MIL-STD-810G_(pages_501_-_1086).pdf

MIL-STD-810G

w/CHANGE 1

METHOD 514.7, ANNEX F, APPENDIX E

514.7F-E1

Theoretical Aspects of Maximum Response

1. INTRODUCTION.

When a vibration excitation is applied to a mechanical system with one degree of freedom, the maximum value of the response of this system for a deterministic signal, or the probability of a maximum value for a random signal, can be calculated. This value is called the ‘maximum’ or the ‘extreme’ value. The maximum response spectrum is the curve that represents variations of the ‘maximum’ response value as a function of the natural frequency of the system with one degree of freedom, for a given damping factor ξ.

2. SINUSOIDAL EXCITATION.

Given a sinusoidal excitation with the form:

)ft2(sinx)t(x m π=

The relative response displacement z(t) of a linear system with one degree of freedom is expressed:

0 f f4 f f1

)t(x)t(z ξ+

−ω

For given values for f and f0, z(t) is a maximum when )m(x)t(x = :

f f f f mxzMRS m ξ ω

The MRS is the curve representing the variations of m

0 zω versus f0, for given value of ξ. The positive and negative spectra are symmetric. The positive spectrum goes through a maximum when the denominator goes through a minimum, i.e.:

212 ξξ − = mxMRS

As an initial approximation, it can be considered that:

mxQMRS =

Example: MRS for a fixed sine excitation at 500 Hz with Q=5 (Figure 514.7F-E.1).

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514.7F-E2

Figure 514.7F-E.1. MRS for a fixed sine excitation at 500 Hz with Q=5.

3. SWEPT SINE EXCITATION.

The MRS is extrapolated from the MRS of fixed sinusoidal signals at frequencies corresponding to the limits of the domain of sweeping.

Example: MRS for a swept sine from 300 Hz to 1200 Hz (Figure 514.7F-E.2).

Figure 514.7F-E.2. MRS for a swept sine from 300 Hz to 1200 Hz.

4. RANDOM VIBRATION EXCITATION.

The MRS is calculated by considering the average number of times a threshold of the response z = a is exceeded with a positive slope for a time T. This number is given by the following equation for a Gaussian vibration:

eff a aa eTTnN π++ ==

0 500 1000 1500 2000 2500 3000

M .R

.S (m

/s ²)

Frequency (Hz)

0 500 1000 1500 2000 2500 3000

M R

S (m

/s ²)

Frequency (Hz)

MRS

Excitation

514.7F-E3

Considering a threshold which is exceeded only once on the average, and setting 1Na =+

)Tn(ln2za 0eff which provides:

)Tn(ln2zf4zf4R 0eff 02s

+π=π=

Example: MRS for a random vibration (Figure 514.7F-E.3) defined by:

100 – 300 Hz 0.5 g2/Hz 300 – 600 Hz 1 g2/Hz 600 – 1200 Hz 0.2 g2/Hz

Q = 10

Figure 514.7F-E.3. MRS for a random vibration.

0 500 1000 1500 2000

M .R

.S (g

²/H z)

Frequency (Hz)

514.7F-E4

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METHOD 514.7, ANNEX F, APPENDIX F

514.7F-F1

References

a. NATO Allied Environmental Conditions and Test Publication (AECTP) 100 Environmental Guidelines For Defence Materiel, Edition 4; May 2009

b. International Test Operating Procedure (ITOP) 1-1-050. Development of Laboratory Vibration Test Schedules. 6 June 1997. DTIC AD No B227368

c. International Test Operating Procedure (ITOP) 1-2-601, FR/GE/UK/US. Laboratory Vibration Schedules. 17 June 2009.

d. Harris, Cyril M., Allan G. Piersol, , Shock and Vibration Handbook, McGraw-Hill Book Company, Fifth Edition, ISBN 0-07-137081-1, 2002

e. Bendat, Julius S., Allan G. Piersol, Random Data Analysis and Measurement Procedures Wiley Interscience, Third Edition, ISBN 978-0-471-31733-3, 2000.

f. Henderson, George R. and Allan G. Piersol, Fatigue Damage Related Descriptor for Random Vibration Test Environments, Sound and Vibration, October 1995.

g. Piersol, Allan G., Determination of Maximum Structural Responses From Predictions or Measurements at Selected Points, Proceedings of the 65th Shock and Vibration Symposium, Volume I, SAVIAC, 1994.

Shock and Vibration Exchange; 1104 Arvon Road; PO Box 165; Arvonia, VA 23004;

http://www.savecenter.org.

h. Curtis, Allen J., Nicholas G. Tinling, Henry T. Abstein, Selection and Performance of Vibration Tests, Shock and Vibration Information Center, SVM-8 1971.

i. Structural Alloys Handbook, Mechanical Properties Data Center, Belfour-Stulen, Inc., Traverse City, MI, 1977 edition.

j. Robinson, John A. and Harry T. Cline, Methodology Investigation, Final Report, Cargo Configuration and Restraint in Military Ground Vehicles, TECOM Project No. 7-CO-R08-AP1-002; APG Report No.

APG-MT-5319, Nov 1979.

k. Baily, Richard D. and Spectral Dynamics Research Corporation, Methodology Investigation, Final Report, Determination of the Effective Mass of Cargo Beds, TECOM Project No. 7-CO-RD9-AP1-001, APG Report APG-MT-5572, Dec 1981.

l. Handbook for Dynamic Data Acquisition and Analysis, IEST-RD-DTE012.2; Institute of Environmental Sciences and Technology, Arlington Place One, 2340 S. Arlington Heights Road, Suite 100, Arlington Heights, IL 60005-4516; http://www.iest.org.

m. NATO Allied Environmental Conditions and Test Publication (AECTP) 240, Mechanical Environmental Testing. July 2009

n. Connon, William, To ‘b’ or Not to ‘b’ - What was the question?, IEST ESTECH Proceedings May 2009, Institute of Environmental Sciences and Technology, Arlington Place One, 2340 S. Arlington Heights Road, Suite 100, Arlington Heights, IL 60005-4516; http://www.iest.org.

o. ETBX Strain-Life Fatigue Analysis Module, 11/5/2008.

p. Defence Standard (Def Stan) 00-35, Environmental Handbook for Defence Materiel, Issue 4, 18 September 2006.

http://www.savecenter.org/ http://www.iest.org/ http://www.iest.org/

514.7F-F2

METHOD 515.7

515.7-i

ACOUSTIC NOISE

CONTENTS

Paragraph Page

1. SCOPE

1.1 PURPOSE

1.2 APPLICATION

1.3 LIMITATIONS

2. TAILORING GUIDANCE

2.1 SELECTING THE ACOUSTIC NOISE METHOD

2.1.1 EFFECTS OF THE ACOUSTIC NOISE ENVIRONMENT

2.1.2 SEQUENCE AMONG OTHER METHODS

2.2 SELECTING PROCEDURES

2.2.1 PROCEDURE SELECTION CONSIDERATIONS

2.2.2 DIFFERENCE AMONG PROCEDURES

2.3 DETERMINE TEST LEVELS AND CONDITIONS

2.3.1 GENERAL

2.3.2 USE OF MEASURED AND RELATED DATA

2.3.3 TYPES OF ACOUSTIC EXCITATION

2.3.3.1 DIFFUSE FIELD

2.3.3.2 GRAZING INCIDENCE ACOUSTIC NOISE

2.3.3.3 CAVITY RESONANCE

2.3.3.4 ADDITIONAL TECHNICAL GUIDANCE

2.4 TEST ITEM CONFIGURATION

3. INFORMATION REQUIRED

3.1 PRETEST

3.2 DURING TEST

3.3 POST-TEST

4. TEST PROCESS

4.1 TEST FACILITY

4.2 CONTROLS

4.2.1 CONTROL OPTIONS

4.2.1.1 SINGLE POINT NOISE CONTROL

4.2.1.2 MULTIPLE POINT NOISE CONTROL

4.2.1.3 VIBRATION RESPONSE CONTROL

4.2.2 CONTROL METHODS

4.2.3 OVERALL ACCURACY OF CONTROL

4.2.4 CALIBRATION AND TOLERANCE

4.3 TEST INTERRUPTION

4.3.1 INTERRUPTION DUE TO LABORATORY EQUIPMENT MALFUNCTION

4.3.2 INTERRUPTION DUE TO TEST ITEM OPERATION FAILURE

4.4 TEST SETUP

4.5 TEST EXECUTION

4.5.1 PREPARATION FOR TEST

515.7-ii

CONTENTS - Continued

4.5.1.1 PRELIMINARY STEPS

4.5.1.2 PRETEST STANDARD AMBIENT CHECKOUT

4.5.2 INSTALLATION OF THE TEST ITEM

4.5.2.1 DIFFUSE FIELD

4.5.2.2 GRAZING INCIDENCE ACOUSTIC NOISE

4.5.2.3 CAVITY RESONANCE ACOUSTIC NOISE

4.5.3 PROCEDURE I - DIFFUSE FIELD ACOUSTIC NOISE TESTING

4.5.3.1 PROCEDURE Ia - UNIFORM INTENSITY ACOUSTIC NOISE TESTING

4.5.3.2 PROCEDURE Ib - DIRECT FIELD ACOUSTIC NOISE TESTING

4.5.4 PROCEDURE II - GRAZING INCIDENCE ACOUSTIC NOISE TESTING

4.5.5 PROCEDURE III - CAVITY RESONANCE ACOUSTIC NOISE TESTING

5. ANALYSIS OF RESULTS

6. REFERENCE/RELATED DOCUMENTS

6.1 REFERENCED DOCUMENTS

6.2 RELATED DOCUMENTS

TABLE

TABLE 515.7-I. TEST TOLERANCES

METHOD 515.7 ANNEX A

GUIDANCE FOR INITIAL TEST SEVERITY

1. BROADBAND RANDOM AND INCIDENCE NOISE TESTING....................................................... A-1

1.1 OVERALL SOUND PRESSURE LEVEL (OASPL) ............................................................................................. A-1

1.2 TEST SPECTRUM ........................................................................................................................................... A-2

1.3 SIMULATION OF AERODYNAMIC TURBULENCE ............................................................................................ A-3

2. CAVITY RESONANCE TESTING ........................................................................................................ A-3

3. EXTERNAL STORES TESTING ........................................................................................................... A-4

3.1 TEST SPECTRUM ........................................................................................................................................... A-4

3.2 TEST PARAMETERS ...................................................................................................................................... A-5

ANNEX A FIGURES

FIGURE 515.7A-1. APPLIED TEST SPECTRUM ........................................................................................................... A-2 FIGURE 515.7A-2. TYPICAL STORE PROFILE ............................................................................................................ A-4 FIGURE 515.7A-3. ONE-THIRD OCTAVE BAND SPECTRUM FOR ASSEMBLED EXTERNALLY CARRIED AIRCRAFT STORES ....................................................................................................................................... A-4

515.7-iii

ANNEX A TABLES

TABLE 515.7A-I. OVERALL SOUND PRESSURE LEVELS AND DURATIONS ................................................................ A-1 TABLE 515.7A-II. ONE-THIRD OCTAVE BAND LEVELS FOR FIGURE 515.7A-1 ......................................................... A-2 TABLE 515.7A-III. CAVITY RESONANCE TEST CONDITIONS (SEE PARAGRAPH 6.1, REFERENCE A)............................ A-3 TABLE 515.7A-IV. SUGGESTED ACOUSTIC TEST LEVELS FOR ASSEMBLED EXTERNALLY CARRIED AIRCRAFT STORES ....................................................................................................................................... A-5

METHOD 515.7 ANNEX B

ADDITIONAL TECHNICAL GUIDANCE

1. REVERBERATION CHAMBERS .......................................................................................................... B-1

2. PROGRESSIVE WAVE TUBES ............................................................................................................. B-1

3. ACOUSTIC NOISE CHARACTERISTICS ........................................................................................... B-1

4. CONTROL STRATEGIES ...................................................................................................................... B-1

5. DEFINITIONS .......................................................................................................................................... B-2

5.1 SOUND PRESSURE LEVEL ............................................................................................................................. B-2

5.2 THIRD OCTAVE FILTERS .............................................................................................................................. B-2

6. DIRECT FIELD ACOUSTIC TEST CHARACTERISTICS ............................................................... B-2

515.7-iv

515.7-1

ACOUSTIC NOISE

NOTE: Tailoring is essential. Select methods, procedures, and parameter levels based on the tailoring process described in Part One, paragraph 4.2.2, and Annex C. Apply the general guidelines for laboratory test methods described in Part One, paragraph 5 of this Standard.

1. SCOPE.

1.1 Purpose.

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.

1.2 Application.

This test is applicable 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 (Method 523.4).

1.3 Limitations.

Technical limitations restrict production and control of laboratory acoustic environments. Therefore, 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. For example, diffuse field acoustic noise (see paragraph 2.3.3.1) better represents acoustics in internal cavities where local reflection and re-radiation from vibrating structures predominate. For external skins exposed to aerodynamic turbulence or jet noise, grazing incidence acoustic noise (see paragraph 2.3.3.2) more closely represents flow/acoustic wave propagation along skin surfaces.

2. TAILORING GUIDANCE.

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.

515.7-2

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) Ia – 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.

515.7-3

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 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.

515.7-4

c. Overall sound pressure level.

d. Duration of the test.

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.

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 2.3.3.1.1 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.

3. INFORMATION REQUIRED.

The following information is necessary to properly conduct the acoustic test.

515.7-5

3.1 Pretest.

a. General. See the information listed in Part One, paragraphs 5.7, 5.8, 5.9, 5.11 and 5.12; and Part One, Annex A, Task 405 of this Standard.

b. Specific to this Method.

(1) Establish test levels and durations using projected Life Cycle Environmental Profiles, available data or data acquired directly from an environmental data-gathering program. When these data are not available, use the guidance on developing initial test severities in Annex A. Consider these overall sound pressure levels (OASPL) (Annex A, Table 515.7A-I) as initial values until measured data are obtained. The test selected may not necessarily be an adequate simulation of the complete environment and consequently a supporting assessment may be necessary to complement the test results.

(2) If the test item is required to operate during the test; the operating checks required are pretest, during the test, and post test. For the pre- and post test checks, specify whether they are performed with the test item installed in the test facility. Define the details required to perform the test, including the method of attachment or suspension of the test item, the surfaces to be exposed, effect of gravity and any consequent precautions. Identify the control and monitor points, or a procedure to select these points. Define test interruption, test completion and failure criteria.

c. Tailoring. Necessary variations in the basic test procedures to accommodate LCEP requirements and/or facility limitations.

3.2 During Test.

a. General. See the information listed in Part One, paragraph 5.10, and in Part One, Annex A, Tasks 405 and 406.

b. Specific to this Method.

(1) Collect outputs of microphones, test control averages, test item operating parameters and any other relevant transducers at appropriate test times.

(2) Collect log/records of materiel operating parameters.

(3) Give particular attention to interactions of the input excitation (diffuse, directional or tonal).

(4) Record transient behavior in the input representing a test anomaly.

3.3 Post-Test.

The following post test data shall be included in the test report.

a. General. See the information listed in Part One, paragraph. 5.13; and in Part One, Annex A, Task 406 of this Standard.

b. Specific to this Method.

(1) Identify any indication of failure under specified failure criteria. Account for tolerance excesses when testing large materiel, the number of simultaneous test items in Procedure I, and any other environmental conditions at which testing was carried out, if other than standard laboratory conditions.

(2) Ensure detailed data analysis for verification of the input to the test item, i.e., the acoustic field and the response monitoring of the test item, are in accordance with the test plan.

(3) Any deviations from the test plan.

4. TEST PROCESS.

4.1 Test Facility.

Ensure the apparatus used to perform the acoustic test has sufficient capacity to adequately reproduce the input requirements. Diffuse acoustic field apparatus that produce uniform acoustic fields above 165 dB are rare. For high

515.7-6 level acoustic input (above 165 dB), consider testing using grazing incidence acoustic noise. For measured data that indicates tonal input, consider a facility that can be configured to produce a cavity resonance condition.

4.2 Controls.

The control strategy depends upon the type of test and the size of the materiel.

4.2.1 Control Options.

4.2.1.1 Single Point Noise Control.

Define the single point, providing an optimum control position in the chamber or progressive wave tube.

4.2.1.2 Multiple Point Noise Control.

Select the control points to define a controlled volume within the reverberation chamber. Base control upon the average of the sound spectrum levels at each microphone. Where the range of measurements at the monitoring positions does not exceed 5 dB (OASPL) a simple arithmetic average of the sound spectrum levels (in dB) may be used. For a range of 5 dB or greater, use an average of the non-logarithmic sound spectrum levels (i.e., µPa or microbar), then convert to dB.

4.2.1.3 Vibration Response Control.

Where it is necessary to achieve a given vibration acceleration response on the test item, adjust the acoustic test spectrum to achieve the required response that may be monitored at either a single point or as the average from multiple monitoring points. Refer to Method 514.7 for further guidance.

4.2.2 Control Methods.

Control can be by either open or closed loop. Open loop control is adequate for progressive wave tubes and for small chambers having a single noise source. Closed loop control is more effective for large chambers having multiple noise sources that cover different bands in the test frequency range.

4.2.3 Overall Accuracy of Control.

Ensure the uncertainty of measurement of the total measurement system, including statistical errors, does not exceed one-third of the specified tolerance for the overall sound pressure level.

4.2.4 Calibration and Tolerance.

Test tolerances are given in Table 515.7-I. Ensure the test tolerance and calibration procedures for test control are generally consistent with the guidance provided in Part One, paragraphs 5.2 and 5.3.2, respectively.

Table 515.7-I. Test tolerances.

PARAMETER TOLERANCE

Overall sound pressure level averaged over all control microphones, ref specified overall sound pressure level

+3 dB -1 dB

Overall sound pressure level at each control microphone, ref specified overall sound pressure level

+4 dB -2 dB

Averaged test spectrum from all control microphones at levels above -15 dB in 1/3 octave bands, ref specified 1/3 octave band sound pressure levels.

±4 dB

Averaged test spectrum from all control microphones at levels below -15 dB and above -25 dB in 1/3 octave bands, ref specified 1/3 octave band sound pressure levels.

±6 dB

Averaged test spectrum from all control microphones at levels -25 dB and below in 1/3 octave bands, ref specified 1/3 octave band sound pressure levels.

±10 dB

Duration ±5 % or ±1 min whichever is less

515.7-7

4.3 Test Interruption.

Test interruptions can result from two or more situations, one being from failure or malfunction of test laboratory equipment. The second type of test interruption results from failure or malfunction of the test item itself during required or optional performance checks.

4.3.1 Interruption Due To Laboratory Equipment Malfunction.

a. General. See Part One, paragraph 5.11 of this Standard.

b. Specific to this Method. Interruption of an acoustic noise test is unlikely to generate any adverse effects.

Normally, continue the test from the point of interruption.

4.3.2 Interruption Due To Test Item Operation Failure.

Failure of the test item(s) to function as required during mandatory or optional performance checks during testing presents a situation with several possible options.

a. The preferable option is to replace the test item with a “new” one and restart from Step 1.

b. A second option is to replace / repair the failed or non-functioning component or assembly with one that functions as intended, and restart the entire test from Step 1.

NOTE: When evaluating failure interruptions, consider prior testing on the same test item and any consequences of such.

4.4 Test Setup.

a. General. See Part One, paragraph 5.8.

b. Unique to this Method. Tests will normally be carried out with the test item mounted in the correct attitude, unless it is shown that the performance of the test item is not affected by gravity.

4.5 Test Execution.

The following steps, alone or in combination, provide the basis for collecting necessary information concerning the test item in an acoustic environment.

4.5.1 Preparation for Test.

4.5.1.1 Preliminary Steps.

Before starting the test, determine the test details (e.g., procedure variations, test item configuration, cycles, durations, parameter levels for storage/operation, etc.) from the test plan. (See paragraph 3.1 above.)

4.5.1.2 Pretest Standard Ambient Checkout.

a. Unless otherwise specified, allow the test item to stabilize at standard ambient conditions.

b. Perform a physical inspection and operational checks before and after testing. Define the requirements for these checks in the test plan. If these checks are required during the test sequence, specify the time intervals at which they are required.

c. Ensure that all test environment monitoring instrumentation and test item function monitoring instrumentation is consistent with the calibration and test tolerance procedures, and are generally consistent with the guidance provided in Part One, paragraphs 5.3.2 and 5.2, respectively.

515.7-8

4.5.2 Installation of the Test item.

4.5.2.1 Diffuse Field

4.5.2.1.1 Uniform Intensity Acoustic Noise.

Suspend the test item (or as otherwise mounted) in a reverberation chamber on an elastic system in such a manner that all appropriate external surfaces are exposed to the acoustic field and no surface is parallel to a chamber surface.

Ensure the resonance frequency of the mounting system with the specimen is less than 25 Hz or 1/4 of the minimum test frequency, whichever is less. If cables, pipes etc., are required to be connected to the test item during the test, arrange them to provide similar restraint and mass as in service. Locate a microphone in proximity to each major different face of the test item at a distance of 0.5 meter (1.64 ft) from the face, or midway between the center of the face and the chamber wall, whichever is smaller. Average the outputs from these microphones to provide a single control signal. When the chamber is provided with a single noise injection point, place one microphone between the test item and the chamber wall furthest from the noise source. The orientation of the microphones in such a facility is not critical, but do not set the microphone axes normal to any flat surface. Calibrate the microphones for random incidence.

4.5.2.1.2 Direct Field Acoustic Noise (DFAN).

The test item should be surrounded by a circular array of acoustic drivers to a height of at least 1meter (3.28 ft) above the test article. The arrangement should avoid symmetry to reduce the potential for adverse coupling of plane waves. The test article can be mounted on a platform or suspended. Multiple microphones, eight to sixteen, should be used for control with either the SISO or MIMO methods (see annex B, paragraph 6). The microphones should be placed randomly around the test article. The distance from the surface of the drivers to the surface of the control microphones should be between 1 meter (3.28 ft) and 1.5 meter (4.92 ft). The distance from the control microphones to the surface of the test article should be between 0.5 meter (1.64 ft) and 0.75 meter (2.5 ft). The height of the control microphones should be centered at mid-height of the test item and randomly varied up and down by about one-eighth of the test item height. The orientation of the free-field microphones in a DFAN test arrangement is not critical. However, reflections from the test article can be minimized with the microphone oriented toward the sound source with a 0 degree incidence (see Paragraph 6, reference c, figure 3.7). Most modern day, quality measurement, free-field microphones are factory adjusted to compensate for incident angle. This phenomenon is most pronounced at high frequencies, above 10kHz for a 1/4" microphone, and is inversely proportional to microphone diaphragm diameter.

4.5.2.2 Grazing Incidence Acoustic Noise.

Mount test items such as panels in the wall of the duct such that the required test surfaces are exposed to the acoustic excitation. Ensure this surface is flush with the inner surface of the duct to prevent the introduction of cavity resonance or local turbulence effects. Suspend test items (such as aircraft external stores) centrally within the duct, on an elastic support. Orient the test item such that the appropriate surfaces are subjected to progressive acoustic waves. For example, orient an aircraft external store parallel to the duct centerline so that the acoustic waves sweep the length of the store. Ensure the rigid body modes of the test item are lower than 25 Hz or 1/4 of the lowest test frequency, whichever is less. Ensure that no spurious acoustic or vibratory inputs are introduced by the test support system or by any ancillary structure. Mount the microphone(s) for control and monitoring of test conditions in the duct wall opposite the test panel. Select other positions within the duct assuming the microphone is positioned so that it responds to only grazing incidence waves, and that the necessary corrections are applied to the measured level. Calibrate the microphones for grazing incidence.

4.5.2.3 Cavity Resonance Acoustic Noise.

Suspend the test item (or as otherwise mounted) in a reverberation chamber such that only that part of the cavity to be tested is exposed to the direct application of acoustic energy. Protect all other surfaces so that their level of acoustic excitation is reduced by 20 dB. Do not use protective coverings that provide any additional vibration damping to the structure. Do not locate the microphone for control of the test within the cavity to be tested.

515.7-9

4.5.3 Procedure I – Diffuse Field Acoustic Noise Testing

4.5.3.1 Procedure Ia - Uniform Intensity Acoustic Noise Testing.

Step 1 Install the test item in the reverberation chamber in accordance with paragraph 4.5.2.1.1

Step 2 Select microphone positions for control, monitoring, and control strategy in accordance with paragraph 4.5.2.1.1.

Step 3 When using open loop control, remove the test item and confirm the specified overall sound pressure level and spectrum can be achieved in an empty chamber, then replace the test item in the chamber.

Step 4 Precondition the test item in accordance with paragraph 4.5.1.2.

Step 5 Conduct initial checks in accordance with paragraph 4.5.1.2.

Step 6 Apply the test spectrum for the specified time. If required, carry out inspections and operational checks in accordance with paragraph 4.5.1.2. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure.

Step 7 Record the test acoustic field at each microphone, any average used in test control, and other pertinent transducer outputs. Make the recordings at the beginning, midpoint, and end of each test run. Where test runs are longer than one hour, record every one-half hour.

Step 8 Carry out the final inspection and operational checks, and see paragraph 5 for analysis of results.

Step 9 Remove the test item from the chamber.

Step 10 In all cases, record the information required.

4.5.3.2 Procedure Ib - Direct Field Acoustic Noise Testing.

Step 1 Build a test setup using a test item simulator in accordance with paragraph 4.5.2.1.2

Step 2 Select microphone positions for control, monitoring, and control strategy in accordance with paragraph 4.5.2.1.2.

Step 3 Perform a pre-test using the simulator to confirm the specified overall sound pressure level and spectrum can be achieved. Also verify any special control features to be used such as; abort tolerances, response limits, field shaping and emergency shut-down procedures. Monitor the resulting field for uniformity, coherence and structural response, if available. Then replace the simulator with the actual test item in the speaker circle.

Step 4 Precondition the test item in accordance with paragraph 4.5.1.2.

Step 5 Conduct initial checks in accordance with paragraph 4.5.1.2.

Step 6 Apply the test spectrum for the specified time. Use multiple runs if the allowable audio system full level ON (duty cycle as discussed in Annex B, paragraph 6) time is less than the total test time. If required, carry out inspections and operational checks in accordance with paragraph 4.5.1.2. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure.

Step 7 Record the test acoustic field at each microphone, any average used in test control, and other pertinent transducer outputs. Make the recordings at the beginning, midpoint, and end of each test run. Where test runs are longer than one hour, record every one-half hour.

Step 8 Carry out the final inspection and operational checks, and see paragraph 5 for analysis of results.

Step 9 Remove the test item from the circle.

Step 10 In all cases, record the information required.

515.7-10

4.5.4 Procedure II - Grazing Incidence Acoustic Noise Testing.

Step 1 Install the test item in accordance with paragraph 4.5.2.2.

Step 2 Select microphone positions for control, monitoring, and control strategy in accordance with paragraph 4.5.2.2.

Step 3 Precondition the test item in accordance with paragraph 4.5.1.2.

Step 4 Conduct initial checks in accordance with paragraph 4.5.1.2.

Step 5 Apply the test spectrum for the specified time. If required, carry out inspections and operational checks in accordance with paragraph 4.5.1.2. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure.

Step 6 Record the test acoustic field at each microphone, any average used in test control, and other pertinent transducer outputs. Make recordings at the beginning, end and midpoint of each test run.

Where test runs are longer than one hour, record every one-half hour.

Step 7 Carry out the final inspection and operational checks, and see paragraph 5 for analysis of results.

Step 8 Remove the test item from the duct.

Step 9 In all cases, record the information required.

4.5.5 Procedure III - Cavity Resonance Acoustic Noise Testing.

Step 1 Install the test item into the chamber in accordance with paragraph 4.5.2.3.

Step 2 Locate the control microphone in accordance with paragraph 4.5.2.3.

Step 3 Precondition the test item in accordance with paragraph 4.5.1.2.

Step 4 Conduct initial checks in accordance with paragraph 4.5.1.2.

Step 5 Apply the sinusoidal acoustic excitation at the required frequencies (see Annex A, Table 515.7A- II). Adjust the test parameters to the specified levels and apply for the specified time. If required, carry out inspections and operational checks in accordance with paragraph 4.5.1.2. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure.

Step 6 Record the test acoustic field at each microphone, any average used in test control, and other pertinent transducer outputs. Make recordings at the beginning, midpoint, and end of each test run. Where test runs are longer than one hour, record every one-half hour.

Step 7 Perform the final physical inspection and operational checks, and see paragraph 5 for analysis of results.

Step 8 Remove the test item from the chamber.

Step 9 In all cases, record the information required.

5. ANALYSIS OF RESULTS.

Refer to Part One, paragraphs 5.14 and 5.17; and Part One, Annex A, Task 406.

6. REFERENCE/RELATED DOCUMENTS.

6.1 Referenced Documents.

a. AFFDL-TR-76-91; Volume II: "A Review of Methods for Estimation of Aero-acoustic Loads on Flight Vehicle Surfaces". February 1977.

b. NATO Allied Environmental Engineering and Test Publication (AECTP) 400, Mechanical Environmental Testing, Method 401, Vibration.

c. Beranek, Leo L., "Noise and Vibration Control" Revised Edition, 1988, Institute of Noise Control Engineering, ISBN: 0-9622072-0-9.

515.7-11

d. Larkin, Paul A. and Smallwood, David O. "Control of an Acoustical Speaker System in a Reverberant Chamber", Journal of the IEST, V. 47, No. 2, 2004.

e. Larkin, Paul A. and Hayes, Dann "Status of Direct Field Acoustic Testing", 27th Aerospace Testing Seminar, October 16-18, 2012.

f. Maahs, Gordon, "Direct Field Acoustic Test (DFAT) Development and Flight Testing of Radiation Belt Storm Probe (RBSP) Satellites", 27th Space Simulation Conference, November 5-8, 2012.

6.2 Related Documents.

a. NATO STANAG 4370, Environmental Testing.

b. NATO Allied Environmental Engineering and Test Publication (AECTP) 400, Mechanical Environmental Testing, Method 402, Acoustic Noise.

c. ISO 266, Acoustics – Preferred Frequencies, International Organization for Standardization, 27 March 1997; http://www.iso.org.

d. Egbert, Herbert W. “The History and Rationale of MIL-STD-810 (Edition 2)”, January 2010; Institute of Environmental Sciences and Technology, Arlington Place One, 2340 S. Arlington Heights Road, Suite 100, Arlington Heights, IL 60005-4516.

e. IEST RP-DTE040.1, High-Intensity Acoustics Testing, Institute of Environmental Sciences and Technology, Arlington Place One, 2340 S. Arlington Heights Road, Suite 100, Arlington Heights, IL 60005-4516; .January 2003; http://www.iest.org.

f. NASA-STD-7001, Payload Vibroacoustic Test Criteria, National Aeronautics and Space Agency, 21 June 1996, http://standards.nasa.gov.

(Copies of Department of Defense Specifications, Standards, and Handbooks, and International Standardization Agreements are available online at https://assist.dla.mil, or the Information Handling Service, or from the Standardization Document Order Desk, 700 Robbins Avenue, Building 4D, Philadelphia, PA 19111-5094.)

Requests for other defense-related technical publications may be directed to the Defense Technical Information Center (DTIC), ATTN: DTIC-BR, Suite 0944, 8725 John J. Kingman Road, Fort Belvoir VA 22060-6218, 1-800- 225-3842 (Assistance--selection 3, option 2), http://www.dtic.mil/dtic/; and the National Technical Information Service (NTIS), Springfield VA 22161, 1-800-553-NTIS (6847), http://www.ntis.gov/.

https://assist.dla.mil/ http://www.dtic.mil/dtic/ http://www.ntis.gov/

515.7-12

515.7A-1

METHOD 515.7, ANNEX A

GUIDANCE FOR INITIAL TEST SEVERITY

1. BROADBAND RANDOM AND INCIDENCE NOISE TESTING.

1.1 Overall Sound Pressure Level (OASPL).

From the known area of operation for the materiel, the test overall sound pressure level and duration may be obtained from Table 515.7A-I.

Table 515.7A-I. Overall sound pressure levels and durations.

TYPICAL APPLICATION

TEST

LEVEL

(OASPL)

dB

DURATION

(Min)

Transport aircraft at locations not close to jet exhausts 130 30

Transport aircraft, in internal materiel bays close to jet exhausts

High performance aircraft at location not close to jet exhausts

High performance aircraft in internal materiel bays close to jet exhausts

Air-to-air missile on medium performance aircraft (i.e., dynamic pressure (q)<57456 Pa (1200 psf)).

Air-to-ground missile on medium performance aircraft (i.e., q<57456 Pa (1200 psf)).

Ground materiel in enclosed engine run-up areas

High performance aircraft in internal materiel bays close to reheat exhaust and gun muzzles or in nose cones

Airborne rocket most locations but excluding booster or engine bays

Air-to-air missile on high performance aircraft (i.e., q<86184 Pa (1800 psf)).

Air-to-ground missile on high performance aircraft (i.e., q<86184 Pa (1800 psf)).

Airborne rocket booster or engine bays

515.7A-2

1.2 Test Spectrum.

The applied test spectrum associated with these levels is shown on Figure 515.7A-1 with breakpoints defined in Table 515.7A-II. Achieve the test spectrum while maintaining the test parameters within the tolerances given in Table 515.7-I.

Figure 515.7A-1. Applied test spectrum.

Table 515.7A-II. One-third octave band levels for Figure 515.7A-1.

1/3 octave center frequency

Hz

Upper tolerance limit dB

Nominal level dB

Lower tolerance limit dB

1/3 octave center frequency Hz

Upper tolerance limit dB

Nominal level dB

Lower tolerance limit dB

50 -19 -29 -39 800 -7 -11 -15 63 -15 -25 -35 1000 -7 -11 -15 80 -15 -21 -27 1250 -7 -11 -15

100 -11 -17 -23 1600 -8.5 -12.5 -16.5 125 -9 -13 -17 2000 -10 -14 -18 160 -8 -12 -16 2500 -9.5 -15.5 -21.5 200 -7 -11 -15 3150 -11 -17 -23 250 -7 -11 -15 4000 -12.5 -18.5 -24.5 315 -7 -11 -15 5000 -16.5 -22.5 -28.5 400 -7 -11 -15 6300 -16.5 -26.5 -36.5 500 -7 -11 -15 8000 -20.5 -30.5 -40.5 630 -7 -11 -15 10000 -24.5 -34.5 -44.5

-50

-40

-30

-20

-10

10 100 1000 10000

1/ oc ta ve S

PL

re la tiv e to

O A

SP

L

Frequency - Hz

Upper and and lower tolerance bands Nominal levels

Tolerances apply to averaged test spectrum from all control microphones (see Table 515.7-I)

515.7A-3

1.3 Simulation of Aerodynamic Turbulence.

Where a broadband noise test is required for the simulation of aerodynamic turbulence, derive the test levels and durations in conjunction with those for the complementary mechanical test.

2. CAVITY RESONANCE TESTING

For cavity resonance testing, the sound pressure level Bo, frequencies fN and duration T will be as calculated or defined in Table 515.7A-III.

Table 515.7A-III. Cavity resonance test conditions. (See paragraph 6.1, reference a.)

Test level

B0 = 20 log(q) + 76.4 dB (ref 20 µPa)

Hz Equation (1)

For Equation (1):

- For cavities that have a length/depth (L/D) > 2

- For applications where 0.4 < M < 1.5

Equation (2)

For Equation (2)

- For cavities that have a length/depth (L/D) < 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

N

U N 0.25f ML 0.57 γ 11 M h c

1 2N f N

515.7A-4

3. EXTERNAL STORES TESTING.

3.1 Test Spectrum.

A typical store profile is shown on Figure 515.7A-2. The applied test spectrum is shown on Figure 515.7A-3.

Figure 515.7A-2. Typical store profile.

Figure 515.7A-3. One-third octave band spectrum for assembled externally carried aircraft stores.

1/

O ct av e

Ba nd

S ou nd P re ss ur e

Le ve l d B

(R ef . 2 µP a (0

.0 m ic ro ba r)

Frequency (Hz)

Lo

200 100 2000 fo

6 db/Octave -A db/Octave

515.7A-5

3.2 Test Parameters.

For acoustic testing of external stores, the associated levels and…