Direct-field acoustic testing
# 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. 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 125–147 dB for more than one minute over a frequency range from 25 Hz to 10 kHz.

## 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 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.

## Process

The process requires the transport to and assembly of a speaker circle around the test article. The size of the circle is dependent on the size of the test article. Generally, a circle 12 feet (3.7 m) in diameter larger and 4 feet (1.2 m) taller than the test article is required. 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. 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 1.0–1.5 meters (3.3–4.9 feet). The distance from the control microphones to the surface of the test article should be 0.5–0.75 meters (1.6–2.5 feet). 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 DFAT 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. 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 10 kHz for a 1/4" microphone, and is inversely proportional to microphone diaphragm diameter.

The speakers are driven by a series of audio amplifiers that are usually powered by a portable diesel generator. The system is safely and accurately controlled by a closed-loop feedback control system that can be used to limit and/or abort if an over-test condition is detected.

A pretest is usually performed using a simulator to confirm the specified overall sound pressure level and spectrum can be achieved. The pre-test is also used to verify any special control features such as; abort tolerances, response limits, field shaping and emergency shut-down procedures. The microphone responses should then be examined to evaluate the resulting field for uniformity, coherence and if available, structural response. Then the simulator is replaced with the actual test item in the speaker circle and the test process is repeated.

The entire operation is usually completed in four days, and only requires the test article for one of those days. All equipment is brought to the test article, assembled, pretested and performance-checked before testing the flight article. The flight article is generally required for only one day of testing depending on the complexity of the test plan. On completion of the flight test, the article is removed and all equipment is disassembled and transported from the site.

    Features
* Portability: can be set up almost anywhere
* Modularity: adapts to multiple configurations
* Controllability: safe, repeatable, real-time control
* Narrow-band control: can provide control to a spec defined at constant, narrow-band (~3 Hz) increments from 25 to 10 kHz
* Response limiting: limit on SPL, acceleration, force or stress response values
* Over-test protection: abort on peak or rms input values

## Capabilities

The convenience, low cost, and mobility of this method distinguish it from conventional testing and are the primary reasons for its growing popularity. The method is convenient because all required sound system, power generation and distribution and data acquisition and control equipment is brought to the test site. Equipment is usually leased for each test event. There is no large investment in a facility, equipment or personnel required on the part of the customer. A diesel generator is the preferred power source, therefore providing clean on-site electrical power in a consistent configuration for connection to the MSI power distribution equipment. This removes the demand for large quantities of power from the test facility. In addition, testing can be performed at a much lower cost per test compared to the installation, operation and maintenance of a more standard high intensity reverberant acoustic chamber system. Finally, mobility allows this test method to be performed at almost any time and place in the normal test article integration and test flow. The test equipment is completely portable and no special facility or infrastructure is required.

