Contents The Noise Problem Effects of Noise 1. Hearing Loss 2. Noise Interference 3. Sleep Disturbance 4. Noise Influence on Health Noise Sources 5. Jet Noise 6. Turbomachinery Noise Noise Measurement and Rules 7. Noise Effectiveness Forecast (NEF) 8. Effective Perceived Noise Level (EPNL) Noise Certification 9. Noise limits Calculations 10. Tupolev 154M Description 11. Noise calculations 1. Take-off Noise Calculation 2. Landing Approach Noise Claculation Noise Suppression 12. Jet Noise Suppression 13. Duct Linings 1. Duct Lining Calculation 1 The Noise Problem Though long of concern to neighbors of major airports, aircraft noisefirst became a major problem with the introduction of turbojet-poweredcommercial aircraft (Tupolev 104, Boeing 707, Dehavilland Comet) in thelate 1950s. It was recognized at the time that the noise levels produced byturbojet powered aircraft would be unacceptable to persons living under thetake-off pattern of major airports. Accordingly, much effort was devoted todeveloping jet noise suppressors, with some modest success. Take-off noiserestrictions were imposed by some airport managements, and nearly all first-generation turbojet-powered transports were equipped with jet noisesuppressors at a significant cost in weight, thrust, and fuel consumption. The introduction of the turbofan engine, with its lower jet velocity,temporarily alleviated the jet noise problem but increased the high-frequency turbomachinery noise, which became a severe problem on landingapproach as well as on take-off. This noise was reduced somewhat bychoosing proper rotor and stator blade numbers and spacing and by usingengines of the single-mixed-jet type. 2 Effects Of Noise Noise is often defined as unwanted sound. To gain a satisfactoryunderstanding of the effects of noise, it would be useful to look brieflyat the physical properties of sound. Sound is the result of pressure changes in a medium, caused byvibration or turbulence. The amplitude of these pressure changes is statedin terms of sound level, and the rapidity with which these changes occur isthe sound's frequency. Sound level is measured in decibels (dB), and soundfrequency is stated in terms of cycles per second or Hertz (Hz). Soundlevel in decibels is a logarithmic rather than a linear measure of thechange in pressure with respect to a reference pressure level. A smallincrease in decibels can represent a large increase in sound energy.Technically, an increase of 3 dB represents a doubling of sound energy, andan increase of 10 dB represents a tenfold increase. The ear, however,perceives a 10-dB increase as doubling of loudness. Another important aspect is the duration of the sound, and the way itis distributed in time. Continuous sounds have little or no variation intime, varying sounds have differing maximum levels over a period of time,intermittent sounds are interspersed with quiet periods, and impulsivesounds are characterized by relatively high sound levels and very shortdurations. The effects of noise are determined mainly by the duration and levelof the noise, but they are also influenced by the frequency. Long-lasting,high-level sounds are the most damaging to hearing and generally the mostannoying. High-frequency sounds tend to be more hazardous to hearing andmore annoying than low-frequency sounds. The way sounds are distributed intime is also important, in that intermittent sounds appear to be somewhatless damaging to hearing than continuous sounds because of the ear'sability to regenerate during the intervening quiet periods. However,intermittent and impulsive sounds tend to be more annoying because of theirunpredictability. Noise has a significant impact on the quality of life, and in thatsense, it is a health problem. The definition of health includes totalphysical and mental well-being, as well as the absence of disease. Noise isrecognized as a major threat to human well-being. The effects of noise are seldom catastrophic, and are often onlytransitory, but adverse effects can be cumulative with prolonged orrepeated exposure. Although it often causes discomfort and sometimes pain,noise does not cause ears to bleed and noise-induced hearing loss usuallytakes years to develop. Noise-induced hearing loss can indeed impair thequality of life, through a reduction in the ability to hear importantsounds and to communicate with family and friends. Some of the othereffects of noise, such as sleep disruption, the masking of speech andtelevision, and the inability to enjoy one's property or leisure time alsoimpair the quality of life. In addition, noise can interfere with theteaching and learning process, disrupt the performance of certain tasks,and increase the incidence of antisocial behavior. There is also someevidence that it can adversely affect general health and well-being in thesame manner as chronic stress.2.1 Hearing Loss Hearing loss is one of the most obvious and easily quantified effectsof excessive exposure to noise. Its progression, however, is insidious, inthat it usually develops slowly over a long period of time, and theimpairment can reach the handicapping stage before an individual is awareof what has happened. Prolonged exposure to noise of a certain frequency pattern can causeeither temporary hearing loss, which disappears in a few hours or days, orpermanent loss. The former is called temporary threshold shift, and thelatter is known as permanent threshold shift. Temporary threshold shift is generally not damaging to human’s earunless it is prolonged. People who work in noisy environments commonly arevictims of temporary threshold shift. Figure 2.1 Temporary threshold shift for rock band performers. Repeated noise over a long time leads to permanent threshold shift.This is especially true in industrial applications where people aresubjected to noises of a certain frequency. There is some disagreement as to the level of noise that should beallowed for an 8-hour working day. Some researchers and health agenciesinsist that 85 dB(A) should be the limit. Industrial noise levellimitations are shown in the Table 2.1. Table 2.1 Maximum Permissible Industrial Noise Levels By OSHA (Occupational Safety and Health Act)|Sound Level, dB(A) |Maximum Duration || |During Any || |Working Day || |(hr) ||90 |8 ||92 |6 ||95 |4 ||100 |2 ||105 |1 ||110 |Ѕ ||115 |ј | Noise-induced hearing loss is probably the most well-defined of theeffects of noise. Predictions of hearing loss from various levels ofcontinuous and varying noise have been extensively researched and are nolonger controversial. Some discussion still remains on the extent to whichintermittencies ameliorate the adverse effects on hearing and the exactnature of dose-response relationships from impulse noise. It appears thatsome members of the population are somewhat more susceptible to noise-induced hearing loss than others, and there is a growing body of evidencethat certain drugs and chemicals can enhance the auditory hazard fromnoise.Although the incidence of noise-induced hearing loss from industrialpopulations is more extensively documented, there is growing evidence ofhearing loss from leisure time activities, especially from sport shooting,but also from loud music, noisy toys, and other manifestations of our«civilized» society. Because of the increase in exposure to recreationalnoise, the hazard from these sources needs to be more thoroughly evaluated.Finally, the recent evidence that hearing protective devices do not performin actual use the way laboratory tests would imply, lends support to theneed for reevaluating current methods of assessing hearing protectorattenuation.2.2 Noise Interference Noise can mask important sounds and disrupt communication betweenindividuals in a variety of settings. This process can cause anything froma slight irritation to a serious safety hazard involving an accident oreven a fatality because of the failure to hear the warning sounds ofimminent danger. Such warning sounds can include the approach of a rapidlymoving motor vehicle, or the sound of malfunctioning machinery. Forexample, Aviation Safety states that hundreds of accident reports have many«say again» exchanges between pilots and controllers, although neither sidereports anything wrong with the radios. Noise can disrupt face-to-face and telephone conversation, and theenjoyment of radio and television in the home. It can also disrupteffective communication between teachers and pupils in schools, and cancause fatigue and vocal strain in those who need to communicate in spite ofthe noise. Interference with communication has proved to be one of the mostimportant components of noise-related annoyance. Interference with speech communication and other sounds is one of themost salient components of noise-induced annoyance. The resultingdisruption can constitute anything from an annoyance to a serious safetyhazard, depending on the circumstance.Criteria for determining acceptable background levels in rooms have alsobeen expanded and refined, and progress has been made on the development ofeffective acoustic warning signals.It is now dear that hearing protection devices can interfere with theperception of speech and warning signals, especially when the listener ishearing impaired, both talker and listener wear the devices, and whenwearers attempt to locate a signal's source.Noise can interfere with the educational process, and the result has beendubbed «jet-pause teaching» around some of the nation's noisier airports,but railroad and traffic noise can also produce scholastic decrements.2.3 Sleep Disturbance Noise is one of the most common forms of sleep disturbance, and sleepdisturbance is a critical component of noise-related annoyance. A studyused by EPA in preparing the Levels Document showed that sleep interferencewas the most frequently cited activity disrupted by surface vehicle noise(BBN, 1971). Aircraft none can also cause sleep disruption, especially inrecent years with the escalation of nighttime operations by the air cargoindustry. When sleep disruption becomes chronic, its adverse effects onhealth and well-being are well-known. Noise can cause the sleeper to awaken repeatedly and to report poorsleep quality the next day, but noise can also produce reactions of whichthe individual is unaware. These reactions include changes from heavier tolighter stages of sleep, reductions in «rapid eye movement» sleep,increases in body movements during the night, changes in cardiovascularresponses, and mood changes and performance decrements the next day, withthe possibility of more serious effects on health and well-being if itcontinues over long periods.2.4 Noise Influence on Health Noise has been implicated in the development or exacerbation of avariety of health problems, ranging from hypertension to psychosis. Some ofthese findings are based on carefully controlled laboratory or fieldresearch, but many others are the products of studies that have beenseverely criticized by the research community. In either case, obtainingvalid data can be very difficult because of the myriad of interveningvariables that must be controlled, such as age, selection bias, preexistinghealth conditions, diet, smoking habits, alcohol consumption, socioeconomicstatus, exposure to other agents, and environmental and social stressors.Additional difficulties lie in the interpretation of the findings,especially those involving acute effects. Loud sounds can cause an arousal response in which a series ofreactions occur in the body. Adrenalin is released into the bloodstream;heart rate, blood pressure, and respiration tend to increase;gastrointestinal motility is inhibited; peripheral blood vessels constrict;and muscles tense. Even though noise may have no relationship to danger,the body will respond automatically to noise as a warning signal. 3 Noise Sources All noise emanates from unsteadiness – time dependence in the flow. Inaircraft engines there are three main sources of unsteadiness: motion ofthe blading relative to the observer, which if supersonic can give rise topropagation of a sequence of weak shocks, leading to the “buzz saw” noiseof high-bypass turbofans; motion of one set of blades relative to another,leading to a pure-tome sound (like that from siren) which was dominant onapproach in early turbojets; and turbulence or other fluid instabilities,which can lead to radiation of sound either through interaction with theturbomachine blading or other surfaces or from the fluid fluctuationsthemselves, as in jet noise.3.1 Jet Noise When fluid issues as a jet into a stagnant or more slowly movingbackground fluid, the shear between the moving and stationary fluidsresults in a fluid-mechanical instability that causes the interface tobreak up into vortical structures as indicated in Fig. 3.1. The vorticestravel downstream at a velocity which is between those of the high and lowspeed flows, and the characteristics of the noise generated by the jetdepend on whether this propagation velocity is subsonic or supersonic withrespect to the external flow. We consider first the case where it issubsonic, as is certainly the case for subsonic jets. Figure 3.1 A subsonic jet mixing with ambient air, showing the mixing layer followed by the fully developed jet. For the subsonic jets the turbulence in the jet can be viewed as adistribution of quadrupoles.3.2 Turbomachinery Noise Turbomachinery generates noise by producing time-dependent pressurefluctuations, which can be thought of in first approximation as dipolessince they result from fluctuations in force on the blades or from passageof lifting blades past the observer. It would appear at first that compressors or fans should not radiatesound due to blade motion unless the blade tip speed is supersonic, buteven low-speed turbomachines do in fact produce a great deal of noise atthe blade passing frequencies. 4 Noise Measurement and Rules Human response sets the limits on aircraft engine noise. Although thelogarithmic relationship represented by the scale of decibels is a firstapproximation to human perception of noise levels, it is not nearlyquantitative enough for either systems optimization or regulation. Mucheffort has gone into the development of quantitative indices of noise.4.1 Noise Effectiveness Forecast (NEF) It is not the noise output of an aircraft per se that raisesobjec--PAGE_BREAK--tions from the neighborhood of a major airport, but the total noiseimpact of the airport’s operations, which depends on take-off patterns,frequencies of operation at different times of the day, populationdensities, and a host of less obvious things. There have been proposals tolimit the total noise impact of airports, and in effect legal actions havedone so for the most heavily used ones. One widely accepted measure of noise impact is the NoiseEffectiveness Forecast (NEF), which is arrived at as follows for anylocation near an airport: 1. For each event, compute the Effective Perceived Noise Level (EPNL) by the methods of ICAO Annex 16, as described below. 2. For events occurring between 10 PM and 7 AM, add 10 to the EPNdB. 3. Then NEF =, where the sum is taken over all events in a 24-hour period. A little ciphering will show that this last calculation is equivalent to adding the products of sound intensity times time for all events, then taking the dB equivalent of this. The subtractor 82 is arbitrary.4.2 Effective Perceived Noise Level (EPNL) The perceived noisiness of an aircraft flyover depends on thefrequency content, relative to the ear’s response, and on the duration. Theperceived noisiness is measured in NOYs (unit of perceived noisiness) andis plotted as a function of sound pressure level and frequency for randomnoise in Fig. 4.1. Figure 4.1 Perceived noisiness as a function of frequency and sound pressure levelPure tones (frequencies with pressure levels much higher than that of theneighboring random noise in the sound spectrum) are judged to be moreannoying than an equal sound pressure in random noise, so a “tonecorrection” is added to their perceived noise level. A “durationcorrection” represents the idea that the total noise impact depends on theintegral of sound intensity over time for a given event. The 24 one-third octave bands of sound pressure level (SPL) areconverted to perceived noisiness by means of a noy table. Figure 4.2 Perceived noise level as a function of NOYsConceptually, the calculation of EPNL involves the following steps. 1. Determine the NOY level for each band and sum them by the relation, where k denotes an interval in time, i denotes the several frequency bande, and n(k) is the NOY level of the noisiest band. This reflects the “masking” of lesser bands by the noisiest. 2. The total PNL is then PNL(k) = 40 + 33.3 log10N(k). 3. Apply a tone correction c(k) by identifying the pure tones and adding to PNL an amount ranging from 0 to 6.6 dB, depending on the frequency of the tone and its amplitude relative to neighboring bands. 4. Apply a duration correction according to EPNL = PNLTM + D, where PNLTM is the maximum PNL for any of the time intervals. Here, where (t = 0.5 sec, T = 10 sec, and d is the time over which PNLT exceeds PNLTM – 10 dB. This amounts to integrating the sound pressure level over the time during which it exceeds its peak value minus 10 dB, then converting the result to decibels.All turbofan-powered transport aircraft must comply at certification withEPNL limits for measuring points which are spoken about in the nextchapter. 5 Noise Certification The increasing volume of air traffic resulted in unacceptable noiseexposures near major urban airfields in the late 1960s, leading to a greatpublic pressure for noise control. This pressure, and advancing technology,led to ICAO Annex 16, AP-36, Joint Aviation Regulation Part 36 (JAR-36) andFederal Aviation Rule Part 36 (FAR-36), which set maximum take-off, landingand “sideline” noise levels for certification of new turbofan-poweredaircraft. It is through the need to satisfy this rule that the noise issueinfluences the design and operation of aircraft engines. A little moregeneral background of the noise problem may be helpful in establishing thecontext of engine noise control. The FAA issued FAR-36 (which establishes the limits on take-off,approach, and sideline noise for individual aircraft), followed by ICAOissuing its Annex 16 Part 2, and JAA issuing JAR-36. These rules have sincebeen revised several times, reflecting both improvements in technology andcontinuing pressure to reduce noise. As of this writing, the rules areenunciated as three progressive stages of noise certification. The noiselimits are stated in terms of measurements at three measuring stations, asshown in Fig. 5.1: under the approach path 2000 m before touchdown, underthe take-off path 6500 m from the start of the take-off roll, and at thepoint of maximum noise along the sides of the runway at a distance of 450m. Figure 5.1 Schematic of airport runway showing approach, take-off, and sideline noise measurement stations. The noise of any given aircraft at the approach and take-off stationsdepends both on the engines and on the aircraft’s performance, operationalprocedures, and loading, since the power settings and the altitude of theaircraft may vary. The sideline station is more representative of the intrinsic take-offnoise characteristics of the engine, since the engine is at full throttleand the station is nearly at a fixed distance from the aircraft. The actualdistance depends on the altitude the aircraft has attained when it producedmaximum noise along the designated measuring line. Since FAR-36 andinternational rules set by the International Civil Aviation Organization(ICAO annex 16, Part 2) which are generally consistent with it have been inforce, airport noise has been a major design criterion for civil aircraft. Stricter noise pollution standards for commercial aircraft,established by the International Civil Aviation Organization, came intoeffect worldwide on 1 April. Most industrialized countries, including allEU states, enforced the new rules and the vast majority of airliners flyingin those states already meet the more stringent requirements. But someEastern European countries are facing a problem, especially Russia. Eightypercent of its civilian aircraft fall short of the standards, meaning itwill not be able to apply the new rules for domestic flights. Even moreworrisome for Moscow is the fact that Russia could find many of its planesbanned from foreign skies. Enforcement of the new rules could force Russiato cancel 11,000 flights in 2002, representing some 12 percent of thecountry's passenger traffic. The new rules have been applied only to subsonic transports, becauseno new supersonic commercial aircraft have been developed since itspromulgation.5.1 Noise Limits As mentioned above, all turbofan-powered transport aircraft mustcomply at certification with EPNL limits for the three measuring stationsas shown in Fig. 5.1. The limits depend on the gross weight of the aircraftat take-off and number of engines, as shown in Fig. 5.2. The rule is thesame for all engine numbers on approach and on the sideline because thedistance from the aircraft to the measuring point is fixed on approach bythe angle of the approach path (normally 3 deg) and on the sideline by thedistance of the measuring station from the runway centerline. Figure 5.2 Noise limits imposed by ICAO Annex 16 for certification of aircraft.On take-off, however, aircraft with fewer engines climb out faster, so theyare higher above the measuring point. Here the “reasonable and economicallypracticable” principle comes into dictate that three-engine and two-engineaircraft have lower noise levels at the take-off noise station than four-engine aircraft. There is some flexibility in the rule, in that the noise levels canbe exceeded by up to 2 EPNdB at any station provided the sum of theexceedances is not over 3 ENPdB and that the exceedances are completelyoffset by reductions at other measuring stations. 6 Noise Level Calculations17 Tupolev 154M Description For most airlines in the CIS, the Tupolev Tu-154 is nowadays theworkhorse on domestic and international routes. Figure 6.1 Tupolev 154M main look It was produced in two main vesions: The earlier production modelshave been designated Tupolev -154, Tupolev -154A, Tupolev -154B, Tupolev-154B-1 and Tupolev -154B-2, while the later version has been calledTupolev -154M. Overall, close to 1'000 Tupolev -154s were built up to day,of which a large portion is still operated. Table 6.1 Tupolev 154M main characteristics|Role | |Medium range passenger aircraft ||Status | |Produced until circa 1996, in wide || | |spread service ||NATO Codename | |Careless ||First Flight | |October 3, 1968 ||First Service | |1984 ||Engines | |3 Soloviev D-30KU (104 kN each) ||Length | |47.9 m ||Wingspan | |37.5 m ||Range | |3'900 km ||Cruising Speed | |900 km/h ||Payload Capacity | |156-180 passengers (5450 kg) ||Maximum Take-off | |100'000 kg ||Weight | | | The Tu-154 was developed to replace the turbojet powered Tupolev Tu-104, plus the Antonov — 10 and Ilyushin — 18 turboprops. Design criteria inreplacing these three relatively diverse aircraft included the ability tooperate from gravel or packed earth airfields, the need to fly at highaltitudes 'above most Soviet Union air traffic, and good field performance.In meeting these aims the initial Tupolev -154 design featured threeKuznetsov (now KKBM) NK-8 turbofans, triple bogey main undercarriage unitswhich retract into wing pods and a rear engine T-tail configuration. The Tupolev -154's first flight occurred on October 4 1968. Regularcommercial service began in February 1972. Three Kuznetsov powered variantsof the Tupolev -154 were built, the initial Tupolev -154, the improvedTupolev -154A with more powerful engines and a higher max take-off weightand the Tupolev -154B with a further increased max take-off weight. Tupolev-154S is a freighter version of the Tupolev -154B. Current production is of the Tupolev -154M, which first flew in 1982.The major change introduced on the M was the far more economical, quieterand reliable Solovyev (now Aviadvigatel) turbofans. The Tupolev — 154M2 isa proposed twin variant powered by two Perm PS90A turbofans.6.2 Noise Calculaions Noise level at control points is calculated using the Noise-Power-Distance (NPD) relationship. In practice NPD-relationship is used in theparabolic shape:,where coefficients А, В, С are different for different aircraft types andengine modes. For Tupolev-154M the coefficients А, В, С are shown in thetable 6.2 in respect to Tupolev-154. Table 6.2 Noise-Power-Distance coefficients of similar aircraft.| |Tupolev-154 |Tupolev-154M ||Weight, kg |80000 |76000 |72000 |68000 |68000 ||Vapp, m/s |74,8 |72,91 |70,964 |68,965 |66,91 ||Thrust, kg |8445,63 |8024,67 |7601,88 |7179,66 |6758,58 ||LA, dBA |96,74 |96,05 |95,35 |94,66 |93,97 ||EPNL, EPNdB |112,17 |111,32 |110,48 |109,64 |108,79 ||?LA, dBA |0 |0,69 |0,7 |0,69 |0,69 ||?EPNL, EPNdB |0 |0,85 |0,84 |0,84 |0,85 ||SQRT (Wing |21,082 |20,548 |20 |19,437 |18,856 ||Load) | | | | | ||Thrust To |0,10557 |0,105588 |0,105582 |0,105583 |0,105603 ||Weight rt. | | | | | | Tupolev 154M has the same aerodynamics as Tupolev 154, thus thenecessary thrust for both of them during approach is almost the same.Tupolev 154M has more powerful engines and it can carry more payload. Itsmaximum landing weight is 2 tons greater than that one of 154. Noiseparameters are different for these aircraft (table 6.2), and the calculatednoise levels slightly differ as well. 7 Noise Suppression7.1 Suppression of Jet Noise Methods for suppressing jet noise have exploited the characteristicsof the jet itself and those of the human observer. For a given total noisepower, the human impact is less if the frequency is very high, as the earis less sensitive at high frequencies. A shift to high frequency can beachieved by replacing one large nozzle with many small ones. This was onebasis for the early turbojet engine suppressors. Reduction of the jetvelocity can have a powerful effect since P is proportional to the jetvelocity raised to a power varying from 8 to 3, depending on the magnitudeof uc. The multiple small nozzles reduced the mean jet velocity somewhat bypromoting entrainment of the surrounding air into the jet. Some attemptshave been made to augment this effect by enclosing the multinozzle in ashroud, so that the ambient air is drawn into the shroud. Certainly the most effective of jet noise suppressors has been theturbofan engine, which in effect distributes the power of the exhaust jetover a larger airflow, thus reducing the mean jet velocity. In judging the overall usefulness of any jet noise reduction system,several factors must be considered in addition to the amount of noisereduction. Among these factors are loss of thrust, addition of weight, andincreased fuel consumption. A number of noise-suppression schemes have been studied, mainly forturbofan engines of one sort or another. These include inverted-temperature-profile nozzles, in which a hot outer flow surrounds a cooler core flow,and mixer-ejector nozzles. In the first of these, the effect is to reducethe overall noise level from that which would be generated if the hot outerjets are subsonic with respect to the outer hot gas. This idea can beimplemented either with a duct burner on a conventional turbofan or with anozzle that interchanges the core and duct flows, carrying the latter tothe inside and the former to the outside. In the mixer-ejector nozzle, theidea is to reduce the mean jet velocity by ingesting additional airflowthrough a combination of the ejector nozzles and the chute-type mixer.Fairly high mass flow ratios can be attained with such arrangements, at theexpense of considerable weight. The most promising solution, however, is some form of “variable cycle”engine that operates with a higher bypass ratio on take-off and in subsonicflight than at the supersonic cruise condition. This can be achieved tosome degree with multi-spool engines by varying the speed of some of thespools to change their mass flow, and at the same time manipulatingthrottle areas. Another approach is to use a tandem-parallel compressorarrangement, where two compressors operate in parallel at take-off andsubsonically, and in series at a supersonic conditions.7.1.1 Duct Linings It is self evident that the most desirable way to reduce engine noisewould be to eliminate noise generation by changing the engine design. Thecurrent state of the art, however, will not pro продолжение
--PAGE_BREAK--vide levels low enough tosatisfy expected requirements; thus, it is necessary to attenuate the noisethat is generated. Fan noise radiated from the engine inlet and fan discharge (Fig. 7.1)of current fan jet airplanes during landing makes the largest contributionto perceived noise. Figure 7.1 Schematic illustration of noise sources from turbofan engines Figure 7.2. shows a typical farfield SPL noise spectrum generated by aturbofan engine at a landing-approach power setting. Below 800 Hz, thespectrum is controlled by noise from the primary jet exhaust. The spectrumbetween 800 and 10000 Hz contains several discrete frequency components inparticular that need to be attenuated by the linings in the inlet and thefan duct before they are radiated to the farfield. Figure 7.2 Engine-noise spectrum The objective in applying acoustic treatment is to reduce the SPL atthe characteristic discrete frequencies associated with the fan bladepassage frequency and its associated harmonics. Noise reductions at thesefrequencies would alleviate the undesirable fan whine and would reduce theperceived noise levels. A promising approach to the problem has been the development of atuned-absorber noise-suppression system that can be incorporated into theinlet and exhaust ducts of turbofan engines. An acoustical system of thistype requires that the internal aerodynamic surfaces of the ducts bereplaced by sheets of porous materials, which are backed by acousticalcavities. Simply, these systems function as a series of dead-endlabyrinths, which are designed to trap sound waves of a specificwavelength. The frequencies for which these absorbers are tuned is afunction of the porosity of flow resistance of the porous facing sheets andof the depth or volume of the acoustical cavities. The cavity is dividedinto compartments by means of an open cellular structure, such as honeycombcells, to provide an essentially locally reacting impedance (Fig. 7.3).This is done to provide an acoustic impedance almost independent of theangle of incidence of the sound waves impinging on the lining. The perforated-plate-and-honeycomb combination is similar to an arrayof Helmholtz resonators; the pressure in the cavity acts as a spring uponwhich the flow through the orifice oscillates in response to pressurefluctuations outside the orifice. Figure 7.2 Schematic of acoustic damping cavities in an angine duct. The size of the resonators is exaggerated relative to the duct diameter.The attenuation spectrum of this lining is that of a sharply tunedresonator effective over a narrow frequency range when used in anenvironment with low airflow velocity or low SPL. This concept, however,can also provide a broader bandwidth of attenuation in a very high noise-level environment where the particle velocity through the perforations ishigh, or by the addition of a fine wire screen that provides the acousticresistance needed to dissipate acoustic energy in low particle-velocity orsound-pressure environments. The addition of the wire screen does, however,complicate manufacture and adds weight to such an extent that otherconcepts are usually more attractive. Figure 7.3 Acoustical lining structure. Although the resistive-resonator lining is a frequency-tuned deviceabsorbing sound in a selected frequency range, a suitable combination ofmaterial characteristics and lining geometry will yield substantialattenuation over a frequency range wide enough to encompass the discretecomponents and the major harmonics of most fan noise.7.1.2 Duct Lining CalculationFirst we have to determine the blade passage frequency:,where z is number of blades, n is RPM.Blade passage frequencies for different engine modes are given in table 7.1Next we determine the second fan blade passage harmonic frequency, which istwo times greater than the first one:. Table 7.1 Fan blade passage frequencies for different engine modes. |Take-off |Nominal |88%Nom |70%Nom |60%Nom |53%Nom |Idle | |RPM |10425 |10055 |9878 |9513 |9315 |8837 |4000 | |1st harmonic freq., Hz |5386,25 |5195,083 |5103,633 |4915,05 |4812,75 |4565,783 |2066,667 | |2nd harmonic freq., Hz |10772,5 |10390,17 |10207,27 |9830,1 |9625,5 |9131,567 |4133,333 | |Using experimental data, we determine lining and cell geometry:For the first harmonic, parameters will be:. Distance between linings 28.5 cm;. Lining length 45 cm;. Lining depth 2.5 cm;. Cell length 2 cm…For the second harmonic, parameters will be the following:. Distance between linings 4.5 cm;. Lining length 5 cm;. Lining depth 2.5 cm;. Cell length 0.4 cm.Figure 7.4 shows the placement of the lining in engine nacelle. Figure 7.4 Lining placement in the nacelle.