FAQ: Direct Field Acoustic Testing (DFAT?)

Q: What is DFAT??

A: DFAT?, or Direct Field Acoustic Test?, is an approved test method using high-intensity sound as a vibro-acoustic load applied to a Unit Under Test (UUT). DFAT? involves recreating a sound field using loudspeakers arranged in a circular formation around the UUT. The scope of such a test is to excite the test article with a diffuse and uniform sound field. The term DFAT? is interchangeable with Direct Field Acoustic Noise Testing, or “DFAN Testing”.

Q: Where is DFAT? applied?

A: Primarily used in the space industry, DFAT? tests spacecraft, space payloads, satellites, and even space stations, ensuring they can withstand extremely harsh vibrations caused by rocket launches. It's a cost-effective and economical alternative to traditional reverberant chamber acoustic testing facilities (RATF).

Today’s acoustic testing standards require a DFAT? test to closely approximate a reverberant field acoustic test. True Multiple-Input Multiple-Output (MIMO) control of the sound field, as is provided by MSI-DFAT?, is the only proven way to achieve this requirement. It is noteworthy that DFAT? has also been used for testing rocket engines, missile components and even aircraft electronics. The versatility of the DFAT? system allows tailoring to meet specific test requirements.

Q: Where is DFAT? conducted?

A: This technology does not require any special facilities. It is conducted wherever it is most convenient for the customer, as long as there is enough floor space to setup the loudspeakers around the UUT. Most typically, tests take place inside of cleanroom facilities, often directly onsite at the location of the UUT. The portable nature of DFAT? eliminates the need for off-site reverberant chamber acoustic testing facilities, and further reduces risks and inefficiencies associated with shipping expensive and mission-critical test articles.

Q: What makes a DFAT? system?

A: There are 3 main parts of a DFAT? System:

1. The Noise Generation System (NGS), composed of speakers and amplifiers that generate sound. Special speakers and amplifiers are designed by MSI-DFAT? to produce these unique sounds.
2. The Noise Control System (NCS), which controls the loudness of the speakers, as well as the frequencies of sound generated. This creates the desired unique sound field with its required diffuseness.
3. A team of experienced professionals. The testing of space hardware requires a team of highly-trained specialists. Given the multi-disciplinary nature of DFAT? (a mix of acoustic, control and aerospace engineering), experts are needed to design, run, and improve DFAT? methods and systems over time.

With expertise in all of these areas, MSI-DFAT? has invented and perfected DFAT? over the past 25 years. MSI-DFAT? has qualified over 200 spacecraft and launch vehicle structures!

Q: Does MSI-DFAT? provide equipment for conducting DFAT??

A: Yes, MSI-DFAT? provides specialized loudspeakers and amplifiers designed for DFAT?, offering flexibility and modularity to meet diverse frequency spectrums. MSI-DFAT? has chosen a flexible, modular, and tailored design approach where the subwoofers and the mid/high-frequency drivers are housed within different cabinets. These speakers and amplifiers allow the widest range of launch frequency spectrums to be met. For instance, some frequency spectra need more low-frequency power, while others require more high-frequency power. In other words, MSI-DFAT? systems allow recreation of the most realistic sounding and widest range of rocket launch sound pressure environments.

Q: Can you provide some advantages of MSI-DFAT? equipment for DFAN testing?

A: MSI-DFAT? equipment allows precise testing across various frequency ranges, adapting to specific testing needs. Take, for example, if one must run a test from 20 Hz – 30 kHz (yes, we have done that for a customer!), additional speakers and amplifiers are used to provide sound above 20 kHz. Standard audio devices only provide frequencies from 20 Hz to 20 kHz, within the range of human hearing. In some cases, there may not be any need to use subwoofers. The NGS must be modular to meet all the different possible combinations of requirements.

Additionally, the MSI-DFAT? Noise Control System (NCS), based on true Multi-Input Multi-Output (MIMO) control, ensures recreation of a realistic acoustic field, capable of matching reverberant chamber conditions. The MIMO controller allows one to define the desired acoustic field to be created by setting a target matrix. This target matrix includes the Sound Pressure Level (SPL) at each control location, and the relationship between each pair of microphones (e.g., coherence) to ensure that the field approaches either a diffuse field, or other desired target field.

Q: Has DFAT? technology and/or techniques changed over time?

A: Yes, DFAT? has improved significantly over time. MSI-DFAT? pioneered DFAN testing in the ‘90s. Back then, the NGS and NCS had technical limitations. For example, acoustic fields were not loud enough (unable to achieve louder than 143 dB) or diffuse (uniform) enough. Modern DFAT? systems produce louder (greater than 150 decibels Overall Sound Pressure Levels) and more diffuse (uniform) sound fields.

MSI-DFAT? systems are also now able to continuously run tests for extended periods of time. For instance, the MSI-DFAT? system can run acoustic fatigue tests, which require hours of exposure to acoustic loading - for example, a 140 dB test for 2 hours, without interruption.

DFAT? technologies and techniques keep advancing to meet the testing needs of industry. MSI-DFAT? works very closely with customers, industry partners, and academia to further the evolution and innovation of DFAT? technology.

Q: Are all DFAT? technologies the same?

A: No, MSI-DFAT? stands out as the inventor and pioneer of DFAT?. MSI-DFAT? owns more than 17 patents on this proprietary technology. Alternative test methods and technologies have limitations, as compared to DFAT? by MSI-DFAT?.

Our technology, utilizing true narrowband MIMO control, closely matches a reverberant chamber, providing the most realistic testing environment. In other words, true MIMO control is the only way to achieve an acoustic field very close to that of a reverberant chamber, and this provides the most realistic launch environment possible for testing.

Finally, the experience and knowledge of the experts at MSI-DFAT? is unrivaled. MSI-DFAT? has more than 30 years of Research, Testing, and Development (RTD) and hands-on experience conducting Direct Field Acoustic Tests globally.

Q: What is a MIMO controller?

A: A MIMO, or Multi-Input Multi-Output, controller is an advanced sound control system used in DFAT?. It allows precise specification of sound levels and relationships between microphones, ensuring a diffuse (uniform) sound field. With a MIMO controller, the targets include the sound pressure level at each microphone location as well as the desired coherence between each pair of control microphones (of course that can be physically realizable).

Only by controlling the coherence to a specific shape (called the sinc-squared function) can a diffuse sound field be generated. This is only possible using narrowband control, since both magnitude and phase information are necessary.

In summary, a narrowband MIMO control system is the best way of coordinating all the speakers and mics together to achieve the desired diffuse acoustic field. This is significantly better than just having individual speakers run independently using an octave-based control system.

NOTE: While it is possible to have multiple microphones and multiple speakers all running independent control loops, this is NOT a MIMO setup. This is because there is no target to define the relationship between the two microphones. The result is not a diffuse (uniform) field. Instead, this results in a set of uncorrelated drives, and the final acoustic field depends only on the test area and the test article. This is not the appropriate or ideal conditions to conduct vibro-acoustic qualification testing.

Q: Where should sound control microphones be placed?

A: Control microphones should be spread randomly throughout the test area at different heights, distances, and angles. Vibroacoustic modeling software can be used to optimize their locations, ensuring the most uniform sound field before test setup, further enhancing accuracy and efficiency. MSI-DFAT? engineers understand the acoustic field and expertly place microphones to ensure a reliable and safe test.

Q: Are there areas where sound is louder or quieter during a Direct Field Acoustic Test (DFAT?)?

A: DFAT? minimizes natural variations in sound levels in the acoustic field by using real-time MIMO control and monitoring microphones throughout the test area. This helps to minimize louder "hot spots" and quieter "cold spots". Unlike reverberant acoustic chambers, DFAT? does not suffer from spatial non-uniformities at low frequencies, instead creating the most uniform sound field possible. The advanced control methods used in DFAT? aim to create the most uniform sound field possible at all points - not just at the control mics locations. Monitoring mics placed throughout the test area helps to validate the uniformity of the sound field.

Q: If the sound field is controlled at specific microphone locations, what happens at other untreated locations within the test area?

A: Monitoring microphones placed randomly throughout the test area ensure uniformity in untreated locations. Advanced DFAT? systems use optimization and predictive algorithms, ensuring monitoring mics measure the same sound levels as control mics in treated areas.

Q: Can DFAT? be used if my test article has certain components that cannot exceed a certain vibration level?

A: Yes, MSI-DFAT? can attach accelerometers to monitor vibration levels on sensitive parts of the test article. The accelerometers serve as limit monitoring channels, with defined limit profiles for their vibration response over specified frequency ranges. If limits are exceeded, the MIMO controller adjusts vibro-acoustic excitation at only the specific narrow frequency bands by modifying the drive spectral density. This protects the sensitive components of the UUT, while also minimizing changes to the overall sound field.

Q: What does it mean when the sound field has low coherence?

A: Coherence in DFAT? indicates how similar sound is at different microphone locations. For DFAT?, the goal is to produce a "diffuse sound field" that closely matches reverberant acoustic chambers. For a diffuse acoustic field, coherence should vary with frequency and distance between mics. Low coherence everywhere is not ideal. An analysis method called "Sinc Indicator Function" (SIF) quantifies how close the field is to ideal diffuse conditions. In a "Diffuse Acoustic Field" (DAF), coherence is a function of frequency and the distance between 2 microphones being analyzed, and a diffuse field is what the space industry generally requires (as written in launcher manuals).

A diffuse field, such as that approximated in reverberant chambers, has a coherence with a specific mathematical form, sinc2(kr), where k is the wavenumber and r is the separation distance. For that reason, it is most appropriate to plot coherence against kr (k contains the frequency and r the distance). So, the coherence in a diffuse field should not always be low. Rather, at high values of kr (high frequencies and microphones farther apart), the coherence will approach zero, and at low values of kr (low frequencies and microphones closer together), the coherence will approach 1.

In a diffuse acoustic field, the coherence should not be low when kr is low. The "Sinc Indicator Function" (SIF) is another metric that can help to quantify how closely a specific acoustic field is to a theoretical diffuse acoustic field. A SIF of 1 means the measured acoustic field is diffuse.

Q: What is next in DFAT??

A: Sorry, we can’t tell you yet! However, since MSI-DFAT? invented it over 25 years ago, we continue to perfect it. Our R&D department is continually working on innovating this technology.

Stay tuned!

For additional questions, please contact [email protected]

?? ?????????? ????