Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. “Acoustic Noise” mentioned in this test method is examining the effects of “Noise” upon EUT, not the other way around. The acoustic noise test is performed to determine the adequacy of materiel to resist the specified acoustic environment without unacceptable degradation of its functional performance and/or structural integrity.
Acoustic noise tests are performed to systems, sub-systems, and units, hereafter called materiel, that must function and/or survive in a severe acoustic noise environment. This test is also applicable for materiel located where acoustic noise excitation is used in combination with, or in preference to mechanical vibration excitation for the simulation of aerodynamic turbulence. Laboratory acoustic fields can be significantly different from many of the real fluctuating pressure loadings classed as "acoustic." Consider these limitations when choosing a test type and test facility, as well as in interpreting test results.
Acoustic tests are conducted inside a room called "reverberation chamber" instead of the "anechoic" room many of you are familiar with. Reverberation of sound pressure waves are desired to amplify the effects of noise upon EUT.
Procedure I has a uniform intensity-shaped spectrum of acoustic noise that impacts all the exposed materiel surfaces. A diffuse field is generated in a reverberation chamber. Normally, wide-band random excitation is provided and the spectrum is shaped. This test applies to materiel or structures that have to function or survive in an acoustic noise field such as that produced by aerospace vehicles, power plants and other sources of high-intensity acoustic noise. Since this test provides an efficient means of inducing vibration above 100 Hz, the test may also be used to complement a mechanical vibration test, using acoustic energy to induce mechanical responses in internally mounted materiel. In this role, the test applies to items such as installed materiel in airborne stores carried externally on high-performance aircraft. However, since the excitation mechanism induced by a diffuse field is different from that induced by aerodynamic turbulence when used in this role, this test is not necessarily suitable for testing the structural integrity of thin shell structures interfacing directly with the acoustic noise.
Procedure II includes a high-intensity, rapidly fluctuating acoustic noise with a shaped spectrum that impacts the materiel surfaces in a particular direction—generally along the long dimension of the materiel. Grazing incidence acoustic noise is generated in a duct, popularly known as a progressive wave tube. Normally, wide-band random noise with a shaped spectrum is directed along the duct. This test is applicable to assembled systems that have to operate or survive in a service environment of pressure fluctuations over the surface, such as that which exists in aerodynamic turbulence. These conditions are particularly relevant to aircraft panels, where aerodynamic turbulence will exist on one side only, and to externally carried stores subjected to aerodynamic turbulence excitation over their total external exposed surface. In the case of a panel, the test item will be mounted in the wall of the duct so that grazing incidence excitation is applied to one side only. An aircraft carried store, such as a missile will be mounted co-axially within the duct such that the excitation is applied over the whole of the external surface.
In Procedure III, the intensity and, to a great extent, the frequency content of the acoustic noise spectrum is governed by the relationship between the geometrical configuration of the cavity and the materiel within the cavity. A resonance condition is generated when a cavity, such as that represented by an open weapons bay on an aircraft, is excited by the airflow over it. This causes oscillation of the air within the cavity at frequencies dependent upon the cavity dimensions and the aerodynamic flow conditions. In turn, this can induce vibration of the structure and of components in and near the cavity. The resonance condition can be simulated by the application of a sinusoidal acoustic source, tuned to the correct frequency of the open cavity. The resonance condition will occur when the control microphone response reaches a maximum in a sound field held at a constant sound pressure level over the frequency range.
• Air-conducted acoustic noise creates physical vibration along the corresponding surface of EUT. In the essence of all prevention methods are based on this information.
• Vibration caused by high air pressure acoustics along the EUT can and will be transferred thoroughly inside and outside of EUT.
• Bigger the outer surface means more susceptibility of acoustic air pressure hence the vibration. Consider making surfaces such as taps, covers etc. smaller in design. Usage of vibration absorbers in joining points of such surfaces with EUT.
• Wires are susceptible to such vibration effects in general terms. Vibration between wires, surfaces causes friction. Friction causes heat and abrasion. This phenomenon is known as “Wire Chaffing”. It is important which can be resulted in disruption of signal, noise in the signal, and fire in case of shorts.
• There is an extensive knowledge for prevention of “Wire Chaffing”. It is highly recommended taking precaution. Applications would be different depending on device.
• Printed circuit boards may crack over time. Selecting proper material for board it self will help.
• Wire connections may be effected from vibration. And intermittent operation may occur. Selection of cable connectors with locking mechanism is advised. There are models with shock absorption available in market.
• Small loose particles such as screws, washers may become an obstruction for mechanisms and short/open circuit for electrical circuitry. Application of your favorite brand thread locking fluid is advised.
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