April 2012 

NASA/TM-2012-217563 

NESC-RP-09-00597

 

Assessment of Microphone Phased Array for 

Measuring Launch Vehicle Lift-off Acoustics 

Roberto Garcia/NESC 

Langley Research Center, Hampton, Virginia 
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April 2012 

NASA/TM-2012-217563 

NESC-RP-09-00597

 

Assessment of Microphone Phased Array for 

Measuring Launch Vehicle Lift-off Acoustics 

Roberto Garcia/NESC 

Langley Research Center, Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 

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NASA Engineering and Safety Center 

Technical Assessment Report

Document #:

NESC-RP09-00597 

Version:

1.0 

Title:

Assessment of Microphone Phased Array for Measuring Launch 

Vehicle Lift-off Acoustics 

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NESC Request No.: 09-00597 

Assessment of Microphone Phased Array for Measuring Launch 

Vehicle Lift-off Acoustics 

March 22, 2012 

 
NASA Engineering and Safety Center 

Technical Assessment Report

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Title:

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NESC Request No.: 09-00597 

Report Approval and Revision History 

NOTE: This document was approved at the March 22, 2012, NRB. This document was 

submitted to the NESC Director on April 4, 2012, for configuration control. 

Approved: Original Signature on File 4/10/12 

NESC Director Date 

Version Description of Revision Office of Primary 

Responsibility Effective Date 

1.0 Initial Release Mr. Roberto Garcia, 

NASA Technical 

Fellow for Propulsion, 

MSFC 

3/22/12 
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Table of Contents 

Technical Assessment Report

1.0 Notification and Authorization ........................................................................................ 5

2.0 Signature Page ................................................................................................................... 6

3.0 Team List ........................................................................................................................... 7

3.1 Acknowledgements ................................................................................................ 7

4.0 Executive Summary .......................................................................................................... 8

5.0 Assessment Plan ................................................................................................................ 9

6.0 Background ....................................................................................................................... 9

7.0 Data Analysis ................................................................................................................... 12

7.1 Fundamentals of Beamforming using a Microphone Phased Aarray .................. 12

7.1.1 Conventional Beamforming ................................................................................. 13

7.1.2 Matlab® Implementation ...................................................................................... 15

7.2 Phased Array Hardware ....................................................................................... 16

7.3 Test Facility ......................................................................................................... 20

7.4 Results and Discussions ....................................................................................... 23

7.4.1 Validation of the Array Performance ................................................................... 23

7.4.2 Horizontal Rocket Motor Only Test .................................................................... 26

7.4.3 Vertical Tests with Model of Ares I Vehicle ....................................................... 28

8.0 Findings, Observations, and NESC Recommendation ................................................ 37

8.1 Summary and Conclusions .................................................................................. 37

8.1.1 Findings ............................................................................................................... 37

8.1.2 Observations ........................................................................................................ 38

8.1.3 NESC Recommendation ...................................................................................... 38

9.0 Alternate Viewpoint ........................................................................................................ 38

10.0 Other Deliverables .......................................................................................................... 38

11.0 Lessons Learned .............................................................................................................. 38

12.0 Definition of Terms ......................................................................................................... 38

13.0 Acronyms List ................................................................................................................. 39

14.0 References ........................................................................................................................ 40

15.0 Appendices ....................................................................................................................... 41

List of Figures

Figure 6.0-1. Model of Noise Source Distribution during Lift-off ............................................ 11

Figure 6.0-2. Modifications for Predicting Ares I Lift-off Level .............................................. 12

Figure 7.1-1. Schematic of the Steering Vectors for Beamforming .......................................... 13
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Figure 7.1-2. (a) Microphone Pattern on the Array Plate; (b) psf for the Two Indicated 

Frequencies for a Point Source Located at the Origin and z = 240 inches 

from the Plate ....................................................................................................... 15

Figure 7.2-1. Phased-Array Inside an Anechoic Chamber ........................................................ 18

Figure 7.2-2. Phased-Array during a Solid Motor Firing in a Horizontal Test Stand ............... 18

Figure 7.2-3. Setup of the Microphone Phased Array at the ASMAT Test Stand .................... 19

Figure 7.3-1. Front View of the ASMAT Test Setup ................................................................ 20

Figure 7.3-2. Side View of the Test Setup................................................................................. 21

Figure 7.3-3. Sound Suppression System Employed for the SSP ............................................. 22

Figure 7.4-1. Beamformed Maps of Three-Speaker Sources in an Anechoic Chamber ............ 24

Figure 7.4-2. (a) Time Trace of Pressure Fluctuations from one of the Microphones in the 

Horizontal Motor Test; (b) Coherence Spectra Between indicated Pairs of 

Microphone .......................................................................................................... 25

Figure 7.4-3. Validation of the Phased Array Operation via Identification of a Single Speaker 

Source (at the bottom-left corner; the four plots are for the indicated four different 

frequencies) .......................................................................................................... 26

Figure 7.4-4. (a) The Plume of a Solid-Rocket Motor Fired Horizontally over a Concrete Pad; 

and (b) Noise Source Distribution at indicated Frequency as Measured by the 

Phased Array ........................................................................................................ 27

Figure 7.4-5. Beamformed Noise Sources at Four indicated Frequencies. ASMAT3: Model 

is at Hold-down Position, without Water Injection, and LM is in Place ............. 28

Figure 7.4-6. Flow of Hot Gas through the Trench ................................................................... 29

Figure 7.4-7(a). Sparks Fly Off the Plume Impingement Point on LM and MLP in ASMAT4: 

Model Lifted by 3.9Dexit above the MLP ............................................................. 30

Figure 7.4-7(b). Closer View showing Drift to the Right of the LM/MLP Hole ........................... 30

Figure 7.4-8. Beamformed Noise-Sources at the indicated Frequencies Measured in 

ASMAT 4 ............................................................................................................ 31

Figure 7.4-9. Side View of the Model and the Launch Pad in ASMAT 17 

(no water, elevation=8.6 Dexit, drift = 0) .............................................................. 32

Figure 7.4-10. Beamformed Noise Sources from ASMAT 17 at indicated Two Frequencies .... 32

Figure 7.4-11. Photograph of ASMAT 11 (drift = 0.9Dexit, elevation = 8.6Dexit, hole and 

trench water) ........................................................................................................ 33

Figure 7.4-12. Beamformed Noise Source Maps at indicated Frequencies from ASMAT 11 .... 33

Figure 7.4-13. Photograph from ASMAT 12: Rainbird (on-deck) Water Injection .................... 34

Figure 7.4-14. (a, b) Time Traces from an Absolute Transducer on the Array Plate; 

(c) ASMAT 11, (d) ASMAT 12 .......................................................................... 34

Figure 7.4-15. Beamformed Noise Source Maps at the indicated Frequencies from ASMAT 12 

(drift = 0.9Dexit, elevation = 8.6Dexit, hole, trench, and on-deck water) ............... 35

Figure 7.4-16. Comparison of Average Spectra from the Indicated Tests; Average Overall 

70 Microphones of the Phased Array ................................................................... 36

List of Tables 

Table 7.3-1. Part of the Test Matrix where Phased Array was Used ....................................... 23
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Technical Assessment Report 

1.0 Notification and Authorization 

A NASA Engineering and Safety Center (NESC) assessment was approved as an out-of-board 

activity on November 4, 2009. The assessment plan was approved by the NESC Review Board 

(NRB) on December 10, 2009. The final report was presented for approval on March 22, 2012. 

The key stakeholders for this assessment are the lift-off acoustic community at Marshall Space 

Flight Center (MSFC), the mobile launch platform (MLP) acoustics engineering community at 

the Kennedy Space Center (KSC), the NASA Aerosciences Technical Discipline Team (TDT), 

and industry launch vehicle analysts interested in the development of this technique and 

application for lift-off environment. Additional stakeholders are the Multi-Purpose Crew 

Vehicle (MPCV) and Space Launch System (SLS) Programs. 
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2.0 Signature Page 

Submitted by: 

Team Signature Page on File – 4/16/12 

___________________________________ 

Mr. Roberto Garcia Date 

Significant Contributors: 

___________________________________ 

Mr. Jayanta Panda Date 

Signatories declare the findings, observations, and NESC recommendations compiled in the 

report are factually based from data extracted from program/project documents, contractor 

reports, and open literature, and/or generated from independently conducted tests, analysis, and 

inspections. 
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3.0 Team List 

Name Discipline Organization 

Core Team 

Roberto Garcia NASA Technical Fellow for Propulsion MSFC 

Jayanta Panda Primary Acoustician, NESC AeroSciences 

and Propulsion TDTs ARC

Robert Mosher Software & Test Support Aerospace Computing, Inc. 

Janice Houston MSFC Engineering support Jacobs Engineering 

Dennis Strickland MSFC Technical support MSFC 

Loutricia Johnson MTSO Program Analyst LaRC 

Administrative Support 

Tina Dunn-Pittman Project Coordinator LaRC/ATK 

Erin Moran Technical Writer LaRC/ATK 

3.1 Acknowledgements 

The assessment team greatly acknowledges the constant help from a large number of engineers 

and technicians led by Mr. Dennis Strickland (MSFC) in setting up the array, Ms. Allison Lee 

(MSFC) for help with the logistics of the operation, and Ms. Janice Houston of Jacobs 

Engineering for coordinating the test operations. The team would like to thank the MSFC team 

for the photographs used in Figures 7.2-2, 7.2-3, 7.3-1, 7.4-9, and 7.4-11. 

 
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4.0 Executive Summary 

Usefulness of a microphone phased array to identify noise sources in applications involving 

rocket motors was verified using a 70-element, 0.25-inch-diameter sensor array in seven 

different tests of the Ares I Scale Model Acoustics Test (ASMAT). Many precautions were 

taken to protect the sensitive condenser microphones and camera equipment from the 

environmental elements during prolonged outdoor stay, exposure to the intense vibro-acoustics 

environment, debris, and water-spray during a motor horizontal hotfire test. In the first phase of 

the study, the array was placed in an anechoic chamber for calibration and validation of the 

indigenous Matlab®-based beamform software. In the next phase, the equipment was shipped to 

the Marshall Space Flight Center (MSFC), where the array hardware was rebuilt to safeguard 

electronic components. The first test involved a motor-only burn. The beamformed noisesource map was superimposed on an image of the rocket plume to readily identify the source 

distribution. It was found that the horizontally-fired motor plume made an exceptionally long, 

greater than 30-nozzle-diameter, noise source over a large frequency range. The array was found 

to be most effective in the frequency range of 2- to 10-kHz. For the next six tests, the array was 

set up in the vicinity of a 5-percent-scale model of the Ares I vehicle in a launch or vertical 

configuration. The tests involved static-firing at different model configurations representing 

various stages of lift-off, such as: (a) model at hold-down position; (b) model at different 

elevations with the accompanying lateral drift; (c) absence or presence of various water injection 

systems; and (d) absence or presence of a launch mount (LM). The beamformed plots show that, 

in almost all cases, impingement by the plume on various regions of the pad constitutes the 

primary noise sources. The scenario is different from the current models and expectations based 

on NASA SP-8072 [ref. 5] and its variations, which assumes the plume as the noise source and 

does not take into account the importance of impingement. It was found that another parameter, 

the sidewise drift of the vehicle with elevation, influenced the strength of the impingement noise 

sources, yet was not considered in the prediction methods. It was observed that the addition of 

water in the trench and in the hole of the mobile launch platform (MLP) attenuated noise sources 

radiating from these places. Water injection on the top of the MLP, “rainbird” system, produced 

some relief, but the impingement source remained active. The noise source maps suggest that 

the minimization of plume impingement by limiting the vehicle drift in the early part of lift-off, 

covering-up the trench as much as possible, and by removing extraneous components, such as 

the LM will lead to a reduction of the lift-off acoustics level. 
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5.0 Assessment Plan 

The scope of this work was divided into two efforts. The first effort was the application of the 

phased array in a horizontal solid rocket firing to verify sensor survivability, to improve 

familiarity operations in the MSFC test area, and to verify the analysis software. The second 

effort was the application of the microphone phased array during six ASMAT firings. 

The first effort was scheduled to be completed in fiscal year (FY) 2010, Year 1, while the second 

effort was for the FY2011, Year 2. In Year 1, the existing array was augmented, calibrated, 

tested, and verified at the Ames Research Center (ARC) acoustics facility. The data processing 

software was upgraded to a version that used deconvolution of the point-spread function (psf) to 

improve the imaging resolution. After completion of these tasks, the array was shipped to MSFC 

for participation in the horizontal motor firing test and the ASMAT firings. 

6.0 Background 

Significant advancements in sensor, data acquisition, and computing technologies, and recent 

progress in sharpening the noise source distribution via advanced processing techniques, have 

widened the usage of microphone phased array in recent years. The aircraft industry uses phased 

array for the identification of noise sources on airframes, engines, landing gear, and other aircraft 

parts [refs. 7, 8, 15]. Arrays are used to detect noise sources in automotive and manufacturing 

industries. The goal of the work was to extend the usage of this technology to spacecraft 

applications. 

Perhaps the most obvious application is in launch acoustics where every part of a launch vehicle, 

launch pad, and a large number of components used for the ground operations are subjected to 

the high acoustics loads generated during lift-off. Even a couple of decibel (dB) reductions of 

the acoustic level translate into a sizable reduction of cost, weight, and risk of failure during 

qualification testing of a large number of components. The first step towards that end is the 

identification of the noise sources. The existing knowledge-base, available flight data, and the 

model-scale tests almost exclusively rely on single microphones that are unable to detect noise 

sources. Single microphones measure the local sound pressure; they are incapable of identifying 

the distant locations where the sound waves have originated. Alternatively, a phased array of a 

microphone is a “tuned listener” that can directly identify the locations of the noise sources. 

The specific purpose of the present work was to demonstrate the suitability of a microphone 

phased array for launch acoustics applications via participation in selected firings of ASMAT. 

ASMAT is a part of the discontinued Constellation Program Ares I Project, but the basic 

understanding gained from this test is expected to help development of the SLS vehicles. 

Correct identification of sources not only improves the predictive ability, but provides guidance 

for a quieter design of the launch pad and optimization of the water suppression system. 
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The phased array operations need to be accomplished on a “least interference” basis relative to 

the baseline ASMAT activities. It was planned to mount the array on a removable stand that can 

be mounted close to the test stand and locked into its designated place prior to a test firing. 

Typically, the array needs to be placed in close vicinity to the subject affected by the noise. For 

the lift-off test, the critical component is the Orion crew module, which lies at the top-most part 

of the model. Therefore, it was desirable to identify sources radiating noise/ignition overpressure (IOP) waves towards the top part. This assessment included support from MSFC 

personnel to help with various pre- and post-test setting up work. 

The baseline plan for ASMAT was about 17 firings over the duration of 7 months. The team 

suggested acquiring phased array data in six of these firings to include the dry (i.e., no water) 

firing, several of the water injection optimization tests, a test with the scale vehicle at some 

elevation above the pad, and the worst ground acoustics environment case. Problems 

encountered with either data analysis software, instrumentation survivability, or the test-to-test 

instrument accuracy were documented and improvements demonstrated as much as possible 

during the test campaign. Verified data from each of these tests was analyzed and provided to 

the ASMAT team. 

The launch acoustics part of the Ares I Project started with at least two different predictions of 

the environment. An examination of the prediction methodologies exposes the debates and 

doubts that arise from a poor understanding of the noise sources. Haynes & Kenny (2009) 

[ref. 2] used the procedure described by Eldred & Jones (1971) [ref. 5] with a small modification 

of the length of the plume-potential-core suggested by Varnier (2001) [ref. 4] for horizontal 

firing and a significant modification of a truncated plume length in the case of a deflected 

exhaust. Plotkin et al. (2009) [ref. 3] used a computer code that seems to carry most of the 

elements of the Eldred method (with additional features of scattering by the launch tower, and a 

different empiricism for the total noise [ref. 11]. The engineering methodologies were 

fundamentally pinned to an older model of noise-source distribution along a free plume. The 

presence of the elaborate launch pad was to make the plume bend at the location of the flame 

deflector (Figure 6.0-1). 
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Figure 6.0-1. Model of Noise Source Distribution during Lift-off [ref. 5] 

The shortfall of this model was that it overlooked the significant impingement zone on the flame 

deflector where, in almost all situations, impingement is known to cause much louder noise 

sources. Computational fluid dynamics analysis supported this deficiency [ref. 6]. Past efforts 

to compare the predicted spectra to those measured in flight indicated the true noise sources lay 

closer to the nozzle exit plane [ref. 12]. To overcome this shortcoming, empirical efforts have 

looked for an equivalent single point: the end of the supersonic core. Varnier (2001) [ref. 4] 

states, “... the sound power peak location is related to the supersonic length of the flow, which 

appears to be the adequate reference length for a future jet noise model.” Sutherland (1993) 

writes “The model applies the widely accepted concept that the dominant sound source for 

supersonic jet flow is close to, and downstream of, the supersonic tip.” The statement may have 

merit for a free-flowing plume, but is highly doubtful in a launch pad configuration where the 

plume is forced through a set of MLP holes, a deflector, and the trench. Nonetheless, the 

empirical effort to improve matching with the flight data led to the model of Haynes & Kenny 

(2009) [ref. 2], where the plume just after deflection was assumed to be the peak noise source 

(Figure 6.0-2). 
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Figure 6.0-2. Modifications for Predicting Ares I Lift-off Level [ref. 2] 

Typically, the level of confidence on these predictions was not very high. Therefore, the 

ASMAT series using a 5 percent scaled model of the launch configuration was deemed necessary 

to improve the confidence in the predictions. During the test campaign, 18 static firings 

(including one motor-only burn) were conducted. However, the model positions were varied to 

represent vehicle locations at different time instances of lift-off. The original goal of ASMAT 

was to measure the acoustic and IOP levels on the vehicle, process-tower, and launch pad via 

individual microphone sensors. This report does not show data from these sensors. Instead, 

attention is confined to the results obtained from the deployment of the phased array 

instrumentation. 

The first use of a microphone phased array to launch acoustics was by Gély et al. (2000) [ref. 9]. 

They used a 24-microphone, ring-array in a small-scale test of the Ariane 5 launch pad to 

identify sources responsible for the excessive low frequency noise in the payload fairing. No 

rocket motors were used for this test; instead, heated compressed air was used to simulate the 

rocket plumes. The beamformed maps correctly identified the sources, which led to various 

modifications, such as extending flue trenches and optimization of the water injection. The 

present application extends the array use from scaled simulations using heated air-to-direct 

measurement of solid-rocket plumes in launch pad configurations. 

7.0 Data Analysis 

7.1 Fundamentals of Beamforming using a Microphone Phased Aarray 

A phased array images the sound sources by analyzing the wave phase front sensed by a large 

number of microphones. The individual microphone signals are delayed in time (or equivalently, 

in phase) and then added together. The first step in the imaging process is to divide the spatial 

zone (where the sound sources lie) into a large number of grid points, which are called the 

interrogation points (Figure 7.1-1). If there lies a noise source at one of the grid points, then the 
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estimated time delays (phase differences) for sound waves to reach the microphones are correct, 

and the summing process gives a large output. Alternatively, if the interrogation point is not a 

noise source, then the final sum is much lower. This principle makes what is known as the 

conventional beamforming, whereby a beamformed map is created over the interrogation region. 

Figure 7.1-1. Schematic of the Steering Vectors for Beamforming 

7.1.1 Conventional Beamforming 

As shown in Figure 7.1-1, let j =1,2,3....N be the interrogation points in the region containing the 

noise source(s), and a phased array that contains l = 1,2,3, ...M number of microphones is used 

for the test. The radial distance from an interrogation point to an individual microphone rjl

determines the phase shift and the relative amplitude measured by the microphone. The steering 

vector is a [Mx1] column matrix defined to incorporate these properties. It is given by the 

expression: 

࢐࢝ ൌ ఞ

௥ೕκ

݁ିఉ௥ೕκ (1)

where χ is a constant to normalize the steering vectors ࢝௝

ற࢝ ௝ൌ ͳ, β is the acoustic wavenumber 

ൌ ଶగ௙

௖ , f is the frequency, and c is the speed of sound. The † symbol represents a complexconjugate and transpose operation. The time traces of pressure fluctuations from individual 

microphones are Fourier transformed to obtain individual spectra Pl(f). The cross-spectra 

between every pair of microphones are calculated and stored in a cross-spectral matrix (CSM) G. 
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Every element of the CSM is calculated as: 

ܩκκȀ ൌ ۃܲκ

כ

 (2) ۄκȀܲ

The * represents a complex conjugate and the ۃ ۄ represents an expected value (i.e., a time 

average over several seconds). Typically, the Fourier transform is performed over multiple 

overlapping blocks of data, which are then multiplied using equation 2 and finally averaged to 

obtain individual elements of the CSM. The diagonal elements of the CSM G11, G22, G33, etc., 

contain the auto-spectra, which are contaminated by various spurious noise sources, such as that 

from the electronic. Setting the diagonal elements to zero improves the beamformed map. 

The summing of the microphone signals (i.e., the elements of CSM) need to be preceded by a 

phase adjustment. These two steps are combined in a matrix manipulation, which leads to the 

beamform map, expressed by: 

ܾ௝ሺ݂ሻ ൌ࢐࢝

 (3) ࢐࢝ࡳற

The microphone weighting procedure was found to be useful in improving spatial resolution 

[ref. 8]. A linearly increasing weight Wl based on the radial location Rl of the microphone was 

used: 

ܹκ ൌ ܹଵ ൅ ሺͳെܹଵሻܴκȀܴ଴ (4)

The constant W1 was 0.1; R0 represented radius of the outermost microphone. The final 

beamform equation utilizing the weight function is the following: 

ܾ௝ሺ݂ሻ ൌ  ࢐࢝

࢐࢝಩ࢃ૙సࢍࢇ࢏ࢊࡳࢃ಩

ሺσ ௐ೘ሻమିሺσ ௐ೘ሻ (5)

where W is a row matrix containing the terms Wl. It is noted that equation 1 for the steering 

vector assumes that the noise sources radiate as monopoles. This basic assumption of the 

beamforming process has been under scrutiny for the history of the phased array technology. 

The justification for the assumption has been that little change in the beamformed map is 

obtained by assuming other types of sources. Moreover, in a lift-off environment there exist a 

variety of distributed noise sources and reflective surfaces, each with different directivity 

patterns. The possibility of making an error is minimized by assuming that the noise sources 

consist of a large number of monopoles. Therefore, the noise source results presented in this 

report are subjected to the inherent assumptions of the conventional beamforming process, 

namely the complex noise sources can be modeled as the sum of many monopoles, and sound 

propagation is strictly linear. 

Interpretation of the noise-source maps obtained from a phased array requires familiarization 

with the fundamental property of the array response, described by its psf (Figure 7.1-2b). As the 

name implies, a psf describes how a point source becomes spread out in the beamform map. The 

spreading or smearing is dependent on the frequency of interest, the solid angle subtended by the 
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array, and the design of the microphone pattern. The psf at any point k in the beamformed region 

is calculated as: 

ݏ݌݂௞ሺ݂ሻ  ൌ  ห࢝௝

ற

ห௞࢝

ଶ

 (6)

An examination of Figure 7.1-2(b) shows that at the low frequency end, the summed-up array 

output decreases slowly from the locations of the actual source to the neighboring regions. The 

extent of smearing is inversely proportional to the frequency of interest: worst for the lowest 

frequency. However, the strength and number of side lobes (that falsely makes the appearance 

of adjacent sources) is directly proportional to the frequency. More discussion on the 

measurement uncertainty is presented later in the report. 

Figure 7.1-2. (a) Microphone Pattern on the Array Plate; (b) psf for the Two Indicated Frequencies for 

a Point Source Located at the Origin and z = 240 inches from the Plate 

Recently, there have been multiple efforts to improve the array resolution by deconvolving the 

psf from the beamformed output. A summary of these methods applied for the present 

application and sample results are discussed in Appendix A. 

7.1.2 Matlab® Implementation 

The beamforming schemes were implemented in the commercially-available Matlab® platform. 

A quad-core personal computer (PC) and the Matlab® Distributed Computing Toolbox were 

used. Once the CSM was calculated, the rest of the beamforming operation was accomplished 
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quickly (a few seconds for each frequency bins). The CSM calculation took between 1 to 

5 minutes, depending on the duration of the microphone data. 

To facilitate direct identification of the noise sources, the interrogation grid for beamforming was 

created over an image of the region of interest. A camera was mounted at the center of the array 

and was used to capture video (at 7.5 frames per second) simultaneously over the duration of the 

microphone data recording. When the noise source is stationary, only one frame of the captured 

video was sufficient for further processing. The first step after selecting the frame was to 

perform a correction for barrel distortion due to the fish-eye lens used with the camera. The low 

focal length lens was useful to capture a large view angle. The expression used for making the 

correction was s = r + ar3

, where s is the true radius and r is the measured radius of each pixel 

from the center of the image. The correction factor ‘a’ was obtained by trial and error. In the 

next step, the images region was divided into a uniformly spaced grid. To select the number of 

grid points, the Rayleigh criterion was applied to the highest frequency of interest. According to 

this criterion, the minimum resolvable detail is due to the diffraction-limit of the array when the 

first diffraction minimum of the image of one source point coincides with the maximum of 

another. For a circular aperture of diameter d, the angular resolution in radians θR is directly 

related to the wavelength O of interest. 

ݏ݅݊ߠோ ൌ ͳǤʹʹ ఒ

ௗ

 (7)

For the present estimate, the maximum separation between any microphone pairs was assumed to 

be d. This equation shows that for a given array size, the angular resolution improves with 

frequency. Conversely, the highest frequency of interest dictates resolution of the interrogation 

grid. For the present application, a uniformly spaced grid of 1/10th the array resolution at the 

highest frequency of interest was used. The conventional beamform calculations were applied 

on individual narrow bands of the CSM. 

The beamform code was extended to create movies of the time-evolution of the noise sources. 

After ignition, the steady part of the motor burn lasted a couple of seconds. To capture the 

changes in source distribution over the entire burn, the time-series data from each microphone 

was pieced into 0.15-second segments, which corresponded to the camera frame rate of 

7.5/second. The CSM was calculated on each segment followed by the conventional beamform 

calculations. The beamform map was plotted to the individual video frames and the created 

frames were animated to create a movie of the noise source evolution. Sample movies were 

presented at the NRB as supplementary information to this report. 

7.2 Phased Array Hardware 

The basic array plate containing the 70-element condenser 0.25-inch-diameter microphone array 

was designed for wind tunnel tests [ref. 15]. The present use in an outdoor rocket test facility 

required additional instrumentations and protection from exterior environmental conditions. The 
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array plate was modified to place a video camera at the center, with two dynamic pressure 

transducers (Kulite®) to measure impulse loading from the IOP pulse. The video camera at the 

center of the array, with a wide-angle 3.5-mm lens, was used to directly image the physical noise 

sources. The aperture of the video camera lens was remotely adjusted via a servo-controlled 

actuator; this allowed for adjustment for the variable day-light conditions and plume glare. 

A weather-resistant case was built around the plate to protect the equipment from rain, snow, 

wind, and other elements. The case was made of three components: a cover plate for the front 

end, a back-chamber to protect the back side, and two junction boxes attached to the backchamber to make a water-tight connection for the large number cables. The weather-resistant 

case, constructed of an aluminum frame, sheet metal, and polycarbonate boards, was sealed 

using weather strips and silicone sealants. The rear chamber held desiccants to maintain a dry 

environment. However, during a simulated rain test, it was found that the case was not 

completely water proof. This led to an addition of two more preventive measures. A plastic bag 

was placed over the weather-resistant case, and the rear chamber was continuously purged using 

dry missile grade air. All of these preventive measures were found to be effective through 

subsequent rain tests and ultimately during prolonged exposure at the test site when several rain, 

snow, and thunderstorms passed over the array hardware. 

After the initial testing, calibration, and software validation at ARC (Figure 7.2-1), the entire 

phased array hardware was shipped to MSFC test stand TS116. There, the setup was rebuilt in 

two parts: the array box with all microphone and other sensors were mounted close to the model, 

and the electronic components (e.g., computer, data acquisition system, amplifiers, etc.) were 

placed indoors in an environmentally controlled room away from the test stand. There were 

164-foot-long cables used to connect microphone preamplifiers to the amplifiers. At the array 

case, the cables were passed through rubber grommets at the junction-box to make water-tight 

connections. The long cable bundle was protected by split pipes. 

The entire array setup was subjected to a non-interference criterion, where the operation of the 

phased array had to be completely independent and separate from the ASMAT primary test 

objectives. The non-interference condition prohibited locating the array closer to the model, 

which would have provided a straightforward means of comparing fluctuations measured on the 

model with that measured by the array. Nevertheless, the same sound sources identified by the 

array were expected to be radiating towards the vehicle model. 

For the horizontal fire test, the array with the protection chamber was placed 15 feet away from 

the nozzle centerline (Figure 7.2-2). The primary reason for that location was to avoid 

interference with all other pole-mounted microphones and IOP sensors used in the test. The 

array bottom edge was elevated 2 feet above the ground surface, and the array was rotated about 

its y-axis by 30 degrees, so that the camera had a clear view of the entire plume. For all other 

ASMAT tests involving the model and the launch pad, the array chamber was mounted on a 

tower (Figure 7.2-3) about 15 feet away from the test stand and 16 feet above ground. The tower 
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was secured via outriggers bolted to the concrete floor. Additionally, tether cables were used to 

increase torsion rigidity of the assembly. The tower with the array chamber was erected 

typically on the morning of each test and taken down after the test. Before erecting the tower, 

the array protection chamber was opened and each microphone was inspected and calibrated. 

Figure 7.2-1. Phased-Array Inside an Anechoic Chamber 

Figure 7.2-2. Phased-Array during a Solid Motor Firing in a Horizontal Test Stand 
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Figure 7.2-3. Setup of the Microphone Phased Array at the ASMAT Test Stand 

ASMAT3: Model at Hold-Down Position 

A 72-channel, 16-bit VXI system was used for simultaneous acquisition of the microphone 

signals. The PC interfacing with the data acquisition system was accessed over a local network 

from the nearby control room. The same PC was used for capturing video signal from the arraycamera, which was connected via a Fireware® extender. The data acquisition was started at 

t-10 second and lasted for 30 seconds. Each microphone channel was simultaneously sampled at 

49152/second. The motor burn lasted for about 6 seconds, of which the first 2 seconds was fullpower. Analysis of signals from the Kulite® transducers showed that the array plate was 

subjected to IOP wave with pressure rise of ~0.6 psi. The average acoustics level varied test-totest in the range of 135 to 150 dB. The array plate experienced water-spray and debris 

impingement. However, the setup performed remarkably well over the year and a half of test 

duration; a total of about five sensors were lost. The most problematic part was that the VXI 

data acquisition system was found to be sensitive to temperature fluctuations requiring various 

components to be changed frequently. Problems due to vibration and variable light condition 

were fixed using better mounts and a camera aperture control system. 
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7.3 Test Facility 

ASMAT was conducted at MSFC test stand TS116. It used a uniformly-scaled (5 percent) 

launch pad, MLP, launch tower, and a simplified model of the Ares I vehicle (Figure 7.3-1 and 

Figure 7.3-2). The launch pad was similar to that used for the Space Shuttle Program (SSP) with 

two trenches and a center flame deflector. The single plume of the Ares I solid rocket booster 

(SRB) impinged on one side (SRB-side) of the deflector. The MLP configuration was somewhat 

different from that of the SSP with one exhaust hole and the addition of a LM between the hole 

and the SRB nozzle exit. Among other purposes, the LM was expected to confine the plume 

within the exhaust hole. Part way through the test, it was concluded that the LM did not help the 

acoustic environment and reduced the effectiveness of the on-deck water injection system. This 

led to the LM removal. 

Figure 7.3-1. Front View of the ASMAT Test Setup 
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Figure 7.3-2. Side View of the Test Setup 

The launch tower stood between the Ares I vehicle and the exhaust plume flowing through the 

trench. The intention of this arrangement was to provide some blockage to the acoustic path. 

This perhaps germinated from the noise source map model in common use [ref. 5] (Figure 6.0-1). 

The in-line vehicle was modeled by a cylinder with a variable diameter corresponding to the 

different zones of the full-scale vehicle. A solid rocket motor with ~10,000 lbf thrust was used 

to model the first stage booster. At the hold-down condition, the plume from the booster flowed 

through the LM and MLP exhaust holes to one side of the flame deflector, and finally flowed out 

of the trench. Note that similar to the SSP launch pad, there is a gap between the bottom of MLP 

and the top of the trench. No part of the plume came out of the rectangular cutout in the metallic 

structure of the MLP as seen in the side view of Figure 7.3-2. 
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The effectiveness of a variety of different water suppression systems was evaluated during the 

ASMAT series. There were two sets of water systems: one specifically targeting the IOP that 

appeared as soon as the solid motor was ignited, and a second system to reduce the overall 

acoustic levels. The second system is relevant for the present purpose. Similar to the SSP 

launch pad (Figure 7.3-3), there were three parts to the acoustic suppression system: “hole 

water”: water injection through the hole in the MLP; “trench water”: water injection inside on 

the deflector and inside of the flame trench; and “rainbird”: water injection on the top of the 

MLP. 

Figure 7.3-3. Sound Suppression System Employed for the SSP 

The system used for the ASMAT test had similar subsystems. The primary differences were: 

only the SRB side of the deflector was used for passing the single plume of the Ares I booster; 

there was only one hole on the MLP; the rainbird system was relocated around the MLP hole; 

and the “trench water” was utilized. 

Besides the horizontal firing, there were 17 vertical tests. The array was used in six vertical 

tests, plus the horizontal motor only test. The first vertical test, ASMAT 3 represented a holddown condition when the vehicle was on the pad during the ignition of the first stage booster. 

For the rest of the ASMAT tests, the model was elevated to replicate different vertical positions 

of the vehicle. A critical element of the lift-off process is the sidewise drift that accompanies 

vehicle elevation. ASMAT replicated maximum expected drift of Ares I for most of the test 

conditions. In Table 7.3-1, the elevation and drift values are non-dimensionalized by the nozzle 

exit diameter Dexit. In ASMAT 17, a situation was tested where the vehicle elevation involved 

no drift, such that the plume mostly passed through the hole in the MLP deck. Additionally, all 

water injection systems were turned off. The hole and the trench water were turned on in 

ASMAT 11, and all three systems (hole, trench, and rainbird) were used in ASMAT 12. Note 
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that in ASMAT 11 and ASMAT 12, the model was moved sidewise (drifted). Therefore, a 

straightforward comparison with the ASMAT 17 was not possible. There were several cases 

tested to verify the effectiveness of the on-deck water injection, where the phased array could be 

used with ASMAT12. 

Table 7.3-1. Part of the Test Matrix where Phased Array was Used 

*Ratio of mass fluxes: water flow to propellant flow 

7.4 Results and Discussions 

7.4.1 Validation of the Array Performance 

The initial system check, calibration, and software validation were performed in the ARC 

acoustics laboratory. Additional in-situ validations were performed before every test at the 

ASMAT test stand. Sample results from these two sets of validations are discussed below. 

For the initial system check the array was mounted inside an anechoic chamber (Figure 7.2-1). 

At first, the camera was calibrated to associate angular positions in x-z and y-z planes with the 

individual pixels. Knowledge of the perpendicular distance (z-distance) from the object plane to 

the phased array provided the x- and y- coordinates of every pixel in the image. The camera 

calibration was followed by an end-to-end calibration of individual microphones using a 

pistonphone. Finally, validation data were obtained by positioning a set of three speakers and 

driving them using separate broadband white noise generators. Each microphone channel was 

checked for phase anomaly. Figure 7.4-1 shows a sample result from a three-speaker setup 

where the speakers were equally spaced at 31-inches apart and at z = 176 inches from the array 

center. The beamform-map is superimposed on a video frame to help identify individual 

sources. The “drawn curtain” appearance of the image was due to the correction applied to 

remove barrel distortion. An examination of Figure 7.4-1 shows that, for the present 

arrangement, the conventional beamforming was unable to resolve individual speakers at the two 

lower frequencies, while they were identified at the two higher frequencies. At the highest, 

15-kHz frequency side lobes appeared, which gave the appearance of pseudo-sources. It is 

Test Name Elevation 

H/Dexit

Drift Y/Dexit Trench H20 

(gpm) 

Hole H20 

(gpm) 

On-deck H20

(Ww/Wp)* Comment 

Horizontal n/a n/a n/a n/a n/a 

ASMAT 3 0 0 0 0 0 With LM 

ASMAT 4 3.9 0.61 873 291 0 With LM 

ASMAT 7 7.9 0.9 873 291 0 With LM 

ASMAT 11 8.6 0.9 873 291 0 No LM 

ASMAT 12 8.6 0.9 873 291 4.5 No LM 

ASMAT 17 8.6 0 0 0 0 No LM 
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interesting to note that Rayleigh criterion of equation 7 places the minimum resolvable frequency 

at around 2.3 kHz for the speaker positions; the present array architecture and the beamforming 

software were found to provide a small improvement (i.e., 85 percent of Rayleigh resolution). 

Figure 7.4-1. Beamformed Maps of Three-Speaker Sources in an Anechoic Chamber 

The color scales represent source strength in a 96-Hz wide frequency band centered at the indicated 

frequencies. 

Attempts were made to apply some of the advanced beamforming algorithms to improve the spot 

size at the low frequency and to reduce the side lobes at high frequencies (see Appendix A). 

Typically, such procedures were found to work well for compact noise sources. However, other 

difficulties arose for different algorithms when attempts were made to use with the rocket test 

data. The test stand created a reverberant environment and the noise sources were distributed; 

both of these seemed to create various problems in the attempt to deconvolve psf. Other 

investigations are in progress to achieve this goal. For this report, only conventional beamform 

data are presented. 

Finally, there is a need to discuss uncertainties in the absolute magnitude of the noise sources. 

Note that the color scale next to each figure of the beamformed map shows the magnitude of the 

noise sources as measured at the array location. Typically, the color scale covers the peak 

10-dB range of the sources. The uncertainties in these numbers arise due to three reasons: the 

inherent convolution of the true noise source with the array psf; the short duration of the motor 

burn; and a loss of coherence among microphone pairs. The first reason was discussed earlier. 

The steady part of the motor burn typically lasted for 2.2 seconds (Figure 7.4-2a), which was 

used for most of the beamformed results. The relatively short duration of the sample implied 
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fewer available cycles at the lower frequencies and increased side-lobes at the high frequencies. 

This is the second reason for uncertainties. The third reason is the inherent nature of the 

distributed noise sources. Data gathered from ASMAT tests showed a progressive loss in 

coherence for sensor pairs with longer separation distances. This loss was found to increase with 

frequency (Figure 7.4-2b). Therefore, the averaging process of the beamforming technique led 

to source strengths that were lower than the levels seen in the auto-spectra. No attempts were 

made to compensate for this loss. Nevertheless, the limitations are similar between two ASMAT 

configurations. Although there is a higher level of uncertainty in the absolute magnitudes of the 

noise sources, the relative differences between any two cases is believed to be reliable, and 

within a fraction of a dB. Therefore, the beamformed maps should be used for a comparative 

study among various ASMAT configurations. 

Figure 7.4-2. (a) Time Trace of Pressure Fluctuations from one of the Microphones in the Horizontal 

Motor Test; (b) Coherence Spectra Between indicated Pairs of Microphone 

Figure 7.4-3 shows in-situ validation obtained from the ASMAT test stand where a single 

speaker was located at a corner of the launch pad. Recall that the array was about 16 feet above 

ground with a ~45-degree angle. The background image was captured by the video camera at the 

center of the array box. Similar to the prior figure, smearing of the source was progressively 

reduced with an increase of the beamform frequency while the number of the pseudo-sources 

increased with frequency. A large number of reflective surfaces were around the model. 

Besides the concrete floor and the metallic pad, there were steel blast curtains on the backside of 

the model. 
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Figure 7.4-3. Validation of the Phased Array Operation via Identification of a Single Speaker Source 

(at the bottom-left corner; the four plots are for the indicated four different frequencies) 

The color scales show sound pressure levels over a 48-Hz wideband centered at the indicated 

frequencies.

The presence of these reflective surfaces created images of the noise source and increased the 

population of pseudo-sources (side-lobes). The “sweet range” was found to be between 2 and 

6 kHz, where the array resolution was reasonably good and the pseudo sources were mostly 

lower than 10 dB. Most of the ASMAT test data presented in this report is in this frequency 

range. Using plume properties for Strouhal frequency calculations, 2.5 and 5 kHz correspond to 

St = fD/U of 0.2 and 0.4, respectively. Strouhal numbers are used for the remainder of the 

figures in this report. 

7.4.2 Horizontal Rocket Motor Only Test 

The setup used in the horizontal test was discussed with Figure 7.2-2. Unlike the image in that 

figure, the camera at the center of the array plate captured a better image of the plume 

(Figure 7.4-4a). This frame, from the initiation of the burn, shows a clear periodic shock pattern 

present in the under-expanded plume. The glow in the plume was due to afterburning. The 

smoke was believed to be mostly made of aluminum oxide powder. The three vertical lines are 

the poles used for holding microphones for a separate suit of instrumentation. 

Figure 7.4-2 shows a microphone time signal, and auto and cross-spectra collected from the 

horizontal test. Note that the spectra have double humps, separated by a dip, such features are 

indicative of interference from distinct sources. Conventional beamform results are shown in 
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Figure 7.4-4(b). The first observation is that there exist two distributed noise sources: one along 

the plume and the other is a reflection on the concrete test floor. The concrete pad acted as a 

mirror to the plume sources. It is obvious that any attempt to make free-field acoustic 

measurements in a horizontal rocket stand needed to pay attention to this reflection that nearly 

doubled the true amplitude. 

Figure 7.4-4. (a) The Plume of a Solid-Rocket Motor Fired Horizontally over a Concrete Pad; and 

(b) Noise Source Distribution at indicated Frequency as Measured by the Phased Array 

Another interesting observation was the long spatial extent (greater than 30 diameters) of the 

noise sources that extended to the end of the visible afterburning core of the plume. The quasiperiodic shock pattern of the plume was found to create periodic modulation of the noise source. 

An examination of the source maps at other frequencies (not shown) showed the peak in the 

source distribution progressively moved closer to the nozzle exit with an increase in frequency. 

However, the downstream extent of the sources remained almost independent of frequency. 

Supplementary material, video file “Horizontal.avi” 

The time-lapsed video showed evolution of the noise sources from ignition to termination of the 

Rocket-Assisted Take Off (RATO) motor. It substantiated the effectiveness of the phased array 

in following the time varying noise source. At the end of the burn, as the plume became shorter, 

so did the distribution of noise sources. The reflected noise source followed suit. 
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7.4.3 Vertical Tests with Model of Ares I Vehicle 

ASMAT 3 

Figure 7.4-5 shows the distribution of the noise sources at four different frequencies for a holddown condition (i.e., at zero elevation). No water was used for this test. An image of this test 

showing the locations of the hot plume and the position of the array is shown in Figure 7.2-3. 

The background image of Figure 7.4-5 was collected before the test by the camera at the center 

of the phased array. As expected, the source distribution at the lowest frequency, St = 0.08 

(1 kHz), was excessively smeared due to the low resolution of the array. While at the highest 

frequency, St = 0.8 (10 kHz), the multitudes of side-lobes were artifacts of the beamforming 

process. The best balance between the two extremes, the “sweet-spot,” are the maps at 

St = 0.2 (2.5 kHz) and 0.4 (5 kHz). 

Figure 7.4-5. Beamformed Noise Sources at Four indicated Frequencies. ASMAT3: Model is at 

Hold-down Position, without Water Injection, and LM is in Place 

The color scales show sound pressure levels over a 48-Hz wideband centered at the indicated 

frequencies. 

Figure 7.4-5 identifies two strong sources: the gap between the MLP and the trench, and the open 

part of the flame trench on the right-side of the launch tower. An image of these two regions can 

be seen in the side view of Figure 7.4-6. Additional video footage taken during the test verified 

the flow of hot gas through the trench in absence of trench water. This hot plume and its 

impingement on the deflector are believed to be the origin of the two sources. There was no 

leakage of hot plume coming out of the gap between the MLP and the trench. Therefore, it is 
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believed that the first part of the identified sources originated inside the trench, and then spilled 

out through the gap between the MPL and the trench. Note that this gap existed on all four sides. 

Therefore, the vehicle model was expected to be exposed to sound radiation from all sides. A 

phased array detects only those sources radiating towards the array surface; that is why the 

beamformed plots of Figure 7.4-5 identifies only the front side of the MLP as the source. 

Closing the gap between the MLP and the trench should reduce the acoustic level experienced by 

the vehicle at the hold-down condition. 

An important observation can be made by comparing the noise source model of Eldred-Jones 

(Figure 6.0-1) [ref. 5] and the measured source distribution of Figure 7.4-5. The rocket plume 

shot out of the right-hand-side trench exhaust. The Eldred-Jones model [ref. 5] assumes the 

entire plume length to be the noise source, while the beamformed map shows that the peak 

10-dB range of the source are confined within the trench. The presence of the second source at 

the MLP pad gap could not be envisioned from the Eldred-Jones formulation [ref. 5]. The 

difference between the Eldred-Jones model [ref. 5] and the source distribution became more 

distinct in the subsequent tests. 

Figure 7.4-6. Flow of Hot Gas through the Trench 

(Image Fusion visualization, courtesy: Louise Walker, UARC and ARC) 

Supplementary material, video file “ASMAT3.avi”

The video footage confirmed the description of the sources during the steady part of the burn. 

During the later part of the video, when the motor progressively tapered off, the noise sources 

weakened. First, the source at the open part of the trench disappeared, followed by the sources 
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around the MLP pad gap. At the end of the burn, the peak source moved to the nozzle exit, on 

the top of the LM, but the source strength was lowered by >30 dB.

ASMAT 4 

An image of the ASMAT 4 test is shown in Figure 7.4-7(a). For this test, the model was lifted 

by 3.9Dexit above the LM, thereby exposing a part of the plume above the launch pad. More 

importantly, the model was drifted towards the launch tower by about 0.61Dexit. This allowed 

part of the plume to spill out of the hole and impinge on the LM, MLP, and the base of the 

launch tower. The lift-off trajectory for the Ares I vehicle dictated this drift; the model-scale 

static test replicated this scenario. Figure 7.4-7(b) shows the off-center location of the nozzle 

exit with respect to the LM. The other significant part of the test configuration was the water 

injection. A large volume of water was injected inside the trench and the MLP hole, and on the 

top of the flame deflector via ducts built on the pad. The water flow was initiated before motor 

ignition and continued after the completion of the burn.

Figure 7.4-7(a). Sparks Fly Off the Plume Impingement Point on 

LM and MLP in ASMAT4: Model Lifted by 3.9Dexit above the MLP 

Figure 7.4-7(b). Closer View 

showing Drift to the Right of the 

LM/MLP Hole 

Figure 7.4-8 shows the noise source locations identified at three different frequencies. 

Unfortunately, the video obtained from the array-camera was underexposed. However, the 

outline of the trench and the MLP were visible. The bottom part of the model is visible as the 

white strip at the top center of each image; as marked, and the nozzle exit lies just below this 
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strip. An examination of the source maps shows that the primary noise source extended from 

one side of the LM to the base of the tower. This was the region where the plume from the 

rocket motor impinged on the pad. The beamformed source map shows that the impingement of 

the plume is responsible for most of the noise generation. Note that a large part of the plume 

passed through the trench. However, unlike ASMAT 3, water injection inside the MLP hole and 

the trench weakened the source so much that it was not visible in the top 10-dB range shown. 

Finally, the noise map has no resemblance to the Eldred-Jones model [ref. 5] of Figure 6.0-1, 

which missed the impingement zones as lift-off noise sources. The team believes that the 

Eldred-Jones model misses many other aspects of the noise path and source distribution. To 

improve comparison with flight and test data, the model makes an empirical assumption of 

reduction in noise generation efficiency of a deflected plume. 

Figure 7.4-8. Beamformed Noise-Sources at the indicated Frequencies Measured in ASMAT 4 

Supplementary material, video file “ASMAT4.avi” 

The noise sources during the steady part of the burn are the same as previously described. As the 

plume weakened, so did the impingement source. At the very end of the burn, the source moved 

up and stayed around the nozzle exit. 
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ASMAT 17 

In ASMAT 17, the sound suppression systems were turned off (completely dry), and the model 

was held at 8.6Dexit above the MLP platform. Additionally, there was no sidewise drift, so that 

the maximum possible part of the plume was passing through the MLP hole. Figure 7.4-9 is a 

frame from a video camera that captured a side view of the motor burn. This figure shows the 

bare part of the plume (seen through the support tower) above MLP, and the exhausting hot 

gases through the trench. The beamformed maps of Figure 7.4-10 show a U shape. A closer 

examination revealed that the noise sources were at three connected regions: like ASMAT 3, the 

gap between the MLP and the trench; like ASMAT 3, the open part of the flame trench on the 

right-side of the launch tower; and unlike ASMAT 3, the region around the MLP hole. The 

absence of hole and trench water brought the first two sources back to prominence. The third 

region was due to the plume impingement around the hole of the MLP deck. The hole was 

designed to be small to minimize IOP reflection. Despite no sidewise drift of the model, the 

edges of the plume impinged on the rim of the hole and created the loudest noise source. Note 

that the background image used to superimpose the beamformed map in Figure 7.4-10 was from 

a video taken before the motor ignition. Note the ladder, man-lift, amplifiers, etc., were removed 

before the test. 

Figure 7.4-9. Side View of the Model and the 

Launch Pad in ASMAT 17 

(no water, elevation =8.6 Dexit, drift = 0) 

Figure 7.4-10. Beamformed Noise Sources from 

ASMAT 17 at indicated Two Frequencies 
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Supplementary material, video file “ASMAT17.avi” 

This video provided the clearest indication that the plume was a weaker (at least by 10 dB) 

source than the rest described above. At the end of the burn, when all three of the identified 

sources disappear, the plume region increased in the source map. This showed that the prime 

part of the burn plume created noise sources that were below the 10-dB plot range. 

ASMAT 11 

The hole and the trench water system were turned on in ASMAT 11. The model was kept at the 

same elevation, but was drifted per trajectory information. Figure 7.4-11 is a front view of the 

test articles with the phased array setup in the foreground. The beamformed maps, shown in 

Figure 7.4-12, provide the clear indication of the effectiveness of the water system: the trench 

and the gap around the MLP are no longer in the source maps and the gap is no longer the 

primary issue. However, compared to ASMAT 17, a larger part of the plume impinged on the 

deck resulting in an increase of the peak source strength (see color bar). In other words, 

compared to ASMAT 17, the spatial extent of the sources is much smaller, yet the peak level is 

higher. 

Figure 7.4-11. Photograph of ASMAT 

11 (drift = 0.9Dexit, elevation = 8.6Dexit, 

hole and trench water) 

Figure 7.4-12. Beamformed Noise Source Maps at indicated 

Frequencies from ASMAT 11 
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Supplementary material, video file “ASMAT11.avi” 

Similar observations, as described with the ASMAT17.avi file, can be made from this video. 

ASMAT 12 

All three parts (hole-water, trench-water, and rainbird) of the water suppression system were 

turned on in ASMAT 12. The vehicle elevation and drift were the same as in ASMAT 11. A 

large flow of water was used for this test. The mass flow of water was 4.5 times the propellant 

mass flux. Unlike a nominal lift-off situation, the rainbird system was turned on before the 

motor ignition. So, there was a pool of water accumulated on the top of the MLP when the 

motor was ignited. Figure 7.4-13 shows the plunging of the plume in this pool resulted in a large 

splash of water. A comparison of the microphone time traces between this and ASMAT 11 is 

shown in Figure 7.4-14 and shows observable attenuation of the acoustic level. The 

0.02-second pressure traces (shown in Figure 7.4-14 (b) and (d)) show that the “N-waves” in the 

Mach wave radiation were particularly affected; the high positive peaks of the sound waves show 

good attenuation from the rainbird water. Figure 7.4-14(c) indicates a time-dependent 

attenuation. After ignition, when the plume plunged into the accumulated pool, there was a large 

attenuation. However, the levels increased significantly (after a fraction of a second) when the 

accumulated pool evaporated and the plume impinges on the MLP deck. In a nominal launch, 

the distance between the nozzle exit and the MLP deck would continuously increase and the 

time-dependent interaction between the plume and water deluge is likely to be different. The 

beamformed maps shown in Figure 7.4-15 identify compact noise sources around the 

impingement point. A benefit of the rainbird system is that it attenuates peripheral sources that 

may be caused by the deflected part of the plume re-impinging on the service tower. 

Nonetheless, the peak source strength in Figure 7.4-15 is only a couple of dB lower than that of 

no rainbird case (Figure 7.4-12). 

Figure 7.4-13. Photograph from ASMAT 12: 

Rainbird (on-deck) Water Injection 

Figure 7.4-14. (a, b) Time Traces from an Absolute 

Transducer on the Array Plate, 

(c) ASMAT 11, (d) ASMAT 12 
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Figure 7.4-15. Beamformed Noise Source Maps at the indicated Frequencies from ASMAT 12 

(drift = 0.9Dexit, elevation = 8.6Dexit, hole, trench, and on-deck water) 

Supplementary material, video file “ASMAT12.avi” 

Similar observations as described with ASMAT17.avi file can be made from this video. 

Discussions are centered on the location and strength of the noise sources as seen by the phased 

array. The acoustic levels are due to an integration of radiation from all such sources. A 

comparison of the acoustic spectra from the five ASMAT tests discussed is shown in 

Figure 7.4-16. Each spectrum in this figure is an average from all 70 microphones present in the 

array. It was difficult to make a direct comparison since more than one parameter was changed 

between most pairs. However, a comparison between the no-LM cases (i.e., ASMAT 17, 

ASMAT 12, and ASMAT 11) showed that the water suppression system produced attenuation in 

almost all frequency ranges. Compared to ASMAT 12, the addition of the rainbird water in 

ASMAT 11 produced a 3.4-dB reduction of the overall level at the array location. A 

comprehensive discussion of the effectiveness of the water suppression system on the model and 

the launch tower is expected from the MSFC testing team. Finally, all spectra of Figure 7.4-16 

have double or multiple humps that are tell-tale signs of strong reflection from various parts of 

the test stand. Some of these reflective surfaces, such as the pad and the ground, are 
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representative of the launch environment; while the blast curtain is not. It is difficult to separate 

the impact of these two types of reflections. 

Figure 7.4-16. Comparison of Average Spectra from the Indicated Tests; Average Overall 

70 Microphones of the Phased Array 

 
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8.0 Findings, Observations, and NESC Recommendation 

8.1 Summary and Conclusions 

A microphone phased array, comprised of 70-element condenser 0.25-inch-diameter 

microphones, was successfully used with a minimal loss of sensors in seven ASMAT hotfires. 

Part of the hardware was custom-built for the test. The processing software was written in the 

Matlab® platform. The existing array was ruggedized for use in an outdoor rocket test facility. It 

was built and tested in the ARC acoustics facility, and then shipped to MSFC for rebuilding 

around test stand TS116. The ASMAT series of static tests used a 5-percent-scaled model of the 

Ares I vehicle and the launch pad. 

8.1.1 Findings 

The following significant findings were made: 

F-1. The beamformed noise source maps, measured from the horizontal, motor-only test, 

showed long coherent noise sources modulated by the shock cells that were present in the 

plume. 

F-2. The plume impingement zones were found to be the primary noise-sources in all vertical 

tests that simulated various altitudes during lift-off. 

- As the vehicle was elevated, a part of the plume spilled outside the MLP hole 

creating an impingement zone. 

- Lateral drift of the vehicle further strengthened plume impingement. 

F-3. The uncovered part of the trench, the deflector, and the gap around the MLP were major 

noise sources when no water was used for attenuation. 

F-4. The noise sources inside the trench were found to be significantly attenuated via water 

injection in the MLP hole, on the top of the deflector, and inside the trench. 

F-5. Results obtained from one test involving injection of the on-deck water (rainbird) initially 

showed effectiveness in source mitigation. The motor plume was found to displace water 

quickly and to reduce the water’s effectiveness; albeit the spatial extent of the source was 

reduced. 

F-6. It was found that the current lift-off model (SP8072) fails to account for the impingement 

noise-sources. There is a need to update this model. 

F-7. The phased array (and its sensors), if properly designed and protected, will survive the 

weather and motor hotfire environments reliably. 

F-8. Attempts were made to apply some of the advanced beamforming algorithms to improve 

the frequency range of the test array. Different sets of difficulties arose for different 

algorithms when attempts were made to use with the rocket test data. More work is 

needed to achieve this goal. 
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8.1.2 Observations 

The following observations were identified: 

O-1. With the right sets of safeguards, sensitive electronic components of a microphone 

phased array were found to withstand the harsh environment of an outdoor rocket test 

facility without any significant problem. In return, the phased array was found to provide 

previously unavailable insights into the noise sources. 

O-2. The 40-inch-diameter array was found to be most effective in resolving noise sources 

above 2 kHz, which was satisfactory for the present model scale test. 

- For application in a full-scale launch where frequencies of interest are below 

2 kHz, the array either needs to be placed close to the pad or needs to be larger in 

size. 

8.1.3 NESC Recommendation 

The following NESC recommendation was identified and directed towards future use of phased 

array in lift-off applications: 

R-1. For the present test, the array was placed away from the model. Since the noise sources 

are directional in nature, it is recommended to place the array closer to the model. This 

will make the noise source maps more relevant for the vehicle development program. 

(All Findings) 

9.0 Alternate Viewpoint 

There were no alternate viewpoints identified during the course of this assessment by the NESC 

team or the NRB quorum. 

10.0 Other Deliverables 

No unique hardware, software, or data packages, outside those contained in this report, were 

disseminated to other parties outside this assessment. 

11.0 Lessons Learned 

No applicable lessons learned were identified for entry into the NASA Lessons Learned 

Information System. 

12.0 Definition of Terms 

Corrective Actions Changes to design processes, work instructions, workmanship practices, 

training, inspections, tests, procedures, specifications, drawings, tools, 

equipment, facilities, resources, or material that result in preventing, 

minimizing, or limiting the potential for recurrence of a problem. 
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Finding A conclusion based on facts established by the investigating authority. 

Lessons Learned Knowledge or understanding gained by experience. The experience may 

be positive, as in a successful test or mission, or negative, as in a mishap 

or failure. A lesson must be significant in that it has real or assumed 

impact on operations; valid in that it is factually and technically correct; 

and applicable in that it identifies a specific design, process, or decision 

that reduces or limits the potential for failures and mishaps, or reinforces a 

positive result. 

Observation A factor, event, or circumstance identified during the assessment that did 

not contribute to the problem, but if left uncorrected has the potential to 

cause a mishap, injury, or increase the severity should a mishap occur. 

Alternatively, an observation could be a positive acknowledgement of a 

Center/Program/Project/Organization’s operational structure, tools, and/or 

support provided. 

Problem The subject of the independent technical assessment. 

Proximate Cause The event(s) that occurred, including any condition(s) that existed 

immediately before the undesired outcome, directly resulted in its 

occurrence and, if eliminated or modified, would have prevented the 

undesired outcome. 

Recommendation An action identified by the NESC to correct a root cause or deficiency 

identified during the investigation. The recommendations may be used by 

the responsible Center/Program/Project/Organization in the preparation of 

a corrective action plan. 

Root Cause One of multiple factors (events, conditions, or organizational factors) that 

contributed to or created the proximate cause and subsequent undesired 

outcome and, if eliminated or modified, would have prevented the 

undesired outcome. Typically, multiple root causes contribute to an 

undesired outcome. 

13.0 Acronyms List 

ARC Ames Research Center 

ASMAT Ares I Scale Model Acoustics Test 

CSM Cross-Spectral Matrix 

DAMAS Deconvolution Approach for the Mapping of Acoustic Sources 

dB Decibel 

FY Fiscal Year 

IOP Ignition Over-Pressure 
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KSC Kennedy Space Center 

LaRC Langley Research Center 

LM Launch Mount 

MLP Mobile Launch Platform 

MPCV Multi-Purpose Crew Vehicle 

MSFC Marshall Space Flight Center 

MTSO Management Technical Support Office 

NESC NASA Engineering and Safety Center 

NRB NASA Review Board 

PC Personal Computer 

psf Point-Spread Function 

RATO Rocket-Assist Take Off 

SLS Space Launch System 

SRB Solid Rocket Booster 

TDT Technical Discipline Team 

14.0 References 

1. Panda, J. and Mosher, R., 2011, “Use of a Microphone Phased Array to Determine Noise 

Sources in a Rocket Plume,” AIAA paper 2011-974, presented at 49th Aerospace Sciences 

Meeting. 

2. Haynes, J., and Kenny, J. R., 2009, “Modifications to the NASA SP-8072 Distributed Source 

Method II for Ares I Lift-off Environment Predictions,” AIAA paper 2009-3160, presented at 

the 15th AIAA/CEAS Aeroacoustics Conference, Miami, Florida. 

3. Plotkin, K. J., Sutherland, L. C., and Vu, B., 2009, “Lift-off Acoustics Predictions for the 

Ares I Launch Pad,” AIAA paper 2009-3163, presented at the 15th AIAA/CEAS 

Aeroacoustics conference, Miami, Florida. 

4. Varnier, J., 2001 “Experimental Study and Simulation of Rocket Engine Free jet Noise,” 

AIAA J. Vol. 39, No. 10, 2001. 

5. Eldred, K. M. and Jones, G. W. Jr., 1971 “Acoustic Load Generated by the Propulsion 

System,” NASA SP-8072. 

6. Tsutsumi, S., Kato, S., Fukuda, K., Takaki, R., and Ui, K. 2009, “Effect of Deflector Shape 

on Acoustic Field of Launch Vehicle at Lift-off,” AIAA paper 2009-328, presented at the 

47th AIAA Aerospace Sciences Meeting, Orlando, Florida. 

7. Underbrink, J. R., 2001, “Aeroacoustics Phased Array Testing in Low Speed Wind Tunnels,” 

in Aeroacoustics Measurements, Springer, Ed. Thomas J. Mueller.

8. Brooks, T. F. and Humphreys, W. M. Jr., 2006, “A Deconvolution Approach for the 

Mapping of Acoustic Sources (DAMAS) Determined from Phased Microphone Arrays,” 

J. Sound & Vib., 294, pp. 856-879. 
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9. Gély, D., Elias, G., Bresson, C., Foulon, H., Radulovic, S, 2000, “Reduction of Supersonic 

Jet noise. Application to the Ariane 5 Launch Vehicle,” AIAA paper 2000-2026, 

6th AIAA/CEAS Aeracoustics Conference, Hawaii. 

10. Sankaran, S., Ignatius, J. K., Ramkumar, R., Satyanarayana, T. N. V., Chakravarthy, S. R., 

and Panchapakesan, N.R., 2009, “Suppression of High Mach Number Rocket Jet Noise by 

Water Injection,” J. Spacecraft & Rockets, Vol. 46, No. 6, pp. 1164-1170. 

11. Sutherland, L.C., 1993 “Progress and Problems in Rocket Noise Prediction for Ground 

Facilities,” AIAA Paper 93-4383. 

12. Dougherty, N. S., Nesman, T. E., Guest, S. H., 1982, “Shuttle SRB Ignition Overpressure: 

Model Suppression Test Program and Flight Results,” presented at the JANNAF 13th Plume 

Technology Meeting, Houston, Texas, April 27-28, 1982. 

13. Dougherty, N. S. and Guest, S. H., 1984, “A Correlation of Scale Model and Flight 

Aeroacoustic Data for the Space Shuttle Program,” AIAA paper 1984-2351, presented at the 

9th AIAA Aeroacoustics Conference, Williamsburg, Virginia. 

14. Fukuda, K., Tsutsumi, S., Fujii, K., Ui, K., Ishii, T., Oimuna, H., Kazawa, J., Minesugi, K., 

2009 “Acoustic Measurement and Prediction of Solid Rockets in Static Firing Tests,” AIAA 

paper 2009-3368. 

15. Burnside, N. J., Jaeger, S. M., Reinero, B. R., Horne, W. C., and Soderman, P. T., 2002, 

“Array Design and Performance for a Large Scale Airframe Noise Study,” AIAA paper 

2002-2576. 

16. Dougherty, R. P., Stoker and R. W., 1998, “Sidelobe Suppression for Phased Array 

Aeroacoustic Measurements,” AIAA-98-2242. 

17. Sijtsma, P., “CLEAN-Based on Spatial Source Coherence,” 13th AIAA/CEAS Aeroacoustics 

Conference 28th AIAA Aeroacoustics Conference), AIAA 2007-3436. 

18. Dougherty, B., “DAMAS, Eigen values, CLEAN-SC and TIDY,” keynote address, AIAA 

Aeroacoustics Conference, May 2009. 

15.0 Appendices 

A. Advanced Beamforming Efforts 
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Appendix A. Advanced Beamforming Efforts 

As discussed earlier, the source map obtained via the conventional beamform process has poor 

resolution at low frequency range, and the presence of many side lobes in the high frequency 

range, which may appear as pseudo sources. Besides, the uncertainty in the absolute source, 

amplitude is high in the conventional beamform results. Many of these difficulties are due to the 

smearing and interference associated with the psf inherent to the array layout. To alleviate such 

difficulties, in recent times, there have been multiple efforts to deconvolve the psf from the 

beamformed output. The most straightforward one, DAMAS [ref. 1], involves large matrix 

manipulation and long computing time. Efforts were made, with limited success, to implement 

DAMAS. There are multiple other approaches [refs. 8, 16, 17, and 18] with their own 

shortcomings, such as DAMAS [refs. 2, 16, and 18] and CLEAN-Spatial Correlation (SC) 

[ref. 17]. Both of these algorithms were attempted during this assessment. In the following 

mathematical steps, Matlab® implementations and sample results are presented. 

A.1 Deconvolution using DAMAS 

If there is a noise source at one point in the interrogation grid, the inherent psf of the array 

spreads its influence to all other grid points. If the distribution of the true source is 

xj, j = 1, 2, ... N, then the conventional beamform level at any grid point j (equation 3) is the 

following sum: 

(8) ேݔே௝ܣ൅ڮ൅ ௝ݔ௝௝ܣ൅ڮ൅ ଶݔଶ௝ܣ ൅ ଵݔଵ௝ܣ ൌ௝ ܾ

Here, Aj1 is the influence of a source at the grid point 1 to the beamformed map at point j, and 

similarly for other terms. Every term of the influence matrix Ajj’ can be calculated based on the 

original assumption of monopole noise source of unit strength at location j/ and the resulting 

steering vectors: 

ܣ௝௝Ȁ ሺ݂ሻ ൌ 

࢐࢝

಩ࢃቀሾሿ

Ȁቁ࢐

૙సࢍࢇ࢏ࢊ

࢐࢝಩ࢃ

ሺσ ௐ೘ሻమିሺσ ௐ೘ሻ (9) 

The term in parenthesis is the modeled CSM corresponding to a unit monopole source. It can be 

calculated using steering vectors from the source point j/ to every microphone locations 

l = 1, 2, ...M as the following:

൫ሾሿ௝Ȁ ൯ൌ൥ሺݓଵ

ଵݓሺ ڮ ଵݓሻכ

ெݓሻכ

ڭ ڰڭ

ெݓሺ ڮ

ெݓሻ כ

൩

௝Ȁ

 (10)

Succinctly, equation 8 above can be expressed as the following: 

ܾ௝ሺ݂ሻ ൌ σ ܣ௝௝Ȁ ௝Ȁ ݔ௝Ȁ (11) 
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This represents a system of linear equations whose solution should provide amplitude of the true 

sources xj. However, the psf varies slowly between grid points, and the rank of the matrix A was 

found to be significantly low, which indicates that equation 11 can accept many different 

solutions (uniqueness problem). Brooks and Humphreys [ref. 8] provides an outline of an 

iterative Gauss-Seidel scheme along with a regularization procedure that was implemented in a 

Matlab® code. Typical success is shown in Figure A-1. For data collected from anechoic 

chamber, the scheme was found to converge to desirable solutions, albeit with some effort 

especially with the multiple speaker data. Finding a converged solution for data taken from the 

ASMAT stand proved to be far more difficult. To check for the convergence of the solution 

scheme, the summed-up rms of source fluctuations (xj) was plotted as a function of the number 

of iterations. Figure A-1 shows that the deconvolution scheme diverged for the data collected 

from the horizontal fire test. ASMAT stand created a semi-reverberation environment. Not all 

reflection sources could be considered into the interrogation region. The monopole assumption 

of the long coherent noise sources may not be appropriate. 

Figure A-1. DAMAS Deconvolution: (top row) a Single Microphone Noise Source in Anechoic 

Chamber; (bottom row) Plume from Horizontal Motor Fire 

 
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A.2 Deconvolution using CLEAN-SC 

The CLEAN-SC is a relatively fast and well-understood methodology among many available 

faster procedures to remove the influence of the psf on the source map. However, one needs to 

trace the underlying sources in the conventional map before applying CLAEAN-SC. The 

deconvolution processes work on the conventional map to sharpen the location of the source and 

to help eliminate the side-lobes. 

CLEAN-SC is an improvement over the original CLEAN methodology that is used in radio 

astronomy. The name CLEAN comes from the progressive buildup of a “clean-map” (Ci

) at the 

expense of a “dirty-map” (Di

). Initially, the dirty-map is the conventional beamform expressed 

as 

(12) ௝ܾ ൌ଴ ܦ

The process begins with the identification of the location ξk and level bk of the peak noise source 

in the conventional beamform. The clean-map Ci is started with this location and level 

ܥଵ ൌ ܾ௞ͳͲିఈหకೕିకೖห

మ

 (13)

Here α is a smearing factor, without which the sharp peaks may not be detectable in the plots. 

Next, the expected contribution of an equivalent monopole (of strength bk located at ξk) is 

subtracted from both the CSM and the dirty-map. The peak locations and levels in this revised 

dirty-map are transferred to the clean-map. The process is continued until a convergence 

criterion is met. 

Peter Sijtsma’s contribution in CLEAN-SC [ref. 17] is an improved procedure to subtract the 

contribution of the already found source at the peak location. The procedure is based on the 

coherence values between the peak source bk at location ξk with all other points in the 

beamformed map. Quintessentially, the psf makes the influence of a point source felt over the 

entire beamform map. This spread-out part is established via the coherence levels between the 

source at ξk and all other points ξj in the map. However, this coherence calculation depends on 

whether the diagonal terms of the CSM are deleted or not. 

When the diagonal elements of the CSM are as measured (not deleted), the coherence is 

calculated based on the degraded CSM as 

ડ୨୩

ଶ ൌ หୠౠౡห

మ

ୠౠୠౡ

ǡ ୨୩ ൌ࢐࢝

றࡳሺ௜ିଵሻ

(14) ࢑࢝

The new degraded dirty-map is calculated by subtracting the coherent part from the earlier 

version by using 

ܦ ௜ൌܦሺ௜ିଵሻ

൫ͳ െ ડ௝௞

ଶ ൯ (15)
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Title:

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NESC Request No.: 09-00597 

The degraded CSM is then established by factoring out contribution of an equivalent monopole 

at ξk by using 

ࡳ ௜ൌࡳሺ௜ିଵሻ െ ܾ௞൫࢑࢝࢐࢝

ற൯ (16)

The convergence is achieved when the newly degraded CSM has more energy than the previous 

iteration. This is given by the expression 

௜ࡳห

ห

ଶ ൒  หࡳሺ௜ିଵሻ

ห

ଶ

 (17)

The final clean-map is simply a sum of peaks found in the earlier iteration plus the final 

degraded dirty-map, or 

௜ܥ σ ൌ௙௜௡௔௟ ܥ

௜ ൅ ܦ௘௥௦௜ௗ௨௔௟ (18)

When the diagonal elements of the CSM are deleted, the coherence value Γ [ref. 2] can become 

unphysical since the beamform values bjj can even be negative. Equations 14, 15, and 16 can no 

longer be applied. Instead of directly calculating the coherence (which was ultimately used to 

establish the degraded CSM, equation 16), Sijtsma specifies an effective steering vector h from 

the source point ξk given by 

ࢎ ൌ ଵ

൯࢑࢝ࡴ಩࢑࢝൫ଵା

భȀమ ቀ

࢑࢝ࡳ

௕ೖ

(19) ቁ࢑࢝ࡴ ൅

which is then used to establish the degraded CSM, expressed as 

ࡳ ௜ൌࡳሺ௜ିଵሻ െ ܾ௞ሺࢎࢎற െ ࡴሻ

(20) றࢎࢎ݂݋ݏݐ݈݈݊ܽ݁݁݉݁݊݋݀݅ܽ݃ ൌ ࡴ݁ݎ݄݁ݓ

In the above two equations, h is calculated iteratively starting with an initial value of h = wk. 

The converged h values are obtained within a few iterations. Unlike the equation 15, the 

degraded dirty-map is calculated from the degraded CSM as 

࢐࢝ൌ௜ ܦ

࢏ࡳற

(21) ࢐࢝

which is then used to find out the new peak. The process is continued until the convergence 

criterion of equation 18 is met. Typically, an acceleration factor (between 0.5 and 0.95) is used 

to help extract distributed sources with equations 15, 16, and 20. 

The above scheme was implemented in Matlab® and at first applied to data from individual 

speaker sources in the anechoic chamber. Figure A-2 shows a comparison between results from 

the conventional beamform and CLEAN-SC. The deconvolution procedure significantly 

localizes the source by removing the influence of the psf. However, neither of them can isolate 

the three speakers at the lower frequencies, well below the Rayleigh resolution limit. A closer 

look into the methodologies clarifies this deficiency. The deconvolution procedures start with 

the peak noise identified in the conventional map (equation 6 above). The location and level of 
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Title:

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NESC Request No.: 09-00597 

the peak are due to a sum of effects from all three sources. For frequencies below the Rayleigh 

resolution, the peak in the conventional beamform does not coincide with a speaker location. 

This makes the deconvolution procedure start with an incorrect assumption of the peak noise 

source that ultimately led to incorrect distribution of sources. Fundamentally, for sources spaced 

closer than the Rayleigh resolution, the summed up field may not have a unique solution for 

noise sources; that is, multiple combination of locations and amplitudes may provide the same 

final distribution. Therefore, CLEAN-SC has difficulties overcoming the limitation imposed by 

the Rayleigh criterion. For frequencies higher than the Rayleigh resolution, Figure A-2 shows 

that the deconvolution procedures significantly reduce side lobes and dramatically improve 

beamform results, at least for the point source in an ideal anechoic environment. 

The success, however, becomes less expected when applied to the distributed noise source of the 

horizontal motor test. Figures A-3 and A-4 show comparison between conventional and 

CLEAN-SC. CLEAN-SC converts the distributed sources into spotty blobs that are unphysical. 

In addition to removing the side-lobes, which are coherent with the principle noise source, the 

procedure also takes away the coherent parts of the true sources. No attempts were made to 

apply CLEAN-SC to the rest of the ASMAT data. 

Figure A-2. Comparative Study of Beamforming at the Indicated Frequencies via the Two Indicated 

Methodologies 
NASA Engineering and Safety Center 

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Figure A-3. Conventional Beamform Image of the Noise Source Distribution on the Rocket Plume at 

the Indicated Frequencies 

Figure A-4. CLEAN-SC Beamform Images at the Indicated Frequencies 
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Assessment of Microphone Phased Array for Measuring Launch Vehicle 

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The specific purpose of the present work was to demonstrate the suitability of a microphone phased array for launch acoustics 

applications via participation in selected firings of the Ares I Scale Model Acoustics Test. The Ares I Scale Model Acoustics 

Test is a part of the discontinued Constellation Program Ares I Project, but the basic understanding gained from this test is 

expected to help development of the Space Launch System vehicles. Correct identification of sources not only improves the 

predictive ability, but provides guidance for a quieter design of the launch pad and optimization of the water suppression 

system. This document contains the results of the NASA Engineering and Safety Center assessment.

15. SUBJECT TERMS

Ares I Scale Model Acoustics Test; microphone phased array; vibro-acoustics; NASA Engineering and Safety Center

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