MIL-STD-810G covers Acoustic Noise Susceptibility in Method 515.6. This Method tests material’s response to very loud noise such as from a jet engine, rocket engine (think Shuttle launch) or power plant. This is not a test to determine how much noise an item generates. Method 515.6 is 8 pages with two Annexes totaling 7 pages.
As with most of the Methods in 810G, tailoring is essential for this Method to have any meaningful results.
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. It is applicable to systems, sub-systems and units that must function and survive in a severe acoustic environment.
The acoustic noise environment is produced by any mechanical or electromechanical device capable of causing large airborne pressure fluctuations. In general, these pressure fluctuations are of an entirely random nature over a large amplitude range (5000 Pa to 87000 Pa), and over a broad frequency band extending from 10 Hz to 10000 Hz. When pressure fluctuations impact materiel, generally, a transfer of energy takes place between the energy (in the form of fluctuating pressure) in the surrounding air to the strain energy in materiel. This transfer of energy will result in vibration of the materiel, in which case the vibrating materiel may re-radiate pressure energy, absorb energy in materiel damping, or transfer energy to components or cavities interior to the materiel.
Some of the effects of a high acoustic environment include:
- Wire chafing
- Component acoustic and vibratory fatigue
- Component connecting wire fracture
- Cracking of printed circuit boards
- Failure of wave guide components
- Intermittent operation of electrical contacts
- Cracking of small panel areas and structural elements
- Optical misalignment
- Loosening of small particles that may become lodged in circuits and mechanisms
- Excessive electrical noise
As with Method 514.6 (Vibration), loud acoustic environments can have an effect on the material that may change the material’s response to other tests. For example, noise can cause a vibration is a panel that cracks the finish which would cause a failure of the Salt Fog test. This test should be performed first or in conjunction with vibration testing.
The Method provides three procedures:
Procedure I – Diffuse Field Acoustic Noise.
A uniform intensity shaped spectrum of acoustic noise that impacts all the exposed materiel surfaces is provided. This would be a typically noisy environment with broadband widespread noise such as produced by aerospace vehicles or power plants.
Procedure II – 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. An example is high speed airflow over the skin of an aircraft or airflow around a missile hanging on a wing pylon.
Procedure III – Cavity Resonance Acoustic Noise.
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. An example would be an open weapons bay on a fighter where the air in the cavity is turbulent. This can induce vibration in adjacent aircraft structures and equipment such as bombs or missiles still in the bay.