515.8-2 CAVITY RESONANCE TESTING
For cavity resonance testing, the sound pressure level B₀, frequencies fₙ and duration T will be as calculated or defined in Table 515.8A-III.
| Test level | |
| B0 = 20 log(q) + 76.4 dB (ref 20 µPa) | |
| fN = | (U∞ / L) [ (N - 0.25) / M + 0.57 / √((1 + M2 γ-1) / 2) ] Hz Equation (1) |
| For Equation (1): | |
| - For cavities that have a length/depth (L/D) ≥2 | |
| - For applications where 0.4 ≤M≤1.5 | |
| For Equation (2) | |
| - For cavities that have a length/depth (L/D) <2 | |
| fN = | ((2N - 1) / 4) * (c / h) Equation (2) |
| Where | |
| c = | speed of sound |
| h = | cavity depth |
| fN = | Resonance frequency for the Nth mode (where N=1, 2, 3, ) up to 500 Hz (where f1>500 Hz use only this mode) |
| N = | Mode number |
| L = | Length/radius of opening exposed to air stream (m). |
| M = | Mach number |
| U∞ = | Free-stream velocity |
| γ = | Ratio of specific heats of gas (1.4 for air) |
| Test duration: | T=1 hour per resonance frequency |
MIL-STD-810H METHOD
515.8-2 PROGRESSIVE WAVE TUBES
A parallel sided duct usually forms the working section of such a progressive noise facility. This may be circular or rectangular in section to suit the test requirements. For testing panels, a rectangular section may be more suitable while an aircraft carried store may be more conveniently tested in a duct of circular section. Noise is generated by an air modulator coupled into one end of the working section by a suitable horn. From the opposite end of the plain duct another horn couples the noise into an absorbing termination. Maximum absorption over the operating frequency range is required here in order to minimize standing wave effects in the duct. Noise then progresses along the duct and is applied with grazing incidence over the surface of the test item. The test item itself may be mounted within the duct in which case the grazing incidence wave will be applied over the whole of its external surface. Alternatively, the test item may be mounted in the wall of the duct when the noise will be applied to only that surface within the duct, e.g., on one side of a panel. The method used will depend upon the test item and its in-service application.
2.1 Selecting the Acoustic Noise Method
After examining the requirements documents and applying the tailoring process in Part One of this Standard to determine where acoustic noise may be encountered in the life cycle of the materiel, use the following to confirm the need for this Method and to place it in sequence with other methods.
2.1.1 Effects of the Acoustic Noise Method
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) (0.73 psi to 12.6 psi), and 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 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. Because of the large amplitude and broad frequency range of the fluctuating pressure, measurement of materiel response is important. The following list is not intended to be all-inclusive, but it provides examples of problems that could occur when materiel is exposed to an acoustic noise environment.
- a. Wire chafing.
- b. Component acoustic and vibratory fatigue.
- c. Component connecting wire fracture.
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METHOD 515.8
- d. Cracking of printed circuit boards.
- e. Failure of wave guide components.
- f. Intermittent operation of electrical contacts.
- g. Cracking of small panel areas and structural elements.
- h. Optical misalignment.
- i. Loosening of small particles that may become lodged in circuits and mechanisms.
- j. Excessive electrical noise.
2.1.2 Sequence Among Other Methods
- a. General. Use the anticipated life cycle sequence of events as a general sequence guide (see Part One, paragraph 5.5).
- b. Unique to this Method. Like vibration, the effects of acoustically induced stresses may affect materiel performance under other environmental conditions, such as temperature, humidity, pressure, electromagnetic, etc. When it is required to evaluate the effects of acoustic noise together with other environments, and when a combined test is impractical, expose a single test item to all relevant environmental conditions in turn. Consider an order of application of the tests that is compatible with the Life Cycle Environmental Profile (LCEP) and sequence guidance in the individual methods.
2.2 Selecting Procedures
This Method includes three acoustic noise test procedures. Determine which of the following procedure(s) to be used.
- a. Procedure I (Diffuse Field) 1a – Uniform Intensity Acoustic Noise, Ib - Direct Field Acoustic Noise.
- b. Procedure II (Grazing Incidence Acoustic Noise)
- c. Procedure III (Cavity Resonance Acoustic Noise).
2.2.1 Procedure Selection Considerations
The choice of test procedure is governed by the in-service acoustic environments and test purpose. Identify these environments from consideration of the Life Cycle Environmental Profile (LCEP) as described in Part One, Annex A, Task 402. When selecting procedures, consider:
- a. The operational purpose of the materiel. From the requirements documents, determine the functions to be performed by the materiel in an acoustic noise environment, the total lifetime exposure to acoustic noise, and any limiting conditions.
- b. The natural exposure circumstances.
- c. The test data required to determine if the operational purpose (function and life) of the materiel has been met.
- d. The procedure sequence within the acoustic noise method. If more than one of the enclosed procedures is to be applied to the same test item, it is generally more appropriate to conduct the less damaging procedure first.
2.2.2 Difference Among Procedures
While all procedures involve acoustic noise, they differ on the basis of how the acoustic noise fluctuating pressure is generated and transferred to the materiel.
- a. Procedure I - Diffuse Field
Ia - Uniform Intensity Acoustic Noise.
Procedure Ia has a uniform intensity shaped spectrum of acoustic noise that impacts all the exposed materiel surfaces.
- Ib - Direct Field Acoustic Noise (DFAN).
Procedure Ib uses normal incident plane waves in a shaped spectrum of acoustic noise to impact directly on all exposed test article surfaces without external boundary reflections. Depending on
MIL-STD-810H
METHOD 515.8
the geometry of the test article this could produce magnitude variations on surfaces due to phasing differences between the plane waves. In the case of large surface area, low mass density test articles the phasing difference may excite primary structure modes in a different way than the diffuse reverberant field. This fundamental difference and its impact on the structure must be weighed against the advantages of the DFAN method. See annex B, paragraph 6 for more detailed information.
- b. Procedure II - Grazing Incidence 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.
- c. Procedure III - Cavity Resonance Acoustic Noise. 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.
2.3 Determine Test Levels and Conditions
2.3.1 General
Having selected this Method and relevant procedures (based on the materiel’s requirements and the tailoring process), it is necessary to complete the tailoring process by selecting specific parameter levels and special test conditions/techniques for these procedures based on the requirements documents, Life Cycle Environmental Profile, and information provided with this procedure. From these sources of information, determine test excitation parameters and the functions to be performed by the materiel in acoustic noise environments or following exposure to these environments.
2.3.2 Use of Measured and Related Data
Wherever possible, use specifically measured data to develop the test excitation parameters and obtain a better simulation of the actual environment. Obtain data at the materiel location, preferably on the specific platform or, alternatively, on the same platform type. In general, the data will be a function of the intended form of simulation. In some cases, only microphone sound pressure levels will be useful, and in other cases materiel acceleration response measurements will be useful.
2.3.3 Types of Acoustic Excitation
2.3.3.1 Diffuse Field
2.3.3.1.1 Uniform Intensity Acoustic Noise
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 in internally mounted materiel. In this role, the test is applicable 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. A practical guideline is that acoustic tests are not required if materiel is exposed to broadband random noise at a sound pressure level less than 130 dB (reference 20 µPascal) overall, and if its exposure in every one Hertz band is less than 100 dB (reference 20 µPascal). A diffuse field acoustic test is usually defined by the following parameters:
- a. Spectrum levels.
- b. Frequency range.
- c. Overall sound pressure level.
- d. Duration of the test.
MIL-STD-810H
METHOD 515.8
2.3.3.1.2 Direct Field Acoustic Noise
A direct field is generated by audio drivers arranged to encircle the test article. Two different control schemes can be used to perform a direct field test. One method, known as single input, single output or SISO, uses a single drive signal to all acoustic drivers with multiple control microphones averaged to produce the control measurement. This method will produce a set of correlated plane waves that may combine to produce large magnitude variations creating local fluctuations on the test article surface. Magnitude variations as much as ±12dB can be experienced. This variation may not be acceptable for some applications. The second method, known as Multiple Input, Multiple Output or MIMO, uses multiple independent drive signals to control multiple independent microphone locations. This method produces a more uncorrelated field that is much more uniform than the SISO field. Magnitude variations in the range of ±3dB are typical when using MIMO control. All other characteristics of diffuse field testing described in 2.3.3.1 also apply to the direct field method.
2.3.3.2 Grazing Incidence Acoustic Noise
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. A grazing incidence acoustic noise test is usually defined by the following parameters:
- a. Spectrum levels.
- b. Frequency range.
- c. Overall sound pressure level.
- d. Duration of the test.
2.3.3.3 Cavity Resonance
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. A cavity resonance test is defined by the following parameters:
- a. Noise frequency.
- b. Overall sound pressure level within the cavity.
- c. Duration of the test.
2.3.3.4 Additional Technical Guidance
Additional Guidance related to the various types of Acoustic Excitation is given in Annex B
2.4 Test Item Configuration
(See Part One, paragraph 5.8.) Where relevant, function the test item, and measure and record performance data during each test phase and/or each acoustic level applied.