Case Study

GTRI: A World Leader in Acoustics Research and Control

Published: September 19, 2012

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The renovation of its two anechoic chambers underscores the Georgia Tech Research Institute's commitment to maintaining its position as a world leader in aeroacoustics.

The large echo-free rooms (anechoic means "without echoes"), located at GTRI's Cobb County research complex, provide the ultra-quiet environment essential for conducting high-level research in sound generation, propagation, detection, prediction and, especially, noise control.

The chambers' interior surfaces are completely covered with hundreds of sound-trapping wedges made from specially fabricated sound-absorbing foam, according to Krishan K. Ahuja, Ph.D., a Regents' professor, Regents' researcher and chief of the Aerospace and Acoustics Technologies Division.

"Over the years, the cumulative effects of temperature changes and humidity impair the foam wedges' ability to absorb sound," he said. "We have to replace them to restore the chambers to their original condition and usefulness."

The foam wedges, measuring about 19 inches in length and installed with the pointed end facing the interior space, are expected to absorb 99 percent of sound waves above 200 Hz.

The anechoic chambers are major components of the extensive testing facilities, equipment and acoustical diagnostic capabilities available to GTRI scientists. These include:

  • Anechoic Flight Simulation Laboratory. A free-jet wind tunnel housed within an anechoic chamber, this large facility allows GTRI researchers to examine a particular item — model-scale propeller, rotor, wing, heated jet engine or even an automobile component — under conditions simulating movement of up to 300 feet per second to determine if the object's forward motion produces changes in the observed noise. Researchers are thus able to measure and identify the source of the noise and develop ways to mitigate it. Measuring 14 x 14 x 20 feet, the room rests upon four massive springs and is physically isolated from the surrounding building. The facility is equipped with two heavy-duty doors, and the floor is constructed of wire mesh, rather than solid, sound-reflecting materials. So sensitive is the facility to sound that when researchers are making measurements, even the air conditioning system outside the facility is shut down.

Significant noise mitigation measures ensure low noise levels in the test section. The flight-simulating free jet exhausts into a collector, downstream of which are a diesel fan and an ejector jet that provide the flow stream. The 6-by-6-foot collector itself is completely lined with 8-inch-thick sound-absorbing foam. The collector duct is a sandwich-type structure filled with several tons of sand to eliminate the possibility of a flanking path for noise or vibration from outside the facility. There are no vibrations associated with the boundary layer developed on the duct wall or as the jet flow impinges on the turning vanes, which are themselves acoustically treated. Additionally, duct silencers located just upstream of the diesel fan and the ejector ensure that even with the ejector jet running supersonically, no jet noise escapes upstream into the test section.

  • Static Jet Acoustic Test Facility. This 22 x 20 x 28-foot anechoic room is equipped to run model-scale jet engines while GTRI engineers measure the sound generated by the machines. The data they collect helps isolate the sources of particular sound frequencies, and enables them to develop quieter engine designs.

Two independently controlled and heated air supply ducts allow for the measurement of both single and coannular jet noise. The facility is equipped with an acoustically lined exhaust collector, which ingests entrainment and room cooling air through the outer channel of a rectangular coaxial duct in quantities dictated by the jet operating conditions, with no special forced-air injection or fan system.  After passing through an air gap between the concrete wall and false wall on the collector side of the room, this entrainment air is distributed symmetrically around the jet axis, thereby keeping the air flow circulation velocities in the room, particularly around the microphones, to a minimum.

Microphones may be placed anywhere in the room but at least 15 inches away from the wedge tips, so as to be beyond near field effects from the foam wedges.

A remotely operated cherry-picker crane, which retracts under an anechoic cover during test procedures, provides access to instrumentation and test installations for calibration, test setup modifications and maintenance, thus avoiding the need for access platforms and their attendant sound-reflection problems.

  • Acoustic Sensors. Instruments in this category include condenser microphones, hearing devices, probe microphones and infrasound sensors. Techniques developed at GTRI allow the use of these sensors at high temperatures and in high-speed flows. The sensors can also educe acoustic signals buried in unrelated background noise, and may be used in conjunction with accelerometers and geophones. In addition, acoustic and seismic sensors allow active imaging of underground facilities.
  • Outdoor Sound-Level Meters. Long-term, continuous acoustic measurements are recorded with GTRI's suite of all-weather sound level meters. The devices have been employed in projects ranging from measuring neighborhood and airport noise to quantifying the sound levels inside nursing homes.
  • Impedance Tubes. GTRI maintains a number of these instruments, which are used to determine the sound-absorbing properties of various materials. These tubes are also used to calibrate acoustic sensors.
  • Sonic Boom Simulator. A high-amplitude sound generator specially built for GTRI produces 40,000 watts of acoustic power. This powerful, one-of-a-kind device has been used to produce sonic booms as well as to calibrate special wind screens for infrasound sensors, thereby eliminating the need for long and cumbersome soaker hoses previously used in the procedure. The broad sensitivity of the sensors can be configured to monitor a secure area by detecting the sounds made by people moving, or even the tremendous sound produced by tornadoes.
  • Beam-Forming Acoustic Array. Consisting of multiple acoustic sensors, this instrument aids in detecting the location of a given noise source.
  • Particle Imaging Velocimeter. This instrument measures the movement of particles in a fluid non-invasively as a way of determining the fluid's flow speed.
  • Hot-Jet-Flow Facility. Equipped with free jets and various nozzle configurations in a non-anechoic room, this facility provides flow visualization and flow diagnostic capabilities.
  • Computational Resources. GTRI is fully equipped with graphics workstations, real-time acquisition computers and dozens of desktop machines. Software includes ANSYS structural analysis programs; various aerodynamic codes; data processing, data base and statistical software; contour and spectra plotting programs; various noise prediction programs, including ANOPP; and free-jet shear-layer correction programs.
  • Other Equipment. Acoustics research at GTRI is supported by a number of amplifiers, signal generators, transducers to modify sound traveling through walls and windows, various traverses, A/D converters, spectrum analyzers (including a 24-channel spectrum analyzer with an 80 kHz bandwidth), hot wires, frequency analyzers, high-temperature nozzles, a two-microphone impedance tube, a flow-duct facility for impedance measurements in the presence of cold and heated flows, and an elliptical mirror for locating sound sources.

Microphones on hand include Brüel and Kjær one-inch, half-inch and quarter-inch models; infinity tubes and probe microphones; miniature Knowls microphones; and Kulite transducers equipped with water jackets for heated flows. Also, a high-speed camera, capable of 7,500 frames per second at 720p resolution and up to 1 million frames per second at reduced resolutions, is available for flow visualization studies and similar tasks. Additional acoustic diagnostic tools provide for flow visualization and velocity fluid determination with PIV and helium bubble flow visualization.

The Aerospace, Transportation and Advanced System Laboratory at GTRI has performed research, development and testing for both government agencies and private industry. One recent effort examined ways to add a valuable stealth component to unmanned aerial vehicles or drones, used extensively by the military, by enabling them to fly as silently as possible. GTRI's expertise in "quiet" technologies has also found applications in wind turbines, military helicopters and jet engines.

A sampling of projects conducted for the government and private industry appears below:


  • Screech Phenomenon of Supersonic Heated Plumes
  • Sources of Noise in Plumes
  • High-Temperature Sound Absorbers
  • Airframe Noise Reduction using Pneumatic Flow Control
  • Distributed Exhaust Nozzles
  • Personal Air Vehicle Noise
  • Noise Characterization of Distributed Exhaust Nozzles under Static Conditions
  • Noise Characterization of Distributed Exhaust Nozzles under Flight  Simulation
  • Thrust Performance of Distributed Exhaust Nozzles
  • The Role of Nozzle Geometry Modifications in Jet Noise Suppression 
  • Propeller Noise
  • An Innovative Aero-engine Combustion Noise Measurement Methodology Using Coherence based Signal Processing 
  • Cruise-Efficient Short Take-Off and Landing (CESTOL) aircraft acoustics
  • Environmentally Responsible Aviation Tasks
  • Coupling between Plume Instabilities and Duct Resonant Modes
  • The Impact of Helicopter Noise on Communities
  • Noise Control in Nursing Homes
  • Quiet Jet Engine Test Facilities
  • Noise of Tip-Jet-Driven Rotors
  • En-Route Noise


  • Various Aspects of Automobile Noise
  • Blended Wing-Body-Airframe Noise
  • Jet-Flap Interaction Noise
  • Jet Impingement Noise and Control
  • Forward Flight Effects on Heated and Unheated Rectangular Jets 
  • Forward Flight Effects the Internal Noise of an Ejector Nozzle
  • Validating Warning Siren Performance

For more information contact Krish K. Ahuja.