NASA/TM—2013–216625 

 

IDENTIFICATION OF NOISE SOURCES 

DURING ROCKET ENGINE TEST FIRINGS 

AND A ROCKET LAUNCH USING A 

MICROPHONE PHASED-ARRAY 

 

Jayanta Panda 

 

Ames Research Center, Moffett Field, California

Robert N. Mosher 

NASA Langley Research Center, Hampton, VA 

Barry J. Porter 

Aerospace Computing Inc., Mountain View, CA 

 

 

 

December 2013
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This page is required and contains approved text that cannot be changed. 
NASA/TM—2013–216625 

 

IDENTIFICATION OF NOISE SOURCES 

DURING ROCKET ENGINE TEST FIRINGS 

AND A ROCKET LAUNCH USING A 

MICROPHONE PHASED-ARRAY 

 

Jayanta Panda 

 

Ames Research Center, Moffett Field, California 

Robert N. Mosher 

NASA Langley Research Center, Hampton, VA 

Barry J. Porter 

Aerospace Computing Inc., Mountain View, CA 

 

 

National Aeronautics and 

Space Administration

Ames Research Center 

Moffett Field, CA 94035-1000 

December 2013
Acknowledgments 

 

The authors would like to thank Mr. Roberto Garcia of NASA Marshall Space Flight Center and 

Mr. Nans Kunz of Ames Research Center for partially funding this work. Additional support was 

provided by NASA ASP innovative measurements program. Mr. Jeff Reddish and Mr. Mark 

Cording of the Wallops Flight Facility, Mr. Kevin Meehan of NASA Johnson Space Center, and 

Mr. Kurt Eberly and Mr. Tony Gaio of the Orbital Sciences Corporation were instrumental in 

coordinating this multi-agency effort. We benefited greatly from the engineering advice that Mr. 

Mark Cording provided. We would also like to thank Mr. Ted Garbeff for performing the 

modeling work for optimizing the microphone locations, Ms. Laura Kushner for photogrammetric 

verification of these locations, and Mr. Greg Zilliac for allowing us to setup the array during the 

hybrid rocket tests. Finally, the credit for the high-speed camera photos of Fig 11 goes to the 

NASA imagery group at Kennedy Space Center (KSC-KX). 

 

Available from: 

 Click here: Press F1 key (Windows) or Help key (Mac) for help 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 
1

 

IDENTIFICATION OF NOISE SOURCES DURING ROCKET ENGINE TEST FIRINGS AND 

A ROCKET LAUNCH USING A MICROPHONE PHASED-ARRAY 

J. Panda 

NASA Ames Research Center, Moffett Field, CA 

R. N. Mosher 

NASA Langley Research Center, Hampton, VA 

B. J. Porter 

Aerospace Computing, Inc, Mountain View, CA 

A 70 microphone, 10-foot by 10-foot, microphone phased array was built for use in 

the harsh environment of rocket launches. The array was setup at NASA Wallops launch 

pad 0A during a static test firing of Orbital Sciences’ Antares engines, and again during 

the first launch of the Antares vehicle. It was placed 400 feet away from the pad, and was 

hoisted on a scissor lift 40 feet above ground. The data sets provided unprecedented 

insight into rocket noise sources. The duct exit was found to be the primary source 

during the static test firing; the large amount of water injected beneath the nozzle exit 

and inside the plume duct quenched all other sources. The maps of the noise sources 

during launch were found to be time-dependent. As the engines came to full power and 

became louder, the primary source switched from the duct inlet to the duct exit. Further 

elevation of the vehicle caused spilling of the hot plume, resulting in a distributed noise 

map covering most of the pad. As the entire plume emerged from the duct, and the ondeck water system came to full power, the plume itself became the loudest noise source. 

These maps of the noise sources provide vital insight for optimization of sound 

suppression systems for future Antares launches. 

 

Nomenclature: 

b = Beamformed output (dB) t = time from engine ignition (s)

w = Steering vector α = strength of monopole sources 

i=-1 

M = Total no of microphones Superscripts: 

Γ = Coherence † = Complex-conjugate and transpose 

G = Cross-spectral matrix * = Complex conjugate only 

R = Radial position of microphone from array center (in) T = Transpose only 

S = Weight applied to individual microphones mod = model 

θ = Angular position from the image center (rad) Subscripts: 

 = Wavelength (ft) c = center of the array 

d = Aperture of the array (ft) j = index for interrogation grid 

f = Frequency (Hz) m = microphone index 

< > = Expected value R = Rayleigh resolution 

c = speed of sound (ft/s) 

k = wavenumber = 2f / c (1/ft) Acronym: 

N, Ns = No of grid points in the viewing region TEL = Transporter Erector Launcher 

r = distance of a microphone from a source 

(interrogation point)

psf = point-spread-function 

I. INTRODUCTION 

 Every part of a launch vehicle, launch pad and ground operation equipment are subjected 

to the high acoustic load generated during lift-off. The acoustic load is a major contributor to the 

vibro-acoustics environment to which every payload, vehicle structure, propellant storage and 

handling devise, electronics and navigational components have to be designed, tested and 
2

certified. Even a couple of decibel reduction of the acoustic level translates into a sizable 

reduction of cost of certification, operation and the weight of vehicle. The same is true for every 

payload that vehicle carries. Therefore, the lowering of the acoustic level via various mitigation 

schemes is an important aspect of a launch pad design. The first step towards the evaluation of 

any mitigation process is an identification of the sources responsible for noise generation. The 

present work is part of an instrumentation development program to identify launch acoustic 

sources. Typically, single microphones are placed at different locations on a launch pad to 

measure acoustic fluctuations. Such microphones, however, are unable to determine the 

sources. Single microphones provide a measure of the absolute level on the vehicle and other 

places, leaving the cause of noise generation to speculations. In contrast, a phased array of 

microphones along with beam-form processing directly identifies the location and strength of the 

noise sources, and is therefore capable of providing significant insights into the underlying 

cause. 

The present effort is a continuation of the prior work [1, 2] where a smaller size microphone 

phased array was used in model-scale launch acoustic tests. A 5% scaled model of the ARES I 

vehicle was tested in a static firing to determine the noise sources with different types of water 

injection and different elevations of the vehicle. The results provided significant insight into the 

source distribution which was found to be very different from that used for traditional models [3, 

4]. The goal of the present work was to take the phased array development to the next level, 

i.e., application to a full-scale rocket vehicle. The array was used for both a static test fire and 

actual launch. The work presented in this report was a multi-year effort that started with the 

design of a large aperture phased array suitable for the lower frequencies of interest in a fullscale vehicle. The larger aperture is also suitable for placing the array at a greater distance from 

the launch pad. The design of the new large-aperture phased array had conflicting requirements 

for a lightweight structure for ease of transportation, and a sturdy structure to house sensitive 

equipment in the harsh environment of a launch site. The newly built array was then tested in an 

anechoic chamber, and then in a small rocket engine test facility. New data processing software 

were developed to perform beamforming for the transient noise sources seen during the rocket 

launch. Finally all parts were sent to the NASA Wallops Flight Facility for setup near the Antares 

launch pad where the array was rebuilt and deployed during the Antares engine test fire and 

launch. The present paper is a summary report from all these activities; although more 

emphasis is placed on results from Antares launch. 

The only prior application of a set of microphones used for beamforming during an actual 

rocket launch was by Gély et al [5]. However, the premise was vastly different. They attached 

12 microphones distributed around the large payload fairing of the Ariane V vehicle during one 

of its first launches, and applied beamform processing to the flight data. The noise maps, albeit 

of poor resolution, confirmed the source distribution found during a model-scale static-fire test. 

In principle, any set of microphones can be used as a phased array as long as the time signals 

are synchronized, and the microphone locations are known within a fraction of the sound 

wavelength. The present work differs significantly from the prior efforts. Much higher resolution 

maps of the noise sources were obtained by an optimized planar array which was placed on the 

launch pad rather than on the flight vehicle. This also reduced the cost of operation; ground 

sensors cost a small fraction of that of the flight sensors. A phased array resolves sources of 

sound waves that impinge on the array. Ideally a payload fairing mounted array is suitable to 

find sources that specifically radiate towards the payload which perhaps was the motivation of 

Gély et al [5]. However, the cylindrical shape of the fairing does not allow for optimal placement 

of the microphone sensors, leading to poor resolution. Most noise sources of interest to rocket 

launches radiate in all directions (albeit non-uniformly).The results obtained from the present 

effort are expected to be applicable to all parts of launch operation, and particularly suitable to 

optimize the acoustic attenuation systems. Other goals of the present effort are to verify the 

noise source models used in the traditional empirical prediction methods [3, 4] and to provide a 

comparison case for the more recent efforts to predict launch acoustics via computational aeroacoustics [6, 7]. 
3

Ia. Antares Vehicle and NASA Wallops pad 0A: 

Antares is a two-stage, medium class (5000kg) commercial launch vehicle, built and 

operated by Orbital Sciences Corporation. The first stage is comprised of twin NK-33 (AJ-26) 

based liquid oxygen and kerosene (RP1) engines that produced 3265kN of thrust, and the 

second stage is a single ATK castor solid motor. The hot-fire static engine test and the maiden 

launch occurred from pad 0A of Mid-Atlantic Regional Spaceport located at NASA Wallops 

Flight Facility. The launch pad was extensively modified for the engine test and the launch 

which were first ever from this modified pad. A wheeled Transporter Erector Launcher (TEL) 

was used to roll out and erect the rocket on its launch pad. The TEL also acted as the rocket’s 

gantry and was tilted back to release the vehicle during launch. 

Figure 1 presents a collage of photographs showing the location of the phased array with 

respect to the launch pad. An examination of the photos shows the details of launch operation 

relevant to the present discussion. Figure 1(a) was from the instance when Antares was on the 

verge of lift-off; the engines came to full power and the glow of the plume was visible through 

the holes of the launch mount. The TEL was tilted back and the vehicle was released. The 

vehicle was mounted on the top of a flame duct with a single hole and a J-deflector. Plumes 

from the twin engines flowed over the deflector and exit out through the duct outlet away from 

the vehicle, towards the ocean. In order for the plume to go over a nearby sea-wall the trench 

outlet was slanted upward by a nominal 10 angle. Figure 1(a) shows the ejection of a large 

amount of steam, water and plume gases through the duct exit. Figure 1(b) is from a later 

instant of time when the vehicle lifted off with a long trail from the plumes. Figure 1(c) was from 

an earlier static-fire test of the first stage engine. The first stage and a part of the TEL were on 

the pad. The engines were mounted on the same duct but on a different, taller, engine mount 

suitable for the long static burn. Additional details of the launch will be proved later in the text. 

The pad was equipped with an acoustic suppression system made of three different sets of 

water injection ports. A large tank (Fig 1a) was built next to the pad to supply this water. Figure 

2 (a) shows the first set of ports around the duct inlet. The second one was inside the duct and 

sprayed water on the J-deflector. The ‘duct water’ started to flow before engine ignition. The 

third set of nozzles, called “rainbirds”, sprayed water on the top of the launch pad (Fig 2b). The 

“rainbird” system also helped to protect the deck from plume impingements. The individual 

rainbird heads are also visible in Fig 2(a). There were two sets of rainbirds, the shorter ones 

started early while the longer ones were delayed to avoid deluging the bottom of the vehicle. 

II. DESIGN AND CONSTRUCTION OF THE MICROPHONE PHASED ARRAY 

 A rule of thumb to select the array aperture for a desirable source resolution can be found 

from the Rayleigh criterion. For a circular aperture of diameter d, the minimum resolvable 

angular separation (in radians) θR between two sources is directly related to the wavelength via 

the following relation: 

ݏ݅݊ߠோ ൌ ͳǤʹʹ ఒ

ௗ

 (1)

The above equation is mostly satisfied by the conventional beamform procedure, although 

various authors have claimed superior resolution using different deconvolution procedures. 

Judicious positioning of the microphones also helps to improve the resolution. Nonetheless, Eq 

1 provides guidance in selecting the size of the array. The new 10-foot by 10-foot array has an 

aperture three times larger than the older one used in the model scale test [1, 2]. Figure 3 

shows the layout of the microphones on the array face. 
4
5

Fig. 2 Water injection systems used for acoustic attenuation in launch pad 0A. (a) Duct inlet 

water (view from the south-west side); (b) on-deck “rainbird” water (view from the east side). 

Fig. 3 (a) Microphone layout and (b) a photograph of the front surface of the 10-foot by 10-foot 

array. 
6

 The goal of the microphone layout was to maximize the array resolution at the lower 

400Hz ≤ f ≤ 2 kHz range, while also reducing the side lobes. The total number of microphones 

was limited to 70, and the available area limited to a 10ft X 10ft square. Two other constrains 

were necessary: adjacent pairs of microphones could not be closer than 0.5 inches, and no 

microphones could occupy the space within a six-inch diameter circle at the center of the plate. 

The first criterion was for ease in microphone placement, and the second was to reserve the 

array center for two cameras. To design the layout, the first 56 microphones were placed on five 

different logarithmic spirals [8]. The starting points of the spirals were placed on an ellipse 

centered at the middle of the plate. Since the spirals did not fill the corners of the square plate, 

additional 16 microphones were placed along the outer edges of two ellipses, whose major axes 

were along the diagonals of the 10’X10’ square. The pattern turned out to produce a fairly 

narrow point-spread function (psf). The optimization of these initial locations was performed via 

a “simulated annealing process” using Matlab® codes that provided further improvements. The 

cost function used for the optimization procedure was a sum of the angular extent for 3dB 

resolution plus side-lobe rejection (i.e., the difference between the primary peak and the next 

highest peak in psf) at 750Hz. The optimization process distorted the original spirals and 

ellipses and was found to sharpen the psf. interestingly the center starting ellipse was optimized 

to a circle. Table I shows the final microphone coordinates. At 750 Hz, the 3dB resolution of the 

array is 4.12 and the side-lobe reject is 14.8dB. The microphone layout provided a greater 

resolution at low frequency (400 Hz) in exchange for increased side-lobes at high frequency (5 

kHz). Since the goal was for an improved resolution for f 2 kHz, the design suits our purpose. It 

is believed that the same coordinates can be linearly scaled to any other array size, as long as 

the number of microphones is kept to 70. The optimum design frequency of 750Hz is also 

expected to scale accordingly. The theoretical psf at two different frequencies are shown in Figs 

4(a) and 4(c). Figure 4(b) and 4(d) present the measured response in an anechoic chamber 

from a single speaker source placed 61.5 feet away from the array. The source maps are 

superimposed on a photograph taken by the video camera at the center of the array. The 

drawn-curtain shape of the image is due to the barrel correction applied on the photo. Figure 4 

show favorable comparison between the theoretical and measured psf. This provides 

confidence in the array setup and the software used for data analysis. 

 The 10-foot by 10-foot array chamber housed all sensors and the cameras (Fig 3b). To 

provide protection from the environmental elements and the debris field the amplifiers, data 

acquisition systems and computers were packaged for a separate housing. The two units were 

connected by a 164 feet long weather-protected cable bundle that was passed through 

conduits. The data and image acquisition computers were accessed over a local area network 

from a remote location using separate laptop computers. The 70 G.R.A.S. ® 1⁄4 inch, condenser 

microphones with protection grids were flush mounted on the front side of the array box. Every 

microphone was calibrated using a piston phone, and phase-checked with respect to a fixed 

microphone. The array box also held one visible wavelength camera, one infrared wavelength 

thermal camera, and one two-axis accelerometer. The 1.3Mpixel visible wavelength camera 

was useful to determine the look-angle of the array and also for remote observation. The 

640X480 pixels, long infra-red, thermal imaging camera provided a view of the flow path for the 

hot-plume which otherwise was obscured by the steam and debris generated during launch. 

The two cameras were at the center of the array behind windows. The accelerometer provided 

a measure of the vibration levels of the array chamber, and was mounted in one of the cable 

junction boxes. The thermal imaging camera was found to be prone to failure from excessive 

vibration. A special vibration isolation mount was created for the launch; the camera ultimately 

failed but provided data for the first 15sec which was of interest for the present investigation. 

While maintaining sufficient sturdiness, the requirement was to keep the labor required to 

assemble, transport, disassemble, pack, and ship the array to a minimum. To satisfy these 

goals the array chamber was made of lightweight aluminum honeycomb sandwich panels and a 

lightweight metal frame. No individual frame piece or panel weighed more than 60 lbs., allowing 
7

full assembly without the use of cranes. Weather stripping and silicone sealants were applied to 

the panels to help waterproof the enclosure. In order to mount the array box enclosure in 

position for a test, clamps were bolted through the internal aluminum frame, and were attached 

to aluminum trusses (similar to those used in mounting stage lights). The trusses could be 

mounted to either flat steel plate bases or to mobile bases having casters and supporting 

outrigger assemblies for stability. 

Table I. Coordinates of the microphone sensors on the array

Mic no X (in) Y (in) Mic no X (in) Y (in) Mic no X (in) Y (in) Mic no X (in) Y (in) 

1 39.038 54.320 19 37.832 -2.063 37 -17.522 -57.649 55 -48.177 -0.957 

2 19.888 54.582 20 25.713 -2.580 38 -3.475 -46.473 56 -7.293 8.372 

3 50.969 42.057 21 13.167 -6.431 39 -22.849 -45.299 57 -8.114 7.340 

4 27.719 44.828 22 7.012 -1.638 40 -34.087 -52.037 58 -15.909 6.079 

5 0.240 40.952 23 1.291 -7.824 41 -49.729 -45.119 59 -3.087 17.649 

6 36.110 37.818 24 56.511 -20.142 42 -6.053 -33.179 60 -15.571 18.629 

7 15.207 34.928 25 45.617 -18.029 43 -21.375 -34.385 61 -28.823 11.610 

8 57.806 25.570 26 33.317 -15.873 44 -9.896 -20.907 62 -39.169 16.442 

9 36.930 22.191 27 14.705 -11.003 45 -17.660 -22.011 63 -29.618 25.641 

10 26.158 21.465 28 16.208 -22.206 46 -30.982 -20.231 64 -11.293 31.465 

11 11.676 21.531 29 4.540 -23.467 47 -47.917 -23.374 65 -27.496 40.753 

12 3.615 24.247 30 56.983 -35.982 48 -54.667 -27.759 66 -42.338 37.879 

13 44.746 5.035 31 38.695 -30.531 49 -2.614 -15.641 67 -57.622 30.862 

14 30.054 7.361 32 24.473 -36.881 50 -48.234 -11.071 68 -9.501 51.605 

15 18.791 10.272 33 13.829 -35.359 51 -5.466 -6.682 69 -24.369 54.563 

16 10.809 12.377 34 45.139 -46.618 52 -16.420 -4.635 70 -46.127 52.126 

17 6.875 10.035 35 24.375 -55.247 53 -24.660 -7.091 

18 53.553 -3.738 36 7.556 -56.978 54 -35.912 -1.146 

 

Fig. 4 Comparison between calculated and measured point spread function (psf) at two 

indicated frequencies. Calculations are for a source at x=y=0°, and z = 740 inches. 
8

 Additional precautions had to be taken to protect the cameras and microphone sensors 

from exposure to moisture during long outdoor stay, and from steam generated during launch. 

The array chamber was continuously purged with low pressure Nitrogen (Fig 1d shows the 

nitrogen skids). When nitrogen was not available, a dehumidifier was placed inside the 

chamber. However, the most useful protective elements were the thin windows made of Mylar®

film that were glued in front of each microphone (Fig 3b). In addition to providing protection from 

rain, the Mylar® films blocked the storm of sand and other debris, created during the launch, 

from impinging on the sensors. All sensors survived multiple rainstorms and the Antares launch, 

proving effectiveness of the protective devices. The particular Mylar® film was chosen after a 

test of transmission loss in an anechoic chamber where the loss was measured to be less than 

0.1dB. 

 An 80 channel, 24-bit PXI system was used for simultaneous acquisition of the 

microphone and accelerometer signals. In addition, a time pulse at the ignition of the Antares 

engines (t = 0) was recorded to synchronize the microphone signals with the launch events. 

Each microphone channel was simultaneously sampled at 20480 samples/s; providing a usable 

bandwidth of greater than 8 kHz. Personal computers using all solid-state devices were used for 

data and image acquisitions. The amplifiers, data acquisition system and the computers were 

kept in a concrete building next to pad 0A. For the Ames hybrid rocket test they were kept in a 

separate enclosure behind the array box. The personal computers interfacing with the data and 

video acquisition systems were accessed over a local network from a remote station. The data 

acquisition was started manually at t-30s and lasted for 60s. 

III. PHASED ARRAY DATA PROCESSING

IIIa. Conventional beamforming (CB): 

To create maps of the noise sources first, the region where such sources are expected to be 

present is divided into a set of grid points. Typically a planar phased array is somewhat 

insensitive in the depth direction (i.e. along the normal to the array plane); therefore a twodimensional set of grid points (also called interrogation points) is setup. Let j =1, 2, 3...N be the 

interrogation points, and m = 1, 2, ...M be indices for microphones. The radial distance rjm, from 

an interrogation point to an individual microphone, determines the phase shift and the relative 

amplitude measured by the microphone. The steering vector is a column matrix defined to 

incorporate these properties: 

ݓ௝ǡ௠ ൌ ௥ೕ೘

௥ೕ೎

݁ି௜௞௥ೕ೘ (2)

Here rjc is the distance of each interrogation point to the center of the array. Note that: 

௝࢝

ற࢝ ௝ൌ ͳ. The above equation is valid if the noise source radiates as a monopole. This basic 

assumption of the beamforming process has been under scrutiny for the entire history of the 

phased- array technology. The justification has been the little change in the beam-formed map 

obtained by assuming other type of sources. Additionally, a lift-off environment is made of a 

variety of sources, some of which are distributed and some of which are compact, whose pole 

types may not be known a priori. The possibility of making a significant error is minimized by 

assuming that the noise sources are made up of an agglomeration of many monopoles. 
9

Fig. 5 Schematic of beamforming process. 

The time traces of pressure fluctuations from individual microphones are Fourier 

transformed to obtain individual spectra Pm(f). The phase and amplitude relations between 

every pair of microphones are calculated and stored in a cross-spectral matrix G. Every element 

of the cross-spectral matrix (CSM) is calculated as: 

כ௠ܲۃ ൌ Ȁ௠௠ܩ

 (3) ۄȀ௠ܲ

 Typically the Fourier transform is performed over multiple overlapping blocks of data which 

are then multiplied using the above equation 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 electronic noise and wind 

generated noise. Zeroing out the diagonal elements improves the beamformed map [9]. 

 The summing of the microphone signals, i.e. the elements of cross-spectral matrices, 

needs to be preceded by a phase adjustment. These two steps are combined in a matrix 

manipulation which leads to the conventional beamform map: 

ܾ௝௝ሺ݂ሻ ൌݓ௝ǡ௠றܩ௠ǡ௠Ȁݓ௝ǡ௠Ȁ (4) 

 To improve resolution, a linearly increasing weight Sm based on the radial location Rm of 

the microphone was used with the conventional beamforming process [10]: 

ܵ௠ ൌ ܵଵ ൅ ሺͳെܹଵሻܴ௠Ȁܴ଴ (5)

The two constant S1 and W1 were set to 0.1; R0 represented radius of the outermost 

microphone. The row matrix S containing weight factors were used in the following fashion. 

ܾ௝௝ሺ݂ሻ ൌ ௪ೕǡ೘಩൫ௌீௌ೅൯௪ೕǡ೘Ȁ

ሺσ೘ ௌ೘ሻమ (6)

It is worth mentioning here that the noise source results presented in this paper are 

subjected to the inherent assumptions of the conventional beamforming process: that the 

complex sources can be modeled as sum of many monopoles, and sound propagation is strictly 

linear. 

IIIb. Direct Spectral Estimate – Spectral Element Method (SEM) [11]: 

A well-known fact about conventional beamforming process is that it causes excessive 

smearing of the noise sources at the low frequency end, and produces pseudo-sources (side 

lobes) at the high frequency end. This is due to the inherent instrument function (also called the 

point-spread-function) of the phased array. The instrument function, however, can be calculated 

if the sound radiation character of the individual sources is known a priori. The discussion above 

assumes that the sound sources are made of uncorrelated monopoles, which is also followed in 

the SEM method. However, instead of making phase adjustment via Eq 4 above, the direct 

spectral method attempts to find the strength of the monopoles directly from the measured 

cross-spectral matrix G. To this end the noise source region is divided into Ns interrogation 
10

points (also called grid points) and a model cross-spectrum between two microphones m and m/

is calculated as following. The unknowns in the model are the strengths of the monopole 

sources j

2

 at each of the interrogation point j = 1, 2 ... Ns. 

Ȁ௠௠ܩ

௠௢ௗ ൌ σ ݓ௝ǡ௠ߙ௝

ଶݓ௝ǡ௠Ȁ

ே כ௝ ୀଵ ൌ σ ൬

௥ೕ೘

௥ೕ೎

݁ି௜௞௥ೕ೘൰ ߙ௝

ଶ ൬

௥ೕ೘Ȁ

௥ೕ೎

݁

ି௜௞௥ೕ೘Ȁ

൰

ே כ

௝ୀଵ (7)

The positive values of j

2

 assure elimination of the unphysical negative sources. In the 

method proposed by Blacondon & Élias [11] the source distribution is determined via 

minimization of error between the measured and the modeled cross-spectra. 

ܧ൫ߙ௝ǡ ݂൯ ൌ σ ቚܩ௠௠Ȁ െ σ ݓ௝ǡ௠ߙ௝

ଶݓ௝ǡ௠Ȁ

ே כ௝ ୀଵ ቚ

ଶ ௃

௠ǡ௠Ȁୀଵ (8) 

Note that the measured cross-spectral matrix G contains (M2

-M)/2 number of independent, 

non-diagonal, complex values when the diagonal terms are deleted. Counting the imaginary and 

the real numbers separately, the total number of independent variables to be minimized 

becomes J, which is used in Eq 7 above. 

J = M2-M (9)

 Similar to a procedure followed by Casalino et al [12], the calculations were carried out in 

Matlab® using routine lsqnonneg that uses a non-negative least square process. Various 

applications involving plume acoustics have used the SEM method in the past [5, 13]. 

IIIc. Modified SEM 

The modeled cross-spectrum of Eq 7 does not account for an important property of 

distributed noise sources, namely a loss in coherence between microphone pairs. The 

monopole model used for the phased array processing assumes a coherence of unity among 

every microphone pairs. However, the measured data show a progressive lowering in 

coherence as the separation between the microphones is increased [2]. The coherence levels 

drop precipitously with frequency; also the more distributed a source, the weaker is the 

coherence. While sound from a compact point source, such as a single speaker produces 

coherence level of unity over microphones placed at wide angular separations, a distributed 

source such as a jet or a plume creates sound that quickly becomes uncorrelated within a small 

angular separation. This fundamental property was discussed in detail by Tam et al [14]. The 

coherence between every pair of microphones m and m/

 can be calculated easily as following: 

Ȟ௠௠Ȁ

ଶ ൌ ீ೘೘Ȁ

ȁ௉೘ȁቚ௉೘Ȁቚ (10)

The modified SEM used in the present paper simply uses a model cross-spectrum whose 

magnitude is reduced by the measured coherence: 

Ȁ௠௠ܩ

௠௢ௗ ൌ Ȟ௠௠Ȁ σ ݓ௝ǡ௠ߙ௝

ଶݓ௝ǡ௠Ȁ

ே כ௝ ୀଵ (11)

ܧ൫ߙ௝ǡ ݂൯ ൌ σ ቚܩ௠௠Ȁ െ Ȟ௠௠Ȁ σ ݓ௝ǡ௠ߙ௝

ଶݓ௝ǡ௠Ȁ

ே כ௝ ୀଵ ቚ

ଶ ௃

௠ǡ௠Ȁୀଵ (12) 

Since Eq 11 is a more realistic model of the measured cross-spectrum, the least square 

method is found to converge easily with less iteration. However, for higher frequencies, when 

the coherence levels become very poor for a majority of the microphone pairs, the difficulty in 

attaining a converged solution returns. A different array pattern is warranted to resolve higher 

frequencies. 

IV. MATLAB® IMPLEMENTATION: 

 The above beamforming schemes were implemented in the commercially available 

Matlab® platform. A personal computer with a six-core processor was used along with the 

Matlab® Distributed Computing Toolbox. Once the cross-spectral matrix (Eq 3) was calculated 

the rest of the beamforming operation for both conventional and SEM methods happened fairly 

quickly (few seconds for each frequency bin). Depending on the time duration of the microphone 

signals, the CSM calculation took between one-half to three minutes. 
11

 To facilitate direct identification of the noise sources, the interrogation grid was created over 

a photograph of the region of interest captured via either the visible band or the infra-red band 

video camera mounted at the center of the array. When the noise source was stationary, a 

single frame of the captured video was sufficient for further processing. The following section 

describes the process applied to the stationary sources. The additional steps needed for the 

moving launch vehicle are described later in section VII. The first processing step after selecting 

the frame was to perform a correction for barrel distortion due to the fish-eye lens used with the 

camera using the following relation. 

(13) ଷݎߙ ൅ ݎ ൌ ݏ

 Here α is the correction factor, r: radius of each pixel from the center of the image, and s: 

corrected radius. The correction factor, a fixed number for a given lens, was established by trial 

and error. In the next step the photographed region was divided into a uniformly spaced grid. To 

select the number of grid points, Rayleigh criterion was applied to the highest frequency of 

interest (Eq 1). Typically the grid spacing obtained using the Rayleigh criterion was refined by a 

factor between 6 and 20; the higher the factor the slower the processing. 

 The cross-spectral matrix (CSM) of Eq 3 was the first step in data processing. Once 

computed, the CSM can be used with any number of beamforming methods without a need for 

recalculation. In the next step the conventional beamforming was performed (Eq 4), followed by 

a superposition of the beamformed colored maps on a video frame from either the visible band 

or the IR-band camera. 

 Correct registration of the beamformed map and the video frames required a calibration 

process. The look-angle of the video camera changed slightly every time the array was rebuilt. 

To correct for such a small but important change, a speaker was placed at various locations in 

the camera field of view, and the video image was rotated in pitch, yaw and/or roll directions for 

correct superposition of the beamformed maps. The alignment error found from this calibration 

process was used to process test data. This calibration step was applied to both the visible and 

thermal wavelength cameras. 

 Implementation of SEM typically required fewer grid points. Papamoschou [13] 

suggested that the total number of grid points be equal to the total number of unknowns J (Eq 

9). However larger numbers of grid points were found to provide similar results but required 

longer computing time. To limit the computing time to a reasonable duration, the maximum 

number of iterations in lsqnonneg routine was limited to 500. 

 One difficulty with SEM was the excessive localization of a distributed source. A speaker 

source became localized to a single grid point, which provided the impression of a highly 

effective scheme. However, the noise sources in a free plume became localized to a set of 

discrete points, which was unphysical. The localization was so sharp that single, discrete grid 

points were identified as sources. To make such points clearly visible, the Savitzky-Golay 

polynomial smoothing filter was used to smear the discrete point sources. The smeared map 

made the discrete points to appear more distributed. Matlab® routine sgolayfilt was used for this 

purpose. 

V. RESULTS FROM THE STATIC TEST FIRING OF SMALL SCALE HYBRID 

ROCKET ENGINES 

 Figure 6 shows a photograph of the array chamber at the NASA Ames hybrid combustion 

facility. The phased array program took advantage of a series of tests conducted to develop 

paraffin-N2O hybrid rocket engines in the 25,000 lbf thrust range [15]. Participation in this series 

of tests helped to improve familiarity with the newly built array, to fix hardware and software 

related problems, and to provide an in-house preparation stage before the remote-site launch 

related activities. Multiple problems related to exposure to rainstorms and the high vibration 

levels of a rocket firing were observed and corrected. The test stand was built on a concrete and 

steel floor which acted as acoustic reflectors. The top halves of each orange-red noise map of 

Fig 7 are along the actual plume (the actual plumes are not shown as the photographs are from 
12

just before firing) and are the true noise sources. The bottom halves of the orange-red noise 

maps are reflections off the floor. The phased array resolved sources of both direct and 

reflected sound waves that reached its sensors, however; the close proximity of the plume to 

the ground surface did not allow for any separation between the actual and the reflected 

sources. Another interesting observation is the unrealistic noise maps obtained using SEM 

(modified SEM for the present case). The SEM scheme was found to be useful for compact 

sources but not so much for distributed sources. Despite using the smoothing filters on the 

noise maps, the distributed noise source of the long plume was fragmented into multiple spots 

as seen in Figs 7(c) and 7(d). The scenario was analogous to earlier results obtained via 

CLEAN-SC from a different rocket plume where the spatially correlated regions of a distributed 

source were similarly replaced by discrete points [1]. Although the two methods differ in 

implementation, but the central difficulty in treating a correlated noise source seems to be 

similar. 

Fig. 6 The large array setup at 

NASA Ames hybrid combustion facility.

Fig. 7 Noise maps from a hybrid rocket test firing; 

(a), (b) conventional beamform; (c), (d) modifiedSEM beamform.

VI. RESULTS FROM FULL-SCALE STATIC-FIRE TEST OF ANTARES FIRST STAGE 

ENGINES 

After the initial testing at NASA Ames Research Center, the phased array was dismantled 

and shipped to NASA Wallops Flight Facility where it was re-assembled. After connecting all 

sensors, cameras and cable bundles, performing another set of microphone calibration, and an 

overall system check, the array structure was lifted and secured inside the basket of a large 

scissor lift. The scissor lift was driven to a parking lot close to launch pad 0A (Fig 1d). The 

location for the array was decided after much deliberation on accessibility, cost of operation, 

and ease of approval from Orbital Sciences Corporation and Mid-Atlantic Regional Spaceport. 

The participation was subjected to a non-interference criterion, where the operation of the 

phased array could not interfere with the vehicle and launch pad preparation activities. Because 

a phased array identifies the locations and levels of sources of sound waves propagating 

towards its direction, ideally the array would be located near the launch vehicle; however, that 

would have violated the non-interference criteria. The best compromise was found to be a 

location outside a fence surrounding the immediate hazard zone, in the parking lot of an 

adjacent building (z40) that was used for various preparatory activities. The scissor lift stood on 
13

outriggers, and the lift basket carrying the array chamber was hoisted 40 feet above ground. 

The elevation provided a clear view of the launch pad, above various obstacles, and ultimately 

identified noise sources radiating upwards. To secure the lift basket from excessive sway in high 

winds and from the high vibratory loads during launch, four guy wires were tied between the 

corners of the lift basket and ground. The guy wires helped to reduce the vibration which was 

still very high. The specification for the thermal imaging camera was for a maximum level of 2g. 

The camera failed during this test and did not produce any images. However rest of the 

equipment survived and provided good data. Another expected consequence of heavy shaking 

was phase jitter and a loss of coherence among microphones. However, the burn lasted for a 

long time which allowed for sufficient averaging to reduce the net influence. 

 The static fire test (“7K -TEST”) took place on February 22nd, 2013. The vehicle 

configuration for the static-fire test can be seen in Fig 1(c). Only the first stage engines, the 

cryogenic propellant tanks and the bottom part of TEL were erected. The engines were mounted 

on the top of the flame duct with a special test mount for the long, 29-second duration test. 

Water was injected inside the flame duct as well as on the top of the deflector. Figure 1(c) 

shows the hot plume and steam emerging from the duct exit. Traces of the hot plume are also 

visible through the gaps in the engine mount. The phased array is barely visible at the right 

edge of this figure. The engine thrust variation is shown in Fig 8(b). The engines were also 

gimbaled over a wide angular range during the burn. The acoustic signature mostly followed the 

thrust pattern with minor differences from the additional variables. Figure 9 shows the noise 

maps at two different frequencies calculated via beamforming. One frame from the visible 

wavelength camera was used for the background photograph. Note that the conventional maps 

show the top 10dB range of source distribution while the SEM maps show 20dB range. Data 

from the peak 14.5s duration shown in Fig 8(a) was used to calculate the noise maps. 

Interestingly the duct exit was found to be the most prominent noise source at all frequencies. 

The higher dynamic range of the SEM maps show that the engine mount, at the bottom of the 

vehicle was also a source, albeit weaker by 16dB. 

Fig. 8 (a) Time trace of the pressure fluctuations from one microphone, and (b) Engine thrust variation 

from the static fire test of the Antares Engines.

 That the duct exit is the overwhelmingly loud noise source, instead of the plume itself, is an 

interesting observation. The hot gases from the engine plume shot out through the duct. The 

traditional modeling expectation [3] is of a long distributed source along the plume path. The 

beamform maps revealed a very different scenario. It is believed that the large amount of water 

sprayed into the flame duct quenched the plume and significantly reduced its ability to generate 

noise. It is the noise generated inside the closed duct, due to impingement on the deflector, and 

due to the general turbulent mixing, that ultimately radiated out through the exit. The noise maps 

of Fig 9 also indicate a path to further attenuate the acoustic level on engine components during 
14

a static-fire test. Such attenuation can be obtained simply by extending the length of the duct to 

make the exit further away from the vehicle. 

Fig. 9 Noise sources during engine hot-fire test at indicated frequencies calculated via (a), 

(b) conventional beamform and; (c), (d) modified SEM. Note different plot range for the color 

scales. 

VII. RESULTS FROM ANTARES A-ONE LAUNCH 

The array was re-deployed at the same parking lot, and about 400 feet away from the 

vehicle for the maiden launch of Antares (Fig 1a, 1b, 1d). The launch took place on April 21st, 

2013. In this installation, only a lighter weight scissor lift was available, necessitating use of 

additional guy-wires (eight instead of four in the static-fire test) to steady the array. Also an 

improved vibration isolation mount allowed the thermal camera to provide video footage for the 

first 10s after ignition. Unfortunately, six of the seventy microphone channels were inadvertently 

turned off, effectively making a 64 microphone array. Nevertheless, the data provided good 

insights into the acoustic sources. The “speaker calibration” process to determine the look angle 

of the cameras was performed the night before launch by placing a loudspeaker at a couple of 

different locations around the pad. The beamformed maps from this speaker when 

superimposed on a frame of the video footage provided the angular differences between the 

array coordinates and the camera “look angle.” This correction was established for both the 

visible and the thermal imaging cameras. 

The Antares vehicle accelerated somewhat slowly. Two seconds after the first stage’s twin 

AJ26 engines were ignited the rocket lifted off the pad (Fig 10a). Eight seconds later, the rocket 

was traveling at 65 km/h and was just 70 meters off the pad. The slow motion of the vehicle 

made any correction for Doppler shift unnecessary in the beamforming calculations. Just before 

ignition the vehicle was released from the TEL which tilted back to avoid re-contact. In addition, 
15

the vehicle was pitched away from the TEL early in the liftoff to provide additional margin (Fig 

10b). This “TEL avoidance maneuver” significantly affected the noise source distribution. All 

three water injection systems for acoustic attenuation (described earlier) were used during the 

launch. 

Fig. 10 (a) Height and (b) tilt angle of Antares during A-one launch; (c) time-trace of noise 

signal from one microphone in the phased array; (d) time variation of overall sound pressure 

level at the phased array.

The noise sources during a launch are much more time dependent than during a static fire. 

For correct time registration between the beamformed maps and the launch events the 

propagation delay of the sound waves from the pad to the face of the array needed to be 

accounted for. This was simple to determine from the known distance of the array and the 

ambient speed of sound. Figure 10(c, d) shows the propagation delay on the time trace of the 

pressure fluctuations measured from a microphone. A launch site next to the ocean is never a 

quiet place. The wind noise and the noise from various bleed operations were around 90dB 

before engine ignition. The ignition and powering-up of the engines created progressively higher 

levels. However, the loudest level of >150dB was measured when the vehicle was ~120 meters 

above the launch pad, and the entire plume emerged out of the flame duct. 

 To determine the time evolution of the noise sources, the microphone signals were 

divided into 0.2s-long segments and the beamforming process was applied to each segment. 

The beamformed maps were superimposed on individual frames from the thermal imaging 

camera. Figure 11 shows the evolution of the noise sources at different time intervals. The right 

column of Fig 11 is from selected frames of a high speed camera (courtesy of NASA KSC 

imaging group) that provided an excellent view of the launch events. Before embarking upon a 

discussion of the noise maps some elaboration of the individual images shown in the left column 

of Fig 11 is warranted. 

 Each image in the left column of Fig 11 is a composite of three images: (a) a 

beamformed noise map whose levels are shown in the color scale; (b) the bottom part of a 

temperature map from just before the ignition; and (c) the top part of the temperature map at the 

time instance marked on the figure. The merging of the latter two was necessary to provide a 

definition of the pad which was obscured in the actual footage. In Fig 11(a), the thermal image 
16

of the Antares was split by the central dark zone that corresponded to the cold LOX tank. The 

bottom RP1 tank and the top part of the vehicle were warmer and fell within the grey scale of 

the image. As the launch events progressed the acoustic levels became louder and the camera 

started to shake. This produced increasingly blurred image of the Antares vehicle which is 

barely visible in Fig 11(d) onwards. However, these figures correctly show the locations of the 

high temperature plume from the engine exhaust. The color scale for the noise maps show dB 

level calculated from an 80Hz wide band centered at 2 kHz. The 10dB plot range was autoscaled for each of the maps. 

 Figure 11(a) was from just after ignition when the engine was at partial power, and the 

hot plume was visible through the openings of the launch mount. These openings allowed for 

the emergence of the sound waves and were the primary noise source at the beginning. The 

ground reflection of the primary source was also seen as a second source by the array. The 

reflected source persisted in Fig 11(b). 

 Some amount of steam and water from inside the duct started to emerge starting at the 

time of ignition. However, the duct exhaust did not become a prominent source until the engine 

came to nearly full power and hot gases began to emerge (Fig 11c). At this point both the inlet 

and the exit of the duct were noise sources. The next three figures 11 (d) - (f) show a 

problematic spread of the noise sources from the duct ends to the top of the pad. The cause 

was a spilling of the hot plumes from the Antares engines over the pad. An examination of the 

vehicle alignment with the nearby lightning protection tower in the right image of Fig 11(d) show 

the tilting of the vehicle as a part of the “TEL avoidance maneuver.” The thermal camera images 

show white patches on the pad indicative of high temperature gas. The combination of tilting of 

the vehicle and drift towards the east (ocean-side) caused the hot plume to spill out of the duct 

inlet and spread over the pad. Figures 11(e) and 11(f) show an increasing amount of spill and 

spread of hot gas all over the pad. This condition was created by a relatively late start of the ondeck “rainbird” system. Even the earlier-start rainbirds did not come to full flow rate until the 

vehicle was elevated 40 feet above the pad. In the right side images of Fig 11(d) – (f), from the 

visible wavelength camera, the orange glow can be identified with the hot plume, and the white 

glow with steam. Very little steam is found to rise from the top of the pad during the early part of 

lift-off. The plots of Fig 11 clearly show a need for an earlier start of the rainbirds. This will be 

implemented in future Antares launches. The early start will quench the plume and the 

impingement zones, resulting in less damage and a lower acoustic level. 

 As the launch progressed part of the plume emerged from the duct, and the beamformed 

maps correctly identified the plume itself as the dominant source. Figures 11(g) and 11(h) show 

sufficient quenching by the rainbird system which reduced the extent of the noise source on the 

top of the concrete pad, leaving the exposed plume as the primary source. A small amount of 

ground reflection is also visible in the noise map. Figure 11 shows plots from a few selected 

time segments. A movie using all time instances provided some more details; although the 

central conclusions were the same. 

 Beyond the last time frame shown in Fig 11, the plume and the noise maps simply 

became longer until most of the plume crossed the upper edge of the camera images; the 

overall acoustic level started to fall due to an increased forward velocity of the launch vehicle. 

SEM beamformed maps were once again found to discretize the distributed noise sources on 

the long plume. Due to the shorter time duration, phase jitter from the shaking of the array 

chamber, and the inadvertent power loss of 6 microphones, SEM maps showed increased sidelobes in plots covering a wider dB range. Since no new insights were gained via SEM 

calculations such maps are not shown here. 
17

Fig. 11 

Contd.
18

Fig. 11 Concluded. (Left column) noise maps superimposed on frames from IR camera, 

conventional beamform at 2 kHz; (right) corresponding frames from a high-speed camera. 
19

VIII. CONCLUSIONS

The evolution of the acoustic sources during static-fire engine tests and a rocket launch was 

measured using a microphone phased array. Although the present demonstration was 

performed on the Antares rocket, but the technology developed for this work is applicable to any 

launch vehicle. The goal of the work was to advance the phased-array technology for launch 

vehicle applications. The actual noise sources are expected to vary for different launch pads 

and rocket vehicles. The use of phased-array technology, as demonstrated in this paper, may 

lead to improvement of the acoustic suppression systems and lowering of the vibro-acoustics 

load. 

A 10-foot by 10-foot, large yet portable, phased array was built for use at the launch pads of 

rocket vehicles. The array used 70 condenser microphones, and two video cameras that 

operated in the visible and long infra-red wavelengths. The infra-red camera was used to 

visualize the location of the plume and various impingement zones on the launch pad. The 

microphone pattern was designed for optimal resolution in the relatively low (400Hz to 2000Hz) 

frequency range. At first, the array was tested during the static firings of a small scale hybrid 

engine at NASA Ames. It was then shipped to NASA Wallops for set up at launch pad 0A. There 

the array was used during a static test firing of the engines of the Orbital Sciences’ Antares 

rocket, and again during the first launch of Antares. It was placed 400-feet away from the pad, 

and was hoisted on a scissor lift 40-feet above the ground to obtain a good view of the top of the 

launch pad. Data collected from these efforts provided remarkable insight into acoustic sources 

in a full-scale static-fire, and a launch, not previously available. 

For the static fire test the twin LOX-RP1 liquid engines of the Antares vehicle were mounted 

above the flame duct that included a deflector and a water injection system. During the staticfire test it was found that the duct exit was the primary noise source, while the nozzle exit and 

the engine mount were weaker, secondary sources. Unlike most of the models of launch 

acoustics, such as NASA SP 8072 [3] the plume that emerged out of the duct was not found to 

be a source. Fundamentally, the noise generated inside the duct from impingement and turning 

of the plume, and from general turbulent mixing, emerged from the duct exit. The plume was 

sufficiently cooled and slowed inside the duct, and did not radiate significantly as it emerged. 

The noise sources during lift-off of Antares vehicle was found to vary with time. During 

engine ignition the launch mount was the source. As the engine came to full power, and hot 

plume came out of the duct, the exit face of the duct was found to be the most prominent 

source. Effective cooling by duct water limited the extent of this source and kept the acoustic 

levels to a reasonable value. As Antares started to elevate, it tilted east (towards the ocean) and 

drifted away from the duct hole as a part of the maneuver to avoid contact with the service mast. 

This caused the hot plume to spread over the pad. A large region on the top of the pad became 

a loud noise source. Further elevation of the vehicle, and the start of the rainbirds, reduced the 

extent of the source. As more and more plume emerged out of the hole, the plume itself became 

the noise source – duct exhaust was no longer a source. Cooling by the rainbirds attenuated 

sources on the top of the deck. The phased array also identified ground reflection of the primary 

sources at ignition and again when the long plume emerged out of the duct. Based on the 

insights gained from the phased array effort the rainbird timing will be changed for an earlier 

start for the next launch of the Antares vehicle. 

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