Dennis L. Huff

Glenn Research Center, Cleveland, Ohio

NASA Glenn’s Contributions to 

Aircraft Engine Noise Research

NASA/TP—2013-217818

December 2013
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Dennis L. Huff

Glenn Research Center, Cleveland, Ohio

NASA Glenn’s Contributions to 

Aircraft Engine Noise Research

NASA/TP—2013-217818

December 2013

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

Thanks goes to the hundreds of people at the NASA Glenn Research Center who, have dedicated their professional careers to 

improving the quality of life near the world’s airports by developing technologies for reducing engine noise since the early 1950s. 

What is often not obvious to the general public is the amount of time needed to conduct tests over multiple shifts, months, and years 

without taking breaks. The dedication of the test teams comes with a sacrifice to their personal and family lives. The analytical 

work has attracted the finest theoreticians in the world to either work directly at Glenn or to visit the Center to collaborate on 

cutting-edge research. Thanks also go to the academic and industry partners who have helped Glenn advance the state of the art 

by incorporating the technologies into their products.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identification 

only. Their usage does not constitute an official endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by expert reviewer(s). 
NASA/TP—2013-217818 iii

Contents

Summary........................................................................................................................................................................1

Introduction ...................................................................................................................................................................1

History and Key Issues Overview .................................................................................................................................1

Early Years (The NACA).......................................................................................................................................1

NASA.....................................................................................................................................................................2

Noise Reduction Trends.........................................................................................................................................2

Regulations.............................................................................................................................................................4

Facilities and Measurement Methods............................................................................................................................4

Altitude Wind Tunnel.............................................................................................................................................4

Engine Test Stands.................................................................................................................................................4

Large-Scale Fan Rig...............................................................................................................................................4

Hot Jet Rig..............................................................................................................................................................7

Engine Tests With ICDs.........................................................................................................................................7

Anechoic Chamber for Fan Noise ..........................................................................................................................8

9- by 15-Foot Low-Speed Wind Tunnel.................................................................................................................8

The Aero-Acoustic Propulsion Laboratory ..........................................................................................................10

Flight Test Aircraft...............................................................................................................................................12

Noise Prediction Methods............................................................................................................................................13

ANOPP Engine Modules......................................................................................................................................13

Rice Equations......................................................................................................................................................13

Theoretical Aeroacoustics ....................................................................................................................................13

Jet Noise Prediction Code (JeNo).........................................................................................................................13

RSI Fan Noise Code .............................................................................................................................................14

Computational Aeroacoustics...............................................................................................................................14

Programs, Partnerships, and Impact ............................................................................................................................15

Turbojet Noise Reduction.....................................................................................................................................16

Quiet Engine Program (QEP)...............................................................................................................................16

Quiet, Clean, Short-Haul, Experimental Engine (QCSEE) Program....................................................................17

Refan Program......................................................................................................................................................17

Quiet, Clean, General Aviation Turbofan (QCGAT) Program.............................................................................18

Aircraft Energy Efficiency (ACEE) Program.......................................................................................................18

Advanced Turboprop (ATP) Program..................................................................................................................19

High-Speed Research (HSR) Program .................................................................................................................20

Advanced Subsonic Technology (AST) Program ................................................................................................20

Aeroacoustics Research Consortium (AARC) .....................................................................................................28

Quiet Aircraft Technology (QAT) Program.........................................................................................................28

Fundamental Aeronautics Program (FAP) ...........................................................................................................30

Integrated Systems Research Program (ISRP) .....................................................................................................32

Collaboration and Outreach..................................................................................................................................34

The Future ...................................................................................................................................................................34

Concluding Remarks ...................................................................................................................................................35

Appendix—Acronyms.................................................................................................................................................37

References ...................................................................................................................................................................38

NASA/TP—2013-217818 1

NASA Glenn’s Contributions to Aircraft Engine Noise Research

Dennis L. Huff

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Summary

This report reviews all engine noise research conducted at 

the NASA Glenn Research Center over the past 70 years. This

report includes a historical perspective of the Center and the 

facilities used to conduct the research. Major noise research 

programs are highlighted to show their impact on industry and 

on the development of aircraft noise reduction technology.

Noise reduction trends are discussed, and future aircraft 

concepts are presented. Since the 1960s, research results show 

that the average perceived noise level has been reduced by 

about 20 decibels (dB). Studies also show that, depending on 

the size of the airport, the aircraft fleet mix, and the actual 

growth in air travel, another 15 to 17 dB reduction will be 

required to achieve NASA’s long-term goal of providing

technologies to limit objectionable noise to the boundaries of 

an average airport. 

Introduction

Aircraft noise reduction was a research topic long before the 

NASA Glenn Research Center was established.

1 Fundamental 

studies on jet flows and propellers had been carried out at the 

NASA Langley Aeronautical Laboratory in Hampton, 

Virginia, when many of the senior researchers were transferred to Glenn. A close working relationship aimed at 

improving aircraft and engines continued between the two 

centers that still exists today. Glenn established itself as an airbreathing, and later a rocket propulsion center working on 

fundamental and applied research.

Throughout the entire history of Glenn, common themes for 

aeronautics research have been to increase efficiency and to 

reduce the environmental impact of aircraft engines. Aircraft 

noise is a quality of life issue near airports and noise regulations 

were established by, and are enforced by the U.S. Federal 

Aviation Administration (FAA) and the other member states 

 1

The Center has had different names during its 70-year history. It 

began operating in 1942 under the National Advisory Committee for 

Aeronautics (NACA) as the Aircraft Engine Research Laboratory. 

The name changed to the NACA Flight Propulsion Laboratory in 

1947 and to the NACA Lewis Flight Propulsion Laboratory in 1948. 

When the NACA became part of the new National Aeronautics and 

Space Administration (NASA) in 1958, the name changed to the 

NASA Lewis Research Center. Finally, the name was changed to the 

NASA John H. Glenn Research in 1999.

of the International Civil Aviation Organization (ICAO).

According to a report from the ICAO Committee on Aviation 

Environmental Protection (CAEP), the purpose of noise 

certification is “to ensure that the latest noise reduction 

technology is incorporated into aircraft design demonstrated 

by procedures which are relevant to day to day operations, to 

ensure that noise reduction offered by technology is reflected 

in reductions around airports (Ref. 1).” Funding for noise 

research has varied depending on how important it was viewed 

compared to other research areas such as fuel consumption

and emissions reduction. It became an important role for 

government since industry did not give priority to noise 

research and because it can take many years to realize benefits

from investments.

This review provides a brief summary of the work done at 

Glenn on aircraft engine noise. More detail is given for recent 

years, but all of the major work that has been highlighted dates

back to the beginning of the Center. Many of the Center’s 

accomplishments required collaboration with industry, other 

government organizations, and universities. This review is 

focused on Glenn contributions, but much credit also needs to 

go to partnering organizations and the independent research 

done throughout the world. This review includes a historical 

perspective of the Center and the facilities used to conduct the 

research, including development of measurement methods, 

data analysis and analytical predictions. Major research 

program accomplishments are highlighted and show their 

impact on industry and aircraft noise reduction technology 

development. Noise reduction trends are discussed and some 

candidate future low-noise aircraft concepts are presented 

based on studies sponsored by NASA. Acronyms are defined 

in the appendix.

History and Key Issues Overview

Early Years (The NACA)

Under the NACA, Glenn focused primarily on engine 

performance research so that aircraft engines could operate at 

higher altitudes and faster speeds. During World War II, while

Germany and Great Britain were developing the jet engine, the 

United States focused its research on piston engines and was 

significantly behind in the development of jet propulsion 

systems. When the war ended, the U.S. research emphasis

changed from propellers and reciprocating engines to turbojet

development.
NASA/TP—2013-217818 2

The first time that the media and public were allowed inside 

the gates of the Center, in June 1945, they were eager to learn 

more about jet engines. An excellent history book about Glenn 

(Ref. 2) states that visitors “experienced the earsplitting roar of 

a ramjet and other jet propulsion performances....” During this 

same time, Glenn obtained a V–1 “buzz bomb” from Germany 

to study and test. “The noise rattled the windows of nearby 

houses like that of the Guerin family, who lived in the valley 

below the laboratory [Glenn] on what is now the southwest 

portion of the laboratory property....” (This house was acquired 

later by Glenn and used for many social events.) Jet engine 

testing startled the residents surrounding Glenn and led to noise 

complaints that would impact tests at the Center for many years.

The author had the pleasure of communicating with Dr. Leo 

Beranek, a pioneer in acoustics, in 2006 when he was writing 

his autobiography (Ref. 3). Dr. Beranek wrote “I was in 

Washington, DC, on Wednesday, January 18, 1950, testifying 

before a congressional committee about aviation noise at 

military bases, when I was handed a note from my office. I 

must call the Director of the NACA Lewis Flight Propulsion 

Laboratory in Cleveland as soon as possible. He was frantic. 

At about midnight two days earlier,” he said, “the Laboratory 

had put into operation a new jet engine in a supersonic wind 

tunnel. The noise produced was so intense that switchboards 

in police and fire stations, radio stations, and public offices lit 

up nonstop with complaints from neighbors....”

That test facility was the 8- by 6-Foot Supersonic Wind 

Tunnel (8×6 SWT), built in 1949, and was testing a ramjet for 

the first time. According to Beranek what happened next put 

Bolt, Beranek, and Newman, Inc. (BBN), on the map. He was 

asked by Glenn to conduct sound measurements and to 

determine a way to quiet the tests so they could continue 

without community complaints. Glenn did not have any inhouse expertise in acoustics at that time and had to rely on 

external contractors. BBN designed the world’s largest 

Helmholtz resonator by adding a concrete enclosure around 

the wind tunnel diffuser to reduce the 5- to 300-Hz range lowfrequency noise. The low-frequency sound/vibration that 

propagated through the ground for miles turned out to be the 

source of the noise disturbances. The muffler that Dr. Beranek

developed was successful and subsequently was used in a 

well-known acoustics textbook as an example for resonator 

muffler design (Ref. 4). Glenn resumed tests within 1 year 

without further complaints.

When turbojet-powered aircraft were introduced, airports 

around the world received many complaints about aircraft 

noise. Turbojet noise was very different from propeller-driven 

aircraft noise because it had a longer duration with a distinct 

low-frequency rumble. Glenn’s acoustic work started in the 

1950s with noise research using the Altitude Wind Tunnel 

(AWT) and several full-scale engine test stands to evaluate

nozzle suppressors. This was the beginning of acoustics 

research at Glenn.

NASA

During the 1960s, after Glenn became part of NASA, 

aeronautics research was reduced in favor of supporting the 

space program. Some of Glenn’s aeronautics researchers 

began working on solutions for rocket combustor instabilities, 

which also required expertise with unsteady fluid mechanics.

By 1966, a significant portion of the technology development 

for Apollo spacecraft was completed and Glenn’s focus 

shifted back to aeronautics research. Air traffic was growing 

rapidly and more research was needed to address problems 

with airport congestion, noise, and pollution. Glenn concentrated on developing quieter engines and engines for short 

takeoff and landing (STOL) aircraft. Glenn researchers also 

worked on jet noise in support of the Supersonic Transport 

(SST) while Europe developed the Concorde aircraft.

During the 1970s, aeronautics research focused on developing technologies for fuel-efficient aircraft while acoustics 

research shifted away from turbofans and towards advanced, 

high-speed propeller noise reduction. Fundamental jet noise 

research to investigate screech, sonic fatigue, and flow 

injection to control jets was carried out as well as reducing

helicopter transmission gear noise.

In the late 1980s, Glenn combined people performing aerodynamic and acoustics research into one branch recognizing

there was an advantage to having aerodynamicists and acousticians working together to develop noise reduction technologies 

that could be incorporated into aircraft engines with acceptable 

performance. The scope of the work in the Acoustics Branch

has included experimental and analytical research for propellers, 

turbomachinery (fans, compressors, and turbines), and jets.

The aeronautics research programs emphasized working on 

noise, emissions, and fuel burn together as a system. This 

approach helped bring multidisciplines together to look at new 

ways to solve problems. This was recognized in the late 1940s

as important by one of Glenn’s most influential leaders, Dr. 

Abe Silverstein: “The use of panels to cross division and 

disciplinary lines was one of the distinguishing marks of 

Silverstein’s management style. By drawing talent from the 

entire laboratory, Silverstein encouraged greater flexibility and 

interaction between groups.” (Ref. 2).

Noise Reduction Trends

Engine noise has always been the major source of aircraft 

flyover noise levels. Primary sources of propeller-driven aircraft

noise are blade thickness, loading noise, and engine exhaust.

Blade thickness noise is the volumetric displacement of the air 

by the blade and the loading noise is associated with lift forces.

Strategies to reduce propeller noise focus on decreasing the 

strength of these sources by reducing the propeller rotational 

speed and increasing the number of blades to reduce the 

aerodynamic loading per blade. Exhaust noise can be reduced 

by using mufflers, mixers, and decreasing exhaust jet velocity.
NASA/TP—2013-217818 3

Turbofan engine noise sources include the fan, jet, core 

turbomachinery (compressor and turbine), combustor, and 

sometimes bleed valves. Fan noise can be reduced by 

decreasing the rotational speed and the fan pressure ratio

(FPR). The nacelle enclosing the fan rotor and stator usually 

includes acoustic treatment (i.e., liners) to absorb the fan 

noise. Jet noise can be reduced by using mixing devices or by 

lowering exhaust velocity. Turbomachinery noise can be 

reduced by careful selection of the blade and vane numbers to 

prevent sound radiation; spacing blade rows to decrease 

interaction noise; and adding sound-absorbing acoustic 

treatment to duct walls. Combustion noise is usually not a 

major source on modern turbofan engines, but noise can be 

further reduced by adding Helmholtz resonators or by 

adjusting fuel injector staging.

In general, significant aircraft engine noise reduction has 

been realized by changing cycle parameters in a way that 

lowers jet exhaust velocities and lowers the pressure rise 

across the fan or propeller blades. Moving large amounts of air 

at lower velocities to provide the required thrust and increase 

propulsion efficiency, while meeting cruise speed and range

requirements, is preferred. Turbojets have such a high exhaust 

velocity that jet noise is the major noise source. This is why 

early researchers focused only on improving nozzle-mixer 

designs and suppressors. With the introduction of high-bypass 

turbofan engines, jet exhaust velocities were lowered, but the 

fan and jet noise still needed to be addressed. Small reductions 

in the jet velocity provide significant jet noise reduction (jet 

noise is proportional to the eighth power of the jet velocity).

Figure 1 shows the overall aircraft noise reduction trends since 

the turbofan engine was introduced. Noise levels for several 

high-performance military aircraft and the Concorde are 

included to show what the noise levels would be if the jet 

exhaust velocities were not reduced. Noise reduction for 

supersonic aircraft presents a unique challenge since high 

exhaust velocities are needed for cruise and low exhaust 

velocities are needed for low noise at takeoff.

One of the engine parameters used to correlate engine noise 

is the bypass ratio (BPR). Increasing a turbofan engine BPR 

(ratio of the air mass flow through the bypass fan duct to the 

air mass flow through the core gas generator), has been shown 

to correlate with reduced engine noise. This is due to the 

reduction of jet exhaust velocities, lower fan rotational speeds, 

and lower blade loading associated with increasing BPR. 

Figure 1.—Aircraft noise reduction trends.
NASA/TP—2013-217818 4

Early turbofan engines had BPRs of about 1.5. Increasing the

BPR has also helped reduce fuel burn due to higher propulsion

efficiencies, but requires engine weight optimization as the 

engine diameter increases with increasing BPR. This synergistic relationship between noise and fuel burn reduction has led 

to even higher BPR engines with today’s high-thrust engine 

values exceeding 10. Noise reduction technologies such as 

improved acoustic treatment and low-loss exhaust mixers, 

coupled with better knowledge of turbomachinery noise 

sources, and subsequent improved design methods have all

helped reduce engine noise.

Regulations

Concerns about aircraft noise started shortly after airplanes 

first flew. An editorial from AERO magazine in 1911 entitled 

“On the Fitting of Silencers” noted that “the tremendous racket 

that is presently associated with the aero plane plays a considerable part in prejudicing the public against these machines.” 

(Ref. 5). Hearings were held about noise during the 1940s,

especially near military bases (see earlier discussion by Beranek

(Ref. 3)). While noise from military jets was generally accepted

as “the sound of freedom,” commercial jet noise became a 

problem that peaked in the 1960s as the increasing number of 

flights and noise levels started to impact the quality of life near 

airports. Lectures at schools near airports were routinely

interrupted by the roar of the jets.

In 1969, regulations were introduced to limit aircraft noise 

levels. The first noise regulation, called Stage 2, was introduced 

and is still enforced today by the FAA under Federal Aviation 

Regulations (FAR) Part 36. Specific guidelines were established 

for how to measure noise levels at three certification points 

called lateral (takeoff rotation at high engine power, also called 

sideline), takeoff flyover (also called cutback), and approach.

Allowable noise levels vary with aircraft weight and the number 

of engines. The measured noise margins relative to the allowable levels are arithmetically summed across the three certification points to determine the “cumulative” noise reduction 

relative to the rule. More stringent Stage 3 regulations were 

introduced in 1977 and included a schedule to phase out noisier 

aircraft. In 1999, the United States joined the ICAO to coordinate aircraft noise internationally through the ICAO Annex 16 

Noise Certification Standards. The terminology changed from 

Stage to Chapter, although the two are often used interchangeably. In 2006, Chapter 4 noise regulations were introduced (Fig.

1). Regulations are based on what is technically feasible and 

economically viable at the time as well as the need to reduce the 

impact of aviation noise on the community. NASA’s role has 

been to help develop the needed technologies to reduce aircraft 

noise and provide independent technical assessments, but not to 

get involved with establishing regulations. Negotiations are 

currently being coordinated by the ICAO for the next level of 

noise requirements and NASA has participated as a member of

Independent Expert Panels.

Facilities and Measurement Methods

Altitude Wind Tunnel

Under the NACA, noise suppressors were evaluated for 

turbojet noise reduction. The AWT, one of the first facilities at 

Glenn, was built to simulate high-altitude conditions. It shared 

refrigerated air capabilities with the Icing Research Tunnel and 

could simulate Mach 0.50 at 40 000 ft (12 192 m). The test 

section diameter was 20 ft (6 m) and a vertical strut was mounted 

to a turbojet engine with thrust measurement capability. Nearfield measurements of several nozzle suppressors were taken 

(Ref. 6) to investigate jet noise reduction methods. Microphones 

were mounted on the strut. The facility was reverberant and 

noise was evaluated by near-field overall, relative sound level 

measurements for nozzle configurations. Figure 2 shows samples

of noise suppressors installed in the AWT.

Engine Test Stands

Researchers recognized that jet noise directivity was important and testing in a reverberant tunnel was inadequate. An 

outdoor engine test stand was built to obtain free-field noise 

measurements that complemented the AWT results (Ref. 7).

The test stand was located near the Cleveland airport away 

from other buildings (Fig. 3(a)). A moveable microphone was 

used to survey the near-field noise (Fig. 3(b)).

An aircraft with a turbojet engine was modified to perform 

near-field static jet noise tests of various nozzles. Thrust 

measurements were taken using a cable with a strain gage link.

One of the more interesting tests was a slot nozzle that had a 

100:1 aspect ratio. Results showed that the overall sound 

power level (PWL) was only reduced by 3 dB compared to a 

standard nozzle, but the directivity of the sound pressure level 

measurements varied by as much as 30 dB over the nozzle 

circumferential direction (Ref. 8). This was determined by 

rotating the nozzle from a horizontal position (Fig. 4(a)) to a 

vertical position (Fig. 4(b)). A jet flap nozzle was the intended 

application, as shown conceptually in Figure 4(c).

Large-Scale Fan Rig

A large-scale fan test rig was located next to the drive motor 

building for the 10- by 10-Foot Supersonic Wind Tunnel

(10×10 SWT). A shaft was extended from the wind tunnel drive 

motors to a concrete pad with a polar array of microphones

(Fig. 5(a)). Fans from the Quiet Engine Program (QEP) were 

tested there to help isolate fan noise sources. By testing the fan 

in isolation, no core or core jet noise could contaminate the fan 

acoustic spectra. Acoustic splitter rings mounted in the inlet 

were investigated to reduce inlet radiated fan noise (Fig. 5(b)).

Although this facility was useful, reflection problems from 

the side of the building made data interpretation a challenge, 

so absorptive material was added on the side of the building.
NASA/TP—2013-217818 5

Figure 2.—Turbojet noise suppressors in the 

Altitude Wind Tunnel (AWT) in 1958. 

(a) Round. (b) Rectangular. (c) Multitube. 

(d) Mixer-ejector.

Figure 3.—Early engine test stand for free-field noise 

surveys. (a) Engine test stand. (b) Near-field microphone survey.
NASA/TP—2013-217818 6

Figure 4.—100:1 aspect ratio nozzle test. (a) Horizontal. 

(b) Vertical. (c) Jet flap concept.

Figure 5.—Quiet Engine Program (QEP) fan tests. 

(a) Drive motor shaft. (b) Test hardware.

One of the lessons learned from this type of static testing was 

the importance of using an inflow conditioner called an inflow 

control device (ICD). Early static ground test measurements 

ingested vortices from the ground that were not representative 

of undisturbed inlet flow in flight. The ICD broke up inlet 

flow vortices and turbulence to better simulate fan noise under 

flight conditions. Static fan noise measurements that did not 

use an ICD produced extraneous fan tones and overestimated 

the fan noise (Ref. 9). An engine test stand was erected on the 

airport property next to the hangar (Fig. 6). An array of 

microphones was mounted on poles surrounding the engine 

for polar directivity measurements every 10°. The test stand 

supported the Quiet, Clean, Short-Haul, Experimental Engine 

(QCSEE) program (Ref. 10) and investigated high lift systems 

for the Short Takeoff and Vertical Landing program. Wing 

simulations could also be included to assess the noise from 

jets impinging on wing and flap systems. In addition to polar 

microphones, overhead microphones were used by suspending 

them on a cable between two poles.
NASA/TP—2013-217818 7

Figure 6.—Outdoor full-scale Quiet, Clean, Short-Haul, 

Experimental Engine (QCSEE) tests. (a) Microphone 

array. (b) Engine test stand.

Hot Jet Rig

A large jet noise facility (Fig. 7) was built to test potential 

suppressors for the SST. The facility had instrumentation to set 

the nozzle temperature and pressure ratio, and far-field 

microphones to measure directivity. The nozzle, shown in 

Figure 7(a), called the divergent lobed suppressor, reduced jet 

noise by 12 PNdB. Many other nozzles were tested and 

screened for further testing in other facilities that could simulate 

forward flight and measure nozzle performance. The SST 

program was cancelled before the nozzles could be refined 

Figure 7.—Hot Jet Rig. (a) Divergent lobed suppressor. 

(b) Blown flap with augmentor wing.

for flight applications. Blown flap tests were done where a 

nozzle was placed near an augmentor wing (Fig. 7(b)) to 

assess the jet and impingement noise for STOL aircraft 

applications (Ref. 11). A combustion noise rig, located next to 

the hot jet, shared the microphone arena.

Engine Tests With ICDs

Smaller engines could be tested on the Vertical Lift Fan 

facility (Fig. 8). The JT15D, YF–102, TF–34, and engines 

from the Quiet, Clean, General Aviation Turbofan (QCGAT)

program were tested on this stand for noise evaluations.

Engines could be fitted with ICDs and large mufflers mounted 

to the exhaust to separate jet noise and engine turbomachinery 

noise. Polar microphone arrays were standard for engine tests 

and were also used for this facility. Circumferential rings of 

microphones surrounding the exhaust exit plane, semi-infinite 

tubes with microphones attached to the combustor, and farfield microphones were correlated using coherence methods to 

develop combustion noise empirical models. This facility was 

useful for engine source diagnostics experiments aimed at 

better understanding of the noise generation mechanisms.
NASA/TP—2013-217818 8

Figure 8.—Outdoor engine facility with JT15D test engine and 

inflow control device (ICD).

Anechoic Chamber for Fan Noise

Inlet noise levels for 20-in.- (51-cm-) diameter model fans 

were measured in an anechoic chamber in the Engine 

Research Building (Fig. 9(a)) (Ref. 12). Acoustic treatment

was added to the walls, floor, and ceiling to make the chamber 

anechoic. An exhaust collector drew the air out of the test cell 

while an electric motor powered a model fan. An ICD was 

used to control the inlet flow (Fig. 9(b)), and microphones 

were arranged on an arc in the test cell to measure the fan 

noise. Aft-radiated fan noise measurements were not possible 

in this facility. Aerodynamic performance measurements were 

made on the same fans in another test cell.

9- by 15-Foot Low-Speed Wind Tunnel

The 9- by 15-Foot Low-Speed Tunnel (9×15 LSWT) was 

added in the return leg for the 8×6 SWT in the late 1960s.

Acoustic treatment was added to the 9×15 LSWT test section 

so that propeller (Fig. 10(a)) and fan (Fig. 10(b)) acoustic tests 

could be done with the proper flight simulation. Acoustically 

treated boxes were added with perforated metal face sheets on 

the wind tunnel wall and dissipative material inside the boxes 

to absorb sound. Airflows in the test section could reach 

speeds of about Mach=0.22. Simultaneous measurement of

inlet- and aft-radiated noise allowed tone interference to be 

determined in the near field, and was useful for noise prediction validation.

Figure 9.—Anechoic chamber for inlet fan noise measurements. (a) Microphone array. (b) ICD.

The 9×15 LSWT has been used for many years to evaluate 

new turbofan and turboprop designs. The fan and nacelle 

models look like small engines that typically scale between 1/3 

and 1/6 depending on the application, and included multiple 

components such as the fan stage, inlet, nacelle, nozzle, and 

sometimes a powered core that simulates the compressor 

booster stage (Fig. 10(b)). Propeller research investigated highly 

swept blades for increasing cruise speeds beyond conventional 

turboprops. It was important to measure both the aerodynamics 

and acoustics so a complete assessment of noise reduction 

concepts could be made along with performance penalties.

Force balances were used to measure axial thrust and torque.

For ducted-fan configurations, a calibrated bell mouth and 

variable area nozzle were added to measure fan operating maps 

(Fig. 10(c)). Microphones have been placed at various locations

over the years including mounted to walls, on a traverse, and 

on a ring to obtain circumferential directivity. Advanced 

flow measurement methods such as laser Doppler velocimetry 

(LDV), particle image velocimetry (PIV), and hot-wire (HW) 
NASA/TP—2013-217818 9

Figure 10.—Model tests in the 9- by 15-Foot Low-Speed Wind Tunnel (9×15 LSWT). (a) Counterrotation propfan, 

acoustic configuration. (b) Ducted fan, acoustic configuration. (c) Aerodynamic configuration. (d) Laser Doppler 

velocimetry. (e) Suite of flow and noise measurements.
NASA/TP—2013-217818 10

anemometry have been used to measure blade wakes, 

boundary layers, turbulence, and other important flow features 

needed to understand noise generation (Figs. 10(d) and (e)). 

Phased microphone arrays have been used to help locate noise 

sources. A barrier wall extending from the test section floor to 

the ceiling was added to isolate the inlet and aft-radiated noise 

when needed.

Another measurement method developed in the 9×15 LSWT 

was called the rotating microphone rake system (Ref. 13) 

(Fig. 11(a)). The original idea was conceived by Pratt &

Whitney (P&W), but it was developed and tested for the first

Figure 11.—Rotating microphone for fan inlet acoustic 

mode measurements. (a) Test in the 9×15 LSWT. 

(b) Honeywell engine test.

time at Glenn. Acoustic duct modes associated with fan noise

are measured by separating the circumferential and radial 

orders of the spinning modes to take advantage of a Doppler 

shift when the measurements are taken in relative frames of 

reference. Previous methods used a fixed probe that made it 

impossible to separate the fan modes from the modes caused 

by the wake of the probe interacting with the fan. The results 

were useful for validating fan noise prediction and propagation methods. The rotating microphone rake system built and 

tested on a Honeywell TFE731 engine at their San Tan test 

facility in Arizona is shown in Figure 11(b).

The Aero-Acoustic Propulsion Laboratory

The Aero-Acoustic Propulsion Laboratory (AAPL) (known 

as the Dome) was built in 1991 to protect the residential 

community on the south and west sides of Glenn from noise 

produced from the Power Lift Facility and a new jet facility 

being built for the High-Speed Research (HSR) program 

called the Nozzle Acoustic Test Rig (NATR). The original 

concept was to use noise barrier walls, but engineers realized 

that enclosing the entire area with a 130-ft- (39.6-m-) diameter 

geodesic dome was better at containing noise and protecting

test equipment from the weather. This same area was the 

location of the Hot Jet Rig during the 1970s, where many test 

runs were cancelled due to bad weather or noise complaints.

The AAPL interior walls were covered with acoustic wedges 

and the large far-field microphone distance was ideal for jet 

noise research. Currently there are several test facilities 

located inside the AAPL (Fig. 12).

Figure 12.—Aero-Acoustic Propulsion Laboratory (AAPL).
NASA/TP—2013-217818 11

Figure 13.—Nozzle Acoustic Test Rig (NATR).

Figure 14.—Jet turbulence measurements using particle 

image velocimetry (PIV).

The NATR (Fig. 13) was used extensively by the Advanced 

Subsonic Technology (AST) and HSR programs for jet noise 

research. Its 53-in.- (135-cm-) diameter free jet simulates forward 

flight for takeoff and landing (M ~ 0.30). Recall that earlier Glenn 

facilities did not include this important feature. Early jet noise 

suppressors that showed significant noise reduction under static 

conditions were much less effective under flight conditions

therefore it was important to simulate flight to truly assess the 

effectiveness of jet suppressor designs. The NATR has been used 

mostly for acoustic and flow diagnostic measurements. Single-, 

dual-, and three-stream exhaust flows can be tested at temperatures up to 1425 °F (774 °C). Since industry relied on ASE 

FluiDyne for nozzle thrust measurements, NASA commonly sent

the nozzles to the FluiDyne facility once a good nozzle configuration was identified. This added to the credibility of the results and 

expedited the transfer of technology to industry. The first 

successful test of chevron nozzles was completed in the NATR in 

1997. Phased microphone arrays, LDV, PIV, and a large traverse 

for temperature and pressure measurements have been used to 

help characterize the flow and acoustics for nozzles tested in the 

NATR. In addition, pioneering research has been done using PIV 

to measure turbulence (Fig. 14) and phased microphone arrays to 

locate the noise sources.

A smaller jet rig called the Small Hot Jet Acoustic Rig 

(SHJAR) was added to the AAPL to test smaller nozzles and 

because it would be less expensive than testing in the NATR. 

The SHJAR (Fig. 15), comparable to university facilities, 

provided a way to directly compare test results and screen 

concepts before more expensive tests were undertaken in the 

NATR. It also provided an anechoic test environment to test 

nozzles from other Glenn facilities where fundamental physics 

and flow control experiments were performed. A simple thrust 

measurement method used by an earlier small jet facility in the 

1970s was implemented on the SHJAR to provide first-order 

thrust measurements. 

Figure 15.—Small Hot Jet Acoustic Rig (SHJAR).
NASA/TP—2013-217818 12

The Advanced Noise Control Fan (ANCF) rig (Ref. 14) was 

initially added to the AAPL (Fig. 16(a)) to study active noise 

control (Fig. 16(b)). This rig was used in many test programs 

that studied acoustic liners and fan noise generation fundamentals. A rotating microphone was embedded in the nacelle for 

duct mode measurements. The rotor could be tested in isolation 

or with stators that could be moved or changed easily within the 

duct. HW anemometry and microphone array measurements 

complemented the far-field microphone measurements. The 

ANCF could be moved in the AAPL using air pads so the farfield microphones could be shared with NATR.

Flight Test Aircraft

Glenn had several aircraft that supported acoustics research. 

The OV–10 Bronco (Fig. 17) was used for propeller noise 

research. Researchers investigated controlling the relative 

position of the two propellers to reduce cabin noise (active 

noise control).

Figure 16.—Advanced Noise Control Fan (ANCF) rig. 

(a) Installation in the AAPL. (b) Array of actuators 

mounted in stator vanes for active noise control (inlet 

with ICD and fan are located behind the stators).

A Lear 25 aircraft (Fig. 18(a)) powered by General Electric 

(GE) CJ610 turbojet engines was used as a chase plane for 

near-field flight measurements of propeller noise from the GE 

Unducted Fan (UDF) and the Lockheed Propfan Test Assessment (PTA) aircraft. The UDF was installed on a Boeing 727 

and an MD–80 in the late 1980s to validate the propfan 

concept. Pilots from Glenn flew the Learjet around the UDF 

with a boom microphone mounted on the wing to measure 

propeller tones. The measurements reinforced earlier 8×6

SWT wind tunnel data. In 2000, the Learjet was modified with 

chevron nozzles to investigate jet noise reduction (Fig. 18(b)).

Figure 17.—OV–10 Bronco test aircraft.

Figure 18.—Learjet test aircraft. (a) Flyover for acoustic 

measurements. (b) Chevron nozzle.
NASA/TP—2013-217818 13

Noise Prediction Methods

ANOPP Engine Modules

The Aircraft Noise Prediction Program (ANOPP) was initiated at Langley in 1973 to develop a computer program for 

predicting aircraft system noise. A need developed to evaluate 

benefits from noise reduction research programs that could be 

used by the government for independent analyses. Glenn took 

responsibility for the engine modules by developing empirical 

and semiempirical prediction methods. The jet, core, fan, and 

turbine noise models were developed in-house based on data 

from NASA and industry. Reports for the engine noise modules 

(Refs. 15 to 18) were designated “interim” but the final reports 

were never written. This turned out to be appropriate since the 

modules have been continuously updated and improved using 

data from newer engines. The jet noise modules have been 

updated several times as the noise sources have become better 

understood. Tests sponsored by Glenn revealed that using an 

inverted velocity profile for coannular nozzles reduces jet noise.

(An inverted velocity profile occurs when the outer flow stream 

velocity is greater than the primary core flow stream.) The jet 

noise modules were modified based on source distribution 

assumptions that helped provide insight into the physics

(Ref. 19) and further refinements were made as more data 

became available from various engine tests (Ref. 20). This 

module is still used today for empirical jet noise prediction in 

ANOPP and is referred to as the “Stone Jet Noise” code. The 

fan noise module by Heidmann was updated for modern fans 

and is also used today (Ref. 17).

Rice Equations

When fan noise research started at Glenn in the 1970s, analytical methods were pursued in conjunction with experimental 

work to provide physical insight. There was an observation that 

each fan duct spinning mode (mentioned earlier for rotating 

microphone development), has a unique directivity pattern in 

the far field. Experimental data from the QEP was used to show 

that the directivity could be predicted assuming equal energy 

per mode (Ref. 22). Dr. Edward Rice observed that acoustic 

modes with similar cutoff ratios also had similar directivity

patterns (Ref. 23). Cutoff ratios determine whether or not fan 

duct modes within a given frequency (i.e., tone) are propagating 

or decaying. When all modes within a given tone (e.g., blade 

passing tone) are decaying, the tone is considered cut off. Rice

developed an approximation for predicting the far-field

directivity of fan noise using the cutoff ratio, duct geometry,

and simple flow parameters. Theoretical calculations based on 

the Weiner-Hopf method were considered the benchmark for 

comparisons with other prediction methods. Figure 19 (Ref. 24)

shows how well the Rice approximations compare to the theory.

Rice also recognized that the optimum impedance for acoustic 

treatment could be correlated with cutoff ratio (Ref. 25). 

Figure 19.—Fan noise directivity prediction using Rice 

equations.

A method for designing acoustic liners for fan ducts was 

developed by Glenn. P&W and other engine companies used 

the method to predict the far-field sound levels for their 

engines and to design acoustic treatment. Computations took 

much less time than methods that directly computed sound 

propagations. The acoustic treatment research initially done at 

Glenn moved to Langley, where it still resides today.

Theoretical Aeroacoustics

One publication from Glenn that stands out for the guidance 

provided to many noise prediction methods (Ref. 26) first 

came out as a NASA Special Publication, and was later 

published as a book by the McGraw-Hill Company in several 

different languages. Dr. M.E. Goldstein derived fundamental 

equations for aeroacoustics starting with classical acoustics 

and adding the effects of mean flow, solid boundaries, 

compressibility, nonuniform flow, and the direct calculation of 

sound. Portions of the work build on Lighthill’s famous 

acoustic analogy. The equations have been used throughout 

the world to develop fan and jet noise prediction codes.

Jet Noise Prediction Code (JeNo)

Early methods to predict jet noise from first principles were 

only relevant for round jets. Acoustic analogy methods based 

on Lighthill’s theory were popular, but approaches based on 

Lilley’s formulation of the convective wave equation showed 

promise for being more computationally efficient. The 

problem was divided into first determining the mean flow and 

then radiating source terms in the governing equations using 

an appropriate Green’s function. NASA further developed this 

approach using computational fluid dynamics (CFD) to define 

the mean flow and turbulence, modify source terms,

and develop a Green’s function for nonaxisymmetric nozzles

(Ref. 27).

A recent noise prediction assessment compared the Stone 

Jet Module in ANOPP and the JeNo code (Ref. 28)

to experimental data taken at NASA’s jet noise facilities.

Neither model predicted all of the test cases within the 
NASA/TP—2013-217818 14

Figure 20.—Cold subsonic jet predictions. (a) Sound 

pressure level (SPL). (b) SPL (predicted minus 

data). (c) Overall SPL directivity.

experimental uncertainties. Component effective perceived 

noise level (EPNL) predictions could be off by several 

decibels using the Stone Jet module. Acoustic spectra and 

directivity were predicted accurately by JeNo for cold 

subsonic jets (Fig. 20), but they were less accurate for hot, 

high-speed jets (Fig. 21). Sample predictions compared to 

experimental data were also presented for nonaxisymmetric 

cases like chevrons and offset nozzles. While there had been 

great improvement in jet noise prediction methods that include 

geometry and mean flow, considerable work was still needed 

in cases where flows became more complex.

RSI Fan Noise Code

Glenn developed the Rotor-Stator Interaction (RSI) fan 

noise prediction code based on an acoustic analogy approach

(Ref. 29). Several tone prediction methods were developed 

with companies under contracts with Glenn. The methods 

included coupling between the inlet, fan, stator, and bypass 

nozzle. Tone levels for fans had been reduced to the point 

where emphasis was placed on the development of fan 

broadband noise prediction. The RSI code has been used for

broadband noise prediction using CFD input for mean flow 

and turbulence parameters, and solving for the unsteady 

response on stator vanes to simulate the interaction with a 

turbulent fan wake. Total duct PWLs were computed and 

compared to experimental data. A recent fan noise prediction 

assessment compared the RSI code accuracy to experimental 

data from the 9×15 LSWT (Ref. 30). The assessment showed 

that the RSI code could predict fan broadband noise within 

4 dB of the experimental uncertainty band (gray bar) based on 

total PWLs over a range of frequencies (Fig. 22). Other

contributions to fan noise such as rotor alone would need to be 

computed using a different code. The assessment also 

compared predictions from ANOPP and the LINFLUX code,

developed in the 1990s, as a computational approach based on 

the linearized Euler equations. The error was larger when 

more sophisticated prediction codes were compared to the 

empirical methods.

Computational Aeroacoustics

Computational aeroacoustics (CAA) are numerical methods 

for direct computation of sound and are distinguished from 

CFD used for predicting aerodynamics. When CAA is fully 

developed, it will be the ultimate prediction method for

describing sound sources and propagating acoustic waves 

through complex flows and to the far field. NASA has been 

instrumental in supporting pioneering work in CAA. Langley 

sponsored the first CAA workshop on Benchmark Problems in 

1994. Two of the three subsequent workshops were hosted by 

Glenn and held at the Ohio Aerospace Institute (OAI) in 1999 

(Refs. 31 and 32). NASA researchers defined model problems 

that have analytical solutions for comparisons with CAA 

predictions. Figure 23 shows one of the benchmark problems
NASA/TP—2013-217818 15

Figure 21.—Hot high-speed jet predictions: (a) SPL. 

(b) SPL (predicted minus data). (c) Overall SPL 

directivity.

Figure 22.—Fan broadband noise prediction using 

Rotor-Stator Interaction (RSI) code for various rotational speeds. Gray shaded regions shows experimental error.

posed by Dr. Edmane Envia to compute the unsteady pressure 

response of a stator cascade interacting with a blade wake 

simulating the third harmonic of the blade passing frequency.

Perturbation pressures are shown in Figure 23(a), and 

perturbation velocities are shown in Figure 23(b). The 

problems were presented as “blind” test cases and participants, 

both U.S. and international, were invited to test their latest 

CAA algorithms for comparisons with each other and with the 

theory. Each workshop showed improvements with methods 

and computational efficiencies. CAA is still an active research 

topic and, depending on the application, it could take many 

years before it can be applied to real life situations. CAA does

hold promise, however, for currently being able to predict 

noise from aircraft and engines with arbitrary geometry and 

flow conditions. ANOPP, while very useful, is limited to 

predicting noise from aircraft and engines similar to past 

applications. NASA intends to develop ANOPP2 that will 

begin to incorporate CAA methods as they mature.

Programs, Partnerships, and Impact

As noted by Dawson (Ref. 2), Glenn has always struggled 

with finding its role between fundamental research and the 

need to show progress through practical applications. World 

War II forced the Center to work on specific projects in

response to national security needs. After the war, Glenn

returned to basic research on engine components, and 

restructured the organization into corresponding divisions and 

branches. New programs and projects focused on specific 

goals, provided funding, and led with industry through 

contracts and partnerships.
NASA/TP—2013-217818 16

Figure 23.—Benchmark problem for the fourth CAA 

workshop. (a) Contours of perturbation pressure. 

(b) Contours of perturbation velocity.

Turbojet Noise Reduction

In the first major noise reduction project at Glenn, researchers investigated ways to reduce jet noise from turbojets. The 

number of complaints around airports were rising as the thrust 

from turbojets was increasing due to improved gas turbine

engine technologies. Work at the NACA focused on full-scale 

tests on engines with nozzle suppressors. In addition to 

community noise issues, structural fatigue of surfaces near the 

nozzles due to high-amplitude acoustic pressure waves (sonic 

fatigue) presented additional problems. Research during this 

time was exploratory with no specific noise reduction goals in 

mind. Many different nozzles were tested without much 

analytical guidance beyond the work of Lighthill (Refs. 33 and 

34). It was known that reducing the jet exhaust velocity was 

the most beneficial to reducing jet noise. Converting the 

primary jet into smaller jets was beneficial since the peak 

noise level was a function of Strouhal number (St). Smaller 

diameter jets would shift the peak levels to higher frequencies 

that had higher atmospheric attenuation and to a less annoying 

region of the sound spectra.

Several nozzles shown in Figure 2 used multiple tubes and 

chutes to shift frequencies, however, significant thrust loss 

from these nozzles hampered their implementation on aircraft.

Ejectors were added to help recover some of the performance 

losses, but the ejectors were heavy and required variable 

geometry to operate between takeoff and cruise. The NACA 

investigation concluded that a 12-lobe nozzle caused the least

thrust loss (3.2 percent at a flight Mach number of 0.50) and 

could provide 5 to 6 dB reduction in peak noise during takeoff 

(Ref. 6).

Glenn resumed work on nozzle suppressors for the SST in 

the late 1960s. The mixer-ejector concept was considered the 

most promising since thrust loss was the primary suppressor 

concern and ejectors could be used to help recover portions of 

thrust loss. Many nozzles were tested for acoustics and 

performance. Cruise performance was found to be acceptable,

but the low-speed performance during subsonic flight fell 

short of the requirements (Ref. 35).

Quiet Engine Program (QEP)

The turbofan engine introduced in the 1960s reduced the 

noise levels compared with turbojets, but a dramatic increase 

in the number of flights resulted in aircraft noise regulations 

and consequently, the need for additional engine noise 

reduction research. The QEP was initiated to reduce turbofan 

engine noise by 15 to 20 PNdB below the levels from Boeing 

707s or McDonnell-Douglas DC-8s. Noise reduction technologies were developed and applied to an experimental engine 

that was designed, built, and tested under contract by GE on a 

CF6 engine. Three fans, designated A, B, and C, had distinct 

low-noise design features and were delivered to Glenn for 

tests with acoustically treated nacelles (Ref. 36). This was the 
NASA/TP—2013-217818 17

first time that Glenn became involved with fan noise research.

The FAA requested that the program be expanded to include 

full-scale engine demonstrator tests.

Quiet, Clean, Short-Haul, Experimental Engine 

(QCSEE) Program

The QCSEE program was established to apply QEP technology to short-haul configurations. GE was tasked with 

developing fans that could be added to a core engine for fullscale acoustic tests. The engines were designed using lower 

pressure ratio fans for STOL aircraft and could operate from 

smaller airports to help relieve airport congestion. The QCSEE 

program had three specific goals (Ref. 10):

• Develop short-haul propulsion technologies that are 

environmentally and economically acceptable.

• Provide government with data for future rule making.

• Transfer data and technologies to industry.

Research carried out at Glenn used model scale and fullscale tests. Many of the advanced technologies that are being 

considered for turbofans today were initially developed under 

these programs. They include high-bypass-ratio engines (10 to 

12), variable pitch fans with low-pressure ratios (1.27 to 1.34), 

variable area fan nozzles, advanced acoustic liners, the highMach inlet concept, digital electronic controls, clean combustors, reduction gearing, and composite components including 

fan blades, fan frames, and nacelles.

Two QCSEE demonstration engines built and tested at 

Glenn, incorporated short-haul installations. One was the 

under-the-wing (UTW) engine (Fig. 24(a)) and the other was 

the over-the-wing (OTW) engine (Fig. 24(b)). Both engines 

were significantly quieter than the Boeing 707 and Douglas 

DC–8 engines. The OTW technologies were used for modified

YF–102 engines on an experimental airplane (Quiet ShortHaul Research Aircraft (QSRA)) that was first flown in 1978 

(Fig. 25) and managed by the NASA Ames Research Center in 

California. Shovlin and Cochrane provide a summary of the 

QSRA program (Ref. 37), and video was taken at a 1987 air 

show at Ames (Ref. 38).

Refan Program

The Refan program, started in 1972, aimed at advancing

technologies to reduce noise and smoke by retrofitting P&W’s 

JT3D and JT8D engines. Funding cuts in 1973 forced the 

program to focus only on the JT8D since it would impact 

longer service aircraft such as the newer 727, 737, and DC–9.

After the initial design phase, contracts were awarded for 

engine design, fabrication, and full-scale testing, and component tests (Ref. 39). Fan noise reduction technologies included 

increased fan diameters resulting from increased BPRs; a

Figure 24.—Quiet, Clean, Short-Haul Experimental 

Engines (QCSEE). (a) Under the wing (UTW). 

(b) Over the wing (OTW).

Figure 25.—Quiet Short-Haul Research Aircraft (QSRA).
NASA/TP—2013-217818 18

Figure 26.—Refan tests. (a) Altitude engine tests in 

the Propulsion Systems Laboratory (PSL). (b) Flight 

test on a DC–9.

single-stage fan to replace the two-stage fan; increasing the 

spacing between the inlet guide vanes, fan, and the stator 

blades; and optimizing the number of blades to reduce noise.

Three different nacelles were designed with varying amounts 

of acoustic treatment to determine which configuration would 

be built for full-scale tests. Each design used a long duct with 

an acoustically treated tailpipe to reduce the exhaust noise.

The B727–200 and a DC9–200 aircraft were tested after the 

new engines were modified, but the 737 flight test was 

dropped.

Predictions indicated that aircraft noise could be reduced by 5 

to 7 effective perceived noise in decibels (EPNdB) on approach, 

about 9 EPNdB on takeoff, and 14 to 15 EPNdB on sideline.

This would provide a cumulative margin of 28 to 31 EPNdB 

under FAR–36, Stage 2 for a 727 aircraft, or an average of 

10 dB at each certification point. Static engine tests were 

performed at industry facilities and altitude tests were carried 

out at the Glenn Propulsion System Laboratory (Fig. 26(a)).

Flight tests were conducted by Boeing and McDonnell-Douglas 

in 1975. Test results showed an average reduction of 6 to 

10 EPNdB for the DC–9 (Fig. 26(b)) and 7 EPNdB for the 727 

at each certification point (Ref. 40). The older 727 and DC–9 

aircraft went out of production before either aircraft were 

retrofitted with improved engines, however, the noise reduction 

technologies were introduced on the 737–300 and the MD–80.

Figure 1 shows that the noise levels from these aircraft were 

about 9 to 11 dB below Chapter 2 limits and also met the new 

Chapter 3 regulations with sufficient margin.

Quiet, Clean, General Aviation Turbofan 

(QCGAT) Program

The QCGAT program was initiated in 1976 to determine if 

the noise reduction technologies developed for larger engines 

could be successfully applied to smaller turbofan engines.

Program goals included both noise and emissions reductions.

Noise reduction goals depended on the aircraft weight and 

followed closer to the slope of the regulated limits for heavier 

aircraft. The way the regulations are written, aircraft below 

about 100 000 lb (45 359 kg) takeoff gross weight have a 

constant value noise limit for each certification point, meaning

the margin is easier to meet as the aircraft becomes lighter.

NASA’s noise goals were intended to determine if further

noise reduction was possible without significant performance 

loss (Ref. 41). Once the aircraft application was selected, the 

effective noise reduction goals ended up being 15 to 20 PNdB 

below Stage 3 at each certification point.

Contracts were awarded to Garrett-AiResearch Company 

and Avco-Lycoming Corporation (both are now part of 

Honeywell Aerospace, Inc.) to develop candidate engines that 

were tested at both Glenn and their own static engine facilities.

In addition to applying many of the noise reduction technologies from the QEP and Refan programs, use of internal 

exhaust mixers and elimination of fan inlet guide vanes were 

investigated. The Avco-Lycoming engine was tested statically 

and the predicted noise levels, accounting for flight effects,

were shown to exceed NASA goals at all three certification 

points (Fig. 27) (Ref. 42). The Garrett-AiResearch engine was 

also tested and found to meet the goals on a cumulative basis 

(Ref 43).

Aircraft Energy Efficiency (ACEE) Program

In the 1970s, after a few very lean years for aerospace 

because the Apollo space program ended, Glenn returned to 

working on aircraft engine efficiency in response to threats to 

the availability of oil used to produce aircraft fuel. The price 

of fuel tripled from 1973 to 1975 and Congress authorized 

NASA to work on ways to conserve fuel. The ACEE program 

brought the Energy Efficient Engine (EEE) and the Advanced 

Turboprop (ATP) programs to Glenn. Information on the 

history of these programs is included in Reference 44.
NASA/TP—2013-217818 19

Figure 27.—Noise levels from Avco-Lycoming 

engine relative to QCGAT noise goals. 

(a) Sideline. (b) Takeoff. (c) Approach.

The ATP program started in 1978 after Glenn spent several 

years studying propfans and working with Hamilton-Standard 

on advanced propellers under the Reducing Energy Consumption of Commercial Air Transportation (RECAT) program.

The EEE program awarded large contracts to GE and P&W to 

develop advanced higher BPR turbofan engines, while the 

ATP program developed technologies to further reduce fuel 

consumption using propfans.

Advanced Turboprop (ATP) Program

The ATP’s program objective was to find ways to reduce 

aircraft fuel burn. The program projected 25 to 30 percent less 

fuel burn over equivalent technology turbofans (Ref. 45).

Glenn started research in propeller acoustics and aerodynamics, structures for very thin and highly swept composite 

blades, and gearboxes to transmit power from the core engine 

to the propeller. Propfans were selected over conventional 

turboprops because of their ability to fly at a faster cruise 

Mach number than older propeller designs. This required 

making blades thinner and adding sweep to reduce shock 

losses. Unfortunately noise levels increased both inside 

aircraft cabins and for the community. There were also 

concerns about en-route noise since the low-frequency tones 

from the propfans could propagate to the ground even from 

cruise altitudes. People living in quiet areas and never exposed 

to aircraft noise could notice the high-altitude flyovers.

Two basic concepts were investigated during the ATP 

program: the first involved single-rotation propellers and the 

second involved counterrotating blades. Single-rotation 

propellers were investigated by Hamilton-Standard and tested 

in model scale (Fig. 28(a)) and flight tested on a Lockheedmodified Gulfstream aircraft called the Propfan Test Assessment aircraft that first flew in 1987 (Fig. 28(b)). Counterrotating propellers were investigated by both GE and a team from 

Hamilton-Standard, Allison and P&W. GE started to work 

with NASA in 1983 to advance their UDF concept that used a 

gearless drive system and counterrotating turbines. The 

Hamilton-Standard, Allison, and P&W concept was also 

counterrotating, but used a gearbox to drive the rotors.

The UDF engine, also known as the GE–36, flew on modified 727 and MD–80 test aircraft (Fig. 29(a)). Near-field noise 

measurements, taken using the Glenn Learjet, confirmed that 

the cruise tone levels correlated with prior 8×6 SWT data.

Model-scale acoustic data derived in the 9×15 LSWT were

shown to correlate with near-field flight data for both singlerotating and counterrotating propfans (Fig. 29(b)). The UDF

just met Stage 3 regulations for community noise levels. Even 

though improvements were possible by increasing the number 

of blades and optimizing the designs, interest in the program 

waned after the price of fuel plummeted from its 1970s highs.

As a result, NASA stopped all work on propfans in 1991.
NASA/TP—2013-217818 20

Figure 28.—Single-rotation propfan. (a) SR–7A in the 

9×15 LSWT. (b) Propfan Test Assessment (PTA) aircraft with wing-mounted propfan.

Figure 29.—Counterrotation propfan. (a) Unducted fan 

(UDF) engine. (b) Near-field acoustic data.

High-Speed Research (HSR) Program

The SST program abruptly ended in 1971, but NASA 

resumed work on supersonic aircraft for the HSR program in 

1990. Noise levels for subsonic commercial aviation were 

lower, so the target noise levels had to be set even lower than 

they were for the SST program. Figure 30 shows a comparison 

of sideline noise suppression versus gross thrust loss for 

suppressors developed during the SST time period with the 

High-Speed Civil Transport time period for the HSR program.

Model scale results from the best mixer-ejector nozzle concepts 

that were tested by the end of the program in 1999 are also 

shown. The noise reduction goal was 20 dB with acceptable 

thrust loss for supersonic cruise and subsonic operations.

Significant progress was made with the help of CFD and lighter 

weight materials (mixer-ejector nozzles add weight and 

complexity to the engine). Figure 31 shows one of the nozzle 

concepts that was tested in model scale. Notice that substantial 

variable geometry is still required to make this concept work.

As U.S. industry lost interest in large SSTs, new interest 

grew in smaller supersonic business jets. NASA started the

Supersonics project in 2006 and continues today with the goal 

of beginning with a smaller aircraft and developing the 

technologies to enable larger aircraft in the future. The 

European Concorde was taken out of service in 2003 because 

it was not economically viable anymore. Studies show that 

this could change if supersonic flight over land becomes

acceptable to the public (currently it is not allowed except for 

designated corridors reserved for military aircraft). Therefore 

a major research activity in the Supersonics project is the 

sonic boom reduction in addition to jet noise reduction.

Advanced Subsonic Technology (AST) Program

Turbofan noise research resumed at Glenn with the AST 

Noise Reduction program in 1994. NASA’s contracts with 

industry to develop engine technologies for AST and HSR

kept most of the acoustic facilities across government, 

industry, and universities working in fan and jet noise 

research. A high-power air turbine drive rig was built by GE 

and Boeing for the wind tunnels to test turbofan models.

A Technical Working Group (TWG), consisting of representatives from industry and NASA, was formed to provide 

input on noise reduction technology needs. The TWG 

approach worked very well and helped transition the technologies to industry by keeping NASA aware of research areas 

that needed the most attention and reduced work duplication 

or overlap between companies.

Reference noise levels were defined using four classes of 

aircraft of different sizes and designated “1992 Technology”

(Ref. 46). Noise reduction goals requiring technologies to 

reduce aircraft noise by 10 dB relative to 1992 technology were 

set. NASA developed the Technology Readiness Level (TRL) 

scale to help identify and track the maturity of technologies.
NASA/TP—2013-217818 21

Figure 30.—Noise reduction and thrust loss projections for large supersonic aircraft.

Figure 31.—High-Speed Research (HSR) mixer-ejector 

nozzle concept.

Levels range from 1 to 9, with TRL 1 meaning conceptual 

research and TRL 9 meaning that technologies are fully 

implemented into products. Utilizing the TRL scale requires that

each discipline define what they mean for their own development sequence. For engine noise research, TRLs 1 to 3 takes a 

concept through fundamental tests and simulations in a laboratory. TRLs 4 to 5 validate the concept at the component level

followed by a system or subsystem level in a relevant environment (e.g., a scale model wind tunnel test). TRL 6 means a static 

engine validation test or a flight demonstration test.

NASA’s role is to develop technologies from TRLs 1 

through 6, and then it is up to industry to decide if the 

technologies can be used in their products, raising the TRL to 

9. NASA awarded contracts to GE, P&W, and Allison to 

identify noise reduction technologies needed to achieve the 

AST noise reduction goals without sacrificing performance 

(Refs. 47 to 49).

Early in the program, jet noise research was excluded since 

NASA worked on future ultrahigh bypass (UHB) engines with 

low enough exhaust velocities that fan noise was the dominant

source. Because industry and the FAA wanted solutions for 

existing engines, both NASA and the FAA agreed to fund a jet 

noise research project with goals to identify technologies for 

reducing fan and jet noise by 3 EPNdB by 1996. The jet noise 

work ended and work focused on fan noise and engine 

validation tests. It was thought that the only promising 

solution for jet noise, after reducing the velocity, was to use 

long duct internal mixers with acoustic treatment to absorb the 

resulting high-frequency mixing noise. Initial tests were done 

on mixers from the EEE program and JT8D mixers intended 

for hushkits. At one TWG meeting, Boeing urged NASA to 

consider separate flow nozzles with short fan ducts since most 

of their aircraft did not have long duct mixers. NASA issued a 

competitive request for proposals and combined ideas from 

GE, P&W, and Allison into one test that was conducted in the 

AAPL in early 1997. Several concepts were tested including 

“tabs” and “chevrons.” Jet noise was reduced by about 

3 EPNdB by adding chevrons to the core nozzle and bypass 

duct nozzle (Fig. 32(a)), and the core nozzle for short fan 

ducts (Fig. 32(b)). The results came as a pleasant surprise to 

the test team. They repeated reference nozzle tests daily to 

make sure all temperature and humidity corrections were 

properly accounted for and that the “delta” noise reduction 
NASA/TP—2013-217818 22

Figure 32.—Chevron nozzles. (a) First test in AAPL. 

(b) GE CF–34 engine test.

from chevron nozzles was real. This was the first successful 

test of chevron nozzles with minimal thrust loss.

Previous tests of similar mixing devices for separate flow 

nozzles used higher penetration angles into the flow resulting 

in high-frequency noise penalties that negated the overall 

noise reduction. The concept introduced by GE kept the tip of 

the chevron within the nozzle-shear layer to introduce 

sufficient streamwise vorticity to mix the core and bypass flow 

downstream of the nozzle with minimum high-frequency 

mixing noise. NASA knew that industry would not 

acknowledge the test results unless thrust measurements were 

completed and also showed acceptable performance. The AST 

program manager (Mr. William Willshire) added funding to 

send the nozzles to ASE FluiDyne. Results showed that the 

performance losses were less than 0.25 percent for chevron 

nozzles with noise reduction ranging from 2.5 to 3 EPNdB.

This was a major breakthrough for jet noise research since 

many of the previous methods depended on engine cycle 

changes to reduce the exhaust velocities. Chevron nozzles

provided significant noise reduction with no change to the 

engine cycle parameters. A workshop was held at Glenn in 

September 1997 to share the results with all TWG members.

Several companies pursued their own versions of the chevron 

nozzle and GE introduced the first production implementation 

on a CF–34 engine in 2003. Several other aircraft have been 

introduced with chevron nozzles including the Boeing 787 and 

747–8. NASA has continued chevron nozzle research to better 

understand the flow physics and apply the idea to turbojets.

Since analytical methods were not sufficiently reliable to 

guide nozzle designs, NASA focused on flow measurement 

methods to characterize the turbulence using PIV to provide 

the quality of data needed to validate jet noise prediction 

codes. Subsequent publications have shown better correlation

between predicted noise reduction and experimental data (Ref.

28). Another incentive for turbojet applications was to see if 

chevrons could be applied to tactical aircraft for the military.

The Glenn Learjet (Fig. 18(b)) was used to demonstrate up to 

a 4-EPNdB reduction in jet noise. A summary of the chevron 

nozzle development has been recently published and provides 

more details (Ref. 50).

Fan noise research during the AST program included model 

scale tests and development of fan noise prediction methods.

There was more success with these methods to guide experiments than there was for jet noise research. Rotating microphone measurements and detailed flow measurements were 

used to define the needed key input parameters for the 

prediction methods. Many fan tests were conducted in the 

Glenn 9×15 LSWT starting in 1994. NASA purchased a 

slightly modified version of the Universal Propulsion Simulator (UPS) drive rig from GE and Boeing that became the 

workhorse for the fan tests. NASA and all of the participating 

companies built fan and nacelle hardware to fit onto the rig

using a 22-in.- (56-cm-) nominal fan diameter. The following 

list summarizes the most significant tests and their results:

(1) GE UPS tests—Since the NASA drive rig had not been

completed by 1994, GE’s UPS rig and several sets of fan 

blades intended for improvements to the GE–90 engine were 

used and served as a baseline for NASA.

(2) P&W Advanced Ducted Propulsor (ADP) tests—Several 

tests were conducted from 1995 through 1997 to investigate 

the ADP concept for UHB engines with lower fan tip speed 

and lower FPRs (Fig. 33). Earlier tests had been conducted at 

Glenn using P&W’s 17-in.- (43-cm-) fan rig. These tests used

Glenn’s 22-in.- (56-cm-) fan rig aimed at further fan noise 

reduction. Advanced acoustic liners were evaluated with the 

knowledge of fan noise source characteristics from previous 

tests to optimize the liner impedance. For lower speed fans 

where broadband noise dominated, the noise correlated better 

with FPR than with fan tip speed. Sufficient spacing between 

the fan and stator could lower the noise and the fan efficiency

improved to about 95 percent due to lower aerodynamic losses

for low-pressure ratio fans (Refs. 51 to 53).
NASA/TP—2013-217818 23

Figure 33.—Advanced Ducted Propulsor (ADP) fan test in 

1995.

A variable pitch fan with casing treatment was used to 

optimize the performance between takeoff and cruise operations, and to provide reverse thrust on landing. (Reverse thrust 

was needed from the engines to back the aircraft out of the 

passenger loading gates, but this changed due to concerns with 

engine emissions getting into the terminal area. Now all 

aircraft are towed.) However, losses associated with the larger 

hub and tip gaps needed to reverse the pitch of the blades were 

high and the amount of reverse thrust fell short of the goals.

P&W would later go to a fixed-pitch fan, a variable-area

nozzle, and an alternative reverser to address these problems.

(3) Allison Low Noise Fan tests—One of the fan noise 

reduction concepts identified by the Allison Engine Company 

(now Rolls-Royce) in their study contract, was to sweep and 

lean the fan stators to reduce the rotor-stator interaction noise.

NASA conducted studies with fan noise prediction codes 

showing that sweeping vanes by 30° and leaning vanes in the 

direction of rotation of the fan provided favorable acoustic 

phase cancellation from hub to tip as the fan wakes passed the 

stators (Ref. 54). It also increased the distance for the wake 

strengths to decay before they reached the stator. The concept 

was tested in 1996 and was found to reduce the fan noise by 

3 dB (Ref. 55). A forward-swept fan was also tested 

(Fig. 34(a)). This was the first known verification that swept 

and leaned stators were a viable way to reduce fan noise

(Fig. 34(b)). Previous attempts in the 1960s were tested

without an ICD, and any possible noise benefits were likely 

masked by extraneous noise caused by inflow distortion.

Figure 34.—Allison low-noise fan. (a) Forward-swept fan 

(counter-clockwise rotation). (b) Swept and leaned stators.

(4) GE High-Speed Fan tests—The TWG expressed concern 

that all of the technologies being pursued by NASA were only 

relevant for UHB engines and noise reduction was still needed

from moderate bypass ratio engines in the 5 to 6 range. Glenn 

conducted tests with GE to investigate swept and leaned 

stators on a higher speed fan, and to investigate forward sweep 

on the fan to help the aerodynamic performance and noise.

Fan tests used a baseline model representative of a current 

CF–6 engine, and the models used new designs for the fan and 

stator. The tests influenced the design of a newer CFM–56 

engine with swept and leaned stators and successfully reduced 

the fan noise (Ref. 56) (Fig. 35).
NASA/TP—2013-217818 24

Figure 35.—GE swept and leaned stators tested on CFM–56 

engine.

(5) Honeywell Quiet High Speed Fan (QHSF) tests—One of 

the challenges to address for fan noise with higher rotational 

speeds was multiple pure tones (MPTs), also known as a

“buzz saw” noise because of the distinct sound during takeoff 

or cruise. Even slight variations in blade geometry cause the 

bow shocks radiating forward of the fan to be unevenly 

spaced, which causes the sound spectra tonal portion to shift 

from blade-passing frequencies and higher harmonics to shaftorder frequencies. Noise can be reduced by adding acoustic 

treatment to the inlet, but it is more desirable to reduce noise

at the source with alternative blade designs. Work done during 

QEP by BBN (Ref. 57) was revisited to design a fan with 

highly swept blades to reduce the shock strength. Honeywell 

started with this design philosophy and made improvements 

for their baseline TFE731–60 engine fan and stator design

(Refs. 58 and 59). The QHSF tested in 1998 (Fig. 36(a)) 

confirmed that the onset of MPT noise can be delayed to 

higher rotational speeds by using forward sweep (Fig. 36(b)).

The overall reduction in fan noise was also attributed to a 

reduction in the blade passing frequency tone level that was 

prevalent in the baseline fan (Refs. 60 to 62). The technologies 

were used to guide the design of Honeywell’s HTF7000

engine.

(6) NASA Alternative Low Noise Fan (ALNF) tests—A 

different approach was investigated to see if increasing the 

number of fan blades and reducing the number of stator vanes 

(long chord stators) could reduce fan noise. Long chord stators 

were previously used for fan noise reduction during the QEP.

By increasing the number of fan blades, the blade passing 

frequency tone and its higher harmonics could be shifted to 

higher frequencies to reduce human annoyance. Long chord 

stators could be made large enough to add acoustic treatment

inside the vanes, and the unsteady surface pressure response to 

the rotor wake interaction could be reduced resulting from

more phase cancellations due to shorter acoustic wavelengths 

across the stator. In addition, the individual fan wake strengths

Figure 36.—Honeywell Quiet High Speed Fan (QHSF). 

(a) Forward-swept fan. (b) Delay of MPT noise onset.

would be reduced due to the lower aerodynamic loading per 

blade. A fan that had 106 blades and 7 long chord stators, with 

a tip speed of about 1100 ft/s (335 m/s) was tested in 1997 

(Fig. 37) using the Allison Low Noise Fan for baseline 

comparisons. Results showed reductions in both the tones and 

fan broadband noise. The NASA ALNF was about 4 EPNdB 

quieter than the Allison Low Noise Fan for takeoff conditions, 

and about 5 EPNdB lower at approach conditions (Ref. 63).

(7) Source Diagnostics Test—A comprehensive fan test 

campaign was carried out during this time period to provide a 

better understanding of the fan noise generation process and to 

provide an extensive validation database that could be used for 

fan noise prediction methods. A test at Boeing in 1995 using 
NASA/TP—2013-217818 25

an 18-in.- (46-cm-) diameter fan to obtain initial diagnostics 

data was supported by NASA. NASA then worked with GE to

obtain similar data using an early fan design for the GE–90 

engine called the R4 fan. One of the more challenging goals 

was to test in a rotor-alone configuration in the wind tunnel,

which required externally supporting the nacelle (Fig. 38(a))

when the structural exit guide vanes were removed. Since the 

drive rig pitched and yawed as the fan rotational speed 

increased, an active control system was designed so the 

nacelle remained centered over the fan to maintain a uniform 

0.005-in. (0.0127-cm) tip clearance (Fig. 38(b)) during testing.

A comprehensive set of flow and acoustic measurements 

were obtained using instrumentation shown in Figure 10(e).

Results showed that, in general, the inlet-radiated noise was 

dominated by the rotor, and the aft-radiated noise was dominated by the rotor-stator interaction noise. The rotor-alone noise

was up to 4 EPNdB less than the noise with the rotor and stators

and provided insight into how important it was to work on both 

the fan and the stator sources before further significant noise 

reduction could be achieved. Flow surveys, turbulence, acoustic 

duct mode, and rotating microphone measurements, and inlet/aft 

separation and rotor-alone data all used to provide input to fan 

noise prediction codes. A fan broadband noise challenge 

problem was posed to several code developers with blind test 

cases using input on flow parameters from the data and from 

CFD predictions. The BFaNS code developed by P&W and 

United Technologies Research Center under a NASA contract 

predicted the overall noise levels within a couple of decibels. A 

special session at the 2002 AIAA Aeroacoustics Conference 

was organized and papers were presented about all aspects of 

the test (Refs. 64 to 68).

Active noise control of the fan noise was another major 

research effort during the AST program. UHB engines would 

have less acoustic treatment due to shorter nacelles and would 

be more susceptible to inflow distortions in flight due to 

shorter inlets. Active noise control had never been tried for 

fans, but was increasingly popular for headsets and ventilating 

system duct noise. For plane waves in a duct, active noise 

control is achieved by using a sound source to introduce

another plane wave that is 180° out of phase with the target 

plane wave. Rotating microphone measurements showed that 

fan noise sources were complex, varied with speed, and 

became even more complex at higher frequencies. The sound 

sources used for active control needed to be equally complex 

to reduce the amplitudes of target duct acoustic modes without 

exciting other duct modes that could increase the noise.

The ANCF (Fig. 16(a)) was built to study the fundamentals 

of fan noise generation, its propagation in the fan duct, and its

radiation to the far field. The ANCF was also an ideal test bed 

to assess the feasibility of active control of fan. Acoustic 

frequencies were set to match expected UHB engine fan 

frequencies. Many concepts were investigated ranging from

Figure 37.—Alternative Low Noise Fan (ALNF).

Figure 38.—Source diagnostics test (SDT). (a) External nacelle support struts. (b) Fan with no stators; 

view looking toward inlet from aft bypass duct.
NASA/TP—2013-217818 26

duct- and hub-mounted actuators (sounds sources) to cancel a 

single duct mode, to a system including an array of actuators 

embedded inside the stator vanes to cancel multiple acoustic 

duct modes and tones simultaneously (Fig. 16(b)). Actuators 

varied from speakers to piezoelectric devices that could be 

tuned for maximum sound amplitude, but were limited in 

frequency range. It was shown that global reduction of fan 

noise around the engine is possible for several tones containing multiple modes. The control systems were adequate to 

adjust to the changes in fan speeds. However, the complexity 

of the system, number of actuators, and expected cost made it 

difficult to justify in an engine application.

A joint solicitation between Glenn and Langley was issued 

to identify ideas for addressing some of these challenges and 

ways to reduce fan broadband noise. One idea pursued by 

Northrop-Grumman and Hersh Acoustical Engineering was to 

integrate the active control system with the acoustic treatment

to create a hybrid active-passive system. The active system 

directed the sound more efficiently into the liners for better 

absorption. The concept was tested in the 9×15 LSWT as an 

add-on to the P&W ADP fan tests and showed promising 

results. Another idea was to control only the modes that 

significantly contributed to the far-field perceived noise levels.

In either case, the system was still too complex to be considered for an engine application. Another problem was that the 

contributions of tones to the overall fan noise levels were 

small for low-pressure ratio fans. Therefore methods to 

address broadband noise were also needed. It is anticipated 

active noise control concepts will be revisited sometime in the 

future as other technologies mature, such as higher amplitude 

actuators and faster processors to analyze the synthesis 

acoustic signals in real time to control fan broadband noise.

The pioneering work done during this time showed fundamentally that active control of complex fan noise sources is 

possible.

In addition to the model tests, there were engine validation 

contracts with Honeywell and P&W to show that some of the 

noise reduction technologies would work at a higher TRLs.

Static engine tests were done at the company test sites. P&W 

tested a scarf inlet, active/passive inlet liners, cut-on stators, and 

an acoustically treated turbine exhaust (Fig. 39(a)). The scarf 

inlet (Fig. 39(b)) was shown to direct the inlet radiated noise 

away from the community (lower) side of the engine by 2 to 4 

EPNdB relative to a conventional inlet. By treating the turbine 

exit, it was discovered that the aft-radiated noise was more from 

the turbine than from the fan. The idea behind using a cut-on 

stator was to reduce the fan broadband noise due to the lower 

number of stator vanes. The blade passing frequency tone noise

would increase, but it could be reduced with either active noise 

control or a scarf inlet. The overall fan noise would be lower 

than a conventional cut-off fan design. Any effectiveness

of the cut-on stator during the test may have been obscured due 

to the unexpected turbine noise contribution. The active/passive 

inlet had mechanical problems and was never successfully 

tested.

Figure 39.—PW4098 engine test. (a) Pratt & Whitney 

(P&W) C11 test stand. (b) Boeing scarf inlet.
NASA/TP—2013-217818 27

Figure 40.—Honeywell TFE731–60 test. (a) Honeywell

San Tan test stand. (b) Calcor variable-area nozzle.

Honeywell conducted static tests on their TFE731 engine in 

1999 at their San Tan facility in Arizona (Fig. 40(a)), followed 

by flight tests in 2001 on a Falcon 20 test aircraft to investigate chevron nozzles (Fig. 41(a)), and a variable area nozzle

(Fig. 40(b)) for jet noise reduction. Flight test results showed 

the chevron nozzles provided about a 3-EPNdB jet noise 

reduction and were consistent with projections from model 

scale and static engine tests (Ref. 69). The Glenn Learjet was 

also flown with a chevron nozzle during these tests.

Honeywell clearly demonstrated the jet noise reduction

benefits for both aircraft by using a video with an audio track 

that switched between flyovers with the baseline nozzle and 

flyovers with the chevron nozzles. The variable area nozzle 

showed a 1- to 2-EPNdB jet noise reduction, but the fan noise 

increased due to the change in loading from varying the back 

pressure (Ref. 70). A scarf inlet was also tested and showed 

about a 3-EPNdB noise reduction (Fig. 41(b)) (Ref. 71).

Figure 41.—Honeywell Falcon 20 test aircraft. 

(a) Chevron nozzles. (b) Scarf inlet.

The AST program officially ended in 1999, but a few of the 

tests extended into 2001. The new set of NASA “pillar” goals,

announced in 1997, intended to push the technology development toward solving the aircraft noise problem. According to 

the U.S. Environmental Protection Agency (EPA) (Ref. 72), 55 

day night level (LDN) is the outdoor noise exposure level 

“requisite to protect the public health and welfare with an 

adequate margin of safety.” The phrase “health and welfare” is 

defined as “complete physical, mental and social well-being and 

not merely the absence of disease and infirmity.” NASA 

conducted a study to determine what this would mean for a 

single event noise metric like EPNL. Since the aircraft fleet 

mix, number of operations, and size of the airports vary, the 

study looked at 17 major U.S. airports to determine an average 

value that could be used for a goal (Fig. 42). NASA selected 20 

EPNdB at each certification point (60 dB cumulative) as a good 

overall goal to contain the objectionable noise within an average 

airport boundary. The new noise goals were set to develop 

technologies for reducing the perceived noise level by 2 times

(10 EPNdB) in 10 years, and 4 times (20 EPNdB) in 25 years 

relative to 1997 aircraft. The AST program would contribute 

5 dB toward the 10 dB goal, and a new program called Quiet 

Aircraft Technology (QAT) would develop the remaining 

technologies needed to reach the 10 dB goal in 10 years.

Follow-on research programs would be needed to work on the 

longer term goal of 20 EPNdB aircraft noise reduction.
NASA/TP—2013-217818 28

Figure 42.—NASA study for setting noise reduction goals.

Figure 43.—Aft acoustic splitter for fan noise reduction. 

(a) Mach number contours from CFD predictions. 

(b) Test installation in wind tunnel.

Aeroacoustics Research Consortium (AARC)

Funding for aeronautics research was significantly reduced 

after the AST and HSR programs. Historically, the average 

life of a NASA project has been about 5 years and it was a 

challenge to execute long-term research. The AARC was 

started in 2001 to establish an organizational structure to 

promote world-class aeroacoustics research while providing a 

stimulating environment that can attract high-quality researchers in this area and complement the NASA Glenn acoustic 

research workforce (Ref. 73). The consortium still exists today 

and is managed by the OAI, with financial support from 

NASA, Boeing, United Technologies, Rolls-Royce, and 

Honeywell. A peer review panel evaluates proposals and funds 

multiyear research activities that are considered fundamental 

to understanding propulsion system noise. Researchers visit 

supporting organizations to share knowledge and to promote 

collaboration. Seventeen researchers have been supported over 

the past 10 years. OAI produces a “Year End Review” for 

participating organizations and many reports documenting the 

research have been published.

Quiet Aircraft Technology (QAT) Program

The QAT program began in 2001 by conducting studies to 

assess the system-level impact of the noise reduction technologies developed in the AST program. Changing cycle 

parameters, such as reducing the FPR and jet exhaust velocity,

provided about a 75 percent reduction in engine noise, leaving 

about 25 percent to noise reduction technologies such as 

chevron nozzles or advanced acoustic liners that could be 

applied to a fixed engine cycle. Incorporating these changes

meant increasing engine diameters, which could adversely

impact fuel burn due to higher drag, weight, and aircraft 

installation challenges. Higher bypass ratio engines would 

eventually make their way into service, so NASA focused the 

noise reduction research on the most important sources for 

UHB ratio engines. Aft-radiated fan noise was most important.

A second entry of the source diagnostics test was used to 

obtain more flow-field measurements such as time-dependent 

PIV and unsteady surface pressures on the stators. Several fan 

duct nozzles were tested and showed about 2 dB reduction in 

fan noise and a thrust benefit by increasing the exit area

(Ref. 74). CFD and noise prediction codes were used to guide 

the design of an aft-treated splitter. The splitter increased the 

treatment area and was successful in previous programs such as 

QEP (Fig. 5(b)), but the aerodynamic losses were too high for 

practical applications. With CFD considered a mature technology for aerodynamic design, the losses from a splitter could be 

minimized with higher accuracy (Fig. 43(a)). Figure 43(b)

shows the splitter installation using the P&W ADP fan and 

nacelle hardware. Results from this test were disappointing,

showing only a 1.5-dB reduction in sound pressure level from 

12.5 to 20 kHz and about 1 percent loss in thrust.

Acoustically treated soft (sound absorbing) stator vanes

(Fig. 44) were investigated on the ANCF rig and then tested in 

the 9×15 LSWT. Small Helmholtz resonators were embedded 

inside the stators and tuned to reduce dominant fan noise 

frequencies. The resonators would also reduce the unsteady 

aerodynamic response of the stators to passing fan wakes.

Results showed about a 1.5-EPNdB reduction with no 

measureable aerodynamic losses.

Another strategy was to mix or reduce fan wake strengths

before they impinged on the stators. Fan-trailing edge blowing

was suggested by MIT and successfully tested in the ANCF rig
NASA/TP—2013-217818 29

Figure 44.—Sound-absorbing fan stators for fan 

noise reduction.

Figure 45.—Fan trailing edge blowing for fan noise reduction.

(Ref. 75). Then a complex test was carried out in the 9×15

LSWT in 2006 where air was supplied through the drive rig 

and into multiple channels within each fan blade and ejected 

through the trailing edge to fill and mix the wakes (Fig. 45).

The internal flow passages shown in Figure 45 were designed 

using CFD and flow measurements were made behind the fan

at various blowing rates. Acoustic results showed about 2 dB 

reduction in the overall PWL using 2 percent of the bypass 

duct mass flow. Part-span filling was tried to reduce the air 

requirements, but the noise benefits diminished (Ref. 76).

Passive mixing reduction methods were also investigated by 

GE, NASA, and Virginia Tech and used chevrons on the 

trailing edges of the fan blades to mix the wakes.

In 2004, Honeywell tested a second QHSF, called the 

QHSF II, aimed at improving the structural and acoustic

performance of the first QHSF fan tested in 1998 (Fig. 46(a)).

Modifications were made to the fan and the stators. The results 

were used to design a fan for the HTF7000 engine that would be 

used for further noise diagnostic tests. Honeywell was awarded 

the Engine Validation of Noise and Emissions Reduction 

Technology contract. A comprehensive series of tests were 

performed including engine fan wake measurements; caged and 

in-duct phased microphone arrays; cross-correlation of unsteady 

flow measurements in the combustor and turbine exit; far-field 

separation of inlet/aft-radiated noise; in situ acoustic liner 

impedance measurements; rotating microphone measurements;

Figure 46.—Honeywell fan and engine tests. (a) QHSF 

in the 9×15 LSWT. (b) Water brake for testing without a fan.

and engine testing without a fan by using a water brake to 

provide a load (Fig. 46(b)) (Ref. 77). Adaptive HerschelQuincke tubes mounted in the fan duct were tested and

showed a 2-EPNdB cumulative noise reduction. A special 

session was organized at the 2008 AIAA Aeroacoustics 

Conference where papers were presented about all aspects of 

the test.

Jet noise reduction concepts were focused on offset nozzles 

to change the noise directivity; chevrons made from shape 

memory alloys to optimize the penetration angle between 

takeoff and cruise; and fluidic injection to control the breakup 

of the streamwise vorticity from mixing devices such as 

chevrons. Advanced mixers were also tested through a 

cooperative program with Rolls-Royce. Strong emphasis was 

placed on flow measurements to improve the understanding of 

the noise generation process.

The SHJAR rig was built for fundamental tests and screening nozzle concepts. Figure 47 shows some of the test setups 

used to obtain very detailed databases that have been used to

compare and improve jet noise prediction tools (Ref. 26).

Similar tests were done in larger scale using the NATR rig.
NASA/TP—2013-217818 30

Figure 47.—SHJAR tests. (a) PIV. (b) Microphone array 

for instability wave diagnostic measurements.

Measurements included three-component PIV (Fig. 47(a))

(Ref. 78), two-point space-time velocity correlations (Ref. 79), 

and three-dimensional phased microphone arrays (Ref. 80).

Arrays of microphones were used to investigate instability 

waves in the shear layer of the jet and the propagation of 

sound to the far field (Fig. 47(b)). Investigations also excited 

the jet with plasma actuators to promote mixing and jet noise 

reduction. Fundamental experiments were started at The Ohio 

State University and scaled up to larger nozzle diameters for 

tests at NASA. Large Eddy Simulations (LESs) were used to 

model plasma actuators to excite the jet (Ref. 81). Results 

from the simulations looked promising for scaling to larger 

jets, but experiments have yet to be successful. A recent 

review paper (Ref. 82) summarizes the long history of fluidic 

injection methods for jet noise reduction.

Fundamental Aeronautics Program (FAP)

In 2006, NASA completely reorganized aeronautics to focus 

on fundamental research in an attempt to return to the NACA 

roots known for rigorous investigations and comprehensive 

documentation of results. More emphasis was placed on 

prediction methods and validation experiments and less on 

specific aircraft applications. Noise research was split among 

the Subsonic Fixed Wing (SFW), Supersonics (SUP) and 

Subsonic Rotary Wing (SRW) projects under the Fundamental 

Aeronautics Program (FAP). The TWG that was utilized

during the AST and QAT programs continued as a way to 

communicate progress across projects and to keep industry 

aware of NASA’s research in acoustics. The aeronautics 

budget was reduced compared to the AST and QAT programs, 

so NASA placed more emphasis on partnerships and cooperative tests with external organizations. Model test hardware that 

used to be fully funded by NASA was now jointly funded with

industry. NASA Research Announcements (NRAs) were used 

to promote competition by universities and industry for related 

work. Areas of research focused jointly on noise, emissions,

and aircraft performance rather than in separate programs or 

projects. For subsonic aircraft, NASA focused on utilizing 

UHB engines (just as it did at the beginning of the AST 

program). Goals were set based on technologies that would be 

ready for future generations of aircraft.

Table I shows the noise, emissions, and fuel burn reduction 

goals for the SFW project. The N+1 goals aimed to reduce the 

noise for B737- and A320-size aircraft, although the technologies would benefit other aircraft classes. Industry projected

that it could meet a 20-EPNdB cumulative noise level under 

Chapter 4 at a TRL 9 (in service) by utilizing higher bypass 

ratio engines, or approximately 2/3 of the noise goal set by the 

AST program in 1992. NASA studies (Ref. 83) showed that if 

even lower FPRs with noise reduction technologies could be 

justified (to satisfy other design criteria like emissions and fuel 

burn), it would be possible to achieve a 25- to 29-EPNdB 

cumulative below Chapter 4, which almost satisfies the AST 

and SFW N+1 goals.

NASA believed that the N+1 goals could be achieved with a 

conventional “tube and wing” aircraft utilizing advanced 

technologies. However, further noise reduction would be 

difficult to achieve without some configuration change to help 

shield the community from the engine noise. Engine noise 

shielding was observed during flyover tests of a DC–9 in the 

1970s. The wings serve as a barrier to reduce the noise 

radiated from the engine inlet. Studies from noise prediction 

codes and experiments during the planning for FAP showed 

that up to a 10-EPNdB cumulative noise reduction could be 

achieved for radically different commercial aircraft designs 

such as a blended or hybrid wing body (Ref. 84). As a result,

the N+2 noise goals were set to be a 42-EPNdB cumulative 

below Chapter 4, with the expectation that the aircraft 

configuration would change from a conventional tube and 

wing to a configuration such as a hybrid wing body.
NASA/TP—2013-217818 31

TABLE I.—RESEARCH GOALS FOR SUBSONIC AIRCRAFT

Corners of the 

Trade Space

N+1 (2015)a

technology benefits

relative to a single aisle 

reference configuration

N+2 (2020)a

technology benefits 

relative to a large twin aisle 

reference configuration

N+3 (2025)a

technology benefits

Noise

(cum below Stage 4) –32 dB –42 dB –71 dB

LTO NOx emissions

(below CAEP 6) –60 percent –75 percent Better than –75 percent

Performance aircraft fuel burn –33 percentb –50 percentb Better than –70 percent

Performance field length –33 percent –50 percent Exploit metroplexc concepts a

Technology Readiness Level for key technologies = 4 to 6. b

Additional gains may be possible through operations improvements. c

Concepts that enable optimal use of runways at multiple airports with the metropolitan areas.

TABLE II.—RESEARCH GOALS FOR SUPERSONIC AIRCRAFT

N+1

supersonic business class aircraft 

(2015)

N+2 

small supersonic airliner

(2020)

N+3 

efficient multi-Mach aircraft

(beyond 2030) 

Environmental goals

Sonic boom 65 to 70 PLdB 65 to 70 PLdB 65 to 70 PLdB 

low-boom flight

75 to 80 PLdB 

overwater flight

Airport noise (cum below Stage 4) Meet with margin 10 EPNdB 10 to 20 EPNdB

Cruise emissions (cruise NOx g/kg of fuel) Equivalent to current subsonic <10 <5 and particulate and 

water vapor mitigation

Performance goals

Cruise speed Mach 1.6 to 1.8 Mach 1.6 to 1.8 Mach 1.3 to 2.0

Range (n mi) 4000 4000 4000 to 5500

Payload (passengers) 6 to 20 35 to 70 100 to 200

Fuel efficiency (pass-miles per lb of fuel) 1.0 3.0 3.5 to 4.5

The N+2 goals were intended for larger aircraft replacement 

such as the B777. The N+3 goal for noise was established to 

meet NASA’s long-term goal of containing the objectionable 

noise within an average airport boundary. Other studies,

including the MIT/Cambridge Silent Aircraft Initiative 

investigated an aircraft design with noise reduction as the 

primary goal. Based on these studies, NASA identified the 

ultimate goal for noise reduction to be about a 71-EPNdB 

cumulative below Chapter 4 levels, which approximately 

corresponded to the EPA goal of 55 LDN noise contours at an 

average airport boundary. This was about 10 dB more 

aggressive than the 2025 noise reduction that was established 

by NASA in 1997.

Supersonic aircraft research has focused on airport noise 

and sonic boom reduction to enable flight over land. The SUP 

project set similar generational goals starting with smaller 

aircraft and increasing the size over time. Table II shows the 

project’s long-term goals. The sonic boom noise goals are 

specified as perceived noise levels in decibels (PLdB). The 

engines planned for commercial supersonic aircraft will use 

lower BPR turbofans. The engines will need to be compact to 

meet the performance and sonic boom goals, but will also 

need to have reduced exhaust velocities during takeoff and 

landing to meet the airport noise goals. Chevron nozzles used 

for subsonic aircraft would not provide enough noise reduction so emphasis has been placed on developing variable cycle 

engines. Military engines are considering a third flow stream 

that could be closed for cruise, would effectively change the 

BPR. A third flow stream could also provide more control 

over the Mach number distribution in the jet with the possibility of implementing inverted velocity profile noise reduction 

concepts first explored by Glenn in the 1970s (Ref. 19).

NASA awarded contracts to Lockheed and Boeing to study 

the benefits of a variable cycle engine. GE and Rolls-Royce 

developed engine concepts aimed at meeting the N+2 goal of a 

10-EPNdB cumulative noise reduction under Chapter 4 

regulations. Model scale tests were carried out in NATR to 

investigate mixer-ejector concepts with a third flow stream 
NASA/TP—2013-217818 32

and inverted velocity profiles. Predictions from Rolls-Royce,

using a mixer-ejector concept, estimated about an 8-EPNdB 

cumulative noise reduction under Chapter 4 is possible. Test 

results showed that this could be achieved if extraneous tones 

could be removed by improving the third stream nozzle

hardware design (Refs. 85 and 86). Figure 48(a) shows a 

reference nozzle with three flow streams and Figure 48(b)

shows a three stream mixer-ejector nozzle from Rolls-Royce.

Subsonic research remains focused on fan noise reduction, 

but core noise is expected to be a concern for future engines as 

the fan and jet noise are further reduced. Engine data from 

Honeywell (Ref. 77) has been used to develop correlation 

methods for separating noise sources. A time delay method 

(Ref. 87) was developed to separate direct and indirect 

combustion noise. The intention is to take advantage of the 

timescale difference between an entropy disturbance traveling 

with the mean flow from the combustor interacting with the 

turbine, and the direct sound radiating from the combustor to 

the far field. Alternative pressure gain combustion concepts 

have been investigated such as pulse detonation engines 

(PDEs). Just as the V–1 “buzz bomb” testing disturbed the 

community around Glenn in 1945, tests of a single PDE tube 

in the AAPL generated complaints about noise in 2002. The 

acoustic investigations showed that while the direct noise from 

the device was high, a turbine placed downstream with the 

appropriate blade numbers and stages could block enough 

sound to make the alternative combustor competitive with 

commercial engine noise levels. For fan noise, the two 

previous source diagnostics tests in the 9×15 LSWT helped 

quantify when the rotor alone noise would need to be addressed. A method for reducing rotor noise without relying on 

further reduction in tip speed or FPR was to add acoustic 

treatment directly over the tip of the fan. Initial concept tests 

in the ANCF looked promising and follow-on tests were done 

in the 9×15 LSWT and with a FJ44 engine (Fig. 49) from 

Williams International (Ref. 88). Even though the concept 

looked promising in engine tests, the wind tunnel results 

showed substantial aerodynamic loss with small reduction in 

noise due to stator-dominated sources in the test (Ref. 89).

Over-the-rotor acoustic treatment is still being investigated 

along with refinements to the soft stator concept. Fundamental 

measurement methods were developed to identify fan 

broadband noise sources within the fan duct (Ref. 90) and

would be key in identifying additional fan noise reduction 

concepts.

Integrated Systems Research Program (ISRP)

In 2010, the Integrated Systems Research Program (ISRP)

was established, and the Environmentally Responsible Aviation 

(ERA) project was started under ISRP to fund system-level 

validation experiments and concepts that could help meet 

NASA’s N+2 goals (Table 1). Several concepts that showed

Figure 48.—Three-stream nozzles for jet noise reduction. 

(a) Reference nozzle (without mixer). (b) Rolls-Royce 

mixer-ejector nozzle.

Figure 49.—Williams International FJ44 engine test in the 

AAPL.
NASA/TP—2013-217818 33

Figure 50.—P&W geared turbofan. (a) Scale model in 

the 9×15 LSWT test. (b) Full-scale flight test on a 

P&W 747 test aircraft.

promise for fuel burn and noise reduction were moved 

from the SFW project into ERA, including P&W’s Geared 

Turbofan (GTF) and GE’s open rotor. Open rotor was the 

name given to the follow-on development of GE’s UDF

concept. After about 10 years of working exclusively on 

propellers during ATP followed by about 15 years of working 

exclusively on turbofans during AST and QAT, Glenn was 

refining both propulsion systems with hope that these 

technologies would be used in engines for new aircraft.

The GTF was tested in the 9×15 LSWT in 2007 (Fig. 50(a)) 

and built on the successful ADP fan tests done in the 1990s.

The GTF work led to a flight test funded by P&W

(Fig. 50(b)). Several new GTF engines are expected to be 

introduced by P&W over the next few years. The open rotor 

was tested from 2009 through 2012 through a jointly funded 

program between GE and NASA (Fig. 51(a)). Model-scale

data were acquired in the 9×15 LSWT and high-speed cruise 

simulation data were taken in the 8×6 SWT, just as was done 

during the ATP program in the 1980s. Newer blade designs 

have shown significantly reduced noise levels compared to the 

UDF. Valuable flow diagnostic data such as microphone 

Figure 51.—GE open rotor test in the 9×15 LSWT. 

(a) Acoustic traverse microphone measurements. 

(b) Phased microphone array test showing pylon 

interaction with first rotor (white region on front blades).

phased array and PIV helped to identify dominant noise sources

(Fig. 51(b)). NASA conducted a study in 2011 to compare noise 

and fuel burn predictions for the GTF and open rotor systems 

based on model data scaled to a B–737-size aircraft using 

ANOPP (Fig. 52). Results show that at TRLs 4 to 5, the open 

rotor is expected to be about a 13-EPNdB cumulative under 

Chapter 4 regulation and the GTF is expected to be about a 25-

EPNdB cumulative below Chapter 4. While the noise levels are 

about 12 EPNdB cumulative higher for the open rotor, the fuel 

burn was predicted to be about 9 percent less. Industry and 

market demands will determine which engine will be selected 

for future aircraft applications.
NASA/TP—2013-217818 34

Figure 52.—NASA study for UHB turbofans and open rotor 

propulsors: fuel burn versus noise.

Collaboration and Outreach

Cooperative working relationships with other NASA centers 

have been critical for the success of engine noise research and 

the ability to influence technologies for new aircraft. Work with 

the Department of Defense (DOD) has also been important 

since the DOD relies on NASA for environmental research in 

noise and emissions. NASA has participated with the Navy and 

Air Force on many committees over the years to help assess jet 

noise problems and possible solutions. NASA’s working 

relationship with the FAA has led to jointly funded research for 

commercial engines that typically have been focused more on 

near-term applications. The most recent example is the FAA’s

Continuous Low Emissions, Energy and Noise (CLEEN) 

program that will help raise the TRL for aircraft to meet the 

N+1 goals. NASA has provided technical expertise for the 

AIAA Aeroacoustics Technical Committee the Society for 

Automotive Engineers (SAE) A21 Aircraft Noise Measure and 

Noise Aviation Emission Modeling committee; ICAO 

Independent Expert Review Panels; and source evaluation 

boards for government procurements. Glenn employees

routinely speak at schools and community meetings to raise the 

awareness on how Glenn’s technologies are being used and to 

inspire the next generation of acousticians.

The Future

While considerable progress has been made on aircraft 

noise reduction, additional research is needed to contain 

objectionable noise levels to the boundaries of an average size 

airport. The N+3 goals for subsonic aircraft were defined to

push technologies even further. It is anticipated that increasing

engine BPR will reach a limit where there will be no further 

Figure 53.—Advanced aircraft concepts. (a) Engines 

over wings. (b) MIT Double Bubble with aft engines. 

(c) NASA turboelectric.

noise reduction benefit. Nacelle lengths will need to be 

shortened to reduce engine weight, which will reduce the 

available acoustic treatment area. The primary benefit of 

increasing BPR has been jet noise reduction. When jet noise 

levels are significantly below the fan levels and other noise 

sources, the FPR will have more of an effect. Once a practical 

limit for lowering the FPR has been reached and the acoustic 

treatment area cannot be increased, additional engine noise

reduction will be difficult to achieve.

One alternative strategy considered in the past shielded the 

engine noise using the aircraft structure. In 2010, NASA 

initiated a challenge to the aerospace community to study

ways to further reduce the noise, along with fuel burn and 

emissions. The concepts have been documented (Refs. 91 to 

95) and show a general trend of placing engines either above 

the wing (Fig. 53(a)) or above the fuselage (Fig. 53(b)) to 

shield the engine noise. The turboelectric aircraft concept,

shown in Figure 53(c), uses electrically powered fans with 

superconducting motors to distribute propulsors across the 
NASA/TP—2013-217818 35

trailing edge of a flying wing. The boundary layer from the 

wing is ingested to provide additional fuel burn reduction 

benefits. It will take many years before the technologies will 

be mature enough to introduce these advanced aircraft, but the 

noise reduction benefits are expected to come close to the 

ultimate goal of solving the aircraft noise problems that the 

NACA and NASA have been working on for almost 70 years.

Concluding Remarks

Since the 1970s, the common thread of all aircraft engine 

noise reduction research programs has been to find ways to 

move larger amounts of air more slowly through the engine 

using methods that enhance or do not adversely impact 

performance. There are similarities between noise reduction 

methods identified in the 1950s and the concepts being explored 

today. For example, the original high-aspect-ratio nozzles

investigated in the late 1950s are similar to today’s distributed 

propulsion concept. The quest for a low-pressure ratio fan was 

explored in the short takeoff and landing (STOL) research of the 

1970s and Pratt & Whitney’s (P&W’s) Geared Turbofan will 

be the first engine to enter service with these characteristics.

The difference has been the enabling of technologies like 

high-fidelity computational fluid dynamics (CFD), lighter 

weight structures, improved measurement methods, and better 

knowledge of the noise generation physics that make today’s

designs practical and effective. A number of noise reduction 

concepts that initially appeared not to work were due to 

inadequate test procedures (extraneous noise sources and 

masking of sources), and were later found to provide significant noise reduction benefits.

Tremendous progress has been made since the aircraft noise 

problem peaked in the 1960s and regulations were introduced.

The average noise level at each certification point has been 

reduced by about 20 effective perceived noise level in decibels 

(EPNdB) over 50 years (60 EPNdB cumulative). Projections 

for turbofan-powered aircraft show noise levels are expected 

to be at least a 20-EPNdB cumulative under Chapter 4 for new 

aircraft with higher BPR engines. Because the impact of 

aircraft noise also depends on how often noise occurs, the 

amount of noise reduction needed for each aircraft is greater if 

the number of flights increase. To achieve the ultimate goal of 

containing objectionable aircraft noise within the boundaries 

of an average airport, more work needs to be done. Studies 

show another 15 to 17 EPNdB at each certification point may 

be required depending on the size of the airport, aircraft fleet 

mix, and actual growth in air travel. Glenn has made significant contributions toward achieving this goal.

NASA/TP—2013-217818 37

Appendix—Acronyms

AAPL Aero-Acoustic Propulsion Laboratory

AARC Aeroacoustic Research Consortium

ACEE Aircraft Energy Efficiency

ADP Advanced Ducted Propulsor

ALNF Alternative Low Noise Fan

AIAA American Institute of Aeronautics and 

Astronautics

ANCF Advanced Noise Control Fan

ANOPP Aircraft Noise Prediction Program

AST Advanced Subsonic Technology

AWT Altitude Wind Tunnel

ATP Advanced Turboprop

BBN Bolt, Beranek, and Newman, Inc.

BPR bypass ratio

CAEP Committee on Aviation Environmental 

Protection

CAA Computational Aeroacoustics

CFD computational fluid dynamics

CLEEN Continuous Low Emissions, Energy and Noise

DOD Department of Defense

EEE Energy Efficient Engine

EPA Environmental Protection Agency

EPNdB effective perceived noise level in decibels

EPNL effective perceived noise level

ERA Environmentally Responsible Aviation

FAA Federal Aviation Administration

FAR Federal Aviation Regulation

FAP Fundamental Aeronautics Program

FPR fan pressure ratio

GE General Electric

GTF Geared Turbofan

HSR High-Speed Research

HW hot wire

ICAO International Civil Aviation Organization

ICD inflow control device

ISRP Integrated Systems Research Program

JeNo jet noise (prediction code)

LDV laser Doppler velocimetry

LES Large Eddy Simulation

MPT multiple pure tone

NACA National Advisory Committee for Aeronautics

NASA National Aeronautics and Space Administration

NATR Nozzle Acoustics Test Rig

NRA NASA Research Announcement

OAI Ohio Aerospace Institute

OTW over the wing

P&W Pratt & Whitney

PDE pulse detonation engine

PIV particle image velocimetry

PLdB perceived noise levels in decibels

PTA Propfan Test Assessment

PWL sound power level

QAT Quiet Aircraft Technology

QCGAT Quiet, Clean, General Aviation Turbofan

QCSEE Quiet, Clean, Short-Haul, Experimental Engine

QEP Quiet Engine Program

QHSF Quiet High Speed Fan

QSRA Quiet Short-Haul Research Aircraft

RECAT Reducing Energy Consumption of Commercial 

Air Transportation

RSI Rotor-Stator Interaction (prediction code)

SAE Society of Automotive Engineers

SDT source diagnostics test

SFW Subsonic Fixed Wing project

SHJAR Small Hot Jet Acoustic Rig

SPL sound pressure level

SRW Subsonic Rotary Wing project

SST Supersonic Transport

St Strouhal number

STOL short takeoff and landing

SUP Supersonics project

TRL Technology Readiness Level

TWG Technical Working Group

UHB ultrahigh bypass

UDF unducted fan

UPS Universal Propulsion Simulator

UTW under the wing

8×6 SWT 8- by 6-Foot Supersonic Wind Tunnel

9×15 LSWT 9- by 15-Foot Low-Speed Wind Tunnel

10×10 SWT 10- by 10-Foot Supersonic Wind Tunnel
NASA/TP—2013-217818 38

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NASA Glenn’s Contributions to Aircraft Engine Noise Research 

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Huff, Dennis, L. 

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14. ABSTRACT

This report reviews all engine noise research conducted at the NASA Glenn Research Center over the past 70 years. This report includes a 

historical perspective of the Center and the facilities used to conduct the research. Major noise research programs are highlighted to show 

their impact on industry and on the development of aircraft noise reduction technology. Noise reduction trends are discussed, and future 

aircraft concepts are presented. Since the 1960s, research results show that the average perceived noise level has been reduced by about 20 

decibels (dB). Studies also show that, depending on the size of the airport, the aircraft fleet mix, and the actual growth in air travel, another 

15 to 17 dB reduction will be required to achieve NASA’s long-term goal of providing technologies to limit objectionable noise to the 

boundaries of an average airport. 

15. SUBJECT TERMS

Turbofan; Turbojet; Aircraft engine noise; Acoustics; Aeroacoustics; Fans; Jets; Turbomachinery; Noise reduction 

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