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Theoretical information




Laboratory work No 5

Modeling of acoustic condition of residential settlements. Definition of equivalent and maximal noise levels of vehicles

 

Aim of the work: to get acquainted with physical parameters of noise, noise norming principles, measuring techniques and protection methods, assessment of noise isolation facilities acoustic efficiency (ДСН 3.3.6.037-99).

 

Tasks of the work:

1. Investigate the main parameters of acoustic contamination of environment.

2. To get acquainted with the noise classification.

3. To make up analysis of the human ear structure.

4. To study the noise contamination protection measures.

Theoretical information

Reducing of the production noise is one of the most important conditions to Improve the working conditions, production safety. Modernization of equipment and transportation means became the reason high noise intensity exposure. Noise affects the whole human being especially central nervous and cardiovascular systems. Long-term noise impact may lead to hearing ability reduce or even loss in some cases. Production noise negative effects for worker include: attention reduce, tiredness, reaction inhibition. All these factors affect working ability and may lead to accidents. Therefore, noise reduction measures play significant role in all productions.

Noise is the majority of sounds of different frequency and intensity affecting normal working and resting.

Following kinds of noise are classified by source:

- mechanic noise – due to vibrations of details and knots;

- aerodynamic noise – when pressed air release;

- hydromechanic noise – when cavitation or liquids release occcures;

- electromagnetic noise – when electromagnetic equipment operation.

There is an air noise and structural noise (in building constructions or knots).

Noise as the physical phenomena has sound pressure, sound intensity, frequency and other parameters. The space where noise is propagated, is called sound field.

Sound is the elastic oscillations propagated as the wave in solid, liquid or gaseous medium.

Sound pressure P is the difference between sound pressure and atmospheric pressure at certain point of sound field [Pa].

Human ear is able to accept sounds of 16-20000 Hz frequency. Low oscillations of 16 Hz and lower are the infrasounds, oscillations with more than 20000 Hz frequency are called ultrasounds.

Sound wave propagation occurs together with energy transfer. Average energy flow at certain point per time unit per square unit, is called sound intensity at this point I, Wt m-2.



To make up assessment of noise impact logarithmic values of sound levels L are considered on the base of threshold sound intensity or threshold sound pressure, but not absolute values of sound intensity.

One bell unit B corresponds to increase of sound intensity with respect to threshold sensitivity in 10 times.

Human ear is sensitive to 0.1 B sound increment, i.e. 1 dB, therefore dB is the sound pressure level unit.

L = 10lg(I/Iо),

where I is the sound intensity at certain point, Wt m-2;

I0 is the threshold sound intensity at 1000 Hz frequency (I0 = 10-12 Wt m-2).

Sound intensity is proportional to squared sound pressure, therefore sound pressure level may be determined taking into account sound pressure:

L = 10lg(I/Iо) = 10lg(Р2/ Ро2) = 20lg(Р/Ро),

where P is the sound pressure at certain point, Pa;

Ро = 2 ·10-5 Pa is threshold sound pressure.

The main formulas to estimate noise parameters:

- noise pressure level:

,

where Li(f) is the sound pressure level in octave frequency band;

- noise level:

,

where ΔLi(f) is the aural perception correction at certain frequency.

Table 1. Aural perception corrections

f, Гц DLi(f)
26,2
16,1
8,6
3,2
-1,2
-1,0
-1,1

- equivalent noise level:

,

where is the noise level at tj time moment, is the time period from 0 to T at tj time moment.

 


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