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Direct Field Acoustic Testing
Direct field acoustic testing, or DFAT, is a technique used for acoustic testing of aerospace structures by subjecting them to sound waves created by an array of acoustic drivers.〔''MIL-STD-810G'', w/Change 1, October 2012, METHOD 515.7, ACOUSTIC NOISE〕 The method uses electro-dynamic acoustic speakers, arranged around the test article to provide a uniform, well-controlled, direct sound field at the surface of the unit under test. The system employs high capability acoustic drivers, powerful audio amplifiers, a narrow-band Multiple-Input-Multiple-Output (MIMO) controller and precision laboratory microphones to produce an acoustic environment that can simulate a helicopter, aircraft, jet engine or launch vehicle sound pressure field. A high level system is capable of overall sound pressure levels in the 125 dB to 147 dB for more than one minute over a frequency range from 25 Hz to 10 kHz.〔''Status of Direct Field Acoustic Testing'', Paul Larkin & Dann Hayes, 27th Aerospace Testing Seminar, October 16–18, 2012〕
==Overview==
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.
The technique 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 more conventional reverberant field testing. This fundamental difference and its impact on the structure must be weighed against the advantages of the DFAT method.
An advantage of DFAT testing over reverberant testing is the portability of the DFAT system. This allows the test equipment to be transported to any location, setup, calibrated, used to perform a High Intensity Acoustic Test and then removed from the test site. The entire process from load-in to load-out can be accomplished in no more than 4 days for a large satellite or similar aerospace structure. The test system uses a “building block” approach to form combinations of equipment to satisfy the environmental requirements. Systems typically include 500 plus speakers, 2 million plus watts of amplification, at least 8 to 16 control microphones, and a closed-loop MIMO Acoustic Control and Data Acquisition System. The mobility and “building block” approach allows this method to be tailored for each application and to provide a more timely and cost effective test solution. This method can also be useful for testing articles that are too large to fit inside a traditional acoustic reverberant chamber.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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