515.8-44 test process

MIL-STD-810H

METHOD

MIL-STD-810H

METHOD 515.8

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.

4.2.1 Control Options

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 514.8 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 515.8-I. Ensure the test tolerance and calibration procedures for test control are generally consistent with the guidance provided in Part One, paragraph 5.3.2.

MIL-STD-810H

METHOD 515.8

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.

4.5.1 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.

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 control microphones should be no less than 1m (3.28 ft). The distance from the surface of the test article to the control microphones should also be no less than 1m (3.28 ft) unless the pre-test characterization determines there are no structure induced pressure effects on the microphone. 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-half 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.

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.

MIL-STD-810H

METHOD 515.8

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.

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 515.8A-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.