Description: Brigham Young University and the U.S. Office of Naval Research are studying how sound radiates from military airplanes in the hopes to employ significant noise-reduction efforts.
Start: July 1, 2014
End: June 30, 2016
- Sponsor: Office of Naval Research
- Principal Investigator: Kent Gee
- Co-PI: Tracianne Neilsen
- Website: http://acoustics.byu.edu
Aircraft noise is often a neighborhood nuisance that brings down property values. Furthermore, sound pollution is medically cited to cause and magnify a host of health impairments, affect mental processes, and disrupt wildlife. Military aircrafts produce an especially loud blast, which makes acoustic research a priority for many military arms, including the Office of Naval Research (ONR). ONR is working with BYU’s Dr. Gee, specialist in aeroacoustics and propagation, and Dr. Neilsen, specialist in acoustical oceanography, to do just this.
Noise coming from airplanes has three main origins: first, largely generated within the mechanics and, second, in the air flow around its form, and a third, lesser portion sourced by the electrical systems or military equipment inside the cockpit and cabin. First, jet engine essentially functions by drawing in and pressurizing a large volume of air, via a propeller-fan and compressor. Then, the air is mixed with fuel and ignited. When the burning gas is expelled, it creates a powerful thrust to move the vehicle forward. The pressure and volume of jet discharge accounts for much of the mechanical roar associated with military planes. Additional, aerodynamic noise is produced by rapid air flow around the main body of the airship, as well as the vast air movement circling in the propellers. Acknowledging these designs, BYU’s investigation team is dissecting how military aircraft noise is sourced and transmitted to develop stratagems to significantly reduce and prevent the jets’ disruptive clamor.
While some understanding can from using laboratory-scale prototypes and creating numerical simulations, Dr. Gee and Dr. Neilsen report that significant and needed amounts of data can only be gleaned in the field. Much of this project’s exploration builds upon contemporary suggestion that “wavepackets” are the key to understanding the audible radiation. A wavepacket is a localized “disturbance” resulting from the sum of different sound waves traveling together as a group. As they cross over a distance, they seem to extend beyond turbulence— the seemingly unsteady movement and recirculation of air or liquid— and continue with strong direction and energy. Previous researchers have found it difficult to measure wavepackets for high-performance military jets, as they operate in relatively extreme environments. However, this team used a technique called beamforming to record the different propagations of waves by building a probe with 90 microphones set at three different heights, and using dozens of other stationary microphones at ground level to provide further reference points. Additionally, a 3D intensity probe used typically for rocket measurements was attached to the top of the rig. And finally, hydrodynamic pressure measurements (registering the movement and pressure enacted on fluids) were set up to determine the source’s parameters. After setting up the equipment, the team began running extensive tests. With this beamforming technique, Dr. Gee and Dr. Neilsen were able to gather over 6000 sound points, which is to-date the most detailed measurement of the near-field acoustics for military jets.
This approach allowed them to better understand the acoustic decomposition for the varying engine power levels: idle, 25%, 50%, 75%, 100%, 130% and 150%. To describe these engine capacities, they can be generalized into four settings—idle, intermediate, military, and afterburner. The “idle” position is naturally assumed as the low running speed of the engine while the vehicle is at rest. “Intermediate” involves the range of powers that a plane can travel at for an extended period of time. In pilot terminology, “military power” is reaching the maximum engine power level (100%) before engaging the additional “afterburner”, which is a component of some military aircrafts that provides a further thrust, usually for supersonic flights, quick takeoffs, and a few combat maneuvers. Utilizing these microphone references, the collaborative team first created “spatial maps” of the sound dispersal, and then was able to create a holographic model to trace the sound back to its source. Effort was made to develop these techniques as close to origin points as possible to minimize any circumstantial distortion.
One objective of ONR’s research was to confirm what the science community thought they knew about wavepackets. What Dr. Gee and Dr. Neilsen have found is that, concurrent with other studies, the width of each wavepacket decreased as its frequency increased, which accounts for the fluctuation and yet strong directivity; in other words, this reveals that the sound does not disperse so widely as to quickly lose its strength, and actually reforms and intensifies for a time while radiating. Several phenomena that laboratory experiments only hinted at, or excluded all together, also began to unfold. These include: a broader directivity and peak directivity angle for the military and afterburner settings, and a distinct shift in the propagation shape.
This project contributes significantly to the science community’s understanding of acoustic reverberation and wavepacket models, turbulent air characteristics, and specifically about high-performance military jets. Also, this study gives example of the bridging between limited laboratory and model-scale experiments, computer simulations, and the need for on-sight environment testing. Tangent research is still needed to understand the full nature of hydrodynamic wavepackets, as well as research to consider turbulence characteristics that would further affect the audible radiation.. As analysis continues, similar investigation may come to significantly alter airplane structures. The main concern, as noise reduction efforts ensue, is to not decrease military jet performance. Five generations of tactical aircraft designs have lead to maximized flight performance, and so exterior redesigns are not currently being employed. However, their experimentations have proved that sound can definitely be reduced within the aircraft mechanics by certain machinery replacements and alterations, some of which actually improve engine performance.