THE LOOS OF SOUND WAVES

Mel’niсk V. N., Karachun V.V, Chebotarjova I.G.

National Technical University of Ukraine «KPI»

tHE LOOS OF SOUND WAVES

Penetrating through the HAF and the elements of the fuselage acoustic radiation enters the instrument compartment of the MM. Structure and character of the passed into sound field is presented in tabl. 1. As mass-overall equivalent was chosen inertial navigation device class ГA-8.

The nature and extent of the acoustic impact, as well as the choice of mechanical calculation model of perturbed motion, are directly dependent on the ratio of product size and half the length of the sound wave. In that case, when this value is within the unit, then the acoustic load is equivalent to a uniformly distributed, and the calculated model – to a solid body of elastic ties. If the external dimension is several times greater than half the length, then the external impact is considered as a wave, and the whole structure – as a system with distributed parameters.

The concept of "noise" does not impose restrictions on the nature and structure of the vibrational spectrum. This may be periodic oscillations, called the sound, but also the casual or non-stationary processes.

Let’s illustrate the possibility of reducing the level of sound effects inside a given volume using the device, devoid of the deficiencies noted. This device is a system of two cylinders made in the same marker performance as a serial exchange device.

Deformations of front and side surfaces of sound-insulated cylinder measured strain type 2ПКБ-10.100в, ТУ 2506.1382.78, the signals are fed to tenzor amplifier type 8.АНЧ-7М.

The device contains a coaxially placed inside a metal cylinder 1 and the outer cylinder 2 (perforated screen), which has equal number of the longitudinal 3 on lateral surface and of the radial 4 on the end surface 5 cutting slits preset sizes (fig. 1). Both cylinders are separated by air space. Under the influence of intense sound pressure at the outer cylinder 2, about 5 ... 7%, depending on the total area of cracks, of sound energy passes through the slit 3 and 4 and goes directly to the surface of the inner cylinder 1. Further, some of this energy is transferred into the cylinder, while the other, reflecting falls on the inner surface of the outer cylinder, thereby creating a back pressure from this side to intense aerodynamic noise. This significantly reduces the noise level inside the device. The diagram of sound pressure is shown in fig. 2 - dotted line. Maximum noise reduction in this design, it seems, is carried out up to 114 dB, and the average across the frequency range - up to 116 dB, except for local extrema. Thus, the device in question can, in fact, to ensure the internal volume of the device acoustic comfort.

Application of cross-cutting slits on the side of the outer cylinder perpendicular to its generator and an angle radian does not make significant changes in the structure of the sound field.

If to make a synchronized recording of sound pressure level inside the unit to suppress the intense aerodynamic noise and strain measurements of the deformation of surfaces of both cylinders, it turns out that the local extrema in the diagram of the acoustic pressure is accompanied by the time the excitation end surface of the outer cylinder (fig. 3, the outer cylinder with transverse cracks). The lateral surface of the cylinders does not excite, because of its relatively high stiffness in bending. However, as seen from the diagrams, the vibration end surface of the inner cylinder ( in upper graph) has no significant effect on the sound pressure level of internal volume of the smaller cylinder, except for the frequency at which excited both ends of the surface. From this fact the conclusion of the possibility of adjusting the diagram of the acoustic pressure of the inner cylinder with, for example, the value of a variable stiffness end surface of the outer cylinder. Technical realization of this condition can be time-personal.

It is clear that the combination of anticipated construction with other means of sound insulation, including autocompensation, will provide an opportunity to significantly change for the better performance against the aerodynamic noise of the MM. The choice of method depends on the severity of the overall dimensions of the product.

Using slotted resonators and the Helmholtz resonator with resonant cavities made in the foam, as shown by studies, does not qualitatively alter the picture noise damping compared with the system of metallic and homogeneous foam cylinders.

Thus, the most effective in terms of reducing the sound field in a predetermined internal reverberation volume, is the system of two coaxial cylinders, the inner of which is in the form of a shell and the outer one has a cross-cutting fissure defined sizes (fig. 1). This conclusion can be extended to other dimension and weight characteristics of devices of this type.

In gyroscopic instruments role of the inner cylinder can perform the actual body of the device.

The outer cylinder with slits plays the role of the perforated screen, the acoustic properties of which as is known estimates with its resistance to insufflation, which have, generally speaking, complex character. For slits of a width exceeding 1,5·10^-3 m real part of this resistance can be neglected and we can consider that the resistance of the screen is purely imaginary magnitude proportional to the associated air masses in the cracks.

The resistance of the perforated screen with holes of irregular shape, including in the form of slits, is usually determined experimentally.

Tabl. 1

Passed sound pressure inside device for different technical implementations of the body

Sound pressure P, dB