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  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
durations.
      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
unpredictability.
      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
noise.
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
attenuation.

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
objec

tions 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 = , 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  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
                               pressure level


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
chapter.

                            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
m.
                                    
   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
promulgation.

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
                                  aircraft.

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-
engine aircraft.
      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
parabolic shape:
                                   ,
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 pro

vide  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
                                   engines

      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: .

     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.