Mel’niñk V. N., Trivailo M.S., Karachun V.V., Saverchenco V.G.

National Technical University of Ukraine «KPI»

PASSIVE MEANS OF STRUGGLE AGAINST AERODYNAMIC NOISE. NOISE-PROTECTIVE SCREEN

The construction refers to Mechanical engineering, namely, to mechanisms for sound isolation from noise, and can be used in different industries.

There is noise-protective screen (NPS), which includes a post and a frame with elastic stripes that cross each other. In armholes between these stripes there are sound absorption mates and also elements for fastening panels (see e.g. À.ñ. ÑÑÑÐ ¹ 1811633, G 10 Ê 11/00, 1993).

Structural complexity and high capacity of material are the drawbacks of this NPS.

There is also another NPS with post and frame, to which hermetic and perforated sheets are fastened, and sound absorption material, that is placed in a gap between sheets. (see e.g., Ïîãîäèí À.Ñ, Øóìîïîãëîùàþùèå óñòðîéñòâà. - Ì.: Ìàøèíîñòðîåíèå, 1973, Ñ. 44, ðèñ. 16 ).

This NPS is technically and effectively the most similar to the proposed construction.

The drawback of the famous NPS is in a small sound resistance that reduces the acoustic protection efficiency.

To explain advantages and disadvantages, written higher, note that airtight sheet has a smooth surface and does not secure crossing of falling down and reflected sound waves. As the result of this the acoustic protection reduces.

The other reason is rather small inflexibility of airtight sheet.

The main task of the proposed construction is to improve the NPS, by changing the shape of the surface of airtight sheet. It will provide both crossing of falling down and reflected sound waves, and increasing of inflexibility at the same time. As a result sound resistance will increase and thus the efficiency of acoustic protection will raise.

The salvation of the problem consists in a special construction of the NPS. The NPS contains a post with a frame. Hermetic and perforated sheets are fastened to this frame. There is also a sound absorbing material, which is placed between sheets. According to the proposal of construction, the hermetic sheet is equipped with spherical ledges and cavities, allocated in alignment in staggered order on its surface.

The equipment of airtight sheet includes spherical, coaxial cavities and ledges. Such placement provides multiple crossing of falling down and reflected sound waves. As a result, sound resistance increases and thus the acoustic protection efficiency raises.

Accessory growing of the NPS sound resistance can be realized, by increasing hermetic sheet inflexibility. This growing can be obtained, by fitting it (NPS) out with coaxial spherical cavities and ledges.

The construction of mentioned NPS is introduced on the fig. 1.1(a), the general form; on the fig 1.1.(b) - the cross-section A-A on the fig. 1.1(a).

The construction of the NPS includes the post 1. The frame 2 is added to the post. On the frame 2 the hermetic 3 sheet and perforated 4 sheets are fastened, in air gap between them there is a sound absorbing material 5 (e.g., shallow fractional keramzit or glass wool). The hermetic sheet 3 is equipped with coaxial cavities 6 and ledges 7, which are done of sphere shape of r₁ and r₂ radiuses.

In order to get ledges and cavities on the airtight sheet, pressing or rolling in rollers of proper profile are used. This permits to increase inflexibility of sheet, by growth of thickness in places, where ledges and cavities are allocated, and to secure the crossing of sound waves, when waves penetrate NPS.

The principle of the NPS operation is following.

When a sound from the source of noise penetrates the NPS, for example, parallel waves 8, 9, 10, partly weaken in the issue of their energy absorption into sound absorbing material and reach the surface of hermetic sheet 3. After this, one their group reflects with the decrease of amplitude from surface of sheet angularly in the form of waves 8', 9' , 10'. Other group penetrates the sheet in form of waves 8'', 9'', 10'' and obtaines the protected object C, in consequence of appearing bending vibrations of sheet wall. When sound waves 8, 9, 10 cross waves 8' , 9' , 10', then occurs an additional dissipation of sound energy. Due to this, sound pressure of waves 8, 9, 10 , falling on the surface of hermetic sheet, declines, and sound waves 8'', 9'', 10'' energy , that influences protected object C, reduces, securing the decrease of noise, which effects the protected object. Other words, the repeated crossing of waves with different amplitudes, vibration frequency, which are allocated in antiphases, increases the intensity of energy dissipation in sound absorbing layer and also raises the NPS sound resistance.

The NPS sound resistance growing can also (except mentioned higher) be realized, taking into consideration, that hermetic sheet, comparing to prototype, possesses larger inflexibility, consequently of ledges and cavities occurance on the NPS surface that scale down sound deformations of sheet, so the energy of penetrating through the NPS sound waves 8'', 9'', 10'' reduces . Filling the cavities up with sound absorbing material assists the rising of sound resistance.

Owing to the growth of the NPS sound resistance, by securing the crossing of falling down and reflected sound waves, and the decrease of sound deformations of hermetic sheet, by virtue of increasing its inflexibility and also, by filing cavities in with sound absorbing material, the sound protection efficiency grows.

The introduced NPS construction includes a social aspect, because it protects serving staff from a harmful influence of noise.