2.5.6. Artificial illumination standard.
Artificial illumination standard “SNiP II-4-79” provides quantitative (minimum illuminance) and qualitative (glare index, lighting pulsation factors) characteristics.
Given standards are divided to describe different lamps and lighting systems.
Dealing with illumination standards is associated with visual task. Visual Task: conventionally designates those details and objects that must be seen for the performance of a given activity; includes the immediate background of the details or objects and contrast of object against its immediate background.
The standard “SNiP II-4-79” describes 8 categories. First 6 categories are determined by size of objects. Categories I-V have 4 sub-categories where illuminance standard comprises contrast of object against background and reflectance.
For the instance the highest illuminance standard makes 5000 lux for Category Ia and the lowest one – 30 lux (Category VIII c).
2.5.7. Artificial illumination prediction methods
Artificial illumination can be designed using four methods: point, power density, graphic and efficacy methods.
Point method is used to predict focal and combined lighting and also lighting incline planes. It’s based on the equation:
,
where Ia – luminous intensity incident to a point, a – angle of incidence: angle at which light rays strike a surface, measured between the ray and a line perpendicular to the surface; r – distance from the lamp to a point.
Ia value is given in look-up tables.
Power density method is considered to be a most simple but the least precise that’s why it’s used for proximate calculations. It allows finding every lamp power to reach illumination standard on the area:
,
where p – power density, W/m2 (reference value); S – work area, m2; N – number of lamps in lighting system.
Graphic method (method of Prof. A.A. Trukhanov) is the most precise to predict direct lighting. It comprises direct beam component and internally reflected component.
Luminous flux of the lamp is calculated by formula:
,
where E – illuminance standard, lux; S – illuminated area, m2; kr – light-loss factor: a factor used in calculating the illuminance taking into account temperature, voltage variations, lamp depreciation, dirt accumulation on luminaire and room surfaces, maintenance procedures and atmosphere conditions (kr=1.3..1.8); Z – factor of lighting unevenness (Z=1.1..1.15); N – number of luminaries; n – number of lamps in luminarier; - luminous efficacy.
Luminous efficacy is given in look-up tables comprising area index i and reflectance of walls and ceiling. Area index is calculated by formula:
,
where a, b – depth and width of the area, m; hp – luminarier elevation above the work plane, m.
Calculated luminous flux is used to select a lamp resorting to table in SNiP II-4-79, and to calculate power required for whole lighting system.
2.6. Protection from noise and vibration
2.6.1. Noise physical characteristics
Noise produces negative effect on human organism and first of all on central nervous system and cardio-vascular system. Long time exposure to noise may bring to hearing worsening and sometimes to deafness. In occupational conditions long noise exposure can cause accident, occupational disease, work capacity lowering.
In noisy conditions work production lowers down to 60%, and errors grow at 50%. Necessity to speak load effects mental activity. It’s a proven fact that for 30% of people noise is the cause of early aging.
For successful noise control it’s important to know its physical nature, its generating and distribution.
Noise is a type of sound that tends to sound unpleasant.
Occupational noise generates in workplaces caused by machines.
Noise is classified as noise to hinder communication; disturbing noise (causing over fatigue); harmful noise (provokes chronic disease, hypertension, tuberculosis, ulcer, hearing worsening); injurious noise (invades into physiological functions).
As a physical phenomenon noise is chaotic composition made up of many sounds with different unrelated frequencies and intensities.
The basic physical characteristics of noise are frequency, pressure and intensity.
Sound frequency is number of sound wave oscillations per second measured in hertz (Hz). Sound spectrum is divided into three diapasons:
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infra sound with frequency lower 20 Hz;
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sound with frequency within 20 - 20000 Hz;
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ultra sound with frequency over 20000 Hz.
Human ear can respond to sound in frequency spectrum from 20 to 20000 Hz. This sound diapason is divided into:
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low frequency – under 400 Hz;
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medium frequency – from 400 to 1000 Hz;
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high frequency – over 1000 Hz.
Sound wave distribution transfers energy. Energy transferred by sound wave through the surface perpendicular to direction of sound distribution per second is called sound intensity:
,
where P – sound pressure; - density of a substance sound is passing through; V – sound velocity in the substance.
Sound spreading in air is called air sound, in solid body – structural sound. Part of the air captured by oscillations is called sound field. Sound field is called free if sound waves spread in it without obstacles (free land, special acoustic chamber equipped with sound-absorbing walls). Sound field is called diffuse if sound waves come to every point in it with the same probability from all directions (rooms having high sound reflectance).
In real conditions sound wave field can be near ultimate ones – free or diffuse sound field.
The human ear responds to sounds over a very large range of sound intensities. Threshold of hearing (the quietest sound we can hear) is 0.00000000001 watts/m2 (often written 10-12 watts/m2). The sound intensity at the threshold of pain is about 10 watts/m².
To handle this large range we make use of a logarithmic ratio scale called the decibel scale.
In general, a decibel scale for any quantity, I, is defined as:
,
where I – sound intensity whose level is being specified, in watts/m2 and Iref – reference intensity = 10-12 watts/m2 (the threshold of hearing).
Note that the decibel is not an absolute measure but is referenced to a selected quantity, Iref.
Another reason for using this scale is that the ear itself 'hears' logarithmically and humans judge the relative loudness of two sounds by the ratio of their intensities, a logarithmic behavior.
When sound intensity is expressed as a decibel it is referred to as sound intensity level and is given the symbol LI.
Sound pressure level is calculated by formula:
,
where P – rms sound pressure in Pa; Pref = 2 x 10-5 Pa (sometimes written as 20 Pa = 20 x 10-6 Pa, which is the sound pressure at the threshold of hearing at 1000 Hz).
Sound intensity level and sound pressure level are related as following:
,
where 0, V0 - density of a substance sound is passing through and sound velocity in the substance in normal conditions; , V – real measured values.
To assess simple sounds in octave bands the term “sound pressure level” is used; for complicated sounds it is “sound level” and we denote this by writing the unit dB (A) (when the A-weighting networks have been used in a measurement).
Human ear does not have an equal response to sounds of different frequencies. It’s maximum in medium and high frequencies (from 800 to 4000 Hz), minimum – in low frequencies (from 20 to 100 Hz).
It is often necessary to obtain information about the frequency spectrum of a sound to design effective noise control and to select appropriate personal hearing protectors.
In most cases it is sufficient to measure the sound pressure level in bands of frequencies, rather than at individual frequencies. The width of the band usually chosen is the octave band - this is a band where the upper frequency is twice that of the lower. Each band is denoted by its center frequency:
,
where f1 – lower frequency, Hz; f2 – upper frequency, Hz.
Sound power is basic characteristic of any source of noise. It is defined as total energy radiated into environment.
Sound power, W, can also be expressed in decibels and is then referred to as the sound power level, Lw.
,
where W – sound power of the source in watts; Wref – reference sound power = 10-12 watts.