DFAT: The Most Versatile Acoustic Test Facility for the Space Industry

In the late 1990s, engineers from the space industry approached Maryland Sound International (MSI). The engineers had a “crazy” request: they asked MSI to reproduce the harsh acoustic noise environment of a rocket launch. The purpose of this scientific experiment was to ensure that space payloads, such as satellites housed in the fairing of a rocket, could survive the extreme vibrations of a rocket launch sound environment. These Overall Sound Pressure Levels (OASPLs) can reach up to 150 dB inside of the rocket fairing. Fast forward 25 years later, MSI has since successfully acoustically tested and qualified over 200 satellites and counting.

Let’s rewind. Up until this time, MSI was one of the largest providers of audio systems for large, loud and live audio events and entertainment. Pink Floyd, David Bowie, Parliament-Funkadelic, Bootsy Collins and many other musicians relied on the expertise of MSI to rapidly coordinate, ship, rig, setup, run, and tear-down premium audio systems for their events. MSI also developed core competencies in designing and manufacturing amplifiers and subwoofers to achieve the desired acoustical performance required by its customers. Lionel Ritchie once said, “A lot of people have bass… but MSI has the thunder!”. It’s therefore no surprise that the adjacent move into the space industry was a natural extension of the core competencies of MSI.

Ever since that first successful Direct Field Acoustic Test, MSI has revolutionized the way acoustic qualification tests are conducted in the space industry. For this reason, in 2016, MSI created its sister company, MSI-DFAT Services, LLC. - the first and only company in the world solely dedicated to providing Direct Field Acoustic Testing (DFAT?) services and equipment. Maryland Sound International, together with its sister company MSI-DFAT has now covered both sides of the “Dark Side of the Moon”!

Revolutionizing Environmental Acoustic Qualification Testing

Acoustic testing is only one major milestone of an overall satellite mechanical environment qualification testing campaign. The other qualification tests in the overall series also include vibration testing on shaker tables (random vibration, sine vibration), pyro-shock testing, and thermal-vacuum chamber testing (TVAC testing), among others.

When the first space race began in the late 1950s, space agencies faced the issue of ensuring that the severe noise caused by the rockets at launch would not damage the payloads. As the field of environmental testing emerged, large and expensive reverberant acoustic testing facilities (RATF) were purpose-built in select geographies of the United States to serve the industry’s needs for spacecraft vibroacoustic testing. These reverberant acoustic chambers were purpose-built to create a uniform and diffuse acoustic sound field.

However, RATFs are extremely expensive to design and build. For instance, they required (i) a special state-of-the-art floor, isolated from the rest of the building to avoid vibroacoustic contamination to adjacent facilities/buildings, and (ii) very thick walls. Also, to reduce sound absorption, special gases, such as nitrogen or dry air, are required to fill these chambers. The system to store the liquid nitrogen and operate it within the chamber only further increases both the building and the operational costs.

From a size perspective, RATFs are also very large buildings. In fact, the size of the acoustic testing facility is directly correlated to accommodate the size of a given test article. Larger reverberant acoustic testing facilities are required to create a more uniform (diffuse) sound field at the lowest possible frequency ranges. This is because low-frequency sound environments have longer sound waves, thus more physical space is required to have large sound waves deflect and diffuse uniformly throughout the acoustic testing chamber’s volume. This low-frequency range, often referred to as the ‘cut-off’ frequency (or Schroeder frequency), is determined by the size of the acoustic chamber. The relationship is thus: the bigger the chamber, the lower the frequency at which the acoustic field becomes uniform (diffuse). For example, in a 10x10x10m (1000 cubic meter) reverberant acoustic chamber, a good uniformity of sound starts at around 60 Hz.

As the space conquest expanded from national government (or “incumbent space agencies”) to the commercial (or “new space”) sector, the necessity of increasing the speed of spacecraft qualification testing programs pushed engineers to look at more efficient ways to complete the test schedules – simultaneously, without having to compromise on accuracy of test results or validity.

From a geographical perspective, the US is set to experience significantly earlier and more growth in DFAT? than other countries, such as Europe, due to two primary reasons: 1) there are more commercial satellites and spacecraft being built in the United States than in other geographies such as Europe, and 2) the territory of the United States is vaster than in other geographies such as Europe. This means greater distances must be covered for transporting spacecraft, which is highly inefficient in a spacecraft qualification testing program. Crossing state borders, insuring spacecraft, and traveling long distances all represent significantly wasted time and cost.

In the old continent, there are at least four large reverberant facilities within a 1000-mile geographical radius - two of which are “test houses”. As one can imagine, the return on investment in constructing a multi-million-dollar state-of-the-art facility for acoustic testing is extremely long, in terms of the payback period. It takes a long time to recuperate the initial investment, as acoustic qualification tests are infrequent. Interim, these facilities often sit idle and unused. It is therefore no coincidence that all the new test facilities being built do not include a reverberant acoustic testing chamber, but instead consider DFAT? as the preferred method of acoustic qualification testing.

Standards for Direct Field Acoustic Testing

The other key consideration when it comes to qualification testing of spacecraft is the need for test standards, guidelines, and reference specifications, which are issued by national and/or international agencies. Engineers, satellite OEMs, and space launch authorities enforce adherence to mandatory acoustic qualification testing guidelines issued by the agencies as ‘peace-of-mind’ policies. For instance, these guidelines are contained in the NASA or ESA handbooks. NASA-HDBK-7010 is currently the main reference available in 2024. It contains a set of test recommendations, and, importantly, the entire bibliography referenced within the handbook is based on work conducted by MSI-DFAT. ESA’s handbook is currently being produced.

In 2018, during the ECSSMET International Conference organized by ESA-DLR-CNES, Dr. Alessandro “Alex” Carrella chaired a special panel of experts on the topic of DFAT?. Experts from academia, service providers, satellite OEMs, and space agencies discussed the state-of-the-art of DFAT? and agreed to set up a working group, coordinated by the European Space Agency (ESA). This working group will contribute to the forthcoming new ESA handbook for acoustic qualification testing. MSI-DFAT is actively collaborating with the space industry to contribute to this important step forward in acoustic testing science, with its 20+ years of experience in spacecraft acoustic qualification testing. There is no doubt that once the new ESA handbook and updated set of guidelines becomes available, the rapid adoption of DFAT? as the de facto standard method for acoustic qualification testing will accelerate.

The Pillars of DFAT?

Whether provided as-a-service or installed as a test facility, there are two essential “core pillars” for DFAT?: 1) the Noise Control System (NCS), which controls the diffuseness of the created sound field, and 2) The Noise Generation System (NGS), which provides acoustic excitation of the UUT. A third element, Vibroacoustic Analysis Software, is not strictly necessary to run a DFAT?, however it can help to gain additional confidence in pre-test analysis by providing an initial estimated prediction of the structural response of the UUT.

The Noise Control System (NCS)

In the early days, MSI-DFAT, together with engineers from NASA and other companies, conducted exploratory direct field acoustic noise tests. Initially, the only available control strategy was to use the already-existing random vibration control for shakers, except for providing an acoustic PSD profile (in Pa2/Hz) as reference. With this method, a single drive was sent to all the speakers, and the reference profile was compared to the average of several accelerometers. The control loop would adjust the drive so that the average would match the reference. Although initial tests proved that this approach was somewhat feasible, as the average of several microphones was close enough to the reference, a more in-depth analysis showed that the difference of the Sound Pressure Level (SPL) between the various microphones was unacceptably high. This non-uniformity of the acoustic field pushed MSI-DFAT to think further outside-the-box.

Over the years, a more accurate and precise control strategy, based on “Multi-Input-Multi-Output” (“MIMO”), has since been developed. Initially, when using a “square control strategy” - meaning there exists an equal number of drive outputs and control channels - test results showed an incredible improvement in the uniformity of the sound field, especially at the control point locations.

The next evolution in the technology of DFAT? has been to adopt the “rectangular control strategy” – meaning the number of control microphones exceeds the number of drive outputs. In this test method, it is possible to control the acoustic field between any two given microphones, in terms of both magnitude and phase, or Cross Spectral Density (CSD), also referred to as “off-diagonal terms”. This is of paramount importance, since this test method allows control of the coherence between the microphones. If the target Power Spectral Density (PSD) at the control microphones is clearly the spectrum required by the launcher, setting a target for the CSD is much less obvious.

MSI-DFAT has found that the most appropriate test method is to control the coherence and to set it to match the profile of reverberant acoustic chambers. In fact, the coherence can be used as an indicator of the diffuseness – since in an ideal diffuse field the coherence is expressed by a sinc-squared function. The result is that the acoustic field generated using MSI-DFAT technology is similar to that of a large reverberant chamber.

Another major aspect of the MIMO noise controller is the computational aspect. When it comes to a narrow-band, usually 3.125 Hz, with MIMO control over a 20-10,000 Hz frequency range, the system must deal with enormous matrices. Optimized control software has been produced to deal with such a large data set to handle: controlling on 24 microphones using 8 drives is nowadays the norm. In addition, recent doctoral studies have created novel algorithms to provide users with a graphic feedback on the quality of the microphone location. Using this advanced algorithms a very uniform field can be achieve with even less control microphones (with consequent less computational burden).

The Noise Generation System (NGS)

The output drives (voltages) that the NCS produces are sent to the 1) amplifiers, which in turn drive the 2) speakers. It is important to note that these two systems are specially designed for conducting DFAT?. Both the spectrum and acoustic levels to which commercial-off-the-shelf (COTS) speakers and amplifiers are designed cannot meet the levels required for properly conducting DFAT?. In fact, these COTS systems are designed to serve the requirements of the entertainment industry for music, concerts, and speeches.

No entertainment event is required to play music as loud as 146 dB or higher. Additionally, the frequency band in which there is the most acoustic energy during a spacecraft launch environment (and test) is the 150-250Hz range. This is exactly the range that any COTS speaker manufacturer tries to minimize and/or eliminate. This is because these frequencies are not “pleasant to hear” from the perspective of human hearing.

It must be understood that COTS equipment is not made for DFAT? testing. Better performance can be achieved by ensuring the speakers and amplifiers are designed to respond to the demanding task of performing satellite and spacecraft acoustic testing. The first factor of consideration is the quality of the acoustic test. An appropriate NGS will “make life easier” for the NCS. A system with DFAT?-optimized speakers and amps will have enough power to reach the required acoustic test levels without exceeding the technical limitations of the equipment.

Operating a NGS at or near its limits is not ideal or acceptable for a DFAT?. The optimal design approach to a NGS is to have high Acoustic Power Density (APD) so that the NGS performs far away from its limits. The nonlinearity of the amps and speakers, plus the thermal effects, would create unstable conditions. This is because the higher the current transmitted through the coils of the speakers, the hotter the speakers get, and the lower the APD. This undesirable effect is known as “thermal compression”. Detecting a lower SPL, the NCS would generate a high-voltage drive signal, which would then further exacerbate the thermal compression effect. Eventually, it would become impossible to reach a uniform and diffuse sound field at the required OASPL, which is a pre-requisite to ensure accurate test results.

The second benefit of using DFAT?-optimized NGS is the cost-benefit. An optimally designed noise excitation system will require fewer amps and speakers in the test setup, because they will all be operating optimally. In contrast, an oversized noise excitation system, consisting of many more amplifiers and speakers in the test setup, represents purely wasted cost.

MSI-DFAT, with its 20+ years of acoustic Research, Testing, and Development (RTD), has innovated amps and speakers specially designed to operate precisely for their intended task: acoustic testing for the spacecraft industry.

Apart from the purely technical acoustic matters of spectrum and noise levels, there are also safety aspects related to the speakers that cannot be underestimated. When configured for satellite testing, the speakers are arranged in vertical columns, or “stacks,” placed in a circular formation around the test object. Even in standard configurations (i.e., for most mini or small satellites) these stacks are commonly 4-5 meters (13-16 ft.) tall. However, MSI-DFAT has been testing multiple spacecraft objects that require the stacks to be far greater than 8 meters (26 ft.) tall setting the world's record of 41.5 ft stack of loudspeakers!

Take a step back to think about the risks… There are a few hundred-thousand dollars of specialized audio equipment, standing 6-8 meters tall, around a test object, worth a few hundred-million dollars! Nobody would want these towers of speakers to fall onto the satellite or spacecraft and damage it - not even if an earthquake strikes! For this very reason, MSI-DFAT has been designing its durable rigging and support systems for the speaker walls to be resistant to the seismic requirements of California (one of the world's most stringent) ensuring their stability and safety during testing, even in the event of an earthquake.

In conclusion, when it comes to DFAT? for the space industry, MSI-DFAT has learned and demonstrated - from the amplifiers to the speakers, to the support systems – that an optimized and purpose-built NGS provides the most reliable and safest way to conduct DFAT?.

Vibroacoustic Analysis

Finite Element Analysis (FEA) is applied across engineering for two main reasons: 1) to optimize structural design, and 2) to predict the behavior of structures when subjected to forces. However, due to each spacecraft's uniqueness and the high costs of creating validated models, FE Modeling (FEM) isn't widespread in the space industry. Experimental testing remains crucial before launch.

When it comes to new designs or testing methods, engineers also seek peace of mind through predictive calculations to anticipate outcomes - especially with novel techniques like DFAT?. Running simulations before test setup and execution is therefore also a critical step. However, while vibroacoustic analysis can certainly aid predictions, it's not a mandatory step for a DFAT? campaign.

Simulating the acoustic test for satellites and spacecraft to mimic the acoustic field within reverberant chamber conditions can be accomplished whether using FEM or Boundary Element Method (BEM). The historical advantage of a reverberant chamber lies within the uniformity of its sound field. However, modeling a non-perfectly diffuse sound field, such as that produced in a DFAT? test, is more challenging. To tackle this, MSI-DFAT partners with the major providers of simulation software to enhance vibroacoustic analysis and simulate a non-perfectly diffuse acoustic field with confidence.

The accuracy of the structural and acoustic system models significantly influences predictability. The time and effort needed to validate the model can make this step expensive. Vibroacoustic analysis may be part of specific use cases, but is not mandatory for testing.

Research, Testing & Development (RTD)

Apart from these technological components, a successful DFAT? also requires “know-how” in properly running a direct field acoustic test.

Performing DFAT?-as-a-service demands unique knowledge and skills, from managing complex logistics to properly setting up microphones. Think, for example, of the complex logistics of moving trucks full of equipment and personnel to the location of the test, or quickly responding, adapting, and re-meeting the seemingly random changes in testing dates due to program slips.

MSI-DFAT is the trusted DFAT? service provider to the spacecraft industry. Beyond this, MSI-DFAT is also a DFAT? equipment systems provider. However, we are also much more than a service provider or vendor of DFAT? equipment. The 200 (and counting) satellites acoustically qualified by the MSI-DFAT team have created unique and inimitable know-how and expertise that has evolved through the years. MSI-DFAT can therefore best serve the comprehensive acoustic qualification testing needs for the satellite and spacecraft industry.

Conclusive Remarks

The comparison of Reverberant versus Direct Field Acoustic Test has been carried out in multiple studies. MSI-DFAT, NASA, ESA, Airbus, and others have published a NASA technical report based on a comparative study done with the European Service Module (ESM) of the Orion program. Thales Alenia Space and ESA have also run a similar study on a reflector. The common conclusion is that DFAT? is a proven and valid alternative to the traditional reverberant acoustic chamber. It is also important to note that nowadays, direct field acoustic noise tests are also run in the same fashion as in reverberant acoustic testing chambers. Empty-chamber-run (or a circle, in the case of DFAT?), satellite-in, and test run(s) at different levels.

A Comparison of DFAT to RFAT (RATF)

Pros of DFAT

Versatile System

• The "base" DFAT? system setup can be expanded in size and customized to accommodate larger test objects (up to 20 meters tall!).
• The setup is portable, enabling tests to be conducted at multi-purpose facilities.
• Operating costs are minimal, requiring only electricity.
• Operating processes are straightforward.

Better Control, More Possibilities

• MIMO narrow-band control ensures uniformity of the acoustic field from 20 Hz; MSI-DFAT has the only DFAT? system that can control diffusivity, creating a sound field that mimics a reverberant chamber.
• The possibility to include accelerometers in the control helps limit the amount of vibration on the structure.
• The system can perform numerous test runs over a long period of time (i.e., no limitation imposed by the excitation system – switch on and test!).
• Capable of producing and controlling higher sound levels up to 10 kHz.
• Maximum Overall Sound Pressure Level (OASPL) of over 150+ dB is now achievable.

Cons of DFAT

Newer Technology

• Despite its relative novelty, MSI-DFAT has addressed questions raised by the environmental testing community through the qualification of over 200 satellites using DFAT?.
• Agencies are currently developing updated standards and handbooks to aid engineers and provide guidelines; NASA-HDBK-7010 is the initial issue scheduled for an update, with MSI-DFAT actively collaborating with ESA on their guidelines.

Limitations

• Test spectrum and other specifications such as coherence may limit system capability.
• The maximum Overall Sound Pressure Level (OASPL) and test duration are constrained by transducers.

Diffusivity of Sound Field

• Unlike a reverberant chamber, each DFAT? system produces different sound fields and responses, requiring the configuration and selection of the appropriate system.
• Various simulation and modeling tools are only recently being developed.

Pros of RFAT

Proven Technology

• RFAT has been utilized for the past 50 years.
• Requirements for uniformity and diffusivity stem from tests conducted in reverberant facilities.
• Diffuse acoustic fields are easier to model with currently available simulation tools.

Highest Noise Level, Largest Structures

• Reverberant facilities can be designed to reach over 160 dB of Overall Sound Pressure Level (OASPL).
• The room can be designed to accommodate test items of many sizes.

Cons of RFAT

Expensive

• RFAT entails significant upfront investment and ongoing operating costs.
• Constructing a complete reverberant chamber requires a substantial costly initial investment.
• Operating an RFAT facility entails high recurring and ongoing operating costs (i.e., gas, pressure systems, etc.) and demands highly specialized, skilled personnel.
• The facility cannot be utilized for any alternative purpose, resulting in 'dead' space and downtime.

Poor Control at Low Frequencies

• Acoustic modal density only becomes significant enough to ensure a uniform field above a certain cut-off frequency (Schroeder frequency), determined by the size of the chamber; the smaller the room, the higher the cut-off frequency.

To conclude, the purpose of this document is to provide an overview of the why, the what, and the how of Direct Field Acoustic Testing (DFAT?). This has been accomplished by the tremendous collective organizational knowledge and experience that MSI-DFAT has accumulated during the acoustic qualification of more than 200 satellites. Inimitable process and product R&D and expertise has also been acquired during the time in-between direct field acoustic tests, where the MSI-DFAT team uses their in-house lab facilities in Maryland to experiment, improve, and optimize each component of the DFAT? system and science.

Over the coming decades, MSI-DFAT is confident that satellites and spacecraft will be routinely acoustically qualified using DFAT?. The handbook that the agencies are working on will only be a boost to this unstoppable trend, which is a growing reality in the United States, and is becoming increasingly accepted globally.?

Contact:

Robert Goldstein

Founder & President

Dr. Alessandro 'Alex' Carrella

VP Strategy & Growth

[email protected]

Bradley Hope

U.S. Business Development Manager

[email protected]

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https://www.msi-dfat.com/

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