The constant of a premise on medium geometric frequency 1000 Hz

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The method of calculation

1. Determine the constant of acoustically untreated room, m²:

В=В₁₀₀₀ μ,                                               (4.2)

where В₁₀₀₀ – the constant of a premise, m² on medium geometric frequency 1000 Hz, determined depending on the extent of the premises (V) of the following ratios (table 4.3).

Table 4.3

The constant of a premise on medium geometric frequency 1000 Hz

Room

В₁₀₀₀,  m²

With a small number of people

V/20

With rigid furnishings and a large number of people or with a small number of people and upholstered furniture (laboratories, woodworking and weaving workshops, offices, etc.)

V/10

With large numbers of people and upholstered furniture (halls design offices, classrooms, meeting rooms, office, accommodation, etc.)

V/6

Sound-absorbing ceiling cladding with premises and parts of walls

V/1.5

Frequency multiplier μ is in the table 4.4.

Table 4.4

Frequency multiplier μ for spaces of different volumes

Room volume, m³

µ value on the average geometric frequencies octave bands Hz

Less than 200

0,80

0,75

0,70

0,80

1,4

1,8

2,5

200…1000

0,65

0,62

0,64

0,75

1,5

2,4

4,2

More than 1000

0,50

0,50

0,55

0,70

1,6

3,0

6,0

2. With the constant of a premise found in each octave bands calculate the equivalent sound absorption area, m²:

                                  (4.3)

where S is the total area enclosing surfaces of a premise, m².

3. Determine the boundary of the zone of reflected sound (when there are n identical noise sources in the room) in terms of the limiting radius r_lim, that is the distance from the noise source at which the sound pressure level of the reflected sound is equal to the sound pressure level of the direct sound emitted by this source

,                                (4.4)

where В₈₀₀₀ – the constant of a premise at the frequency of 8000 Hz:

B₈₀₀₀ =B₁₀₀₀ ·μ_8000.

     4. Calculate the maximum reduction in sound pressure level, dB, in each octave band when using sound-absorbing coatings at a design point located in the area of reflected sound:

ΔL=10lg(B'/B),                                (4.5)

where В' –  the constant of a premise after installation it sound-absorbing constructions, m², is calculated by the formula:

В'=(A₁ +ΔA)/(α – α₁),                                   (4.6)

where А₁ – equivalent sound absorption area surfaces without sound-absorbing facing, m²,

А₁ = α (S – S_o);                                (4.7)

ΔA –  the total additional sound absorption area, m²; α – average sound absorption in a room to his acoustic processing:

α = В/(В+ S);                                      (4.8)

α₁ – average sound absorption coefficient, acoustically treated room:

α₁= (А₁ + ΔA)/S                               (4.9)

The total additional space of sound absorption of sound-absorbing constructions lining or piece sound absorbers:

ΔА = α₀ ·S₀ = α₀ ·A_pc ·п,                                        (4.10)

where α₀ – coefficient of sound absorption of the cladding design (table 4.5); S₀ – faced surface area, m²; A_pc – sound absorption area one piece of sound absorption, m²; n – the number of piece sinks.

Table 4.5

Sound absorption material coefficients, α₀

Materials, constructions

The value of α₀, with the average geometric frequencies, Hz

Concrete

0,01

0,01

0,02

0,02

0,02

0,04

0,04

0,04

Wood paneling

0,1

0,1

0,1

0,1

0,08

0,08

0,11

0,11

Mats of fiberglass 50 mm thick

0,13

0,13

0,32

0,64

0,62

0,59

0,59

0,59

The perforated panels of size 25×25 cm and 3 cm thick with asbestos wool with a thickness of 6 mm inside

0,21

0,52

0,54

0,54

0,5

0,41

0,33

0,32

Foam rubber

0,2

0,2

0,22

0,3

0,75

0,77

0,71

0,6

Perlite plate thickness of 30 mm

0,15

0,15

0,68

0,79

0,61

0,6

0,63

0,63

Isolative plate thickness 50 mm, length 30 mm

0,52

0,52

0,65

0,5

0,48

0,5

0,59

0,59

Example.  In the lab with dimensions a = 12 m, b = 6 m, h = 3 m without windows, noisy equipment is installed, when operating in octave bands the following sound pressure levels are created: L₆₃ = 74 dB; L₁₂₅ = 78 dB; L₂₅₀ = 81 dB; L₅₀₀ = 75 dB B; L₁₀₀₀ =72 dB; L₂₀₀₀ =69 dB; L₄₀₀₀ = 67 dB and L₈₀₀₀ = 63 dB. You must select a material for sound-absorbing cladding and determine the effective conduct of their application in this area.

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