be exceeded or in terms of the difference between the background level and the offending level. A person appointed to make such measurements makes measurements with an approved sound-measuring device usually on a property boundary line.

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Noise reduction

The numerical difference in decibels of the average sound pressure in two areas or rooms. A complete measurement of the reduction in noise consists of the simple level difference between the two rooms due to the common partition and also takes into account the background level in the receiving room and the amount of sound absorption in the receiving room. The noise reduction is a function of frequency typically increasing with increasing frequency and is measured in third octave bands from at least 100 Hz to 3150 Hz.

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Noise reduction coefficient

A measure of the acoustical absorption performance of a material calculated by averaging its sound absorption coefficients at 250, 500, 10000 and 2000 Hz and expressed to the nearest multiple of 0.05.

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Noise reduction rating

A single number rating value based on the difference between the A and the C weighted overall noise levels. The difference in the readings obtained with a suitably equipped sound level meter will vary depending upon the frequency distribution of the subject noise and its spectrum particularly in the lower frequency bands. For a sound with a lot of low frequency components the C weighted level will be higher than the A weighted sound level. An increase in the A weighted level over the C weighted level indicates significant noise in the middle range of frequencies.

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Nuisance

A legal definition of a noise that offends or upsets the receiver because it is occurring at the wrong time in the wrong place or is of a character that annoys due to excessive tonal components or impulses.

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