The Noise Problem
Effects of Noise
1. Hearing Loss
2. Noise Interference
3. Sleep Disturbance
4. Noise Influence on Health
5. Jet Noise
6. Turbomachinery Noise
Noise Measurement and Rules
7. Noise Effectiveness Forecast (NEF)
8. Effective Perceived Noise Level (EPNL)
9. Noise limits
10. Tupolev 154M Description
11. Noise calculations
1. Take-off Noise Calculation
2. Landing Approach Noise Claculation
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 noise
first became a major problem with the introduction of turbojet-powered
commercial aircraft (Tupolev 104, Boeing 707, Dehavilland Comet) in the
late 1950s. It was recognized at the time that the noise levels produced by
turbojet powered aircraft would be unacceptable to persons living under the
take-off pattern of major airports. Accordingly, much effort was devoted to
developing jet noise suppressors, with some modest success. Take-off noise
restrictions were imposed by some airport managements, and nearly all first-
generation turbojet-powered transports were equipped with jet noise
suppressors 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 landing
approach as well as on take-off. This noise was reduced somewhat by
choosing proper rotor and stator blade numbers and spacing and by using
engines of the single-mixed-jet type.
2 Effects Of Noise
Noise is often defined as unwanted sound. To gain a satisfactory
understanding of the effects of noise, it would be useful to look briefly
at the physical properties of sound.
Sound is the result of pressure changes in a medium, caused by
vibration or turbulence. The amplitude of these pressure changes is stated
in terms of sound level, and the rapidity with which these changes occur is
the sound's frequency. Sound level is measured in decibels (dB), and sound
frequency is stated in terms of cycles per second or Hertz (Hz). Sound
level in decibels is a logarithmic rather than a linear measure of the
change in pressure with respect to a reference pressure level. A small
increase in decibels can represent a large increase in sound energy.
Technically, an increase of 3 dB represents a doubling of sound energy, and
an 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 it
is distributed in time. Continuous sounds have little or no variation in
time, varying sounds have differing maximum levels over a period of time,
intermittent sounds are interspersed with quiet periods, and impulsive
sounds are characterized by relatively high sound levels and very short
The effects of noise are determined mainly by the duration and level
of the noise, but they are also influenced by the frequency. Long-lasting,
high-level sounds are the most damaging to hearing and generally the most
annoying. High-frequency sounds tend to be more hazardous to hearing and
more annoying than low-frequency sounds. The way sounds are distributed in
time is also important, in that intermittent sounds appear to be somewhat
less damaging to hearing than continuous sounds because of the ear's
ability to regenerate during the intervening quiet periods. However,
intermittent and impulsive sounds tend to be more annoying because of their
Noise has a significant impact on the quality of life, and in that
sense, it is a health problem. The definition of health includes total
physical and mental well-being, as well as the absence of disease. Noise is
recognized as a major threat to human well-being.
The effects of noise are seldom catastrophic, and are often only
transitory, but adverse effects can be cumulative with prolonged or
repeated exposure. Although it often causes discomfort and sometimes pain,
noise does not cause ears to bleed and noise-induced hearing loss usually
takes years to develop. Noise-induced hearing loss can indeed impair the
quality of life, through a reduction in the ability to hear important
sounds and to communicate with family and friends. Some of the other
effects of noise, such as sleep disruption, the masking of speech and
television, and the inability to enjoy one's property or leisure time also
impair the quality of life. In addition, noise can interfere with the
teaching and learning process, disrupt the performance of certain tasks,
and increase the incidence of antisocial behavior. There is also some
evidence that it can adversely affect general health and well-being in the
same manner as chronic stress.
2.1 Hearing Loss
Hearing loss is one of the most obvious and easily quantified effects
of excessive exposure to noise. Its progression, however, is insidious, in
that it usually develops slowly over a long period of time, and the
impairment can reach the handicapping stage before an individual is aware
of what has happened.
Prolonged exposure to noise of a certain frequency pattern can cause
either temporary hearing loss, which disappears in a few hours or days, or
permanent loss. The former is called temporary threshold shift, and the
latter is known as permanent threshold shift.
Temporary threshold shift is generally not damaging to human’s ear
unless it is prolonged. People who work in noisy environments commonly are
victims 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 are
subjected to noises of a certain frequency.
There is some disagreement as to the level of noise that should be
allowed for an 8-hour working day. Some researchers and health agencies
insist that 85 dB(A) should be the limit. Industrial noise level
limitations 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 the
effects of noise. Predictions of hearing loss from various levels of
continuous and varying noise have been extensively researched and are no
longer controversial. Some discussion still remains on the extent to which
intermittencies ameliorate the adverse effects on hearing and the exact
nature of dose-response relationships from impulse noise. It appears that
some members of the population are somewhat more susceptible to noise-
induced hearing loss than others, and there is a growing body of evidence
that certain drugs and chemicals can enhance the auditory hazard from
Although the incidence of noise-induced hearing loss from industrial
populations is more extensively documented, there is growing evidence of
hearing 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 recreational
noise, the hazard from these sources needs to be more thoroughly evaluated.
Finally, the recent evidence that hearing protective devices do not perform
in actual use the way laboratory tests would imply, lends support to the
need for reevaluating current methods of assessing hearing protector
2.2 Noise Interference
Noise can mask important sounds and disrupt communication between
individuals in a variety of settings. This process can cause anything from
a slight irritation to a serious safety hazard involving an accident or
even a fatality because of the failure to hear the warning sounds of
imminent danger. Such warning sounds can include the approach of a rapidly
moving motor vehicle, or the sound of malfunctioning machinery. For
example, Aviation Safety states that hundreds of accident reports have many
«say again» exchanges between pilots and controllers, although neither side
reports anything wrong with the radios.
Noise can disrupt face-to-face and telephone conversation, and the
enjoyment of radio and television in the home. It can also disrupt
effective communication between teachers and pupils in schools, and can
cause fatigue and vocal strain in those who need to communicate in spite of
the noise. Interference with communication has proved to be one of the most
important components of noise-related annoyance.
Interference with speech communication and other sounds is one of the
most salient components of noise-induced annoyance. The resulting
disruption can constitute anything from an annoyance to a serious safety
hazard, depending on the circumstance.
Criteria for determining acceptable background levels in rooms have also
been expanded and refined, and progress has been made on the development of
effective acoustic warning signals.
It is now dear that hearing protection devices can interfere with the
perception of speech and warning signals, especially when the listener is
hearing impaired, both talker and listener wear the devices, and when
wearers attempt to locate a signal's source.
Noise can interfere with the educational process, and the result has been
dubbed «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 sleep
disturbance is a critical component of noise-related annoyance. A study
used by EPA in preparing the Levels Document showed that sleep interference
was the most frequently cited activity disrupted by surface vehicle noise
(BBN, 1971). Aircraft none can also cause sleep disruption, especially in
recent years with the escalation of nighttime operations by the air cargo
industry. When sleep disruption becomes chronic, its adverse effects on
health and well-being are well-known.
Noise can cause the sleeper to awaken repeatedly and to report poor
sleep quality the next day, but noise can also produce reactions of which
the individual is unaware. These reactions include changes from heavier to
lighter stages of sleep, reductions in «rapid eye movement» sleep,
increases in body movements during the night, changes in cardiovascular
responses, and mood changes and performance decrements the next day, with
the possibility of more serious effects on health and well-being if it
continues over long periods.
2.4 Noise Influence on Health
Noise has been implicated in the development or exacerbation of a
variety of health problems, ranging from hypertension to psychosis. Some of
these findings are based on carefully controlled laboratory or field
research, but many others are the products of studies that have been
severely criticized by the research community. In either case, obtaining
valid data can be very difficult because of the myriad of intervening
variables that must be controlled, such as age, selection bias, preexisting
health conditions, diet, smoking habits, alcohol consumption, socioeconomic
status, 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 of
reactions 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. In
aircraft engines there are three main sources of unsteadiness: motion of
the blading relative to the observer, which if supersonic can give rise to
propagation of a sequence of weak shocks, leading to the “buzz saw” noise
of 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 on
approach in early turbojets; and turbulence or other fluid instabilities,
which can lead to radiation of sound either through interaction with the
turbomachine blading or other surfaces or from the fluid fluctuations
themselves, as in jet noise.
3.1 Jet Noise
When fluid issues as a jet into a stagnant or more slowly moving
background fluid, the shear between the moving and stationary fluids
results in a fluid-mechanical instability that causes the interface to
break up into vortical structures as indicated in Fig. 3.1. The vortices
travel downstream at a velocity which is between those of the high and low
speed flows, and the characteristics of the noise generated by the jet
depend on whether this propagation velocity is subsonic or supersonic with
respect to the external flow. We consider first the case where it is
subsonic, 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 a
distribution of quadrupoles.
3.2 Turbomachinery Noise
Turbomachinery generates noise by producing time-dependent pressure
fluctuations, which can be thought of in first approximation as dipoles
since they result from fluctuations in force on the blades or from passage
of lifting blades past the observer.
It would appear at first that compressors or fans should not radiate
sound due to blade motion unless the blade tip speed is supersonic, but
even low-speed turbomachines do in fact produce a great deal of noise at
the blade passing frequencies.
4 Noise Measurement and Rules
Human response sets the limits on aircraft engine noise. Although the
logarithmic relationship represented by the scale of decibels is a first
approximation to human perception of noise levels, it is not nearly
quantitative enough for either systems optimization or regulation. Much
effort 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 raises
objections from the neighborhood of a major airport, but the total noise
impact of the airport’s operations, which depends on take-off patterns,
frequencies of operation at different times of the day, population
densities, and a host of less obvious things. There have been proposals to
limit the total noise impact of airports, and in effect legal actions have
done so for the most heavily used ones.
One widely accepted measure of noise impact is the Noise
Effectiveness Forecast (NEF), which is arrived at as follows for any
location 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 = [pic], 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
4.2 Effective Perceived Noise Level (EPNL)
The perceived noisiness of an aircraft flyover depends on the
frequency content, relative to the ear’s response, and on the duration. The
perceived noisiness is measured in NOYs (unit of perceived noisiness) and
is plotted as a function of sound pressure level and frequency for random
noise in Fig. 4.1.
Figure 4.1 Perceived noisiness as a function of frequency and sound
Pure tones (frequencies with pressure levels much higher than that of the
neighboring random noise in the sound spectrum) are judged to be more
annoying than an equal sound pressure in random noise, so a “tone
correction” is added to their perceived noise level. A “duration
correction” represents the idea that the total noise impact depends on the
integral of sound intensity over time for a given event.
The 24 one-third octave bands of sound pressure level (SPL) are
converted to perceived noisiness by means of a noy table.
Figure 4.2 Perceived noise level as a function of NOYs
Conceptually, 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 with
EPNL limits for measuring points which are spoken about in the next
5 Noise Certification
The increasing volume of air traffic resulted in unacceptable noise
exposures near major urban airfields in the late 1960s, leading to a great
public pressure for noise control. This pressure, and advancing technology,
led to ICAO Annex 16, AP-36, Joint Aviation Regulation Part 36 (JAR-36) and
Federal Aviation Rule Part 36 (FAR-36), which set maximum take-off, landing
and “sideline” noise levels for certification of new turbofan-powered
aircraft. It is through the need to satisfy this rule that the noise issue
influences the design and operation of aircraft engines. A little more
general background of the noise problem may be helpful in establishing the
context 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 ICAO
issuing its Annex 16 Part 2, and JAA issuing JAR-36. These rules have since
been revised several times, reflecting both improvements in technology and
continuing pressure to reduce noise. As of this writing, the rules are
enunciated as three progressive stages of noise certification. The noise
limits are stated in terms of measurements at three measuring stations, as
shown in Fig. 5.1: under the approach path 2000 m before touchdown, under
the take-off path 6500 m from the start of the take-off roll, and at the
point of maximum noise along the sides of the runway at a distance of 450
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 stations
depends both on the engines and on the aircraft’s performance, operational
procedures, and loading, since the power settings and the altitude of the
aircraft may vary.
The sideline station is more representative of the intrinsic take-off
noise characteristics of the engine, since the engine is at full throttle
and the station is nearly at a fixed distance from the aircraft. The actual
distance depends on the altitude the aircraft has attained when it produced
maximum noise along the designated measuring line. Since FAR-36 and
international rules set by the International Civil Aviation Organization
(ICAO annex 16, Part 2) which are generally consistent with it have been in
force, 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 into
effect worldwide on 1 April. Most industrialized countries, including all
EU states, enforced the new rules and the vast majority of airliners flying
in those states already meet the more stringent requirements. But some
Eastern European countries are facing a problem, especially Russia. Eighty
percent of its civilian aircraft fall short of the standards, meaning it
will not be able to apply the new rules for domestic flights. Even more
worrisome for Moscow is the fact that Russia could find many of its planes
banned from foreign skies. Enforcement of the new rules could force Russia
to cancel 11,000 flights in 2002, representing some 12 percent of the
country's passenger traffic.
The new rules have been applied only to subsonic transports, because
no new supersonic commercial aircraft have been developed since its
5.1 Noise Limits
As mentioned above, all turbofan-powered transport aircraft must
comply at certification with EPNL limits for the three measuring stations
as shown in Fig. 5.1. The limits depend on the gross weight of the aircraft
at take-off and number of engines, as shown in Fig. 5.2. The rule is the
same for all engine numbers on approach and on the sideline because the
distance from the aircraft to the measuring point is fixed on approach by
the angle of the approach path (normally 3 deg) and on the sideline by the
distance of the measuring station from the runway centerline.
Figure 5.2 Noise limits imposed by ICAO Annex 16 for certification of
On take-off, however, aircraft with fewer engines climb out faster, so they
are higher above the measuring point. Here the “reasonable and economically
practicable” principle comes into dictate that three-engine and two-engine
aircraft have lower noise levels at the take-off noise station than four-
There is some flexibility in the rule, in that the noise levels can
be exceeded by up to 2 EPNdB at any station provided the sum of the
exceedances is not over 3 ENPdB and that the exceedances are completely
offset by reductions at other measuring stations.
6 Noise Level Calculations
17 Tupolev 154M Description
For most airlines in the CIS, the Tupolev Tu-154 is nowadays the
workhorse on domestic and international routes.
Figure 6.1 Tupolev 154M main look
It was produced in two main vesions: The earlier production models
have been designated Tupolev -154, Tupolev -154A, Tupolev -154B, Tupolev
-154B-1 and Tupolev -154B-2, while the later version has been called
Tupolev -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 in
replacing these three relatively diverse aircraft included the ability to
operate from gravel or packed earth airfields, the need to fly at high
altitudes 'above most Soviet Union air traffic, and good field performance.
In meeting these aims the initial Tupolev -154 design featured three
Kuznetsov (now KKBM) NK-8 turbofans, triple bogey main undercarriage units
which retract into wing pods and a rear engine T-tail configuration.
The Tupolev -154's first flight occurred on October 4 1968. Regular
commercial service began in February 1972. Three Kuznetsov powered variants
of the Tupolev -154 were built, the initial Tupolev -154, the improved
Tupolev -154A with more powerful engines and a higher max take-off weight
and 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, quieter
and reliable Solovyev (now Aviadvigatel) turbofans. The Tupolev — 154M2 is
a 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 the
where coefficients А, В, С are different for different aircraft types and
engine modes. For Tupolev-154M the coefficients А, В, С are shown in the
table 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 the
necessary thrust for both of them during approach is almost the same.
Tupolev 154M has more powerful engines and it can carry more payload. Its
maximum landing weight is 2 tons greater than that one of 154. Noise
parameters are different for these aircraft (table 6.2), and the calculated
noise levels slightly differ as well.
7 Noise Suppression
7.1 Suppression of Jet Noise
Methods for suppressing jet noise have exploited the characteristics
of the jet itself and those of the human observer. For a given total noise
power, the human impact is less if the frequency is very high, as the ear
is less sensitive at high frequencies. A shift to high frequency can be
achieved by replacing one large nozzle with many small ones. This was one
basis for the early turbojet engine suppressors. Reduction of the jet
velocity can have a powerful effect since P is proportional to the jet
velocity raised to a power varying from 8 to 3, depending on the magnitude
of uc. The multiple small nozzles reduced the mean jet velocity somewhat by
promoting entrainment of the surrounding air into the jet. Some attempts
have been made to augment this effect by enclosing the multinozzle in a
shroud, so that the ambient air is drawn into the shroud.
Certainly the most effective of jet noise suppressors has been the
turbofan engine, which in effect distributes the power of the exhaust jet
over 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 noise
reduction. Among these factors are loss of thrust, addition of weight, and
increased fuel consumption.
A number of noise-suppression schemes have been studied, mainly for
turbofan 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 reduce
the overall noise level from that which would be generated if the hot outer
jets are subsonic with respect to the outer hot gas. This idea can be
implemented either with a duct burner on a conventional turbofan or with a
nozzle that interchanges the core and duct flows, carrying the latter to
the inside and the former to the outside. In the mixer-ejector nozzle, the
idea is to reduce the mean jet velocity by ingesting additional airflow
through a combination of the ejector nozzles and the chute-type mixer.
Fairly high mass flow ratios can be attained with such arrangements, at the
expense 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 subsonic
flight than at the supersonic cruise condition. This can be achieved to
some degree with multi-spool engines by varying the speed of some of the
spools to change their mass flow, and at the same time manipulating
throttle areas. Another approach is to use a tandem-parallel compressor
arrangement, where two compressors operate in parallel at take-off and
subsonically, and in series at a supersonic conditions.
7.1.1 Duct Linings
It is self evident that the most desirable way to reduce engine noise
would be to eliminate noise generation by changing the engine design. The
current state of the art, however, will not provide levels low enough to
satisfy expected requirements; thus, it is necessary to attenuate the noise
that 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 contribution
to perceived noise.
Figure 7.1 Schematic illustration of noise sources from turbofan
Figure 7.2. shows a typical farfield SPL noise spectrum generated by a
turbofan engine at a landing-approach power setting. Below 800 Hz, the
spectrum is controlled by noise from the primary jet exhaust. The spectrum
between 800 and 10000 Hz contains several discrete frequency components in
particular that need to be attenuated by the linings in the inlet and the
fan 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 at
the characteristic discrete frequencies associated with the fan blade
passage frequency and its associated harmonics. Noise reductions at these
frequencies would alleviate the undesirable fan whine and would reduce the
perceived noise levels.
A promising approach to the problem has been the development of a
tuned-absorber noise-suppression system that can be incorporated into the
inlet and exhaust ducts of turbofan engines. An acoustical system of this
type requires that the internal aerodynamic surfaces of the ducts be
replaced by sheets of porous materials, which are backed by acoustical
cavities. Simply, these systems function as a series of dead-end
labyrinths, which are designed to trap sound waves of a specific
wavelength. The frequencies for which these absorbers are tuned is a
function of the porosity of flow resistance of the porous facing sheets and
of the depth or volume of the acoustical cavities. The cavity is divided
into compartments by means of an open cellular structure, such as honeycomb
cells, to provide an essentially locally reacting impedance (Fig. 7.3).
This is done to provide an acoustic impedance almost independent of the
angle of incidence of the sound waves impinging on the lining.
The perforated-plate-and-honeycomb combination is similar to an array
of Helmholtz resonators; the pressure in the cavity acts as a spring upon
which the flow through the orifice oscillates in response to pressure
fluctuations 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 tuned
resonator effective over a narrow frequency range when used in an
environment 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 is
high, or by the addition of a fine wire screen that provides the acoustic
resistance needed to dissipate acoustic energy in low particle-velocity or
sound-pressure environments. The addition of the wire screen does, however,
complicate manufacture and adds weight to such an extent that other
concepts are usually more attractive.
Figure 7.3 Acoustical lining structure.
Although the resistive-resonator lining is a frequency-tuned device
absorbing sound in a selected frequency range, a suitable combination of
material characteristics and lining geometry will yield substantial
attenuation over a frequency range wide enough to encompass the discrete
components and the major harmonics of most fan noise.
7.1.2 Duct Lining Calculation
First 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 7.1
Next we determine the second fan blade passage harmonic frequency, which is
two times greater than the first one: [pic].
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
| |2nd harmonic freq., Hz |10772,5
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.