Noise is produced by pressure pulses in air generated from any vibrating source. The pulsating energy is transmitted through the air and is heard within the audible frequency range (i. e., 20 to 20,000 Hz). The intensity and frequency of pulsation determine the physical limits of human tolerance. In certain conditions, acoustic (i. e., noise) vibrations can affect an aircraft structure. Noise is perceived as environmental pollution.
The intensity of sound energy can be measured by the sound pressure level (SPL); the threshold of hearing value is 20p, Pa. The response of human hearing can be approximated by a logarithmic scale. The advantage of using a logarithmic scale for noise measurement is to compress the SPL range extending to well over a million times. The unit of noise measurement is a decibel, abbreviated to dB, and is based on a logarithmic scale. One “Bel” is a tenfold increase in the SPL; that is, 1 Bel = logi010, 2 Bel = logi0100, and so on. A reading of 0.1 Bel is a dB, which is antilogi00.1 = 1.258 times the increase in the SPL (i. e., intensity). A twofold increase in the SPL is log102 = 0.301 Bel, or 3.01 dB.
Technology required a meaningful scale suitable to human hearing. The units of noise continued to progress in line with technology demands. First was the “A-weighted” scale, expressed in dB(A), that could be read directly from calibrated instruments (i. e., sound meters). Noise is more a matter of human reaction to hearing than just a mechanical measurement of a physical property. Therefore, it was believed that human annoyance is a better measure than mere loudness. This
Jet takeoff (25 m) (eardrum rupture)
Aircraft carrier deck Jet takeoff (100 m)
Live rock music, chain saw (threshold ofpain) Riveting, car hom (1 m)
Jet takeoff (305 m), lawn mower Busy urban street, food blender Dishwasher, factory, freight train (15 m) Vacuum cleaner, freeway traffic (15 m) Conversation in restaurant, office Quiet suburb, conversation at home Library
Quiet rural area Whisper, rustling leaves Breathing
Threshold of hearing
resulted in the “perceived-noise” scale expressed in PNdB, which was labeled as the associated “perceived noise level (PNL),” shown in Figure 15.1 from various origins.
Aircraft in motion presented a special situation with the duration of noise emanating from an approaching aircraft passing overhead and continuing to radiate rearward after passing. Therefore, for aircraft applications, it was necessary to introduce a time-averaged noise – that is, the effective perceived noise level (EPNL), expressed in EPNdB.
In the 1960s, litigation from damages caused by aircraft noise caused the government regulatory agencies to reduce noise and impose EPNdB limits for various aircraft classes. Many airports have a nighttime curfew for noise abatement and control, with additional fees being charged for using the airfield at night. Through research and engineering, significant noise reduction has been achieved despite the increase in engine sizes that produces several times more thrust.
The United States was first to impose noise certification standards for aircraft operating within that country. The U. S. airworthiness requirements on noise are governed by FAR Part 36. An aircraft MTOM of more than 12,500 lb must comply with FAR Part 36. The procedure was immediately followed by the international agency governed by ICAO (see Annexure 16, Volume I). The differences between the two standards are minor, and there has been an attempt to combine the two into one uniform standard. Readers may refer to FAR Part 35 and ICAO Annexure 16 for further details.
Because existing larger aircraft caused the noise problem, the FAA introduced regulations for its abatement in stages; older aircraft required modifications within
Figure 15.2. Noise measurement points at takeoff and landing
a specified period to remain in operation. In 1977, the FAA introduced noise-level standards in three tiers, as follows:
Stage I: Intended for older aircraft already flying and soon to be phased out
(e. g., the B707 and DC8). These are the noisiest aircraft but least penalized because they are soon to be grounded.
Stage II: Intended for recently manufactured aircraft that have a longer life
span (e. g., the B737 and DC9). These aircraft are noisy but must be modified to a quieter standard than Stage I. If they are to continue operating, then further modifications are necessary to bring the noise level to the Stage III standard.
Stage III: Intended for new designs with the quietest standards.
Stage IV: Further increased stringency was applied for new aircraft certification during 2006.
ICAO standards are in Annexure 16, Volume I, in Chapters 2 through 10, with each chapter addressing different aircraft classes. This book is concerned with Chapters 3 and 10, which are basically intended for new aircraft (i. e., first flight of a jet aircraft after October 6, 1977, and a propeller-driven aircraft after November 17, 1988).
To certify an aircraft’s airworthiness, there are three measuring points in an airport vicinity to ensure that the neighborhood is within the specified noise limits. Figure 15.2 shows the distances involved in locating the measuring points, which are as follows:
1. Takeoff reference point: 6,500 m (3.5 nm) from the brake release (i. e., starting) point and at an altitude given in Table 15.1.
2. Approach reference point: 2,000 m (1.08 nm) before the touchdown point, which should be within 300 m of the runway threshold line and maintained at least at a 3-deg glide slope with an aircraft at least at a 120-m altitude.
3. Sideline reference point: 450 m (0.25 nm) from the runway centerline. At the sideline, several measuring points are located along the runway. It is measured on both sides of the runway.
Make linear interpolations for in-between aircraft masses. For takeoff, make linear interpolations for in-between mass.
The arithmetic sum of noise levels at the three noise measuring position is referred to as the “cumulative noise level”; and the difference between this level and the arithmetic sum of the noise limits allowed at each measuring point is referred to as the “cumulative noise level margin.”
The maximum noise requirements in EPNdB from ICAO, Annexure 16, Volume I, Chapter 3, are listed in Table 15.1 and plotted in Figure 15.3.
This is for any number of engines
MTOM (kg) <35,000
EPNdB limit 98
This is for any number of engines (use linear interpolations for in-between masses).
MTOM (kg) <35,000 >400,000
EPNdB limit 94 103
A typical footprint of the noise profile around a runway is shown in Figure 15.4. The engine cutback area is shown with the reestablished rated thrust for an enroute climb. Residential developments should avoid the noise-footprint areas.
Stage IV requirements for new type designs from January 1,2006 are as follows:
• a cumulative margin of 10 EPNDB relative to Stage 3
• a minimum sum of 2 EPNDB at any 2 conditions
• no trades allowed
The airframe also produces a significant amount of noise, especially when an aircraft is in a “dirty” configuration (e. g., flaps, slats, and undercarriage deployed). Figure 15.5 shows the sources of noise emanating from the airframe. The entire wetted surface of an aircraft – more so by the flow interference at the junction of two bodies (e. g., at the wing-body junction) – produces some degree of noise based on the structure of the turbulent flow causing pressure pulses that are audible to the human ear. The noise is aggravated when the undercarriage, flaps, and slats are deployed, creating a considerable vortex flow and unsteady aerodynamics; the fluctuation frequencies appear as noise. In the conceptual design phase, care must be taken to minimize gaps, provide fillets at the two-body junction, make streamlined struts, and so forth. Noise increases as speed increases. Care must be taken to eliminate acoustic fatigue in structures and to design them to be damage-tolerant; material selection is important.
Figure 15.5. Typical sources of noise emanating from an airframe
Figure 15.6. Relative noise distributions from various aircraft and engine sources
Typical noise levels from various sources are shown in Figure 15.6 at both takeoff and landing. Aircraft engines contribute the most noise, which is reduced at landing when the engine power is set low and the jet efflux noise is reduced substantially. There is more noise emanating from the airframe at landing due to higher flap and slat settings, and the aircraft altitude is lower at the measuring point than at the takeoff measuring point. Because the addition of noise level is in a logarithmic scale, the total noise contribution during takeoff and landing is almost at the same level.
The power plant constitutes the nacelle and is the main sources of noise at takeoff when an aircraft is running at maximum power. All of the gas turbine components generate noise: fan blades, compressor blades, combustion chamber walls, and turbine blades. With an increase in the BPR, the noise level decreases because a low exhaust velocity reduces the shearing action with ambient air. The difference in noise between an AB turbojet and a high-BPR turbofan can be as high as 30 to 40 EPNdB. Figure 15.7 shows that in subsonic-flight speed, noise radiation moves ahead of an aircraft.
To reduce noise levels, engine and nacelle designers must address the sources of noise, as shown in Table 15.2. The goal is to minimize radiated and vibrational noise. Candidate areas in engine design are the fan, compressor, and turbine-blade; gaps in rotating components; and, to an extent, the combustion chamber. Engines are bought-out items for aircraft manufacturers, which must make compromises between engine cost and engine performance in selecting what is available on the market. Aircraft and engine designers communicate constantly to make the best choice without compromising safety.
Figure 15.7. In-flight turbofan noise – radiation profile
Table 15.2. Nacelle and turbofan technological challenges to reduce noise
Fan/compressor and turbine Burner
Internal liner – intake
• absorbs fan noise Internal liner – casing/fan duct
• insulates compressor noise
• absorbs burner noise Internal liner – jet pipe
• absorbs turbine noise
• absorbs burner noise
• mixes hot and cold flows
• improves exhaust-flow mixing
Figure 15.8 shows the positions of noise-suppression liners placed in various areas and the jet-pipe-flow mixing arrangements for noise abatement. Exhaust-noise suppression also is achieved by using a fluted duct (which increases the mixing area) at the exit plane. Many types of liners are available on the market and there is room for improvement in liner technology. Primarily, there are two types of liners: reactive and resistive. The reactive liners have different sizes of perforations to react with matched frequency range of noise and absorb. The resistive type of liner is a noise insulator in layers with screens. The most common type of acoustic liner comprises a combination of both types. It has resistive facing sheet covering a honeycomb structure between the insulator screens with cell sizes matched to the frequency range where noise attenuation is requuired. Nacelle certification is the responsibility of the aircraft manufacturer, even when it is subcontracted to a third party, because it is covered by FAR Part 25 requirements, rather than FAR Part 33, which are for the engine.
Propeller-driven aircraft must consider noise emanating (i. e., radiation and reflection) from the propellers. Here, the noise-reflection pattern depends on the direction of propeller rotation, as shown in Figure 15.9. The spread of reflected noise also depends on the propeller position relative to the wing and the fuselage.
Inside the aircraft cabin, noise comes from the ECS and must be maintained at the minimum level. These problems are addressed by specialists. Cabin-interior design considerations are addressed in Phase 2 of a project.
At this stage of study, design considerations for noise reduction do not substantially affect the aircraft external configuration other than using proper filleting at
Figure 15.9. Noise considerations for propeller-driven aircraft
two-body junctions, streamlining the projected structure, minimizing gaps, and so forth. The finalized aircraft configuration – as obtained in Chapter 6 and sized in Chapter 11 – remains unaffected because the aircraft external geometry is assumed to have accounted for these considerations. The choice of materials (e. g., nacelle liners, cabin insulators, and fatigue-resistant material) can affect aircraft mass. Engine – noise abatement is generally the responsibility of engine designers.
The advancement of CFD capabilities in predicting noise has resulted in good judgments for improving design. Substantiation of the CFD results requires testing.
In the near future (i. e., gradually evolving in about two decades), remarkable improvement in noise abatement may be achieved using a multidisciplinary design approach, taking the benefits from various engineering considerations leading to a BWB shape. Cambridge University and the Massachusetts Institute of Technology have undertaken feasibility studies that show a concept configuration in Figure 15.10 for an Airbus 320 class of subsonic-jet commercial transport aircraft. The engineers predict that the aircraft will be 25 dBs quieter than current designs – so quiet as to name it “silent aircraft.” The shaping of the aircraft is not based solely on noise reduction; it is also driven by general aerodynamic considerations (e. g., drag reduction and handling qualities). Noise reduction results from the aircraft body shielding the intake noise, minimizing two-body junctions by blending the wing and the fuselage, and eliminating the empennage. Of course, reduction in the engine noise is a significant part of the exercise. However, to bring the research to a marketable product will take time, but the author believes it will come in many sizes; heavy-lift cargo aircraft are good candidates.