Gary G. Podboy

Glenn Research Center, Cleveland, Ohio

Jet-Surface Interaction Test: Phased Array 

Noise Source Localization Results

NASA/TM—2013-218085

September 2013

GT2012–69801
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Gary G. Podboy

Glenn Research Center, Cleveland, Ohio

Jet-Surface Interaction Test: Phased Array 

Noise Source Localization Results

NASA/TM—2013-218085

September 2013

GT2012–69801

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for

Turbo Expo 2012

sponsored by the American Society of Mechanical Engineers (ASME)

Copenhagen, Denmark, June 11–15, 2012
Acknowledgments

The NASA Fundamental Aeronautics Program, Subsonic Fixed Wing Project, supported this work. 

Special thanks to Steven A. E. Miller of NASA Langley for his insights regarding the origins of broadband shock noise.

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NASA/TM—2013-218085 1

Jet-Surface Interaction Test: 

Phased Array Noise Source Localization Results

Gary G. Podboy

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

An experiment was conducted to investigate the effect that a 

planar surface located near a jet flow has on the noise radiated 

to the far-field. Two different configurations were tested: 1) a 

shielding configuration in which the surface was located 

between the jet and the far-field microphones, and 2) a 

reflecting configuration in which the surface was mounted on 

the opposite side of the jet, and thus the jet noise was free to 

reflect off the surface toward the microphones. Both 

conventional far-field microphone and phased array noise 

source localization measurements were obtained. This paper 

discusses phased array results, while a companion paper

(Ref. 1) discusses far-field results. The phased array data show 

that the axial distribution of noise sources in a jet can vary 

greatly depending on the jet operating condition and suggests 

that it would first be necessary to know or be able to predict 

this distribution in order to be able to predict the amount of 

noise reduction to expect from a given shielding configuration.

The data obtained on both subsonic and supersonic jets show 

that the noise sources associated with a given frequency of 

noise tend to move downstream, and therefore, would become 

more difficult to shield, as jet Mach number increases. The 

noise source localization data obtained on cold, shockcontaining jets suggests that the constructive interference of 

sound waves that produces noise at a given frequency within a 

broadband shock noise hump comes primarily from a small 

number of shocks, rather than from all the shocks at the same 

time. The reflecting configuration data illustrates that the law 

of reflection must be satisfied in order for jet noise to reflect 

off of a surface to an observer, and depending on the relative 

locations of the jet, the surface, and the observer, only some of 

the jet noise sources may satisfy this requirement.

Introduction

In October 2008, NASA called on industry and academia to 

propose conceptual designs for advanced aircraft which could 

satisfy both future commercial air transportation capacity 

requirements and specific goals related to reductions in fuel 

burn, noise, and air pollution. Compared to an aircraft entering 

service today, NASA’s goals for a 2030-era aircraft are 1) a 

71 dB cumulative reduction below current Federal Aviation 

Administration noise levels, 2) a greater than 75 percent

reduction in nitrogen oxide emissions, and 3) at least a 

70 percent reduction in fuel burn.

The four aircraft shown in Figure 1 were proposed in 

response to NASA’s call. These all share one characteristic: in 

each case the aircraft engines are located near solid surfaces.

In the case of the two turboprop-driven designs, shown in 

Figure 1(a) and (b), the propellers are mounted upstream of 

the wing’s leading edge, while with the aircraft shown in 

Figure 1(c) the engine exhaust would flow under the wing. In 

these three cases the interaction between the flow downstream 

of the engine and the wing could be expected to generate 

increased noise relative to an engine operated in isolation.

With the fourth design shown in Figure 1(d), the aircraft’s 

fuselage would shield some of the forward-propagating fan 

noise from reaching the ground, resulting in decreased noise 

levels relative to an isolated engine.

The idea of using parts of the airframe to shield engine 

noise has been proposed for other aircraft as well. It is one of 

the noise reduction technologies included in the Hybrid Wing 

Body (HWB) concept aircraft developed by the Boeing 

Company with funding from NASA’s Environmentally 

Responsible Aviation Project (Ref. 2) and the Advanced 

Model for Extreme Lift and Aeroacoustics (AMELIA) 

developed by the California Polytechnic State Institute 

through a 3-year NASA Research Announcement grant 

sponsored by NASA’s Subsonic Fixed Wing Project. Both of 

these aircraft are shown in Figure 2. With these designs, the 

wing is used to shield some jet noise from reaching the 

ground. The amount of jet noise reduction would depend on 

the location of the engine nozzle relative to the trailing edge of 

the wing and the distribution and directivity of noise sources 

within the jet plume. Consequently, the location of the engine 

relative to the wing trailing edge is a critical aspect of design.

Another consideration is that if the engine is too close to the 

wing’s surface or embedded in the wing such that the engine 

exhaust flow scrubs across the wing’s surface then lowfrequency trailing edge noise will be created, mitigating some 

of the noise reduction potential of the shielding concept.

Recently, McLaughlin et al. (Ref. 3) demonstrated that 

noise measurements made during field tests of full-scale 

aircraft or aircraft engines can depend significantly on noise 

reflecting off the ground. They point out that when pole 

mounted microphones are used in such investigations that 

noise reaches the microphone from both a direct and a ground 

reflection path, and that the resulting interference leads to both 

positive and negative reinforcement of the sound waves. This 

interference depends on both the acoustic frequency and the 

path length difference and generates multiple humps in the 
NASA/TM—2013-218085 2

(a) GE Aviation’s 20 passenger aircraft (b) Boeing’s SUGAR, Volt

(c) Northrup Grumman’s SELECT (b) MIT’s “double bubble” D8

Figure 1.—Advanced aircraft designs proposed to meet NASA’s goals for a 2030-era aircraft.

(a) Boeing’s Hybrid Wing Body (b) California Polytechnic Institute’s AMELIA

Figure 2.—Advanced aircraft designed to shield engine noise.
NASA/TM—2013-218085 3

measured acoustic spectra and increases the integrated mean 

square value of the acoustic pressure (the area under the 

spectral curve). They obtained data with both cold and hot 

Mach 1.5 jets tested in isolation and near a simulated ground 

plane. Using these data they developed a model that attempts 

to derive “free-field” noise estimates from measurements 

made during full-scale field experiments. They point out that 

the development of such a model “requires reasonably 

accurate information on the distribution of noise sources 

within the jet.”

Based on the above discussion, it is clear that in order reach 

NASA’s stringent noise reduction goal that the interaction 

between the noise and/or flow produced by an engine and any 

nearby solid surfaces will have to be considered. Depending 

on the configuration, the surface effects could lead to either 

increases or decreases in noise relative to an engine operated 

in isolation.

One goal of NASA’s Subsonic Fixed Wing Program is to 

develop computer programs that can accurately predict the 

changes in aircraft noise caused by engine installation effects.

Historically, ANOPP, NASA’s most well-known program for 

predicting the community noise generated by a given aircraft 

configuration, has used rather simple models of jet noise to 

predict the noise benefits associated with shielding. Recently, 

Papamosochou (Ref. 4) used data obtained from subscale 

experiments to show that these simple models do not provide 

accurate estimates of jet noise shielding. He argued that they 

were inadequate because they approximate the jet noise as a 

small number of discrete point sources, when in actuality jet 

noise is a more complicated, distributed source. He developed 

a new model that describes the jet noise source as a 

combination of a wavepacket and a monopole. He points out, 

however, that in order to predict any reduction in jet noise due 

to shielding that a realistic jet noise model is not sufficient; it 

is also necessary to know the axial distribution of noise 

sources in the jet (Ref. 5). Past experiments have shown that 

these distributions can vary significantly depending on both 

the jet operating condition and the angle of the observer 

relative to the jet (Ref. 6).

Despite the importance of installation effects, there is a lack 

of quality experimental data on jets with surfaces nearby 

which could be used to develop and validate noise prediction 

methods. In an effort to fill this need, a series of experiments 

known collectively as the Jet-Surface Interaction Test (JSIT) 

are being conducted at the NASA Glenn Research Center in 

Cleveland, Ohio. Phase 1, completed in February 2011, was 

conducted in an effort to expand the database available 

regarding how a planar surface parallel to the jet centerline 

interacts with a jet to modify the noise propagating to the far 

field. Two different configurations were tested: 1) a shielding 

configuration in which the surface was located between the jet 

flow and the far-field microphones, and 2) a reflecting 

configuration in which the surface was mounted on the other 

side of the jet, and thus the jet noise was free to reflect off of 

the surface toward the microphones. The surfaces were chosen 

to be large relative to the size of the nozzle so that they would 

appear semi-infinite, i.e., extend forever upstream of the 

trailing edge. Three parameters were varied during the test: 

1) the axial distance that the surface extended downstream of 

the nozzle exit, 2) the radial location of the surface relative to 

the jet centerline, and 3) the jet operating condition. Far-field 

microphone, phased array noise source localization, unsteady 

surface pressure, and pressure sensitive paint data were 

acquired during the test. Brown has presented the far-field 

microphone data in a companion paper (Ref. 1). 

The purpose of the present paper is to show some examples 

of the phased array noise source localization data acquired 

during the test. These data were acquired for two reasons. The 

first was to help explain the far-field microphone results. In 

order to understand how a surface interacts with a jet to alter 

the noise propagating to the far-field it is necessary know the 

axial distribution of noise sources within the jet. These 

distributions were measured for each of the jet operating 

conditions set during the test. They can be used to explain, for 

example, why a shield might be effective at blocking noise for 

one jet operating condition, but not for another. The second 

reason was to ensure that the shielding surfaces were blocking 

all measurable noise except for that emanating at or 

downstream of the surface trailing edge. The example data 

presented herein provide useful insights regarding how a 

surface near a jet can alter noise levels depending on the 

relative locations of the jet, the surface and the observer.

Nomenclature

b acoustic source strength estimated from beamforming

c the speed of sound

camb ambient speed of sound

cj speed of sound in jet

C, CSM cross spectral matrix

D nozzle exit diameter

g steering vector 

Ma acoustic Mach number, (Vj/camb)

Mj ideally-expanded Mach number, (Vj/cj)

NPR nozzle total pressure ratio, (Pj total/Pamb)

OB octave band

Pj nozzle plenum pressure

ω angular frequency

SPL sound pressure level

Tj jet temperature

TSR nozzle static temperature ratio, (Tj static/Tamb)

V velocity

Vj ideally-expanded jet exit velocity

w normalized steering vector

x

 grid point location

y

 microphone location
NASA/TM—2013-218085 4

Research Instrumentation

Test Hardware 

This experiment was conducted using the Small Hot Jet 

Acoustic Rig (SHJAR) located at the NASA Glenn Research 

Center (GRC) in Cleveland, Ohio, USA. SHJAR is a singlestream nozzle test rig used for fundamental jet noise research. 

It can accommodate air mass flow rates of up to 6 lb/sec 

(2.7 kg/s), nozzle exhaust temperatures ranging from ambient 

to 1300° F (980 K), and nozzles as large as 3 in. (7.62 cm) in 

diameter. The test rig is located within the Aeroacoustic 

Propulsion Laboratory (AAPL), a 19.8 m radius anechoic 

geodesic dome. Both the floor and the dome’s interior surface 

are covered with sound absorbing acoustic wedges. The 

facility acoustic instrumentation includes a far-field 

microphone array made up of 24 microphones arranged in a 

circular arc at 5° intervals from 50° to 165° from the jet 

upstream axis and located 150 in. (3.81 m, 75 nozzle 

diameters) from the nozzle exit. Brown et al. (Ref. 7) provide 

more information regarding SHJAR and the acoustic 

characteristics of the AAPL facility.

The planar surfaces used during the test were mounted in 

the two configurations illustrated in Figure 3. The upper photo 

shows the shielding configuration in which the surface was 

located between the jet flow and the far field microphone 

array; the lower photo shows the reflecting configuration, 

where the surface was located on the opposite side of the jet.

The surfaces were 6 ft tall and, except near the trailing edge, 

were made of 1⁄2 in. thick aluminum plates. Plates were added 

and removed as necessary during the test to change the axial 

dimension of the surface. A separate, 6 ft tall by 4 in. wide by

1⁄4 in. (1.83- by 10.2- by 0.635-cm) thick aluminum strip was 

used to provide the surfaces with a sharp trailing edge. This 

strip, which was flush mounted to the downstream edge of the 

thicker surface, had its trailing edge cut back at a 39.2° angle 

such that the pointed side was on the side of the jet flow. The 

surfaces were mounted onto a support structure that, in turn, 

was mounted onto a moveable cart. The cart rode along rails 

that were parallel to the jet centerline and was moved 

manually in order to change the axial location of the surface 

trailing edge relative to the nozzle exit. A 1-m, linear traverse 

system mounted to the top of the cart was used to move the 

surfaces in the radial direction relative to the jet. 

The intent was for the surfaces used during the shielding 

configuration to appear semi-infinite, i.e., to block any noise 

coming from upstream of the surface trailing edge from 

reaching the far field microphones. Phased array data obtained 

early on in the test indicated that at times measurable noise 

would leak above or below the surface, or between the gap 

that existed between the upstream edge of the surface and the 

wedge wall. In order to block this noise, multiple layers of 

welder’s blankets were hung from a horizontal support above 

the top edge of the shielding surface. The blankets covered the 

backside of the surface and draped around the side of the 

wedge wall shown in Figure 3. From an acoustic standpoint, 

Figure 3.—Photos showing example shielding (top) and 

reflecting (bottom) surface configurations.

they increased the vertical height of the shield and covered the 

gap between the surface and the wedge wall. All of the 

shielding configuration phased array data presented in this 

report were acquired with the blankets in place. Phased array 

data obtained on subsonic jets after the blankets were installed 

confirmed that the noise coming from downstream of the 

surface trailing edge was always at least 10 dB greater than 

any noise coming around the other three sides of the surface.

Similar data obtained on supersonic jets suggests that some 

screech tones may have either penetrated the surface/blanket 

barrier or, for certain shield locations, may have reflected off 

the backside of the shield. 

Data were acquired using two SMC series nozzles that have 

been tested extensively in the past at GRC, SMC000 and 

SMC016. SMC000 is a convergent nozzle that serves as a 

baseline for most SHJAR tests. SMC016 is a convergentdivergent (C-D) nozzle that was designed using the method of 

characteristics to provide an ideally expanded flow at Mj=1.5.

Both nozzles have a 2 in. (5.08 cm) exit diameter. 
NASA/TM—2013-218085 5

(a) Front view

(b) Back view

Figure 4.—Photos of the Array48 phased array system.

Test Conditions

Table I provides information regarding the jet operating 

conditions set during the test. The eight different jet conditions 

that were tested will be identified by the setpoint listed in the 

table. As indicated, data were acquired using the convergent 

SMC000 nozzle at five different setpoints, four of which 

correspond to subsonic jets. Of these four, two were cold, as 

was the lone supersonic case. Data were acquired using the 

convergent-divergent SMC016 nozzle on three cold, 

supersonic jets, one under-expanded, one ideally-expanded, 

and one over-expanded. 

TABLE I.—JET OPERATING CONDITIONS

Nozzle Setpoint NPR

Pt/Pamb

TSR

Ts/Tamb

Ma

V/camb

Mj,

V/clocal

SMC000 3 1.20 0.95 0.50 0.51

SMC000 7 1.86 0.835 0.90 0.98

SMC000 27 1.36 1.76 0.90 0.68

SMC000 46 1.24 2.70 0.90 0.55

SMC000 9010 3.18 0.74 1.18 1.40

SMC016 11606 2.75 0.76 1.13 1.29

SMC016 11610 3.67 0.72 1.31 1.50

SMC016 11617 4.32 0.76 1.41 1.61

The Phased Array System

Figure 4(a) shows a front view of the OptiNav Array48 

phased array system used during this test. This system consists 

of 48 Earthworks M30 microphones flush-mounted to a 1- by 

1-m aluminum plate. The microphones are arranged in a series 

of log spirals in an effort to reduce sidelobes (errors in the 

phased array data). The microphones have 1⁄4 in. (0.635 cm) 

diameter diaphragms and a flat frequency response over a 

frequency range of 5 Hz to 30 kHz. They can be used to 

measure sound fields with amplitudes as high as 142 dB 

before they begin to saturate (136 dB when flush mounted in 

the array plate).

A photograph of the back of the microphone array is shown 

in Figure 4(b). This photo shows the microphones mounted to 

the back of the array plate and a camera located at the center 

of the plate. There is a hole in the center of the plate through 

which the camera can be used to take a photo of the “field of 

view” of the phased array system. The phased array data 

reduction software superimposes the acoustic source 

localization data on top of the image taken with the phased 

array camera.

The phased array data were reduced using classical 

beamforming in the frequency domain. The first step in the 

data reduction process was to compute the cross spectral 

matrix, C, from the array data using the periodogram method 

with Hanning windowing functions and 50 percent block 

overlap. The diagonal elements of the cross spectral matrix are 

then deleted (yielding C ), and the beamforming result, b, at a 

given grid point, k, is computed using the classical 

beamforming expression

wkCwk = ′ bk

where wk is a normalized version of the steering vector gk, and

b represents the apparent strength of the acoustic source 

located at grid point k as estimated from the beamforming. 

The individual elements of gk represent the Green’s function 

for a monopole located at grid point k as observed by 

microphone i. In free space with no flow

i k

y x cj

ki

y x

e i k

 

 

− =

− ω

g

where xk

 and yi

 denote the locations of grid point k and 

microphone i, respectively. A beamforming result is computed 

for each point in a beamforming grid. The results 

corresponding to an entire grid are then displayed as color 

contour maps (known as beamform maps). Each contour map 

corresponds to a selected frequency band, and shows the 

location of the dominant noise source or sources in the band as 

a two-dimensional color contour map overlaid on top of a 

photograph taken with the phased array camera. The color 

contours correspond to the location and strength of the noise 
NASA/TM—2013-218085 6

sources found within an image plane (the beamform grid) 

parallel to the array plate at some specified distance away 

from the array. The dynamic range of the color contours (max 

value minus min value) is the same for each beamform map 

(i.e., processed frequency band), but the peak value can vary 

from map to map. The color contour maps presented later in 

this paper all have a 7 dB dynamic range.

Phased Array and Surface Locations

All of the phased array data obtained during the shielding 

tests were acquired with the array mounted on a stationary 

support stand located between the base of the moveable cart 

and the far field array (see Figure 5). With this arrangement, 

the shielding surface moved while the phased array and nozzle 

remained fixed. The center of the array was at the same height 

as the jet centerline, 10 ft (3.05 m) above the floor, and the 

array plate was parallel to, and 2.82 m (about 55 nozzle

diameters) away from, the jet centerline. Since the nozzles 

have different lengths, the axial location of the phased array 

relative to the two nozzle exit locations was slightly different.

With the SMC000 nozzle, the array was located such that a 

line normal to the center of the array intersected the jet 

centerline at the nozzle exit, while with the SMC016 nozzle 

the array normal intersected the jet centerline 5 in. (12.7 cm) 

upstream of the nozzle exit. The polar angle between a ray 

directed downstream through the jet axis and a line spanning 

from the array center to the center of the nozzle exit was 90°

when the SMC000 nozzle was tested, and 92° with the 

SMC016 nozzle. The array aperture was roughly 20°.

Table II provides a list of surface locations and setpoints at 

which phased array data were acquired during the shielding

surface tests. Data were acquired at each of the eight setpoints 

listed in Table I with the trailing edge of the surface located 2, 

4, 6, 10, 15 and 20 nozzle diameters downstream of the nozzle 

exit. For each axial location of the surface, data were acquired at 

17 different radial locations ranging from 1 to 16 diameters 

from the jet centerline. In this paper, the symbol “D”, which 

corresponds to a distance equal to the nozzle exit diameter 

(50.8 cm), will be used to designate both how far a surface 

extends downstream of the nozzle exit and how far it is away 

from the jet centerline. For example, a surface extending 6 

nozzle diameters downstream of the nozzle exit will be referred 

to as a 6D surface, and a surface located 4 nozzle diameters 

from the jet centerline will be referred to as being 4D away.

A small subset of the phased array data obtained during the 

reflecting surface tests were acquired with the array at the 

same location it was at during the shielding surface tests 

(shown in Figure 5). As indicated in Table III, data were 

obtained with the array mounted on the stationary stand with 

the jet operating at two cold conditions (7 and 9010) with the 

surface at one axial location relative to the nozzle exit (trailing 

edge 6 diameters downstream of the nozzle exit) and at the 

same set of 17 radial locations set during the shielding tests. 

This array location allows back-to-back comparisons of 

shielding and reflecting configuration data, but also has a 

couple of drawbacks. One is that the noise coming from the

surface is likely to show up in the phased array beamform 

maps at the same location as the noise coming directly from 

the jet, making it difficult to differentiate between them. The 

other is associated with the fact that the noise reflecting off or 

being created at the surface has to pass through the jet before 

reaching the phased array; this is a disadvantage since the 

turbulence in the jet would tend to de-correlate the sound 

waves coming from the surface, making it difficult to image 

the surface noise. 

Figure 5.—Photo showing the phased array during a shielding 

configuration test.

TABLE II.—SURFACE LOCATIONS AND OPERATING CONDITIONS DURING SHIELDING CONFIGURATION PHASED ARRAY TESTS

Axial location of trailing edge 

(nozzle diameters downstream of nozzle exit)

Radial locations relative to jet centerline

(nozzle diameters)

Setpoints

2 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 12, 14, 16 3, 7, 27, 46, 9010, 11606, 11610, 11617

4 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 12, 14, 16 3, 7, 27, 46, 9010, 11606, 11610, 11617

6 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 12, 14, 16 3, 7, 27, 46, 9010, 11606, 11610, 11617

10 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 12, 14, 16 3, 7, 27, 46, 9010, 11606, 11610, 11617

15 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 12, 14, 16 3, 7, 27, 46, 9010, 11606, 11610, 11617

20 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 12, 14, 16 3, 7, 27, 46, 9010, 11606, 11610, 11617
NASA/TM—2013-218085 7

TABLE III.—SURFACE LOCATIONS AND OPERATING CONDITIONS DURING REFLECTING CONFIGURATION TESTS 

CONDUCTED WITH THE PHASED ARRAY LOCATED ON THE STATIONARY SUPPORT STAND

Axial location of trailing edge 

(nozzle diameters downstream of nozzle exit)

Radial locations relative to jet centerline

(nozzle diameters)

Setpoints

6 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 12, 14, 16 7, 9010

TABLE IV.—SURFACE LOCATIONS AND OPERATING CONDITIONS SET DURING REFLECTING CONFIGURATION TESTS 

CONDUCTED WITH THE PHASED ARRAY MOVING WITH THE SURFACE

Axial location of trailing edge 

(nozzle diameters downstream of nozzle exit)

Radial locations relative to jet centerline 

(nozzle diameters)

Setpoints

5 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 12, 14, 16 3, 7, 9010, 11606, 11610, 11617

10 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 12, 14, 16 3, 7, 9010, 11606, 11610, 11617

15 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 12, 14, 16 3, 7, 9010, 11606, 11610, 11617

20 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 12, 14, 16 3, 7, 9010, 11606, 11610, 11617

Figure 6.—Photo showing the phased array mounted on the 

moveable cart during a reflecting configuration test.

All of the other phased array data obtained during the 

reflecting surface tests were acquired with the phased array 

mounted on the moveable cart, fixed with respect to the wall 

rather than the nozzle. As shown in the photo provided in 

Figure 6, the array was mounted on the cart below the jet with 

the array plate angled upward. This was done 1) so that the 

noise coming directly from the jet and the noise scattering off 

of the surface would show up at different locations in the 

beamform maps, 2) to shorten the distance between the array 

and the surface, and thus increase the spatial resolution of the 

phased array, and 3) so that the jet turbulence would not decorrelate the sound waves coming from the surface. As 

indicated in Table IV, reflecting surface data were acquired 

with the array mounted on the moveable cart at each of the 

same six cold setpoints tested during the shielding tests, using 

surfaces that extended 5, 10, 15, and 20 nozzle diameters 

downstream of the nozzle exit. For each axial location of the 

surface, data were acquired at the same set of 17 different 

radial locations set during the other tests. Due to time 

constraints, hot jet data were not acquired while the surfaces 

were mounted in the reflecting configuration.

Results

Bare Jet Results

When a surface interacts with a jet, the noise heard by an 

observer in the far field is dependent not only on the relative 

locations of the surface, the jet and the observer but also on 

the axial distribution of noise sources in the jet. Obviously, the 

closer the sources are to the nozzle exit, the easier it would be 

to shield the noise using a surface. The directivity of jet noise 

is also important. Some jet noise sources, such as large 

turbulent eddy/shock wave interaction, are themselves 

directive, meaning that the noise they produce in the far field 

is dependent on the angle of the observer relative to the jet 

axis. Jet noise also has directionality due to convective 

amplification, the concept that states that sound levels are 

increased when the sources are moving toward the observer 

(in the downstream direction for jet noise) and reduced when 

they are moving away from the observer (in the upstream 

direction). The interaction between sound waves produced 

inside the jet and gradients in the jet mean flow also 

contributes to the directivity of jet noise. This interaction tends 

to bend any downstream propagating sound waves away from 

the jet axis. Because of the directive nature of jet noise, the 

noise source locations measured using a phased array will vary 

depending on the angle between the array and the jet axis.

During this experiment, bare jet phased array data were 

obtained with the array at the same location that it was at 

during the shielding tests, i.e., broadside to the jet, roughly 55 

diameters away, and at an angle approximately 90° to the jet 

axis. This data provides information regarding the distribution 

of noise sources in the jet as observed from this particular 

direction. It can be used to gain insights regarding why a 

shield might or might not be effective at blocking the noise 

propagating in this direction.

Figure 7 shows bare jet phased array data obtained on a 

cold, subsonic jet. These data were obtained using the 

convergent SMC000 nozzle at setpoint 3 (Ma=0.50, 

TSR=0.95), which corresponds to the lowest Mach number set 

during the test. Seven beamform maps are shown at the right 

in Figure 7; the upper six correspond to 1/12th octave bands 
NASA/TM—2013-218085 8

centered at 1, 2, 4, 8, 16, and 32 kHz. The 1/12th octave band 

center frequency and corresponding Strouhal number (St) are 

shown in the upper left corner of each map, and the small, red 

square on each map corresponds to the peak noise source 

location. The spacing of the green grid lines superimposed on 

the beamform maps corresponds to a distance equal to one 

nozzle diameter. The plot in the upper left of the figure shows 

PSD spectra computed from the output of the microphone 

closest to the center of the array. This is conventional singlemicrophone spectra, not a product of the phased array 

beamforming. The single microphone spectra presented in this 

section and in the two shielding configuration sections that 

follow have been scaled to a distance equal to 100 nozzle 

diameters from the jet centerline. The single microphone 

spectra presented later in the reflecting configuration section 

were not scaled in this manner since the distance between the 

array plate and the jet varied during that part of the test. The 

plot at the lower left in Figure 7 shows the distance between 

the peak in the beamform maps and the nozzle exit as a 

function of frequency, in units of nozzle diameters. This type 

of plot will be referred to herein as a peak location plot. Red 

asterisks have been superimposed on each of the two line plots 

at the frequencies represented by the six 1/12th octave band 

beamform maps at the right in the figure. 

The single microphone spectrum provided in Figure 7

indicates that this jet produces significant noise over a wide

frequency range between St=0.17 and St=0.8 (600 and 

2600 Hz), and the peak location plot shows that this noise 

originates between roughly 4 and 9 diameters downstream of 

the nozzle exit. The peak location plot also indicates that the 

peak source occurs about 10 nozzle diameters downstream of 

the nozzle exit at the lowest processed frequency (400 Hz, 

St=0.12), and that it gradually moves closer to the nozzle exit 

with increasing frequency. For frequencies above about 10 

kHz (St=3.1), the peak source location is always within two 

nozzle diameters of the nozzle exit. At this operating condition 

the dominant noise sources occurring within any given 

frequency band do not appear to extend significantly in the 

axial direction. Instead, they tend to be centered about one 

axial location in the jet plume, although this location does vary 

with frequency—it gets closer to the nozzle exit with 

increasing frequency.

The beamform map provided in the lower right hand corner 

of Figure 7 corresponds to the same data processed into one 

wide band ranging from 388 to 45,986 Hz. In that both 

produce a final result based on a very wide frequency range, 

the data processing used here is similar to that used 

to calculate Overall Sound Pressure Level (OASPL).

Figure 7.—Single microphone spectra (upper left), peak location plot (lower left), 1/12th OB beamform maps (right), and OASPL 

beamform map (lower right) for the SMC000 RC nozzle operating at setpoint 3 (Ma=0.50, TSR=0.95)
NASA/TM—2013-218085 9

Consequently, this sort of plot will be referred to herein as an 

OASPL beamform map. A plot such as this does not provide 

as much information as a set of 1/12th OB plots, but is 

practical for use in a technical paper since it shows the

locations associated with the loudest sources in the jet in a 

single plot. OASPL beamform maps will be used later in the

presentation of shielding configuration results. The OASPL

plot shown in Figure 7 indicates that for this cold, subsonic jet

the loudest sources are located between 4 and 9 diameters 

downstream of the nozzle exit.

Figure 8 shows these Ma=0.50 data compared with results 

obtained with the SMC000 nozzle set to provide a higherspeed, subsonic jet (setpoint 7, Ma=0.90, TSR=0.835). As 

shown in the figure, the results for these two cases are similar 

in that they both show 1) broad, rounded spectra, and 2) the 

peak noise source located relatively far downstream at low 

Setpoint 3, Ma = 0.50 Setpoint 7, Ma = 0.90

Figure 8.—Single mic spectra (upper left), peak location plots (upper right), and beamform maps (bottom) for the SMC000 nozzle at 

setpoints 3 (Ma=0.50, TSR=0.95) and 7 (Ma=0.90, TSR=0.835).
NASA/TM—2013-218085 10

frequency and moving gradually closer to the nozzle exit as 

frequency is increased. The main differences are 1) that the 

peak spectra levels are about 20 dB higher, and 2) the peak 

source location starts out further downstream (11.5 versus 10 

diameters) at low frequency in the higher speed jet. A 

comparison of the OASPL beamform maps indicates that the 

loudest noise occurring at a given frequency is generated

about 2 diameters further downstream in the higher speed jet. 

The peak location plot provided in the upper right corner of 

the figure indicates, however, that plotting the peak source 

location versus St (rather than frequency) tends to collapse the 

data from the two jets to a single line, especially for St>0.5. 

The results just discussed (Figure 7 and Figure 8) 

correspond to cold, subsonic jets. Figure 9 provides 

information regarding the changes that occur when heat is 

added to a cold, subsonic jet in such a way that acoustic Mach 

number (jet velocity) is maintained. The beamform maps 

presented in Figure 9 correspond to setpoints 27 (NPR=1.36, 

Setpoint 27, Ma = 0.90, TSR=1.76 Setpoint 46, Ma = 0.90, TSR=2.70

Figure 9.—Single mic spectra (upper left), peak location plots (upper right), and beamform maps (bottom) for the SMC000 nozzle at

setpoints 27 (Ma=0.90, TSR=1.76) and 46 (Ma=0.90, TSR=2.70).
NASA/TM—2013-218085 11

TSR=1.76, Ma=0.90) and 46 (NPR=1.24, TSR=2.70, 

Ma=0.90). The spectra and peak location plots presented at the 

top of the figure show results for both of these hot jets, as well 

as for a cold jet at the same acoustic Mach number (setpoint 7, 

shown previously in Figure 8). The spectra indicate that 

adding heat while maintaining the same jet velocity tends to 

reduce the level of high frequency noise propagating in the 

direction of the array (90° to the jet axis) by as much as 5 dB. 

Meanwhile, the peak location plots show that the noise 

sources corresponding to any given frequency tend to move 

upstream if heat is added while acoustic Mach number is held 

constant. For example, the source location of the lowest 

processed frequency band, 400 Hz (St=0.07), moves steadily 

upstream from 14 to 8 diameters downstream of the nozzle 

exit as TSR is increased from 0.835 to 2.70. In this case 

acoustic Mach number was maintained, but jet Mach number 

decreased when heat was added, and the peak source locations 

corresponding to any given frequency moved upstream. 

Results from a previous test (Ref. 8) indicate that if heat is 

added such that jet Mach number is maintained then the 

source locations corresponding to any given frequency do not 

change. This indicates that jet Mach number (or NPR) dictates 

where the turbulent mixing noise generated at any given 

frequency comes from in the jet—the sources move 

downstream as jet Mach number increases.

All of the subsonic jet results presented above show 

1) broad haystack-like spectra, and 2) the peak noise source 

location moving steadily upstream as frequency is increased. 

All of these results are similar because in subsonic jets there is 

only one source of noise, the turbulent mixing that occurs 

between the jet and the ambient air. 

Figure 10 shows results from data acquired with the 

convergent-divergent SMC016 nozzle at 3 supersonic 

setpoints (11606 over-expanded, 11610 ideally-expanded, and 

11610 under-expanded). The ten points labeled along the 

spectra and peak location line plots correspond to the ten 

1/12th octave band beamform maps shown for each setpoint. 

At low frequency (St<0.3), both the spectra and peak location 

plots generated from the supersonic jets resemble the subsonic 

results discussed earlier, suggesting that turbulent mixing 

noise is dominant in this frequency range. A fundamental 

screech tone occurs at St=0.41 (3000 Hz, pt 2, blue line plots) 

in the over-expanded jet data and appears in the corresponding 

beamform map (#2, left column) predominantly as a reflection 

off of the upstream nozzle hardware. A harmonic of the 

fundamental screech tone occurs at St=0.82 (6000 Hz, pt 5,

blue line plots) and comes directly from a location in the jet 

plume about 5 diameters downstream of the nozzle exit.

Broadband shock noise (BBSN) shows up clearly in the 

spectra presented in Figure 10 for the two off-design operating 

conditions as the elevated region at high frequency (to the 

right of pt 3 for the over-expanded case and to the right of pt 2 

for the under-expanded). A dominant BBSN hump is clearly 

visible in the spectrum for the under-expanded jet between 

St=0.35 and St=0.87 (3000 and 7500 Hz, magenta spectrum, 

between points 2 and 6). The 1/12th octave band beamform 

maps provided for the under-expanded jet (right column) 

indicate that the location of the peak noise source moves 

downstream as frequency increases through this BBSN hump 

(beamform maps 3 through 6). Then, at a frequency beyond 

the hump (to the right of point 6 on the magenta spectrum) the 

peak noise source location jumps back upstream, before this 

pattern (downstream movement followed by a jump back 

upstream) repeats once again (points 7 through 9). This 

upstream-downstream-upstream-downstream movement of the 

peak noise source location shows up in the peak location plots 

as the repeated shark-fin-like pattern. Note that it shows up not 

only in results obtained for the off-design cases (magenta and 

blue), but also at the design condition (green). This sort of 

pattern has also been seen in the peak location plots generated 

from data obtained on other supersonic jets (Ref. 8).

The spectrum provided in Figure 10 for the under-expanded 

case (magenta) shows one dominant hump rising above the 

higher frequency broadband shock noise, to the right of the 

hump. The BBSN theory put forth by Tam (Ref. 9) suggests that 

the elevated region of high frequency noise is actually made up 

of a series of humps centered about different frequencies. These 

humps are illustrated in Figure 11 (taken from Miller (Ref. 10)). 

Sometimes multiple humps are visible in the acoustic spectra 

obtained from shock-containing jets. Normally, however, the 

lowest frequency hump is much easier to identify than the 

others. These humps are thought to be created by turbulent 

eddies which are large enough to span more than one shock cell. 

The interaction of these large eddies with the shocks generates 

highly correlated noise radiating from multiple shocks 

simultaneously. According to Harper-Bourne and Fisher 

(Ref. 11), at the peak frequency in each hump “radiation from 

all sources interferes constructively,” while on either side of the 

peak “this constructive interference is less complete and hence 

lower levels of noise are anticipated.” The lowest frequency 

hump is created by the interference of sound waves coming 

from adjacent shocks (1 shock spacing part), the second hump is 

created by the interference of waves coming from every-other 

shock (2 shock spacings apart), etc.
NASA/TM—2013-218085 12

Setpoint 11606, over-expanded, Mj=1.29 Setpoint 11610, ideally-expanded, Mj=1.50 Setpoint 11617, under-expanded, Mj=1.61

Figure 10.—Single mic spectra (upper left), peak location plots (upper right), and 1/12th OB beamform maps (bottom) for SMC016

CD nozzle at setpoints 11606 (Mj=1.29, TSR=0.76), 11610 (Mj=1.50, TSR=0.72) and 11617 (Mj=1.61, TSR=0.76).
NASA/TM—2013-218085 13

Figure 11.—BBSN spectral humps predicted using Tam’s 

model compared with experimental data. (Figure taken from 

Miller, Ref. 10.)

Figure 12.—Peak frequencies (asterisks) corresponding to the 

two lowest-frequency BBSN humps predicted by Tam’s 

model overlaid on top of spectra and peak location plots 

measured with SMC016 CD nozzle at setpoints 11606 

(Mj=1.29) and 11617 (Mj=1.61).

Using Tam’s model, Miller (Ref. 12) calculated the peak 

frequencies of the humps corresponding to the two off-design 

operating conditions tested with the C-D SMC016 nozzle.

Figure 12 shows the peak frequencies corresponding to the two 

lowest frequency humps (asterisks) overlaid on top of the 

spectra and peak location plots for the over-expanded and 

under-expanded jet data shown previously in Figure 10. As 

shown in the peak location plots, the predicted hump peak 

frequencies occur very close to the centers of the first two 

shark-fin patterns that are shown for each operating condition.

This good correlation suggests that the mechanism that creates 

the spectral humps in Tam’s model (large eddy/shock wave 

interaction) is also responsible for the downstream-upstreamdownstream-upstream movement of the peak noise source 

location that occurs as frequency increases. It also implies that 

the peak location plots show where the large eddy/shock wave 

interaction that produces the humps occurs at in the jet. In the 

over-expanded case, the first hump is created by shocks located 

between 4 and 8 diameters downstream of the nozzle exit; the 

second hump by shocks between 5 and 7 diameters 

downstream. For the under-expanded jet the first hump is 

created by shocks between 8 and 17 diameters downstream, and 

the second hump by shocks between 12 and 15 diameters 

downstream. In both jets, some of the same shocks that produce 

the first hump also produce the second. The individual 

beamform maps shown in Figure 10 corresponding to the first 

hump (the first downstream movement of the peak source, maps 

4 through 7 in the left column, and maps 3 through 5 in the right 

column) indicate that any given frequency in a hump is 

generated from a relatively small axial region in the jet. This 

suggests that the constructive interference of sound waves that 

produces the noise at a given frequency and observer location 

comes primarily from a small number of shocks, rather than 

from all the shocks at the same time.

It is interesting that the repeated shark-fin pattern also 

appears in the peak location plot provided in Figure 10 for the 

design condition (setpoint 11610, green line) even though any 

shocks occurring in the jet at this condition are likely to be 

much weaker than at the two off-design conditions. The 

beamform map given for the 1/12th OB centered at 30 kHz 

(St=3.7, map #10, center column) shows that shocks were 

present in the jet at this operating condition (it was not quite 

ideally-expanded). The presence of the shark-fin pattern 

suggests that the large eddy/shock wave interaction 

mechanism also occurred at this condition, but the spectrum 

shown in Figure 10 (green) indicates that the noise produced 

by this interaction was not loud enough to produce a distinct 

BBSN hump. At least for this case, it appears that the 

downstream movement of the peak source location is a more 

sensitive indicator of the presence of large eddy/shock wave 

interaction than the spectrum itself.

The subsonic jet data presented earlier showed that the peak 

source location associated with any given frequency of noise 

tends to move downstream as NPR (or jet Mach number) 

increases. A comparison of the three peak location plots 

presented in Figure 10 shows this to be true in supersonic jets as 

well. This is indicated not only for the low frequency region 

controlled by turbulent mixing but also for the high frequency 

region where large eddy/shock wave interaction produces the 

shark-fin pattern observed in the peak location plots. This means 
NASA/TM—2013-218085 14

that the shocks which interact with the large turbulent eddies to 

produce the BBSN humps are, on average, located further away 

from the nozzle the higher the nozzle pressure ratio.

The 1/12th OB beamform maps presented in Figure 10 for 

the over-expanded condition (left column) are noticeably 

different than those provided for the two higher jet Mach 

numbers (center and right columns). None of the color 

contours shown in the beamform maps for the over-expanded 

condition extend very far downstream of the peak noise source 

location (the location of the small red square), whereas some 

of the contours provided for the two higher speed jets do. This 

has important implications regarding jet noise shielding. A 

shield that extends a couple of nozzle diameters further 

downstream than the peak noise source location in the overexpanded jet would block almost all of the noise associated 

with that frequency band, while a surface placed in a similar 

manner in the higher speed jets could still allow much of the 

noise to propagate in the direction of the phased array. A 

surface extending 10 diameters downstream of the nozzle exit, 

for example, placed next to the ideally-expanded jet would 

cover the peak source location shown in beamform map #8 

(center column), but a significant portion (about 5 diameters) 

of the noise-producing region of the jet would remain 

uncovered. This means that in order to effectively shield the 

noise produced by a jet it may not be enough to know the 

location of the peak noise source as a function of frequency 

(information provided by the peak location plots). It might 

also be necessary to know how far the noise producing regions 

extend downstream of the peak source location in each 

frequency band (information provided by the 1/12th OB plots).

Figure 13 shows all the peak source location data presented 

previously on the same plot. Unlike the preceding plots, 

however, the peak source location data presented here have

been nondimensionalized by potential core length. The 

potential core length corresponding to each setpoint was 

estimated from the Witze correlation parameter (Ref. 13) 

following the method outlined by Bridges et al. (Ref. 14). As 

shown in the figure, the nondimensional peak source location 

versus nondimensional frequency curves for the four subsonic 

test cases tend to collapse to one distribution. The lowfrequency portion of the curve provided for the ideallyexpanded supersonic case (setpoint 11610, green) also 

collapses, but the low-frequency segment of the curve 

provided for the under-expanded case (setpoint 11617, 

magenta) is slightly higher, and that of the over-expanded case

(setpoint 11606, blue) is much lower than this distribution.

This suggests that this method of nondimensionalizing data 

collapses the low frequency region of supersonic jet peak 

source location data only when strong shocks are not present 

in the flow. The presence of the shocks appears to bias the low 

frequency source location data of under- and over-expanded 

supersonic jets toward the location of the shocks (i.e., closer to 

the nozzle for over-expanded jets and further from the nozzle 

for under-expanded). At higher frequencies, where the sharkfin pattern occurs, none of supersonic source location data 

collapse; this shock noise is generated further downstream as 

jet Mach number increases.

Shielding Configuration, Subsonic Jet Results

A surface placed between a jet plume and an observer can act 

as a shield, but it can also act as a noise source. If the surface is 

located so close to the jet that the flow impinges on the surface 

then additional noise will be generated 1) by the turbulent flow 

“scrubbing” along the surface and 2) at the trailing edge where 

hydrodynamic pressure fluctuations are scattered into acoustic 

waves. Shielding configuration data will be presented below for 

surfaces located 1, 2, and 3 diameters from the jet centerline.

The schematic presented in Figure 14 shows the axial location 

Figure 13.—Nondimensional peak noise source location 

plotted versus nondimensional frequency for each of the 

seven tested setpoints.

Figure 14.—Schematic showing jet/surface intersection points for surfaces 1, 2, and 3 jet diameters from the 

jet centerline and a jet assumed to expand at a 7° angle.
NASA/TM—2013-218085 15

at which each of these surfaces could be expected to first 

intersect a jet that is assumed to expand at a 7° angle. It shows

that surfaces located 1, 2, and 3 diameters from and parallel to 

the jet centerline could be expected to intersect the jet roughly 4, 

11.5, and 20 nozzle diameters downstream of the nozzle exit, 

respectively. This means, for example, that a surface located 1D 

away from the centerline and extending 2D downstream of the 

nozzle exit would not be expected to generate scrubbing noise, 

but a surface at the same radial location and extending 6D 

downstream would.

The bare jet data presented in the preceding section 

indicates that subsonic jets are relatively simple in comparison 

to supersonic jets. In both cold and hot subsonic jets there is 

only one source of noise, turbulent mixing, and the peak noise 

source location occurs relatively far downstream at low 

frequency and moves gradually upstream as frequency 

increases. Therefore, it would be more difficult to shield low 

frequency versus high frequency noise in a subsonic jet. Also, 

the peak noise source location corresponding to any given 

acoustic frequency moves upstream toward the nozzle as jet 

Mach number decreases. Consequently, it would become 

increasingly more difficult to shield the jet noise coming from 

a subsonic jet as the jet Mach number increases.

Of the five subsonic operating conditions set during the test, 

setpoint 7 (Ma=0.90, TSR=0.835) corresponds to the highest 

jet Mach number, and therefore, the most difficult to shield. 

Figure 15 shows shielding configuration phased array data 

obtained at this operating condition with surfaces located 3 

nozzle diameters (3D) away from the jet centerline. OASPL 

beamform maps (right) and single microphone spectra (upper 

left) are provided for the bare jet as well as for surfaces that 

extended 2, 4, 6, 10, 15 and 20 diameters downstream of the 

nozzle exit. The center, left plot shows the difference between 

the single microphone spectrum levels measured with the bare 

jet and with each surface. As such, it shows the noise 

attenuation provided by each surface, and will be referred to 

herein as an attenuation plot (a positive value indicates that the 

surface reduced noise). The peak location plot shown in the 

lower left corner shows only the bare jet (no surface) 

distribution. In order to better visualize the noise source 

locations relative to the nozzle hardware, the phased array 

contour maps are superimposed onto a photo that was taken 

when the shielding surface was not in place. The vertical red 

line shown on all but the top map (the bare jet map) designates 

the location of the trailing edge of the shielding surface used 

in each case. Note that this line does not always match up with 

the grid line corresponding to a given axial location in the jet.

The vertical red line shown for the 6D wall, for example, is 

located about 6.3 nozzle diameters downstream of the nozzle 

exit, while that shown for the 20D wall is about 20.7 diameters 

downstream. These differences are due to parallax. Since the 

array was always upstream of the surface trailing edge, from 

the viewpoint of the array camera the surfaces blocked more 

of the jet than would be suggested by the wall trailing edge 

locations (2D, 4D, etc.). These differences increased as 1) the 

trailing edge of the surface shifted downstream and 2) the 

surface moved outward away from the jet centerline and 

toward the array. 

The bare jet beamform map presented in Figure 15 (top) 

indicates that at this condition the loudest jet noise is 

generated between approximately 4 and 9 diameters 

downstream of the nozzle exit. Based on this, it is not 

surprising that the data presented for surfaces that extend only 

2 or 4 nozzle diameters (2D or 4D) downstream of the nozzle 

exit show only small amounts of noise attenuation (up to 2 dB 

at high frequency), while a surface that extends 6D 

downstream blocks roughly half of the noise-producing region 

of the jet and reduces high frequency noise by about 5 dB. A 

surface that extends 10D downstream blocks the noise coming 

directly from the peak noise source location (designated by the 

red square in the bare jet beamform map) and reduces noise by 

as much as 10 dB at high frequency, while 15D and 20D 

surfaces provide as much as 17 and 20 dB of high-frequency

noise attenuation, respectively.

The data presented in Figure 15 indicate that none of the 

surfaces were effective at reducing low frequency noise, 

below St=0.17 (1 kHz). The bare jet peak location plot 

suggests that the two longest surfaces, 15D and 20D, should 

block the regions in the jet where this low frequency noise is 

generated, but rather than a reduction, the corresponding 

attenuation plots show an increase in noise relative to the bare 

jet. In fact, these data indicate that having the 10D surface 

located 3 jet diameters from the jet centerline also results in 

the production of low frequency noise. The schematic shown 

in Figure 14 suggests that a surface located 3 jet diameters 

from the jet centerline would not be expected to intersect the 

jet plume unless it extended 20 or more jet diameters 

downstream of the nozzle exit. In order to impinge upon a 

surface that extended only 10D downstream of the nozzle exit

the jet would have to expand at an angle of about 15° rather 

than the 7° assumed in the schematic. Huang et al. (Ref. 15)

conducted a similar shielding experiment using a surface next 

to a jet. Using pitot probe surveys they verified that excess 

low frequency noise can be produced even when the jet does 

not come in contact with the surface. Consequently, it may be 

that the additional noise associated with the 10D and 15D 

surfaces shown in Figure 15 is not caused by the flow

scrubbing the surface. Instead, it may be due to the conversion 

of hydrodynamic pressure fluctuations into acoustic waves 

along the surface or at the trailing edge. Regardless of the 

source, these data indicate that it is not correct to assume that a 

surface located just outside of a 7° jet expansion cone would 

not generate any additional low frequency noise. The data 

shown in Figure 15 correspond to surfaces located 3D from 

the jet centerline. Figure 16 and Figure 17 show the same type 

of plots for surfaces located closer to the jet, 2D and 1D from 
NASA/TM—2013-218085 16

the jet centerline, respectively. A comparison of the spectra 

provided in these three figures indicates that when a surface is 

moved inward toward the jet centerline both the level and 

upper frequency of the noise generated by the jet-surface 

interaction increases. This is shown, for example, in the 

spectra provided for the 6D surface (cyan). When this surface 

is 3D from the jet centerline (Figure 15), the spectra is slightly 

(< 2 dB) above that of the bare jet (dark blue) out to about

St=0.25 (1500 Hz). When it is 2D from the jet centerline 

(Figure 16) the noise level is as much as 3 to 4 dB above that 

of the bare jet and noise is produced out to about St=0.35 

(2000 Hz). When this surface is moved to 1D from the 

centerline (Figure 17) the noise level exceeds the bare jet case 

by as much as 10 dB and extends out to St=0.6 (3500 Hz). The 

increase in level is not surprising considering that the flow 

scrubs across more and more of the surface as it is moved 

inward toward the jet centerline. The increase in frequency 

might be associated with the fact that as the surface is moved 

inward the jet/surface impingement point would move 

upstream. As it does the average size of the turbulent eddies 

interacting with the surface would tend to decrease, and the 

frequency of the noise resulting from this interaction would 

tend to increase.

Figure 15.—Single mic spectra (upper left), attenuation plots (center left), peak location plot (lower left) and beamform maps (right) 

for the bare jet and surfaces extending 2, 4, 6, 10, 15, and 20D downstream of nozzle and 3D away from jet centerline for the

SMC000 nozzle at setpoint 7 (Ma=0.90, TSR=0.835).
NASA/TM—2013-218085 17

Figure 16.—Single microphone spectra (upper left), attenuation plots (center left), peak location plot (lower left) and beamform 

maps (right) for the bare jet and surfaces extending 2, 4, 6, 10, 15, and 20D downstream of nozzle and 2D away from jet 

centerline for the SMC000 nozzle at setpoint 7 (Ma=0.90, TSR=0.835).
NASA/TM—2013-218085 18

Figure 17.—Single microphone spectra (upper left), attenuation plots (center left), peak location plot (lower left) and beamform 

maps (right) for the bare jet and surfaces extending 2, 4, 6, 10, 15, and 20D downstream of nozzle and 1D away from jet 

centerline for the SMC000 nozzle at setpoint 7 (Ma=0.90, TSR=0.835).

Shielding Configuration, Supersonic Jet Results

The bare jet data presented earlier suggests that it could be 

much more difficult to shield noise propagating in the 

direction of the array (roughly 90° to the jet axis) coming from 

a supersonic, as opposed to a subsonic, jet. In subsonic jets the

peak noise source location moves gradually toward the nozzle 

exit as frequency increases. In contrast, in cold, shockcontaining jets the source location data tend to show a rather 

complicated behavior in which the peak source location moves 

downstream as frequency increases through a BBSN hump. 

Consequently, it would be more difficult to shield the higher 

frequency noise to the right of the peak in a BBSN hump than 

it would be to shield the lower frequency noise to the left. The 

data presented earlier also show that the peak noise source 

location associated with any given frequency of noise tends to 

move downstream as NPR increases. Consequently, it would 

become increasingly more difficult to shield the jet noise 

coming from a supersonic jet as the jet Mach number 

increases. As discussed earlier, this was also the case for 

subsonic jets.

Figure 18 shows shielding configuration data obtained on 

the SMC016 nozzle at the lowest of the 3 jet Mach numbers 

set using this nozzle (over-expanded setpoint 11606, Ma=1.13, 

TSR=0.76). The bare jet peak location plot provided in this 

figure (lower left) indicates that the source location associated 

with the fundamental screech tone (at St=0.41, 3000 Hz) was 

upstream of the nozzle exit. The 1/12th OB beamform map 

corresponding to this frequency provided in Figure 10 (#2, left 

column), shows that this screech tone reflected off of the

upstream nozzle hardware. Except for this one tone, the peak 

location plot shown in Figure 18 indicates that all of the other 

peak noise sources associated with St > 0.13 (1 kHz) are 

confined to a relatively small axial region in the jet between 4 
NASA/TM—2013-218085 19

and 8 diameters downstream of the nozzle exit. By far the 

loudest source is the harmonic of the screech tone (St=0.82, 

6000 Hz) generated 5D downstream of the nozzle exit. Since it 

is more than 7 dB (the dynamic range of the color maps) 

higher than any other source, it is the only source visible in the 

bare jet beamform map. Based on the bare jet data, it is not 

surprising that the shielding configuration data presented in 

Figure 18 indicates that a surface extending only 4D 

downstream of the nozzle exit provides very little noise 

attenuation (red line). A surface extending only two diameters 

further downstream (to 6D), on the other hand, would 1) cover 

about half of the noise-producing region in the jet, 2) cover the 

locations in the jet producing the lower frequency noise to the 

left of the peak of the lowest-frequency BBSN hump but not 

the higher frequency noise to the right, and 3) completely 

cover the location where the dominant screech tone is 

generated. The data presented for the 6D surface indicates that 

it provided 20 dB of noise attenuation at the frequency of the 

dominant screech tone (St=0.82, 6000 Hz) but only as little as 

3 dB of attenuation at frequencies corresponding to the high 

frequency side of the lowest-frequency BBSN hump. The peak 

location plot indicates that the noise corresponding to the 

high-frequency side of this hump is generated downstream of 

the trailing edge of the 6D surface, between 6 and 7.5 

diameters downstream of the nozzle exit. Consequently, it is 

not surprising that the 6D surface would not provide much 

attenuation in this frequency range. A surface that extends 

greater than 8 diameters downstream of the nozzle exit would, 

however, cover most of the regions in the jet responsible for 

producing the elevated region of BBSN (> St=0.55, or 

4000 Hz). The data presented for the 10D surface indicates 

that it was very effective at reducing this noise (by about 20 

dB). In fact, this 10D surface was so effective at blocking the 

noise that relatively little additional attenuation was achieved 

by employing the two longer (15D and 20D) surfaces.

Figure 18.—Single mic spectra (upper left), attenuation plots (center left), peak location plot (lower left) and beamform maps (right) 

for the bare jet and surfaces extending 2, 4, 6, 10, 15, and 20D downstream of nozzle and 3D away from jet centerline for the 

SMC016 nozzle at the over-expanded setpoint (11606, Mj=1.29, TSR=0.76). 
NASA/TM—2013-218085 20

Similar data obtained using the C-D nozzle at the ideallyexpanded operating condition (setpoint 11610, Ma=1.31, 

TSR=0.72) are shown in Figure 19. The bare jet beamform 

map indicates that the loudest noise is produced relatively far 

downstream in the jet, between 7 and 14 diameters 

downstream of the nozzle exit. This suggests that a surface 

extending at least 15 diameters downstream of the nozzle exit 

would be needed to block the loudest part of the jet. Based on 

this it makes sense that the 10D surface provides < 5 dB of 

attenuation at St<1.4 (11 kHz) and a maximum of 10 dB of 

attenuation at the highest plotted frequency. Unlike the overexpanded case in which the 10D surface completely blocked 

the loudest noise-producing region in the jet and the 15D and 

20D surfaces provided little additional benefit, in this case the 

15D and 20D surfaces provide about 10 and 15 dB, 

respectively, of additional attenuation relative to the 10D 

surface at St>0.6 (6 kHz).

Figure 20 shows the same type of data obtained using the CD nozzle at the high Mach number test case (under-expanded 

setpoint 11617, Ma=1.41, TSR=0.76). At almost all 

frequencies the peak noise source locations plotted in this 

figure are downstream of those provided in Figure 19 for the 

ideally-expanded case. In this higher speed jet a surface 

extending 10D downstream of the nozzle exit would only 

block some of the peak noise source locations associated with 

the noise occurring 1) to the left of the peak of the dominant 

BBSN hump and 2) at St>2.5 (22 kHz). Based on this it is not 

surprising that the 10D surface provides < 5 dB of attenuation 

over the entire frequency range. The 1/12th OB beamform 

maps presented above in Figure 10 corresponding to 

Figure 19.—Single mic spectra (upper left), attenuation plots (center left), peak location plot (lower left) and beamform maps (right) 

for the bare jet and surfaces extending 2, 4, 6, 10, 15, and 20D downstream of nozzle and 3D away from jet centerline for the

SMC016 nozzle at the ideally-expanded setpoint (11610, Mj=1.50, TSR=0.72).
NASA/TM—2013-218085 21

frequencies to the right of the peak of the dominant BBSN 

hump (beamform map #5 and #6, right column) indicate that 

the large eddy/shock wave interaction responsible for 

producing this noise occurs more than 10 diameters 

downstream of the nozzle exit. The spectra shown in Figure 20

for the bare jet and the 10D surface overlap within this 

frequency range, indicating 1) that the 10D surface provides 

no attenuation of this noise, 2) that all of the associated noise 

sources must be located more than 10 diameters downstream 

of the nozzle exit, and therefore, 3) that the 1/12th OB 

beamform map is correct in showing these sources more than 

10 diameters downstream. The peak location plot also 

indicates that a surface that extends 15 diameters downstream 

of the nozzle would be much more effective than a 10D 

surface at blocking the noise produced by this jet. A 15D 

surface would block the peak noise source locations associated 

with 1) the turbulent mixing noise occurring in the range

0.15<St<0.38 (1500<f<3300 Hz), 2) the lower frequency noise 

to the left of the peak of the dominant BBSN hump, and 

3) most and the high frequency noise to the right of the 

dominant BBSN hump. The spectra provided in Figure 20

indicate that the 15D surface was effective at blocking noise 

within these frequency ranges, especially the noise to the left 

of the peak in the BBSN hump (which was reduced by as 

much as 15 dB). These data indicate, however, that even the 

15D surface would not extend far enough downstream to 

block all of the measurable noise coming directly from the jet. 

In particular, it would not block 1) the BBSN to the right of 

the peak in the dominant BBSN hump and 2) some of the 

higher frequency BBSN generated at frequencies to the right 

of the hump. The spectra provided in Figure 20 indicate that 

the 20D surface was effective at blocking noise that was not 

blocked by the 15D surface. For St>0.6 (above the peak in the 

BBSN hump) the attenuation provided by the 20D surface was 

roughly double that provided by the 15D surface. 

Figure 20.—Single mic spectra (upper left), attenuation plots (center left), peak location plot (lower left) and beamform maps (right) 

for the bare jet and surfaces extending 2, 4, 6, 10, 15, and 20D downstream of nozzle and 3D away from jet centerline for the 

SMC016 CD nozzle at the under-expanded setpoint (11617, Mj=1.61, TSR=0.76).
NASA/TM—2013-218085 22

Comment on Source Localization Data Accuracy

Numerous examples have been presented above which show 

that when the source locations measured in a bare jet using the 

phased array were subsequently blocked with a surface that 

the noise measured by the microphone closest to the center of 

the array was reduced. In other words, there is a consistency 

between the single microphone spectra and the phased array 

noise source localization measurements. This consistency 

tends to validate the accuracy of the phased array noise source 

localization data. This is important because the data 

processing used here assumes that the noise sources in the jet 

are stationary, incoherent monopoles. In the past some people 

have dismissed jet phased array data processed in this manner 

because they feel that this source model is too simplistic. They 

argue that a source model must take into account the fact that 

jet noise is produced by extended, moving, coherent 

structures. The data presented above indicates that using a 

simple, incoherent monopole source model provides accurate 

source localization data, at least as in cases such as this, where 

the phased array is 90° to the jet axis.

Reflecting Configuration Results

This section will discuss results obtained with surfaces 

mounted on the opposite side of the jet during the reflecting 

configuration part of the test. These data were acquired with 

the phased array mounted below the jet on the moveable cart 

with the array plate angled upward (see Figure 6). It is 

important to realize that in order for noise reflecting off of a 

surface to be sensed by a microphone in the array that the law 

of reflection must be satisfied. Depending on the relative 

locations of the jet, the surface, and the microphone only some 

of the jet noise sources may satisfy this requirement. This is

illustrated in Figure 21. The schematic provided in this figure 

shows a top view of a reflecting surface configuration test in 

which the surface (depicted by the red line) extends 5D 

downstream of the nozzle exit and is located 4D from the jet 

centerline. The black, vertical line represents a line normal to 

the array plate drawn from the center of the array. The dark 

blue lines labeled 1 and 2 illustrate the law of reflection for 

noise reflecting off of the surface trailing edge to the 

microphone located at the center of the array. With this 

configuration any jet noise sources downstream of line 2 

cannot satisfy the law of reflection, and therefore, cannot 

reflect from the surface in such a way that they can be sensed 

by the microphone at the center of the array, while all sources 

upstream of this line can. This is important because the quality 

of the beamforming images of the reflected noise increases as 

the number of microphones that can sense it increases.

Consequently, for the configuration depicted here it is unlikely 

that the phased array would be able to produce images of the 

Figure 21.—Schematic of a reflecting surface configuration test 

illustrating the law of reflection.

noise reflecting off of the surface from locations in the jet 

downstream of the surface trailing edge.

As mentioned above, reflecting configuration phased array 

data were obtained with surfaces that extended 5, 10, 15 and 

20 diameters downstream of the nozzle exit. For the 5D 

surface, the phased array data were actually acquired twice; 

once with the reflecting surface in place, and once again on 

the bare jet after the surface was taken down. Having both sets 

of data allows the bare jet cross-spectral matrix (CSM) to be 

subtracted from that of the jet/surface combination. Assuming

that the jet noise is the same in both cases (i.e., that the jet 

noise is not modified by the presence of the surface), 

subtracting the bare jet CSM from that of the jet/surface 

combination yields a CSM corresponding to the noise coming 

directly from the surface, and processing the resulting CSM 

provides better images of this noise. This is a way of “turning 

off” the noise coming directly from the bare jet to the array in 

order to better image the noise that is either generated at or 

reflected off of the surface. 

Figure 22 shows data obtained using the SMC000 nozzle at 

setpoint 7 (Ma=0.90, TSR=0.835) that was processed in this 

manner. The configuration was the same as that depicted in 

Figure 21; the surface extended 5D downstream of the nozzle 

exit and was located 4D from the jet centerline. The beamform 
NASA/TM—2013-218085 23

maps shown in this figure were computed from CSMs

corresponding to 1) the bare jet (left column), 2) the 

jet/surface combination (center column), and 3) the jet/surface 

combination minus the bare jet (right column). As such, the 

right column of beamform maps show noise coming from the 

surface in the absence of noise coming directly from the jet. 

Beamform maps are shown for five different 1/3rd octave 

bands. The line plots in the upper left show the beamform map 

peak contour levels as a function of nondimensional frequency 

for the three datasets, and the line plots to the right show 

single microphone spectra for both the bare jet and the 

jet/surface combination.

The beamform map peak spectra (left line plots) provided in 

Figure 22 show two distinctly different frequency regions. For

St<0.6 (3.5 kHz), the jet noise source is stronger than the 

source associated with the surface alone, whereas above this 

frequency they are almost equal. The beamform maps 

presented for the jet/surface combination (center column) can 

be used to explain this difference. They show that the source 

of low frequency jet noise is located downstream of the 

surface trailing edge. Consequently, this low frequency noise

does not satisfy the law of reflection and cannot reflect off of 

the surface towards the phased array. As frequency increases, 

however, the peak noise source location moves upstream, and 

near St=0.6 (3.5 kHz), it moves upstream of the trailing edge 

of the surface. Once it does 1) the law of reflection is satisfied, 

2) the corresponding noise reflects off of the surface 

efficiently toward the array, 3) the amplitude of the reflected 

noise becomes essentially equal to that of the noise coming 

directly from the jet, and 4) the two sources combine together 

to produce the increased noise levels shown in the single 

microphone spectra (as evidenced by the green line being 

higher than the blue in the upper right plot). 

Figure 23 shows results obtained using this 5D surface at 

the same jet operating condition but with surface located 

closer to the jet, only one nozzle diameter from the jet 

centerline. The data provided above in Figure 22 showed that 

when this surface was located 4D from the jet centerline that 

the amplitude of the low frequency noise coming from the 

surface was lower than that coming directly from the jet. In 

contrast, the beamform map peak spectra provided in 

Figure 23 indicate that when the surface is only 1D from the 

centerline the surface is the dominant source of low frequency 

noise (magenta higher than blue). The beamform maps 

provided for the jet/surface combination (center column) show 

this low frequency noise coming from the surface trailing 

edge, rather than further downstream in the jet like it did when 

the surface was further away. This appears to be scrubbing

and/or trailing edge noise created by the jet flow impinging on 

the surface. For St>0.6 the beamform map peak spectra

resembles that shown previously (Figure 22) for the case when 

the surface was further from the jet centerline. This suggests 

that the mechanism responsible for the high frequency noise 

coming from the surface is the same in both cases, and 

therefore, that it is simply a reflection of the jet noise off of 

the surface and not due to flow scrubbing.

The beamform maps provided for the jet/surface combination 

(center column) in Figure 23 show the low frequency scrubbing 

noise coming from the surface trailing edge. This is to be 

expected since as shown in the schematic at the top of Figure 23

this is the only part of the 5D surface that would be impacted by 

the flow. Figure 24 shows data obtained with a surface that 

extended much further downstream, 20 diameters downstream 

of the nozzle exit. The beamform maps at the right correspond 

to jet/surface combination data obtained with this 20D surface 

located 1 diameter from the jet centerline. The line plots at the 

left show single microphone spectra for this case (surface 1D 

from the centerline, blue) and also for this surface located 

outside the jet plume, 6 diameters from the jet centerline 

(green). Once again, the increase in low frequency noise 

generated with the surface in the flow versus out of the flow 

(blue versus green in the spectra) is due to the flow coming in 

contact with the surface. The corresponding beamform maps 

show that the vast majority of this additional noise is generated 

at the surface trailing edge even though much of the surface 

upstream of the trailing edge is also impacted by the jet flow 

(see the schematic presented in the figure). Near 1 kHz the peak 

noise source location jumps from the trailing edge to an 

upstream location, and then gradually moves upstream toward 

the nozzle exit as frequency increases. This gradual upstream 

movement of the peak source location is similar to that which 

occurs in a bare subsonic jet. For frequencies above 1 kHz the 

in-flow versus out-of-flow spectra almost overlap, indicating 

that scrubbing noise does not does contribute significantly to the 

noise produced by the in-flow surface at higher frequencies. 

Instead, this higher frequency noise seems to be just a 

combination of the jet noise 1) coming directly from the jet, and 

2) reflecting off of the surface. When the surface is in the flow 

there appear to be two different competing sources: 1) the 

trailing edge noise, and 2) the jet noise (which reaches the array 

both directly and via reflection off the surface). At low 

frequency, the trailing edge noise dominates, while at high 

frequency the jet noise dominates.
NASA/TM—2013-218085 24

Figure 22.—Beamform map peak spectra (upper left), single mic spectra (upper right) and beamform maps (bottom) for the bare jet 

and the 5D reflecting surface located 4D from the jet centerline for the SMC000 nozzle at setpoint 7. The beamform maps are 

bare jet (left column), jet/surface combination (center), and surface alone (right).
NASA/TM—2013-218085 25

Figure 23.—Beamform map peak spectra (upper left), single mic spectra (upper right) and beamform maps (bottom) for the bare jet 

and the 5D reflecting surface located 1D from the jet centerline for the SMC000 nozzle at setpoint 7. The beamform maps are 

bare jet (left column), jet/surface combination (center), and surface alone (right).
NASA/TM—2013-218085 26

Figure 24.—Single mic spectra (left) for the 20D reflecting surface located 1D (in the flow, blue) and 6D (out of the flow, green) from 

the jet centerline and 1/3rd OB beamform maps (right) for the same surface located 1D (in the flow) from the jet centerline for the 

SMC000 nozzle at setpoint 7

.

Summary

1) Both bare jet and shielding configuration noise source 

localization data were acquired with the Array48 phased 

array system using two nozzles, one convergent and one 

convergent-divergent, over a wide range of operating 

conditions. These data were obtained with the array at an 

angle approximately 90° to the jet axis, and provide 

information regarding the distribution of noise sources in 

the jet as observed from this particular direction.

2) Bare jet phased array data acquired on subsonic jets show 

that the sources of low frequency noise are located 

relatively far downstream and that the sources move 

gradually upstream toward the nozzle exit as frequency 

increases. These data indicate that it would be more 

difficult (i.e., require a longer surface) to shield low 

frequency as opposed to high frequency noise in a 

subsonic jet. These data also show that the source location 

corresponding to a given frequency of noise produced in a 

subsonic jet is controlled solely by jet Mach number 

(changing the jet temperature while holding jet Mach 

number fixed has no effect). These data indicate that it 

would become increasingly more difficult to shield the 

noise coming from a subsonic jet as the jet Mach number 

increases.

3) The source location data obtained on cold, shockcontaining jets tend to show a rather complicated behavior 

in which the peak source location moves upstream, then 
NASA/TM—2013-218085 27

downstream, then upstream again as frequency increases.

The downstream movement correlates well with the 

location of the spectral humps predicted by Tam’s BBSN 

model. This indicates that the mechanism thought to be 

responsible for creating the humps, large turbulent 

eddy/shock wave interaction, is also responsible for this 

downstream movement. Since the source location moves 

downstream as frequency increases through a hump it 

would be more difficult to shield the higher frequency 

noise to the right of the peak in a BBSN hump than the 

lower frequency noise to the left. 

4) The phased array data obtained on cold, shock-containing 

jets also show that the eddy/shock wave interaction which 

produces the spectral humps occurs further and further 

downstream as jet Mach number increases. Accordingly, 

the shielding configuration data showed that it was much 

more difficult to shield the noise produced by an underexpanded, as opposed to an over-expanded, jet.

5) The phased array data obtained on cold, shock-containing 

jets also show that any given frequency in a BBSN hump 

is generated from a relatively small axial region in the jet.

This suggests that the constructive interference of sound 

waves that produces the noise at a given frequency and 

observer location comes primarily from a small number of 

shocks, rather than from all the shocks at the same time.

6) The single microphone spectra measured using a C-D 

nozzle operating very close to its design point (Mach 

1.50) resembled that of a subsonic jet (broad and round, 

with no screech tones or distinctive BBSN humps), but 

the peak noise source location plot did not. Instead of 

moving gradually upstream as frequency increased (like 

in a subsonic jet), there were some frequency ranges over 

which the peak noise source location moved downstream 

(like in the over-expanded and under-expanded jets). This 

suggests that although noise levels tend to decrease when 

a cold, over-expanded supersonic jet is brought closer to 

its design point, the noise source locations may become 

more difficult to shield because they move downstream.

In regards to shielding, jet Mach number appears to be a 

more important variable than how far-off design the jet is 

operating. Like in subsonic jets, the noise produced by 

supersonic jets becomes more difficult to shield as jet 

Mach number increases. 

7) Numerous examples are presented which show that when 

the source locations measured in a bare jet using the 

phased array were subsequently blocked with a surface 

that the noise measured by the microphone closest to the 

center of the array was reduced. This consistency tends to 

validate the accuracy of the phased array noise source 

localization data and the legitimacy of the stationary, 

incoherent monopole source model used to process it. 

8) Reflecting configuration noise source localization data are 

presented for a convergent nozzle operating at a high 

subsonic Mach number. These data illustrate that the law 

of reflection must be satisfied in order for jet noise to 

reflect off of a surface to an observer. Depending on the 

relative locations of the jet, the surface, and the observer

only some of the jet noise sources may satisfy this 

requirement. 

9) The low frequency noise created when the jet flow 

impinges on the surface was found to come primarily 

from the trailing edge regardless of the axial extent of the 

surface impacted by the flow.

References

1. Brown, C.A., “Jet-Surface Interaction Test: Far-Field 

Noise Results,” ASME paper GT2012-69639, June 2012.

2. Collier, F., Thomas, R., Burley, C., Nickol, C., Lee, CM, Tong, M., “Environmentally Responsible Aviation -

Real Solutions for Environmental Challenges Facing 

Aviation,” presented at the 27th International Congress of 

the Aeronautical Sciences, Sept. 2010.

3. McLaughlin, D.K., Kuo, C.W., and Papamoschou, D., 

“Experiments on the Effect of Ground Reflections on 

Supersonic Jet Noise,” AIAA paper 2008-22, Jan. 2008.

4. Papamoschou, D. “Prediction of Jet Noise Shielding,” 

AIAA paper 2010-653, Jan. 2010.

5. Kawai, R.T., “Acoustic Prediction Methodology and 

Test Validation for an Efficient Low-Noise Hybrid Wing 

Body Subsonic Transport,” Final report for NASA 

Contract Number NNL07AA54C, Feb. 2011.

6. Dougherty, R.P., Podboy, G.G. “Improved Phased Array 

Imaging of a Model Jet,” AIAA paper 2009-3186, May 

2008.

7. Brown, C.A. and Bridges, J.E., “Small Hot Jet Acoustic 

Rig Validation,” NASA TM 2006-214234, April 2006.

8. Podboy, G.G., Bridges, J.E., Henderson, B.S., “Phased 

Array Noise Source Localization Measurements of an 

F404 Nozzle Plume at Both Full and Model Scale,” 

ASME paper GT2010-22601, June 2010.

9. Tam, C.K.W., “Stochastic Model Theory of Broadband 

Shock Associated Noise from Supersonic Jets,” Journal 

of Sound and Vibration, 116(2), pp. 265-302, 1987.

10. Miller, S.A.E., “The Prediction of Broadband ShockAssociated Noise Using Reynolds-Averaged NavierStokes Solutions,” Ph.D dissertation, Pennsylvania State 

University, Dec. 2009.

11. Harper-Bourne, M., Fisher, M.J., “The noise from shock 

waves in supersonic jets,” AGARD, 131, pp. 1-13.

12. Miller, S.A.E., E-mail message. (Aeroacoustics Branch 

member, NASA Langley Research Center)

13. Witze, P.O., “Centerline Velocity Decay of 

Compressible Free Jets,” AIAA J 12, 417, 1974.

14. Bridges, J.B., and Wernet, M.P., “The NASA Subsonic 

Jet Particle Image Velocimetry (PIV) Dataset,” 

NASA/TM—2011-216807, Nov. 2011.

15. Huang, C., and Papamoschou, D., “Numerical Study of 

Noise Shielding by Airframe Structures,” AIAA paper 

2008-2999, May 2008.
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Jet-Surface Interaction Test: Phased Array Noise Source Localization Results 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

An experiment was conducted to investigate the effect that a planar surface located near a jet flow has on the noise radiated to the far-field. Two different configurations were 

tested: 1) a shielding configuration in which the surface was located between the jet and the far-field microphones, and 2) a reflecting configuration in which the surface was 

mounted on the opposite side of the jet, and thus the jet noise was free to reflect off the surface toward the microphones. Both conventional far-field microphone and phased 

array noise source localization measurements were obtained. This paper discusses phased array results, while a companion paper (Brown, C.A., “Jet-Surface Interaction Test: 

Far-Field Noise Results,” ASME paper GT2012-69639, June 2012.) discusses far-field results. The phased array data show that the axial distribution of noise sources in a jet 

can vary greatly depending on the jet operating condition and suggests that it would first be necessary to know or be able to predict this distribution in order to be able to 

predict the amount of noise reduction to expect from a given shielding configuration. The data obtained on both subsonic and supersonic jets show that the noise sources 

associated with a given frequency of noise tend to move downstream, and therefore, would become more difficult to shield, as jet Mach number increases. The noise source 

localization data obtained on cold, shock-containing jets suggests that the constructive interference of sound waves that produces noise at a given frequency within a broadband 

shock noise hump comes primarily from a small number of shocks, rather than from all the shocks at the same time. The reflecting configuration data illustrates that the law of 

reflection must be satisfied in order for jet noise to reflect off of a surface to an observer, and depending on the relative locations of the jet, the surface, and the observer, only 

some of the jet noise sources may satisfy this requirement.

15. SUBJECT TERMS

Aircraft noise; Jet flow; Shielding; Reflection; Phased arrays 

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