Design of noise protection measures

Practical work 4

Design of noise protection measures

4.1. Design of acoustic screen

 

Acoustic screens are usually used and installed in case of impossibility of the use of means of protection against other types of noise. It should be kept in mind that the low frequency noise screens almost no effect, so how low frequency noise due to diffraction effect bends the screen.

The method of calculation

 

1. From equation (2.1) we are calculating the coefficient k:

,                            (4.1)

where f is the frequency, Hz; h – screen height, m; l – screen length, m; b –distance from the screen to the workplace, m; a – distance from the screen to a noise source, m.

2. Current value of k determines the efficiency of screen ΔL_s:

k                          0 0,5 1 1,5   2  3   4   5  7 10

ΔL_s, dB    5  8  11 13,5 15 18 20 22 25 30

 

3. Determine the sound pressure level at the workplace ΔL_wp. m after installation of acoustic screen and compare with permissible sound pressure level L_psl.

4. Conclusion:

a) if ΔL_wp < L_psl, acoustic settings of the screen to reduce sound pressure level are selected correctly.

b) if ΔL_wp > L_psl acoustic settings of the screen to reduce sound pressure level are selected incorrectly, you need to change the settings and repeat the calculation.

It should be remembered that screens applied in the case of excess valid values for noise in the workplace is not less than 10 dB and not more than 20 dB at least one medium geometric frequencies.

Table 4.1

Permissible noise levels at various workplaces [15]

Workplaces

Sound pressure levels in octave bands, dB

Sound pressure level, dBA

 

Industrial premises

1. Facilities Design Bureau,  laboratories

for theoretical work and experimental data

processing

 

2. Control rooms, operating rooms

3. Monitoring and remote control cabins:
no voice on the phone connection
a voice communication telephone

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4. Facilities and sites precision assembly

5. Permanent working spaces and work areas in industrial premises and in the territory of the enterprise

-

6. Premises laboratories for experimental work, noisy units of computers

-

Example. Levels of sound pressure generated by noise source at frequencies f₁ = 8000 Hz and f₂ = 4000 Hz are respectively 81 and 84 dB, acceptable sound pressure levels, respectively, 69 and 71 DB.

Define the efficiency of application to eliminate the outer screen  with sizes (h) x (l) = 1 × 1.5 m. The screen is stood at a distance a = 0.5 m from the source of noise, and distance from the screen to the workplace (b) = 1.5 m.

Solution. From equation (4.1) we define coefficient k, corresponding to frequency f₁ = 8000 Hz:

We define the efficiency of the screen on this frequency by interpolation method: ΔL_{s 8000} = 19.5 dB.

Sound pressure level at the workplace:

ΔL_{wp 8000} = ΔL₈₀₀₀ – ΔL_{s 8000} = 81 – 19.5 = 61.5 dB, which is less than L_{psl 8000} = 69 dB.

Similarly, we obtain k₄₀₀₀ = 2.66, ΔL_{s 4000}= 17 dB and ΔL_{wp 4000} = 67 dB which is less than L_{psl 4000} = 71 dB.

Conclusion: the acoustic parameters of the screen to reduce sound pressure level are selected correctly.