Military acoustic testing is performed to determine if material can resist the specified acoustic environment. It must resist this environment without degradation of its functional performance and/or structural integrity. Acoustic noise testing is best done in an acoustic testing lab.
Keystone Compliance is a sound test lab with significant experience in MIL-810 acoustic testing. Our test engineers are able to provide noise testing services for commercial, military, and aerospace products. We have an in-depth knowledge of MIL-STD 810G acoustic noise and MIL-STD 810H acoustic testing standards.
Method 515.8 acoustic noise testing is for systems, sub-systems, and units that function in severe acoustic noise environments. It also applies to material located where acoustic noise excitation is used in combination with mechanical vibration excitation. In some cases acoustic noise excitation may be used in preference to mechanical vibration excitation, for the simulation of aerodynamic turbulence.
The following information is extremely technical in nature. It provides a summary of test method 515.8, as derived from the MIL-810 acoustic noise section. Even though the language is from MIL-810H acoustic, it applies previous versions of the standard. This includes MIL-810G acoustic.
What are the Effects of Military Sound Testing?
The acoustic noise environment is produced by any mechanical or electromechanical device capable of causing large airborne pressure fluctuations. Generally, these pressure fluctuations are completely random, over a large amplitude range (5000 Pa to 87000 Pa). They are also over a broad frequency band extending from 10 Hz to 10000 Hz.
On occasion there may exist very high amplitude discrete frequency pressure fluctuations referred to as ‘tones’. When pressure fluctuations impact material, generally, a transfer of energy takes place. This is between the energy (in the form of fluctuating pressure) in the surrounding air to the strain energy in material.
This transfer of energy will result in vibration of the material. In this case the vibrating material may re-radiate pressure energy, or absorb energy in material damping. Or it may transfer energy to components or cavities interior to the material. Because of the large amplitude and broad frequency range of the fluctuating pressure, measurement of material response is important.
The following list is not intended to be exhaustive. It provides examples of problems that could occur when material is exposed to an acoustic noise environment.
- 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
What are Differences Among Acoustical Testing Procedures?
All procedures involve acoustic noise. However,they differ on the basis of how the acoustic noise fluctuating pressure is generated and transferred to the material.
Procedure I – Diffuse Field includes two methods: method Ia and method Ib.
Ia is called uniform intensity acoustic noise. Procedure Ia has a uniform intensity shaped spectrum of acoustic noise that impacts all the exposed material surfaces.
Ib is called direct field acoustic noise (DFAN), and uses normal incident plane waves in a shaped spectrum of acoustic noise. This is to impact directly on all exposed test article surfaces without external boundary reflections. Depending on the geometry of the article this could produce magnitude variations on surfaces. This would be due to phasing differences between the plane waves.
In the case of large surface area, low mass density test articles may excite primary structure modes in a different way. This fundamental difference and its impact on the structure must be weighed against the advantages of the DFAN method.
Procedure II – Grazing Incidence Acoustic Noise includes a high intensity, rapidly fluctuating acoustic noise. It will have a shaped spectrum that impacts the material surfaces in a particular direction. Generally this will be along the long dimension of the material.
Procedure III – Cavity Resonance Acoustic Noise. Procedure III, is based on the intensity and frequency content of the acoustic noise spectrum. This is governed by the relationship between the geometrical configuration of the cavity and the material within the cavity.
What are the Various Types of Acoustic Excitation?
In Uniform Intensity Acoustic Noise, a diffuse field is generated in a reverberation chamber. Wide band random excitation is provided and the spectrum is shaped. This test is applicable to material or structures that have to function in an acoustic noise field. This may be produced by aerospace vehicles, power plants and other sources of high intensity acoustic noise.
This test provides an efficient means of inducing vibration above 100 Hz. Thus, it may also be used to complement a mechanical vibration test. This occurs by using acoustic energy to induce mechanical responses in internally mounted material. In this role, the test is applicable to items including material in airborne stores carried externally on high performance aircraft.
However, the excitation mechanism induced by a diffuse field is different from that induced by aerodynamic turbulence. In this case, the test is not apt for testing thin shell structures in direct contact with acoustic noise.
A practical guideline is that acoustic tests are not required if material is exposed to broadband random noise. This noise has a sound pressure level less than 130 dB overall. And if it’s exposure in every one Hertz band is less than 100 dB. A diffuse field acoustic test is usually defined spectrum levels, frequency range, overall sound pressure level, and duration of the test.
Two different control schemes can be used to perform a Direct Field Acoustic Noise test. One method is known as single input, single output or SISO. The second method is known as Multiple Input, Multiple Output or MIMO. A direct field is generated by audio drivers arranged to encircle the test article.
SISO uses a single drive signal to all acoustic drivers with multiple control microphones averaged to produce the control measurement. MIMO uses multiple independent drive signals to control multiple independent microphone locations.
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 systems that have to operate in a service environment of pressure fluctuations over the surface.
Cavity Resonance is a resonance condition generated when a cavity is excited by the airflow over it. An example of such a cavity is an open weapons bay on an aircraft. This causes oscillation of the air within the cavity at frequencies dependent upon the cavity dimensions and the aerodynamic flow conditions.
Where is the Best Acoustic Lab to Get Acoustic Certification for Your Product?
Keystone Compliance has been recognized among sound testing companies, as one of the best acoustic testing labs in the country. Our test engineers have in-depth knowledge about acoustic compliance testing. And our acoustical testing lab is equipped according to military acoustic lab testing standards.
Looking for the best acoustic laboratory near you? Talk to our experts to develop a streamlined test plan and receive a professional and affordable quote. Work with certified vibration analysts that understand the requirements of MIL-810G acoustic and MIL-810H acoustic testing. Contact us to learn why so many manufacturers rely on Keystone Compliance to meet their compliance testing needs.
There have been several versions of the acoustic noise testing procedures in MIL-STD-810 vibration testing. Below is a list of each version and the appropriate method number:
- MIL-STD-810A Test Method 515 Acoustic Noise Testing
- MIL-STD-810B Test Method 515.1 Acoustic Noise Testing
- MIL-STD-810C Test Method 515.2 Acoustic Noise Testing
- MIL-STD-810D Test Method 515.3 Acoustic Noise Testing
- MIL-STD-810E Test Method 515.4 Acoustic Noise Testing
- MIL-STD-810F Test Method 515.5 Acoustic Noise Testing
- MIL-STD-810G Test Method 515.6 Acoustic Noise Testing
- MIL-STD-810H Test Method 515.8 Acoustic Noise Testing
