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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.

                                    [pic]

       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.

                                    [pic]
 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
      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.

                                    [pic]

     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.

                                    [pic]
           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
                                   [pic],
      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
                                   [pic],
      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 

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