Continuous noise level

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. 

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. 

The following is a mathematical model based on empirical data (ISO 1999) used to calculate the maximum permissible continuous noise level at the work place that will not lead to permanent hearing loss ... 

Wearing ear protection devices at continuous noise levels greater than 85 dBA can prevent or reduce the danger of permanent hearing damage. 

Are there areas in the workplace where continuous noise levels exceed 85 decibels ... 

Noise dose Is a measure which expresses the amount of noise measured as a percentage, where 8 hours at a continuous noise level of 90dB(A) is taken as 100%. If the work method and noise output is uniform, and the dose measured after 4 hours is 40%, then the likely 8-hour exposure will be less than 100%. However, if the dose reading after 2 hours is 60%, this will be an indication of an unacceptably high exposure. 

Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

The standard rates the offending noise according to its nature 5dB(A) is added where the noise has a definite continuous note and a further 5 dB(A) added for noise of an intermittent nature. The number of occasions that happen in an 8-hour period is then plotted on a graph and the correction for intermittency is derived. When these calculations have been performed, the noise level is compared to the background level. The standard states that where the noise exceeds the background by 5 dB or more, the nuisance is to be classed as marginal, and where the background is exceeded by lOdB(A) or more, complaints are to be expected. 

The main reasons for this lie in feasibility. Conducting fillers are rather expensive and their use increases the cost of an article. Besides, filled polymers have worse physical-mechanical properties, especially impact strength and flexural modulus. The use of fillers is also detrimental to the articles appearance and calls for additional treatment. The continuous development of electronics has also contributed to a loss of interest to conducting composites as screening materials the improvement of components and circuits of devices made it possible to reduce currents consumed and, thereby, noise level a so called can method is practised on a wide scale in order to cover the most sensitive or noisy sections of a circuit with metal housings [14]. 

We have been discussing the question of how noise in a spectrometer affects the observed noise in the spectra we measure. This question was introduced [1] and various known phenomena was presented that contribute (or, at least, can contribute) to the noise level of the observed spectra. Since this is a continuation of the previous chapters, we will continue the numbering of equations, figures, and so on as though it were all one chapter. 

So let us continue. In Chapter 43 [4], which the interested reader may wish to go back and refresh themselves about, we discussed the general descriptions of how and why the equations came about, we noted that the point of departure for investigating what happens when the noise level becomes large enough that it can no longer be ignored was equation 49-5 ... 

The basis of these charts is the discovery that hearing loss is a function of both the noise level and the cumulative time of exposure. In the late 1900s, the normal limit of exposure to continuous noise was 90dB(A) for 8 hours or its equivalent. Now we see companies and governments seeking a limit of 85dB(A) for 8 hours. 

The most important factor of the above-mentioned points is that an SRV is normally closed and in normal designed processes should never open. In my opinion, this means that an SRV cannot be considered as continuously contributing to the environmental noise levels of a plant. 

The ideal on-line detector has versatility, high sensitivity, the capacity for continuous monitoring of the column effluents, low noise level, wide linearity of response, stable baseline, insensitivity to flow rate and temperature changes, and response to all types of compounds. It is rugged, not too expensive, and is able to measure accurately a small peak volume without increasing its volume appreciably. The terms noise, sensitivity, and linearity are typically used in describing detector performance, as discussed below. 

Continuous indicators are tracked by looking at their mean and standard deviation over a series of tests/observations. For example, the noise level associated with our braking system was measured at a mean value of 34 Db with a standard deviation of 1.5. 

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