The equivalent noise level

In general the noise level in the community or inside a factory will vary with time. The equivalent noise level (LAeq) is defined as tire notional steady noise level which, over a given period of time, would deliver the same amount of sound energy as the fluctuating level. Thus to maintain the 1-Aeq when SPL is doubled, i.e. increased by 3 dB, exposure time must be halved Table 3.5.2). The equivalent noise level concept forms the basis of the exposure criteria used in the Control of Noise at Work Regulations 2005 which calls it daily personal noise exposure (LEPd)- Where tire fluctuation 

Lio - the noise level exceeded for 10% of the time (average peak). 

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The exposure criterion used in these Regulations is the equivalent noise level over the working day or shift (Lep ci). Three actions levels are identified, the first action level is an lEP,d of 85 dB(A) when an employee has to be provided with hearing protection on request (Reg. 8(1)), the second action level is an exposure of 90dB(A) or more when hearing protection must be provided and worn (Reg. 8(2)) and the third level is the peak action level when a peak sound pressure level of 200 pascals occurs when hearing protection must, again, be provided and worn. 

Percentiles are expressed as the percentage of time (for the stated period) during which the stated noise level was exceeded, i.e. 5 min Lgo of 80 dB(A) means that for the 5-min period of measurement for 90 per cent of the time the noise exceeded 80dB(A). Therefore Lo is the maximum noise level during any period and Lioo is the minimum. Leq (the equivalent continuous noise level) is the level which, if it were constant for the stated period, would have the same amount of acoustic energy as the actual varying noise level. 

Refractive index detectors are not as sensitive as uv absorbance detectors. The best noise levels obtainable are about 1CT7 riu (refractive index units), which corresponds to a noise equivalent concentration of about 10-6 g cmT3 for most solutes. The linear range of most ri detectors is about 104. If you want to operate them at their highest sensitivity you have to have very good control of the temperature of the instrument and of the composition of the mobile phase. Because of their sensitivity to mobile phase composition it is very difficult to do gradient elution work, and they are generally held to be unsuitable for this purpose. 

Impulse noise Noise comprising rapid increases (of at least 5 to 10dB) of short duration in the sound level. The impulse sound level Lj can be measured with appropriate equipment. If an ordinary sound level meter (not an impulse sound level meter) is used for determining the equivalent continuous level L, the measured value should be increased by 6dB. 

The above-mentioned relationships can also be used in a case where a noise source which could not be representatively measured during the time of its occurrence has to be additionally allowed for in the calculation of the equivalent continuous level, for example traffic noise on the site of a cement works to be added to the noise emitted by the works itself. 

In the "maximum level" averaging procedure described in "TA Larm" (1968) the maximum sound level which occurs in each time interval of duration t (as a rule t = 5 sec but for purposes of audiometry in industry, more particularly for protecting people at work against noise t = 3 sec) is used for determining the equivalent continuous level. Worked examples illustrating this method are given in "TA Larm". 

For the operation of soxmd level meters of different types the safety adviser should refer to the manufacturer s instruction book. The prime requirement for any instrument for noise measurement is that it should not be more sophisticated than necessary and it should be easy to use and calibrate. A typical sound level meter for use by the safety adviser should have the facility for measuring dBA and octave band soxmd pressure levels. More sophisticated meters have facilities for measuring equivalent noise level (Leq, Lepci) (see section 3.5.6) and xmderlaking statistic analysis. 

For certain applications, it is also common practice to further simplify this multidimensional description by eliminating the temporal variable and measuring the equivalent sound level (Leq) or OSHA sound level (Tosha) time average sound level, as observed on standard dosimeters and modern sound level meters. Leq is often used in community noise analysis. It is also used during the analysis of employee noise exposures in many countries other than the United States. Tosha is often used in employee noise exposure analysis in the United States. 

The attainable signal-to-noise ratio V /V which is in principle limited by the noise of the incident radiation, but may in practice be further reduced by inherent noise of the detector. The detector noise is often expressed by the noise equivalent input power (NEP), which means an incident radiation power which generates the same noise level as the detector itself yielding a signal to noise ratio S/N = 1. In infrared physics a figure of merit for the infrared detector is the detectivity. 

To unambiguously identify the presence of a peak and, in addition, be able to give some proximate estimation of its size for quantitative purposes, the peak height needs to be at least 5 times the noise level. The detector sensitivity, or the minimum detectable concentration, (Xd), is defined as that concentration of solute that will give a signal equivalent to twice the noise level and, consequently, the concentration of solute at the limiting (k ) value must be 2.5Xd. 

The disturbance caused by a noise depends on its intensity (equivalent pressure level L in dB(A)), its frequency spectrum (that is its energy distribution), and the acoustic characteristics of the medium in which the listener is kicated. 

All the previous discussions have concerned steady-state noise. It will, however, be apparent that most noises change in level with time. It may therefore be necessary to derive indices which describe how this happens. The most common of these are percentiles and equivalent continuous noise levels. 

Detector Sensitivity or the Minimum Detectable Concentration has been defined as the minimum concentration of an eluted solute that can be differentiated unambiguously from the noise. The ratio of the signal to the noise for a peak that is considered decisively identifiable has been arbitrarily chosen to be two. This ratio originated from electronic theory and has been transposed to LC. Nevertheless, the ratio is realistic and any peak having a signal to noise ratio of less than two is seriously obscured by the noise. Thus, the minimum detectable concentration is that concentration that provides a signal equivalent to twice the noise level. Unfortunately, the concentration that will provide a signal equivalent to twice the noise level will usually depend on the physical properties of the solute used for measurement. Consequently, the detector sensitivity, or minimum detectable concentration, must be quoted in conjunction with the solute that is used for measurement. 

Noise levels will be different for different models of the same type of detector, and for a given model will depend very much on how the detector is used. The noise equivalent concentration refers to a solute with favourable properties, and may be very much higher for other solutes. 

The mass sensitivity of a chromatographic system is that mass of solute (m) that will provide a peak with a height equivalent to twice the noise level. 

The sensitivity of the detector (Xd) (or minimum detectable concentration) is defined as that concentration of solute that will provide a signal equivalent to twice the noise level. Now the concentration of solute at the peak maximum is approximately twice the average concentration of the solute in the peak, volume. Thus, the minimum detectable mass will be that mass (m) that, when dissolved in a volume of mobile phase equivalent to the peak volume, will produce a concentration of Xp/2. 

What is the smallest "linear level Is it the minimum detectable level, or is it a level sufficiently higher than the noise to allow a measurement to be made with a precision equivalent to the allowable deviation from linearity Understandably, instrument manufacturers prefer the former definition, chromatographers with the responsibility of specifying the accuracy of their analyses prefer the latter. 

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