Acoustic Impingement

An acoustic noise impingement test is performed to determine the ability of a Test Specimen to perform in an acoustic field without unacceptable degradation in operation or structural integrity. Different types of acoustic fields include:
Diffuse Field Acoustic Noise
Diffuse field acoustic noise represents acoustics in internal cavities where local reflection and re-radiation from vibrating structures predominate. Diffuse field acoustic noise 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 is applicable 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.
Grazing Incidence Acoustic Noise
Grazing incidence acoustic noise more closely represents flow/acoustic wave propagation along external skin surfaces exposed to aerodynamic turbulence or jet noise. Grazing incidence acoustic noise 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 exist 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.
Cavity Resonance Acoustic Noise
Cavity resonance acoustic noise intensity and frequency content of the spectrum is governed by the relationship between the geometrical configuration of the cavity and the materiel within the cavity.
A cavity 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 resonant frequency of the open cavity.
Intermittent operation Wire chafing
Acoustic fatigue Vibratory fatigue
Connecting wire fracture Failure of wave guide components
Printed circuit board cracking Optical misalignment
Loosening of small parts Excessive electrical noise
Cracking of panels and structural elements  
Energy Distribution Frequency Range
Overall Sound Pressure Level Test Duration
Related Test Methods
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