Structural and Practical Implications of Direct Field Acoustic Testing on Host Facilities: A Comprehensive Overview

Cassandra Dutt – Acoustic Research Systems

Abstract

Direct Field Acoustic Testing (DFAN) has been growing in popularity due to the  logistical advantages offered by the portable acoustic test solution. While a  significant number of studies have been performed to quantify the acoustic  field created using DFAN technology, there has been no discussion regarding  the overall impact of DFAN testing campaigns on host facility operations. This  paper will present a detailed overview of the structural and practical  implications of the acoustic load generated by a DFAN test on a host facility.  Findings presented in this paper serve to inform best practices the protection  of sensitive flight hardware and facility personnel during testing campaigns.

Introduction

Since the early 1990’s Direct Field Acoustic Testing (DFAN) has been adopted  as a method for the acoustic qualification of spacecraft and flight hardware.  DFAN offers significant logistical advantages over traditional reverberant  chamber (RFAT) testing. By utilizing portable acoustic devices that can be  easily transported and rapidly assembled on-site, DFAN transforms any  suitably sized room into an effective acoustic testing facility. This method  eliminates the need to transport flight hardware to an external testing venue,  thereby providing notable benefits in terms of project budget and schedule.

Despite its advantages, the use of DFAN testing raises concerns about the  potential impacts on the host facility’s structural integrity. To date, no studies  have addressed the effects of acoustic load on building structures in the  context of DFAN testing, and there have been no reported structural failures or  damages directly attributable to DFAN. The portable nature of DFAN systems  often means they are used in facilities where other manufacturing or testing  activities are also ongoing. Without proper consideration of facility impacts  during the pre-test planning stage, the use of DFAN systems can lead to  logistical challenges that affect facility operations. For the purposes of this  paper, the term “facility impact” is categorized into three major areas of  concern frequently raised by facility managers and test directors during DFAN  test campaign planning:

• Building Structure: Assessing whether the acoustic energy produced  during testing could affect the building’s structural integrity
• Adjacent Flight Hardware: Evaluating the potential for DFAN testing to  interfere with or damage nearby flight hardware
• Personnel Noise Exposure: Ensuring the safety and comfort of  personnel working in or near the testing area

Each of these influences the overall operations of a host facility during a  testing campaign. Proper consideration and planning are essential to mitigate  the impact on adjacent flight hardware and to safeguard personnel. The  success of a DFAN testing campaign depends on the host facility’s readiness.

This paper aims to provide an overview of the impacts of DFAN testing on host  facilities, offering a framework for consideration in future test campaigns and  guiding beneficial design choices for new testing facilities. Because DFAN test technology and control strategy are discussed thoroughly in many  publications, they will not be the focus of this paper.

3. BUILDING DESIGN CRITERIA   

The development of rocket technologies introduced a set of distinctive  challenges for the structural design of facilities near launch pads and launch  support structures. Blast and acoustic loading generated by the intense,  dynamic environment of the launch became environmental design  considerations for buildings, alongside better-known wind and seismic loads.  Consequently, manuals for sonic and vibration environments were authored  with launch facilities and their surrounding structures in mind [11] [2.]

Most facilities proposed for DFAN testing are not located within the vicinity of  an active launch site. Because modern US and European building codes do  not include requirements for acoustic load in building design, it is likely that  acoustic loading was not a formal consideration when determining the critical  design forces for typical building structures. Thus, it is understandable that the  primary concern of facility managers regarding DFAN testing often pertains to  the structural integrity of the building.

This concern seems to be rooted in misconceptions about the nature of  acoustic loading, standard building design recommendations, and structural  failures due to forced resonance.

The acoustic environment created by a DFAN system fundamentally differs from the over pressures caused by blast or sonic loads. Blast waves are a  continuously propagating pressure pulse with significant bulk air flow. The  acoustic load generated by DFAN testing can be represented as an omnidirectional, randomly oscillating pressure load where little to no bulk flow  of air is imparted by the waves. Consequently, acoustic design pressures are  much lower than blast pressures.

One of the first steps taken in the structural design of buildings is the specification of environmental load criteria for both wind and seismic conditions. The lateral force resisting system for the building will then be designed to meet both material strength and serviceability (i.e. deflection & drift) criteria under the calculated environmental loads. It can be generally accepted that designing for the following lateral and environmental loads effectively addresses any acoustic loading from DFAN testing:

• Wind loads: The frequency spectrum of wind loads has been reported to be between 0.001 and 0.2 Hz. Because the typical resonant frequency of low-rise building structures is greater than 1 Hz, the dynamic nature of wind loads is deemed negligible. This is why wind pressures are then applied as an equivalent static load despite being based on maximum loads during momentary gusts [14.]
• Seismic loads: Earthquake loads are dynamic loads that typically have maximum energy between 0.01 and 10 Hz. This range overlaps with the common natural frequency range for low-rise buildings. Buildings designed to withstand seismic loads will have a more resistant dynamic response to vibro-acoustic loading [1]
• Human induced vibrations: Human induced vibrations, such as walking or rhythmic excitation, are sometimes considered in the vibration serviceability design of building structures. Structure serviceability, stiffness, and resonance are dominant considerations in developing designs that support human comfort by limiting movement due to human induced vibrations [9.]

Structures like those common in aerospace manufacturing applications typically have resonant frequencies above 1 Hz, categorizing them as rigid structures, but below the frequency spectrum of a DFAN system (<20 Hz). Below the first resonant frequency, the motion of the entire building structure is controlled by the static stiffness of its load carrying members and associated foundations. Current speaker technology cannot generate enough sound power to adequately excite building superstructures at their resonant frequency, regardless of overall sound pressure level achieved.

For these reasons, design recommendations for acoustic loading are often specific to walls, ceilings, and roofs which can act as large flexural absorbers. Flat panels exhibit greater deflections under acoustic load and are therefore at greater risk for structural damage and material fatigue. Scale model studies of building superstructures subjected to broadband acoustic loading have shown negligible movement, with the most significant acoustic mobility observed in walls between column supports [14.]

Figure 1 – Diagram of Acoustic Loading Considerations for External Building Wall Design [14]

In summary, while DFAN testing presents specific design considerations for elements susceptible to flexural stresses, the overall impact on building superstructures is negligible

4. SOUND PROPAGATION   

To better understand the sound field generated by DFAN testing and how it propagates within a facility, it is important to review some fundamental concepts of acoustics and sound propagation. While these concepts are extensively documented elsewhere, a brief overview is provided to contextualize their relevance to DFAN testing and discuss how these concepts may be used to make predictions of sound attenuation within host facilities.

4.1 Physical Characteristics   

Directivity refers to how an acoustic source distributes sound energy across different angles. The directivity of a device varies with frequency and is dependent upon the design of the acoustic source or speaker. Ideally, the design of acoustic devices used for DFAN corral the sound energy towards the test article positioned inside of the test circle and limit the loss of energy from rear rejection of the device. Directivity of the acoustic source plays a key role in understanding how sound generated by the source will propagate.
There are two physical acoustic regions to understand when evaluating sound propagation from a DFAN system: the near field and the far field. The near field is defined as the region closest to the acoustic device.

Behavior in the near field is complex as there is no fixed relationship between distance and sound pressure. In this region, sound pressure and intensity will vary unpredictably with distance due to interference patterns and complex interaction of sound waves, making behavior difficult to predict. In the acoustic far field, sound pressure levels decay with distance from the source according to the inverse square law, making sound propagation in the far field is much easier to predict.

• Characterize the sound field in the test area outside of the DFAN test configuration
• Quantify the response of a large flexural absorber, representative of a typical solar array, both inside and outside of the test configuration
• Monitor personnel sound exposure in commonly occupied areas of the facility during DFAN testing

The amount of information gathered as a part of this exercise was significant, to certain findings have been omitted for the sake of brevity. Future publications are planned to expand upon the balance of information collected.

Figure 4 – Neutron System – Top View

Other commercially available DFAN solutions utilize separate low frequency and mid/high frequency devices whose design fundamentally differs from that of the Neutron. As the system configuration, drive methodology, and speaker directivity between systems vary, results reported in this study may not be fully representative of all DFAN methodologies. Further study is required to determine what, if any, variation may exist in the sound propagation and associated facility response from different DFAN technologies.

4.6 Test Facility

This study was performed in a pre-engineered metal panel building approximately 160’L x 120’W x 36’H.

Figure 5 – Facility Photo

The facility superstructure is a structural steel frame with a metal deck and blanket insulation roof and corrugated metal panel walls. This type of construction is commonly used for aircraft hangers, manufacturing facilities, and warehouses. Note that the facility is on an airfield which can contribute to the overall ambient noise within the facility during take-off and landing operations.

4.7 Test Configuration and Item Under Test

A DFAN test configuration comprised of 24 Neutron104 acoustic devices were arranged in 8 stacks of 3 Neutron104s placed at 45-degree radial intervals. The overall height of the Neutron104 stacks was 11’-6”/3.5m. See Figure 4 for test configuration. The center of the test configuration was located as shown in Figure 7.

The item under test (IUT) was a 4’-0”x8’-0” aluminum honeycomb panel. The panel was mounted with offsets at 4 points to a custom test fixture constructed from 80/20 aluminum framing. The total height of the panel on the test fixture was 7’-0” tall. The panel was instrumented with 5 tri-axial accelerometers and 2 surface mounted microphones. A monitor microphone was mounted to the panel frame to quantify the acoustic field at the panel. See Figure 6 for panel instrumentation configuration.

Figure 6 – Panel Instrumentation

One control microphone was placed in front of each Neutron device for a total of 24 control microphones which are identified in Figure 4.

The test profile used had a maximum OASPL of 141.7dB. The total run time at full level was 60 seconds.

To account for dissimilarities between commercially available DFAN methodologies, two discrete control methods were employed during the study. Control strategy A was defined as “incoherent” with each of the 24 Neutrons acting as independent stochastic acoustic sources where each device received a unique drive signal. Control strategy B was defined as “stack coherent” with each of the 8 vertical columns of Neutrons receiving a single drive output.

To study the response of the panel as it moved away from the DFAN configuration, the IUT was placed in 4 discrete locations within the test area as shown in Figure 7. Microphone and accelerometer data were collected for the panel using both control strategies A and B at each location.

Figure 7- Panel and Microphone Locations

A series of 4 microphone “trees” were constructed with 3 microphones spaced about 5’-0” / 1.52m on center vertically. During the test runs, the microphone trees were translated around the space, gathering field measurements in locations denoted with “ML#X” in Figure 7. One additional microphone was mounted to the hoist hook located over the center of the test configuration to quantify the field above the setup.

It should be noted that the temperature was 91°F/33°C with a relative humidity of 50%. As different frequencies attenuate differently through air depending upon temperature and humidity, results from this test could vary in climate controlled facilities.

Noise dosimeter readings were collected in spaces that are commonly occupied during routine DFAN testing, indicated in cyan in Figure 7.

The review of measurements taken at monitor microphone positions outside the DFAN test configuration revealed several key observations. Table 1 summarizes the average OASPL level of the top, middle, and bottom microphones positioned at each monitor location specified in Figure 7. For the sake of clarity, only measurements taken using control strategy A have been included in the table. Monitor locations have been sorted by distance from the center of the test configuration.

The average OASPL at the back face of the Neutron acoustic devices (ML#1 and ML#2) was 9 to 10 dB lower than the test spectrum level. This large reduction in OASPL over a short distance can be partly explained by the directivity characteristics of the Neutron, which restricts sound emission in the rear direction [4]. The average at these locations is skewed as the bottom monitor microphone recorded higher levels than the top monitor microphone with deviations ranging from 3 to 5 dB for control strategy A and 6 to 8 dB for control strategy B. This is likely due to the sound reflecting off the concrete floor near the test configuration. At monitor positions further from the test configuration, the variation in sound pressure levels between top, middle, and bottom microphones were less pronounced.

Table 1 – OASPL at Monitor Locations

Outside of the test configuration, sound pressure levels did not consistently decay per the inverse square law due to the host of previously discussed factors. Generally speaking, at least 15 dB of attenuation was noted about 50 feet from the center of the test configuration. At over 100 feet, the measured attenuation was between 19-20 dB.

Minor discrepancies in sound pressure levels between control strategies A and B were observed. Tests conducted with control strategy B, which employed a vertically coherent drive strategy, consistently resulted in higher OASPL compared to control strategy A, which used a fully incoherent drive strategy. Monitor locations further from the test configuration exhibited less variation in sound pressure levels between control strategies A and B.

Results of this exercise were used to study the transmission of sound between the test area and the technician control room to inform future practices for pre-test predictions.

The Sound Transmission Class (STC) rating was chosen as the metric to evaluate the performance of the partition between the test area and the control area. STC is a laboratory measured, single number metric that defines the anticipated sound transmission loss (TL) across a barrier or partition in decibels at mid-frequencies (125 – 4000 Hz) [7] STC ratings are determined based upon construction type. While the STC value is not a full characterization of transmission loss across all frequencies, it can serve as a basis for determining anticipated sound pressure levels across a partition.

The partition between the test area and the control area is a full height, cold formed metal stud wall with a single layer of gypsum board on each face. This type of wall construction has a published STC value of 37. This effectively means that the anticipated drop in sound pressure level over the partition would be 37 dB for mid frequencies. The lowest STC values published for interior partition and door constructions is 27 dB [12.]

A review of the data illustrates the variation in sound transmission properties for the partition at varying frequencies. At and below 80 Hz, the partition construction is significantly less efficient in blocking sound energy with less than 20 dB of attenuation across the partition. At the highest frequencies, the partition was highly effective in blocking sound as illustrated in Figure 8 below.

It should be noted that the calculated values are unweighted. Sound exposure criteria for personnel is based on A-weighted decibel values which filter out the high and low frequency sounds that cannot be heard by the human ear. This will be covered further in the Personnel Exposure section.

• Approximately 10 dB of attenuation can be expected from the front of the Neutron speaker to the back face of the speaker, meaning that the OASPL just behind the test configuration can be estimated as the OASPL of the test spectrum minus 10 dB. This could vary for more compact test configurations with less space between speaker stacks.
• Control strategy can impact the sound field both inside and outside of the test configuration. As you move further away from the system, control strategy becomes negligible.
• Standard measures of transmission loss, like STC, may not serve as the best tool for understanding how much sound will be lost over building partitions. Attenuation of sound across standard single partition walls can be conservatively assumed to be 20-25 dB. Double-partition arrangements and concrete block walls will provide considerably more transmission loss than single panel walls.
• All predictions should be validated through low level testing, as described later in this paper.

Existing literature suggests that walls and roofs (i.e. large flexural absorbers) are most at risk for damage due to acoustic load [14.] To evaluate the impact of DFAN generated acoustic load on building structures, design wind loads, and internal lateral pressures have been compared to the RMS pressure derived from the OASPL at the panel boundary condition. Design codes allow wind load and internal lateral pressure to be applied conservatively as a as a quasi-static uniform distributed load for rigid (low rise) buildings, typically defined as buildings having a period greater than 1 Hz [1.]

Lateral loads for the test facility used as a part of this study have been calculated per ASCE 7-16. The design wind speed for exterior building walls, based upon a wind velocity of 115 miles per hour for a 3-second gust, have been calculated to be 27 psf (0.19 psi) [ref]. Per the International Building Code (IBC), all interior walls and partitions that exceed 6 feet in height should have adequate strength and stiffness to resist a minimum horizontal load of 5 psf (0.035 psi) to account for differential pressures on either side of the partition [6.]

Note that this is a simplification of an incredibly complex calculation of acoustic loading applied on building walls. The purpose of this calculation is to demonstrate that the acoustic energy generated by DFAN systems is unlikely to exceed the design thresholds for environmental loads.

Historically, more robust comparisons of acoustic and wind loading have been performed for buildings located at Kennedy Space Center in Cape Canaveral, FL. Wind loading was considered to override acoustic load effects from the Saturn V and Space Shuttle launches in the design of surrounding buildings and ground support systems. Note that the acoustic environment experienced by buildings and support systems during launch was louder than that which can be created by commercially available DFAN systems. [2]

Based on the findings of this study, several conclusions can be drawn regarding the structural stability of building structures hosting DFAN testing campaigns:

• Commercially available DFAN systems are currently unable to generate sustained broadband noise levels like those experienced by ground structures during launch (160 dB+). Therefore, specialized analysis considering vibro-acoustic loading is typically not necessary for standard building structures hosting DFAN testing.
• Building lateral resisting systems are designed to withstand worst case environmental loading like wind and seismic load. Because of the overall stiffness and mass of a standard aerospace building superstructure, short periods of acoustic load on the building will not excite it beyond the strength and serviceability design cases.
• The duration of exposure from a single DFAN testing campaign will not negatively impact the fatigue life of building wall and ceiling construction. However, for new buildings equipped for DFAN testing, designers should consider the potential for fatigue loading if the facility will frequently conduct tests with consistent acoustic profiles. Published recommendations suggest that life cycles between 105 and 107 warrant consideration of fatigue [14.]

In summary, while DFAN testing introduces acoustic loads that differ from traditional design considerations, current DFAN systems produce sound levels that are manageable within the structural design limits of typical aerospace facilities.

Obviously, when the panel was outside of the test configuration it was subjected to less acoustic load and therefore exhibited lower levels of structural excitation. When placed at Position 2, located about 20 feet from the center of the circle, a 78% decrease in gRMS from Position 1 and corresponding drop of about 10 to 11 dB was noted for both control strategies. At Position 3, located 75’ from the center of the configuration, an 88% decrease in gRMS from Position 1 and a corresponding drop of 17 to 18 dB was noted for both control strategies.

Table 3 – Panel Monitor Microphone Measurements with A01Z Accelerometer Responses in gRMS

One observation from this data is that the room boundary conditions are evident in the monitor microphone and accelerometer data collected in panel Position 4, located in the room’s far corner. The structural response of the panel in Position 4 is consistent with the response noted in Position 3, despite being located an additional 85’ away from the test configuration.

In room corners, sound waves reflecting off the walls converge resulting in a concentration of sound energy at these points. Corners tend to act as nodes or antinodes for low frequency standing waves, creating areas where base frequencies can become particularly strong or problematic. In Figure 10 below, the general attenuation across the one-third octave bands can be explained by the increased distance from the source. The boost in the 63 Hz octave band, however, is likely due to the panel’s location in the corner of the room.

Figure 10 – Panel Microphone at Positions 03 and 04

Another observation in the data was the difference in panel response, especially in the Z axis, which was the most reactive. In general, acoustic coherence can be defined as a summation of two or more wavefronts. This summation, especially in the low frequencies, can boost structural excitation especially where modes are present in large flexural absorbers, like the panel in this test, because the structural response depends, in part, on the cross spectrum of the acoustic field. This phenomenon is known as joint acceptance. [5]

To illustrate this phenomenon, the difference between the average panel response in the Z-axis direction was compared between both cases. The transfer function of this comparison between test-case A (fully incoherent) and test-case B (partially coherent) can be seen Figure 11 below. In a transfer function, a value of one represents no change at all and is depicted by the black line. The blue line depicts Position 1 (in the direct field) and the red line, Position 4. Values above one indicates that the coherent stacks are providing more excitation and below, less.

Figure 11 – A vs B Coherence at Position 01 and 04

This plot illustrates that the further away the test article is from the direct field, the less the coherence influences structural excitation. It also seems to indicate more spatial variation in the direct field when stacks of speakers are coherent, which is something that has been shown in other studies of this kind.

Based on the findings of this study, several recommendations can be made regarding the placement and protection of sensitive flight hardware during DFAN testing campaigns:

• If adjacent flight hardware is projected to be exposed to acoustic levels that are higher than allowable by the manufacturer, test conductors and facility managers should develop a plan to relocate the sensitive hardware to another location or provide acoustic mitigation for the article prior to the testing campaign.
• If adjacent flight hardware will stay in the test area for the duration of the test campaign, consideration should be given to the placement of the hardware within the room. Hardware should be placed as far from the test configuration as practical. Room corners should be avoided.
• When locating sensitive flight hardware outside of the test area, it is recommended that, if possible, the hardware should be stationed in a room that is separated by a double wall from the test area.

Exposure to elevated sound pressure levels can influence various aspects of human physiology, including stress levels, hearing health, and overall well being. In addition to risks for pain and hearing loss, reports suggest that intense sound can cause nausea or “sea sickness” in certain individuals. The intensity, duration, and frequency spectrum of the sound greatly influence the severity of the effects above. Therefore, understanding the characteristics of the generated sound field and exposure risks to personnel during DFAN testing is essential for selecting adequate hearing protection and determining appropriate system control areas.

Industry recommendations for noise exposure as published by the US Occupational Safety and Health Administration (OSHA) and the European Parliament and Council are geared towards traditional industrial and manufacturing environments where employees are exposed to sustained elevated noise levels for the duration of an 8-hour shift. Noise dose and recommended exposure levels are based on A-weighted sound level measurements, which effectively filter out the lower and higher frequency energy that cannot be heard by the human ear. OSHA and the EU have recommended noise exposure limits of 85 dBA and 87 dBA respectively with a maximum exposure level of 140 dBA for impulse / impact noises [10][13].

MIL-STD-1474E provides a more conservative steady state maximum sound pressure level of 130 dBA after hearing protection has been applied and recommends that personnel should not be exposed to levels exceeding 150 dBA, regardless of hearing protection worn. [15].

The noise dosimeter data collected during the study was used to assess employee noise exposure throughout the course of a standard test day. Tabe 4 below shows the recorded LAeq, a measurement of equivalent continuous sound level measured in A weighted decibels, for the 4 areas where noise dosimeter measurements were recorded within the test facility (as shown in Figure 7).

Table 4 – Noise Dosimeter Measurements

**Note: The discrepancy in measurements for ML_Office S, Control Strategy B is attributed to heavy rain during this test run, which likely contributed to an approximately 4 dB increase in recorded LAeq.

Over the course of both the control A and B test runs, the recorded LAeq never exceeded the OSHA recommended exposure limit of 85 dBA. Despite this, hearing protection was still provided to and worn by test technicians stationed within the control room during all test cases.

Note that in all cases, the LAeq generated using control strategy B was measured to be slightly higher that measured using control strategy A.

It is important to understand that the acoustic environment created by a DFAN test varies from the assumptions used by OSHA and the European Parliament in two primary ways. First, the duration of a typical DFAN test case is between 1.5 and 3 minutes of elevated noise exposure. Even if several test cases are run over the course of a standard 8-hour shift, the short bursts of elevated noise have very little impact on the overall noise dose and time weighted average values calculated per OSHA. Second, noise dosimeter readings taken as a part of this study reinforce the fact that the bulk of the energy that is propagating through the facility is low frequency noise (between 20 and 250 Hz) as illustrated in Figure 12. The illustrated sound pressure levels are unweighted, or Z-weighted, as it is a more accurate representation of the low frequency energy created by DFAN testing.

Figure 12 – Z-Weighted SPL Noise Dosimeter Measurements for South Office

Further study is required to determine the best methodology for accurately determining impacts of the low frequency energy generated on personnel noise exposure limits.

8.4 Recommendations for Managing Personnel Noise Exposure

Based on the findings of this study, several recommendations can be made to limit personnel noise exposure over a DFAN campaign:

• Expected sound pressure levels within the test area and adjacent rooms during a full-level DFAN test should be predicted to determine the OASPL in areas occupied by personnel during DFAN testing. This should be done as a part of pre-test planning.
• Ensure that no personnel are stationed within the test area. System control and test technicians should be positioned in a room adjacent to the test bay, ideally with a significant partition or barrier to reduce sound transmission.
• Use of hearing protection is recommended to attenuate the measured sound pressure level in dBA to the recommended exposure limit of 85 dBA.
• Post noise hazard signs in areas that are anticipated to see levels equal to or above 85 dBA. Hearing protection requirements and distances from the test area at which it must be worn shall be specified clearly on the signage [15].

In cases where sound levels in occupied areas are less than 103 dBA OASPL, a single layer of hearing protection with a minimum noise reduction rating (NRR) of 25 dB (i.e. over-the-ear hearing protection) will be sufficient in attenuating the sound pressure levels to the 85 dBA threshold. Where sound levels are greater than 103 dBA OASPL, both in ear and over-the-ear hearing protection should be provided. Note that this considers a derating of the NRR of commercially available hearing protection by the 7 dBA recommended by the US National Institute for Occupational Safety and Health (NIOSH.) [3]

Low level acoustic tests were performed as a part of a recent DFAN testing campaign to validate the expected sound pressure levels prior to scaling to full test level. As a part of the pre-test planning, expected OASPL were predicted using heritage data and estimated STC ratings for the building construction, including partition walls and doors. This predictive approach aimed to ensure appropriate hearing protection and safety measures for personnel during testing.

The low-level run was performed at 20 dB below the full test level to verify the expected OASPL and validate the adequacy of planned hearing protection requirements. Microphones were positioned at five key locations within the facility to record measurements. These measurements were then linearly scaled to estimate sound pressure levels for a full-level test.

The results from the low-level run showed that the predictions, based on the known construction types, were consistent with the scaled measurements. Notably, the projected sound level in the system control area was found to be 108 dBA, which underscored the need for double hearing protection during full-level tests as discussed in the Personnel Noise Exposure section. During the bare chamber test runs, data was collected at 3 dB increments. The measurements obtained were consistent with the predictions made from the -20 dB low-level run, confirming the accuracy of the initial predictions.

Based on these findings, it is recommended to conduct a low-level verification run that is at least 18 dB below the full test level. This approach ensures that the planned protections for both personnel and adjacent flight hardware are sufficient for mitigating the actual OASPL measured during high-level tests.

The rise of DFAN testing has been based on a need for more cost-effective and schedule friendly environmental testing. However, without adequate consideration of the facility impacts, the touted logistical advantages of DFAN can be lost. It is imperative for test directors and facility managers to review the impacts of testing on the building structure, adjacent flight hardware, and personnel during pre-test planning phases to:

• Define and create plans to maintain “keep out” zones for personnel during test campaigns based on predictions of sound pressure levels away from the test setup.
• Determine what, if any, sensitive hardware needs to be relocated prior to the testing campaign and include timeframe for relocation in test campaign schedule
• Evaluate where and what type of hearing protection may need to be offered for facility personnel
• Perform low level testing to validate OASPL predictions

It should be stressed that, based upon existing evidence and design code requirements, there is no apparent risk of structural failure to standard aerospace manufacturing facilities posed by DFAN testing. The maximum sustained sound pressure levels achievable using DFAN should not exceed calculated environmental design loads used for the strength and serviceability design of the structure.

Each facility impact discussed here merits further analysis as the depth of heritage data and information collected as a part of the study is greater than what could be covered in this comprehensive overview.

The author would like to thank Eric Friedlander, Dale Schick, Daniel Hayes, Geoff Eckert, Nathan Kiefer, and Zachary Crawford for their contributions to this case study.