Anyone who has watched a TV news broadcast of a space shuttle launch has heard it: a loud crackling as the rockets lift the shuttle high into the air.
The noise is also unpleasantly familiar to those who serve on aircraft carriers, as it is generated while an F-18 lunges off the deck. Also, the sharp crackle may strike the ears of northern Utah residents near Hill Air Force Base when F-16s accelerate there.
Brigham Young University's Kent L. Gee says the crackle sound is "a very dominant component of the noise" from military jet engines. "That crackle really dominates the overall noise."
If scientists could determine the source of the crackle, he added, they could "reduce it eventually. ... It could reduce the overall noise impact of these aircraft."
Gee, assistant professor of physics at the Provo university, has a new explanation for crackle, which he presented last week during the meeting of the Acoustical Society of America, held in Salt Lake City.
In this case, the work has focused on F-18s, launched from aircraft carriers; the study was supported by the Office of Naval Research and the Strategic Environmental Research and Development Program.
For about 30 years, acoustics specialists have believed the crackle was related to turbulence in the jet plume radiating from the rear of the engine. The turbulence causes a skewed, or asymmetric, time signature in the jet noise, they believed.
Sounds result from air pressure in a sound wave. When the ear is impacted by part of a sound wave where the pressure is higher, the eardrum momentarily flexes inward. When a lower pressure part arrives, the eardrum vibrates back out. Sensory apparatus inside the ear translate these pressure fluctuations as sound.
Gee and colleagues from Penn State University, State College, Pa., used a computer to try to simulate the pressure wave-form of an F-18. They sculpted a wave-form with the same distribution of overall pressure and frequency, skewing it as the jet engine's sound wave is shaped. But when they had the computer play back the resulting noise, it was different.
"Even though it had everything we thought it needed to have," Gee said, "it did not crackle."
Gee continued the collaboration after leaving graduate school at Penn State and coming to Utah. The team he led discovered that if a waveform were constructed based not only on the F-18's pressure values but included information for the rate of change of the pressure, the sound was replicated.
"If we generate an artificial waveform having the same distribution of pressure changes, we hear crackle," he wrote in a layperson's version of their scientific paper. "Consequently, the 'snap, crackle, pop' in jet and rocket noise is probably the result of our ears responding to the extremely rapid pressure changes in waveform rather than from the simple asymmetry in pressure."
He told the Deseret Morning News that when there is a near-instantaneous pressure change from lower to higher, "our ear perceives it as a pop."
Some things about the crackle remain unknown, he said. "Is it generated right at the engine, or does the crackle start to form as the (sound) wave travels? It's probably a combination of the two."That's something he would like to work on, to develop better ways of measuring the crackle. The research could result in valuable improvements in controlling jet engine noise. Gee noted, "There's a community impact; there's an impact on people on aircraft carrier decks."