For those who do not want to read this long screed - just put your fingas in your ears as hard as you can and shout out loud "Lah Lah Lah Lah" - be a TeleTubby for the day. Others may read some text supporting the NRAC chart on previous page plus dis udda one....
"Appendix A - Terms of Reference - NRAC Tactical Jet Engine Noise Reduction Study
Objective
The noise on the flight decks of our carriers is 20 to 30 dB higher than any technology we have to protect the hearing of our Sailors and Marines. We are not in compliance with OSHA standards, and to quote the DASN for Safety, “We are creating a hearing loss certainty, not just a risk.” The noise problem cannot be solved by only hearing protection devices. The source of the noise must be reduced in addition to finding better ways to decrease the noise exposure times of our Sailors and Marines. The technology does not exist to achieve the needed decreases in engine noise from tactical aircraft jet engines without significant adverse impacts to performance. This study will investigate current technology for reducing tactical jet engine noise and will make recommendations for actions that can be taken to both reduce jet engine noise in existing engines and to be able to achieve lower noise levels in the next generation of tactical jet aircraft.
Background
Progress is being made in developing improved hearing protection devices to replace the current day cranial helmets that were designed in the 1950’s and are still in use on the flight deck. However, there has been no focused effort to reduce tactical aircraft jet engine noise. In fact noise has never been a design parameter for designing a new tactical aircraft, but rather aircraft such as the JSF/ F-35 have a contract specification to only mitigate the noise. No requirement exists for engine noise staying below any threshold noise level. The needed design tools to make such advances do not exist.
F-35A noise levels have undergone some measurement and appear to be comparable to the dB levels of other current tactical aircraft in Mil and afterburner. However, the noise power, watts per square meter, not just dB, generated by the F-35A is two times greater than that generated by the F/A-18 E/F. All tactical aircraft engines grow in thrust over time, and that equates to even greater noise in the future.
THEN... page 9.... "...Although it is desirable to have a single number to measure noise, near-field noise measurements require more than a single dB metric to fully quantify the acoustic pressure levels generated by an engine or to compare one engine to another.
Overall sound pressure levels, i.e., noise, are normally measured in dB, and are a summation of the sound pressure levels across a spectrum of frequencies. Because the human ear is not equally sensitive to all the frequencies of sound across the spectrum, noise levels at maximum human sensitivity between 2 and 4 kHz are factored more heavily into sound descriptions using a process called frequency weighting. Therefore, the noise levels affecting humans are normally shown in dBA (A-weighted decibels), a frequency weighted average.
There were concerns that the Joint Strike Fighter (JSF) F-135 engine would be noisier than existing engines and that hearing protection might possibly be inadequate for speech intelligibility for flight deck personnel. Accordingly, in 2002 the JSF Program Office funded a study of the noise environment during carrier qualification operations aboard USS John F. Kennedy (CV-67) and USS Abraham Lincoln (CVN-72) and during AV-8B operations aboard USS Nassau (LHA-4). This was the first time since a 1971 study that measurements of the noise during flight deck operations were recorded. Note that many of the conclusions and recommendations of reports generated in 1971 and 2002 are similar to those made in this report.
Noise levels approaching 150 dB are generated by today’s tactical aircraft. This chart represents a graphical representation of the peak jet noise levels (in dB) for several modern, high performance tactical jet aircraft. The noise numbers on the chart represent the maximum sound pressure levels (SPL) in dB measured for each aircraft in both Military and Afterburner (A/B) power settings along a 42 ft line parallel to the aircraft (representing the “foul line” on a modern aircraft carrier).
The data were collected by the Joint Strike Fighter Flight Systems IPT Vibroacoustics Team during the late 1990’s and are documented in the reports which are referenced in Appendix C. Additional data from a more recent (Oct 2008) test of the F-35 AA-1 Aircraft was provided by the JSF Program office as part of a brief to the NRAC Panel, and additional data from a test of the F/A-18E aircraft in 2000 was provided by the F/A-18 Program Office and is documented in a report titled “Effect of Jet Blast Deflector on Exhaust Noise of F-18E” also listed in Appendix C.
While the above data are considered the “best” data available, there are some concerns as to their absolute validity and the ability to compare data from one aircraft to another, because of the lack of standards for collecting such data as described previously.
Tests were a “one-off” event, and no attempt to produce repeatable data was documented. The Panel raises this concern because there have been two instances in which later measurements were made of both the F-35 and the F/A-18E/F, and differences of 4 dB and 2 dB, respectively, were measured. This shows that a single test, while an indicator of noise levels, cannot be construed as the true level. This variation could be caused by (at least) several contributing factors such as: test set up and execution, microphone placement, type, calibration, weather conditions, engine variability, etc.
NASA Glenn has estimated that at best a good consistent engine test may be able to yield +/- 1 dB for 1/3 octave spectra and +/- 0.5 dB for overall sound pressure levels with today’s techniques and technology. Flight test data will have larger error bars due to other influences such as aircraft position uncertainties and weather, which includes wind, humidity and temperature. Some, but not all, reports documented these variable conditions; however none of the data in the reports were corrected to a standard condition.
The selected test site can also induce variability, and not all aircraft were tested at the same location. This discussion is not meant to degrade the excellent work and effort done to collect the data which were provided to the NRAC, but it is a further justification for the Panel to believe that a set of standards for the measurement of near-field jet engine noise is required....
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...page 13 ...Eliminating afterburner during takeoff will also provide a significant noise reduction benefit. Afterburners increase the jet noise levels by 5 to 10 dB above military power....
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...page 25-26 ...Unfortunately, acoustic signatures have not been critical performance parameters in military tactical aircraft system development programs. For future aircraft programs, concern should be paid to acoustic signature effects on the hearing of our Sailors and Marines as well as the environmental affects on the local air base communities. The Navy must rethink how to incorporate lower noise signatures into a full system parameter requirement. This new contracting strategy will allow the prime contractor, in concert with the propulsion system contractor, to initially tradeoff the contributions of the various signature elements with the normal system performance elements (e.g., speed, range, and maneuver) and perform a system level optimization taking all elements into consideration. Without integrating all performance and signatures together, there can not be a system of systems optimization. In order to make significant reductions in aircraft noise, aircraft system contracts need to specify a noise requirement. This can be done by establishing noise as a Key Performance Parameter (KPP) and incentivizing the prime contractor and the propulsion system subcontractor to develop designs which meet this KPP.
In preparation for the next generation tactical aircraft, the Panel believes there should be a KPP for noise. The Navy should initiate a competitive design study to identify the technologies critical to minimizing mid-field and far-field noise for the next generation, high performance tactical aircraft. This design study should include the definition of the multidimensional vehicle design space available and the tradeoff factors between vehicle design characteristics and vehicle performance. In addition, the study should indentify the critical technologies, vehicle configuration and integration features to reduce jet noise and the realistic bounds of vehicle KPPs, including key mission performance and noise. Such a competitive design should be one of the steps in order to define a noise KPP for the next generation tactical aircraft...."
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...Another chart from page 32 below....
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...page 37 ...When noise levels exceed 85 dBA, for a period of greater than eight hours, humans run the risk of permanent hearing loss. Even with state-of-the-art protection providing 47 to 53 dBA of attenuation, one is still at risk in the high noise environments (145 – 150 dB) around jet aircraft. The magnitude and impact of noise transmitted via bone and other media such as fluid is largely unknown. The ear canal, if maximally protected, will reduce the noise by 47 – 53 dB. As the sound intensity increases past 110 dB, noise can be transmitted to the hearing apparatus via other routes – most notably bone conduction. In high noise environments noise is transmitted by bone, so attenuating the noise only in the ear canal will never be sufficient at noise levels above 150 dB. The Navy must anticipate that some fraction of the “at-risk” population of Sailors and Marines could lose their hearing, even when outfitted with protection that occludes 100% of the noise. Navy medical research into antioxidant therapy for brief impact noise in Marine subjects, suggests that there may be an effective “pre-exposure” therapy available that will increase noise-level tolerance. Data also suggests that post exposure therapies can potentially re-grow damaged hair cells. This kind of research needs to be expanded to include chronic noise exposure on the flight deck.
Noise levels below 500 Hz are normally not recorded by either dosimeters or medical audiograms. Although in the research environment audiograms routinely record down to 125 Hz. Various anecdotal reports have noted both the presence and absence of subjective discomfort attributed to “low-frequency” noise. Those who have stood near an F-22 or F-35 at high power levels report uncomfortable sensations and believe their internal organs are moving, such as could be caused by low frequency noise. Other occupational environments — undersea sound and human physical vibration — have produced human injury and disease. Critical organs of the body have harmonic resonances ranging from a few Hz to 400 Hz. For these reasons, greater bio-medical research into the adverse effects of low frequency, air-propagated, sound is needed. As is the case with our hearing conservation research recommendations, we recommend that this research be guided by individual, job-specific, noise level exposure data....
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...page 39-40 ...Since 2003, the Navy has invested approximately $15 million in tactical jet noise reduction research. This research has focused primarily on university basic research and subscale/lab demonstrations. One full-scale demonstration was conducted to assess several technologies, and did lead to the current chevron rapid technology transition effort for introducing chevrons into the F/A-18 E/F F414 engines.
The Air Force investments in engine noise have been solely focused on measurement and modeling, largely for community noise which has been and remains the focus for the Air Force. [b]The JSF Program has invested in numerous acoustic surveys of baseline noise data for the F-135 engine and also the F-35A aircraft. However, these efforts were focused on characterizing the noise level for hearing protection, and providing adequate hearing protection for the aircrew and maintenance personnel – a requirement of the JSF contract.
The JSF Joint Program Office initiated a study (by Pratt & Whitney, General Electric Aviation, and National Aerospace Laboratory – funded by the Netherlands) to investigate reducing the F-35 near-field personnel noise and far-field community noise. This study was a low-detail, high-level assessment of noise impacts. It evaluated and estimated the effectiveness and viability of currently available and emerging “public domain” technologies for reducing the propulsion system’s contribution to the F-35 acoustic footprint...."