515.8-6 DIRECT FIELD ACOUSTIC NOISE CHARACTERISTICS
Closed-loop, digital control is preferred for all direct field testing. Since the drivers used are capable of responding over the entire test bandwidth (usually 25 to 10kHz) and beyond, narrow-band drive signals are often used to control the test. Narrow-band control allows all features of modern, random vibration control systems such as; control of local resonances, response limiting, peak limits/aborts based on spectral lines out, and rms limits/aborts to be used for acoustic testing. See paragraph 6.1, references d and e, for more information about narrow-band control.
Single Input, Single Output (SISO) control will produce a well correlated sound field since the same drive signal is delivered to all audio devices. However, sound pressure level (SPL) variations due to wave interference patterns in the SISO field can be as large as + 12dB from the average SPL due to constructive and destructive wave combinations. In addition, multiple microphone averaging can exacerbate the problem by allowing large variations at the control points to result in an apparently well controlled composite when compared to the reference. Typical performance in the SISO environment is + 1dB variation between the reference and the composite control average with + 5dB of control microphone to control microphone variation and + 12dB or more between monitor microphone locations.
The recent application of MIMO (Multiple Input,-Multiple Output) acoustic control to the DFAN process has created a much improved methodology over the earlier practice of using a SISO approach. The MIMO process is based on multiple, independent inputs, multiple references and results in multiple independent drive outputs. Using MIMO control the user can input magnitude, phase and coherence specifications with tolerance bands on each. The system will use those constraints and the independent drives to produce a complying environment at each control point. In effect, this method controls the response of each control microphone to meet its individual requirements based on the input it receives from each independent drive signal. The result can be an incoherent field with minimum variation between control microphones. This represents a huge improvement in field uniformity (spatial variation) as well as providing a sound field with much lower coherence. Typical test tolerances for a MIMO controlled test are + 1dB on
MIL-STD-810H
METHOD
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.
- 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
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a. NATO STANAG 4370, Environmental Testing.
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b. NATO Allied Environmental Engineering and Test Publication (AECTP) 400, Mechanical Environmental Testing, Method 402, Acoustic Noise.
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c. ISO 266, Acoustics – Preferred Frequencies, International Organization for Standardization, 27 March 1997; http://www.iso.org.
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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.
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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.
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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.
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/.
MIL-STD-810H METHOD 515.8
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MIL-STD-810H