EXCERPTS FROM NOISES OF MODERN AIRCRAFT AND METHODS OF DIMINISHING THEM

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP81-00280R000100010010-2
• Release Decision: RIPPUB
• Original Classification: U
• Document Page Count: 13
• Document Creation Date: December 22, 2016
• Document Release Date: April 1, 2011
• Sequence Number: 10
• Publication Date: September 18, 1956
• Content Type: REPORT

Sanitized Copy Approved for Release 2011/04/01
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CIA-RDP81-00280R000100010010-2 b I A I
Sanitized Copy Approved for Release 2011/04/01
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CIA-R DP81-0028OR000100010010-2
 ?1;0I:3a.; 0? 14 OD1_1?t AI.1' .t: _1 STAT
2!z_52 _ .! C' -- .mot :v
of Mode:-. Air-raft L.J Methods of
Dimintsr.:ng Them, Moscow,
Pages 5-8. '-: ':. ?o-.?. 57-59
ei ! ? r..: 1,?rw:e progress made in noise control in aviation,
=hr t.?rn aircraft is quite high.
?:a 4i?: 5 tnr r.:nge2 of ^niie intensity for certain types of
air^rkf? , :r the c ckpit (for commercial aircraft, in the
p?lasent:,?r . --j?..-!?: at
::;r.:.r.ctornaya gruppa
1t
typ,?.
113 1
I4RLE 1
N)1: .::V:'713 OF VARIOUS AIRCRAFT
r? r:f.
120-130
110-128
90-110
:rr;siderable,even in aircraft of the same
.:rcraft is usually less th:.n in craft with
Duch t t. ::uch iev.'la conversation is completely impos-
sibl., n?:s cause a temporary decrement in hearing.
cve^ u,.:,tion instruments (interphones, radios)
d?. t.;n:. r ; .tn t ?r r.olsy conditions do not assure that speech
will : ?!,?:r: ,r,? lower noise levels at which conversation
in p t r: art, t')n tur?ting to persons aubjected to them for
tang , ? r. ?:?
thv author's data, adduces levels of typical sir-
c r .ft r. .x 1Tatt ht:.?nnn response thereto (without helmet
rAi3l. 2
"A'01: OF NOISE' 1NT Fr 1I11G WITH ACTIVITY
ev?Zlu.ition
~? t it 11C noise
.1iiturbing. Voice raised in conversation
irritating
:: : s x'..10:1 impossible even at top of o..e's lungs
opproacive and disoriantirg
..nr; .;f pain.
Noise 13vel, db
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It goes without sayi:ag that the passengers in an aircraf, should be
able to ton.verse without special effort and not be subject to excitation,
by noise. This corresponds to a level of approximately 90 to.105 db,.
although this level exceeds that in well-built trunk-line railway car.
Aircraft noise is more tt:onfortable,? as railway noise is ?sharper" and
more irregular. Thus, in evaluating the reaction to s noise, it is
necessary to rer;kon not only with it, intensity, but its pitch. harmonics.
In military aircraft the crew workers in helmet-eniased headphones,
which considerably ease its work. Here, in evaluating permissible noise
level, it is necessary to allow for maximum possible impairment of the
quality of communication by radio and among crew members. In this situation
a.noise level of 115 to 120 db represents a maximum at which it is today
still pcs'ible to have satisfactory communication.
Of major significance in this respect is reduction of noise at its
sources in the aircraft and provision of soundproof cabin walls.
Table 3 shows the possible limits of noise reduction relative to
the overall level,, n, and to that of various frequency bands,f a f,
with the use of send insulation and diminution of noise at the source
for a twin-engine passenger aircraft.
Table 3 is divided into 2 portions.
The first shows noise levels from major sources with and without
attention to possible maximum possibilities of reduction therein and on
the assumption tha' the cabin wall carries no insulation. Thus, for
engines of this power, the total propeller noise is 122 db, while use
of special low-noise propellers permits the reduction of this factor
to 100 to _10!. db.
NOISE IN DEa FROM MAJOR SOURCES IN AIRCRAFT, WITH AND WITHOUT
C014SIVERATION OF POSSIBILITIES OF REDUCTION THEREIN
Source Noise level in db from various sources
without noise- with maximum reduction
reduction
Propeller
122
100-104
Exhaust
118
100-104
Engine
104
89-99
Ventilation
114
72-76
Vibration of
parts
108
74-79
Aerodynamic
noise
94
79-84
Total noise
level
124
104-108
POSSIBILITIES FOR REDUCTION OF PASSENGER-CABIN NOISE BY INSULATION
Type of Insulatior.
(cabin wall mnnt?er_nl) Total 600-120c 1200-2400c 2400-4800--
No insulat.:on (model with
rainimtr, n31se level) 105 86 84 82
 Typo of tnsulati-n for cabin
Noise reduction with given insul
ation, db
Peneling plus insulation, with
air sp?.ce
3-5
3-6
6-9
10-16
Paneling and multi-ply insulation,
with :,.r space
3-8
4-10
10-16
20-40
The noise level from ventilation is 114 db. Use of rational venti-
lation d(?signs and sound insulation of the air ducts makes it possible
to reduce noise from this source to 72 to 76 db. With m^xian noise
reduction in all setrcos, noise can be brought down to not less than 104
to 107 d:: in all, to, to a level somewhat higher than that from the
largest single eource of noise with maximum reduction. Under these con-
ditions, the noise from ventilation, vibration of parts, etc, may not
exceed 90 to 95 db as further reduction would in any case have no signi-
ficant 'ffect on total noise level (see below, determination of noise
it. and fortaulas L. and 4a).
The r,rr'n.' part of Table 3 permits evaluation of the effect of various
types of ,:.thin :?oundproofing. Here we examine both the reduction in total
cabin n.::_c (:0-) -'b being a value selected on the basis of the findings
in T-,blu i, Part I) and the noise levels in various frequency bands as
the eff,?.: .f ;,:unlproofing is felt with particular force at high frequencies.
It ,??::?i,nt that use of the third type of soundproofing permits
a rcdar:: m ?f t:1 8 db in total noise, and of 20 to 40 db in the 2,400
to !.,8(` : :i
Tai::- rcve,ir? that the total attainable noise reduction, in db from
aircraft n: )n'.ral and the use of cabin soundproofing, is 25 db, (from
122 t: at n txt tum noise reduction on and multi-ply soundproofing)
and to high frequencies.
the total noise level in the aircraft cabin without
:. .? :; ni .:.~ :r re auction of noise from individual sources is 124-2, or
12:_ db, .~., t , :abin walls reduce noise by about 2 db. Assuming that
the noi: redo(:::n ?:t the source is about 20 db, and the cabin sound-'
proof:n,, t:.?)t tier 5, we obtain a total possible noisy reduction of about
25.
Tti:;, redact:un is extremely important to improve intelligibility of
conversation in co.tmuttications lines and for passenger confort in com-
:nercial aircraft
For a rcraft with other noise 1Lvols the possible reduction attaina-
ble is the unite
Int., comparison of Tables 1 and 3 the following questions naturally
(1) *.nat are the causes for the high noise levels in aircraft, and
how can they be reduced to a minimum?
(2) ;,'hat noise levels may be considered permissible, considering
that t:tt, cont.5 of aircraft construction avid operation are affected by
the well, , ht of the soundproofing they carry and the complexity of the
74e35urec1 uaoa for noise control.
 Before proceeding to an examination of these questions, let us
discuss the characteristics of the noises of contemporary aircraft and
the effect of aircraft noise on the human organism.
[Pages 32-351
7 Effect of Noise on the Human Organism
Noise irritates th's human nervous system and induces changes in the
functioning of the apparatus of hearing and speech. In the presence of
noise a person involuntarily speaks louder and strains his vocal chords.
Sometimes this causes a noticeable change in the pitch composition of
speech, a fact that must be considered in planning communications apparatus
designed to function in surrounding noise. Long-term presence in noise
induces fatigue. Beginning at a particular noise level, and depending upon
the pitch composition, one is conscious of a sensation of discomfort, the
irritation of the organism by noise increasing very rapidly as the level
rises (Table 2). In addition, the irritating properties of sounds differing
in frequency are not identical, and the most unpleasant are sounds in the
high-frequency portion of U aural band.
Thus, the irritation caused by 400 and 4,800 c is identical if the
"0-c noise is 20 db higher. It is therefore essential to devote particular
attention to reducing the high-frequency components of noise, thus making
it more "comfortable."
It must also be noted that the intelligibility of speech under con-
ditions of noise depends upon the speech components being higher than
the noise in the range from 250 to 3,500 to 4,000 c. Therefore, for
raising the intelligibility of speech it desirable to reduce the high-
frequency components of noise by all possible means.
All researchers emphasize the importance of taking all possible mea-
sures to reduce noise in general, especially noise in the middle frequencies
(600 to 4,000 cycles) in particular to assure comfort in an aircraft.
Thus far there are no objective criteria for determining the maximua
permissible noise level and the pitch harmonics thereof required to provide
the needed standard of comfort. In passenger aircraft the effort is made
to reduce noise as far as possible, considering that comfort is assured
if passengers sitting next to each other can converse without marked effort.
It has long since been established that noise produces a shift in
the aud?bility threshold due to the masking action of the noise. This is
the equivalent of partial temporary deafness, as cessation of the noise
produces a gradu.ni return of full hearing.
Figure 15 illustrates the audib'.'ity threshold in quiet surroundings
for a person of normal hearing (curve 1). It is evident that sounds of
varying frequency become audible at various levels of intensity. Curve
4 repr?3r :its the audibility level of a person with impaired hearing in
quiet surroun.iings.
In the presence of noise the audibility threshold of a normal person
changes as per curve 5; curve 2 provides the threshold of the feeling
of pain. Thus, in the quiet : person hears all sounds, the intensity of
which are higher than curve 1, while in the noise he hears only those
levels, whoce intensity are higher than curve 5 (the curves adduced
pertaining to the pure sound only).
 The levels of sound producing a shift in the threshold within the
r.r curve' s feeling of pas The:: it is clear that
noise has a de.., erring errect which :.ontributes to pour .,.,..cprenensioali:,
of speech listened to in noisy surroundings.
The curve:s for shift in audibility level - audiograms - are
usually plotte.i on the audibility level in the quiet. Thus, in the
absence of nose an audiogram will take the form of a straight line
coinciding with the horizontal axis. Consequently, in the presence of
noise th,r auli:ngram become.,% elevated, remaining stable when the noise
char ct. n:.::c constant, while after its elimination there is a slow
return ti the :.tr.rting position. The after-effect of the noise depends
on the dur:,t::?r, of the effect of the noise on the ear. After one hour
of subsection to 125 db the hearing returns to normal 3 to 4 hours later.
Some rcne.archors assert the existence of an industrial deafness
uiibseq::?.nt to long term of work in aviation, but this cannot be regarded
as firmly e::tnblish_d.
.:r?: st:.pe of the audiogrnm depends upon the nature of the pitch
har:mo:rir.i. The audiograms of aircraft noise are very similar in shape
if they :ire taken in a single helmet-mounted earphone.
To ietc--irn?e whether the noises of various aircraft affect hearing,
the author r.cor,ied noise audiograms in several aircraft. Toward this
end a special :nstrunent, termed an audiometer, was built; it consisted
of J port::;;ie HC sound frequency oscillator, with output db attenuator.
The metho.i .,f mcasure:nent was the following. Audiograms of several
individuals were t^i:en in a quiet room; this individual audiogram
detenninincc, for .11 practical purposes, the audibility threshold when
the given frequency was transmitted by the audiometer telephone. The
same op=r.:t:o: then repeated' in an aircraft under conditions of noise.
ikon, nssu^:I1 ,' 'hit the air pressure in th3 telephones is
p - kU (19)
in which U 13 th:? voltage in the telephones, we obtain the following
audiogr _n order, ?! ,
14 - 20 log, P - 20 log U Un (20)
J y
in which th.? utscripts, n and t, represent, respectively, noise and
siler,c, .
M (f), in which f is frequency in c, gives us the
nudiogr:LTI we :?e,k.
::~ T tIin i is particularly convenient since it reduces to a minimum
errors h??::ni- t to with conversions, standardization of telephones, etc.
r ig.ir ? 1 i,: ?.?::ertts a noise audiogram for several aircraft with re-
ciproc,tin. ..?:n, ..
L:"r f .n,lysis of the data we obtained permits the follow-
fun I .. 1 : ra.:lttaions:
tt, ? ,1r::::t .Live nature of all the audlogra.ns 13 identical,
me.*Lnin . r h 0,1 1- ::*fact. of the noise of various types of aircraft on
the cf Ii trin^ is identical;
 (b) the quantitative change in the magnitude H (f) is such that
it is possible to find a single design parameter for noise in modern
aircraft regardless of type, something that has already baen done;
(c) changes in noise level make it possible to hold, practically
speaking, that the audlogra.:. shifts parallel to itself,ie, that it rises
with elevation of noise level and declines as noise level declines;
(d) at the measured noise levels and shifts in audibility threshold,
the latter is approximately proportional to the change in the level of
intensity of noise, in db.
As indicate) above, aircraft noise can attain very high levels
and must be reduced to a minimum which is determined separately in each
individual instance
Whr.t are the desirable methods of reducing noise? At what noise
levels is noise c,ntrol most effective?
It must always be remembered that neither noise suppression at the
source (quiet prcpeilers, exhaust manifolds, careful sealing of apertures)
nor sou.id proofing can effect any considerable reduction of cabin noise
independent of each other.
5epiir?_t .' corf nrrt:ance cf any of these measures is meaningful only
if an -iircrift ?tli?nady :n Lx:stence has to undergo corrections of defects
In design or in sounoproofing. For example, cockpit noise in certain
aircraft has been reduced by 5 to 6 db by sealing the landing lights and
elinin:1ting crevice; in the cockpit housing. There are instances in
Which cockpit noise has keen reduced 25 db in the high-fregency components
by selling cracxs An increase in the rigidity of cabin walls close to
th?~ propeller tip: mry ,lso produce a noise redu ion of several db.
i:o.tever, a a:Cnificant reduction in cabin noise is attainable only
by a cemoinaticn of measures to suppress noise at the source, and by
provision cf cairn ~,oundprocfing, starting at the drawing board. It is
absolutely essential that the cabin be "acoustically homogeneous," ie,
th :t virtu%1ly iuerit:cal levels of soundproofing be provided at all
co.-iuon:rits of the cabin surface Otherwise the effect of sound insulation
will be insit,-nif:c:.nt
'L mti t be r.r..embcred that the noise level in the cabin is determined
by its weakest i:nk, ie, if good soundproofing has been provided everywhere
e.:^ept , :i;? , nt t h windows, cabin noise will be high.
Any c:?-.:k, :,iertures, however insignificant, can markedly reduce
soundproof:::;;. iicwever? Food soundproofing of the cabin, even if acousti-
cilly uniforn, will not be effective if noise suppression at the source
i; ncglectci T: ignore this is wrong both from the engineering and
economic 3t,-LnJr_:'s. :.s in m-:ny cases it is simpler to reduce noise at
the :?ource "t..~. to redi..:e its penetration into the cabin. In addition,
:t. i; ecorie-.ic?.1'.; unprofitable to try to deaden high noise levels, as the
effectiveness cf :.cund:reofing is considerably higher at low noise levels.
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It ?..,:.t be rnnzembered that the 1---r the level Of wr.t--nnl n^ ix*
~t..c Greater i.ne auppresslort of noise at its sources), the higher the
efficiency of a giver, type of soundproofing, le, the weight of the sound-
proofing is uapicycd more rationally.
In reality the subjective response tc sound is determined not by
intensity tut by loudness, L. The expression L (p)n is markedly nolinear.
The function d 112 - ?(~,n) is illustrated in Figure 21. Consequently,
d_ n
at various noise levels an identical red'ictionpfn from a given level:
of cabin r)undprv)fing produces a differrnt subjective effect.
Whet f n.-35 dt;, the function y) (P) attains a maximum and declines
continuous:y thereafter with increase infln.
Let us examine the possibilities for reducing noise at its most
important source'.
From Figure 7 :t i clear that cabin noise due to the propeller may
be reduced if the blade velocity, V0, is cut down, and the smallest distance
from prop,.ller-tip to fuselage be increased. Use of multibladed low-
rpm propeller:, can also reduce noise by several db.
Ac.-crd:tip, to Figure 7, an increase in d0 to more than 30 to 40 cm
has very l:tt;r effect on noise in the cabin (other sources claim the
contrary, however, and inaicate that do should not be less than 60 cm).
This is t practicable requirement for new aircraft, a statement
that cannot i~?' r.ad': as far as reduction in propeller speed and number of
blades i:> con -ned, as these litter reduced prop __er efficiency to some
degre.: and involve some serious technical difficulties (changes in engine
gear-rat io, ?:t c) Nevertheless, considering that a well-designed low-
noise propell,!r provides a reduction of up to 20 db with small loss of
efficiency, careful study must be given to the question of the use of
such propellers an ptsrer.ger aircraft in which comfort is a consideration.
To reduce cabin prcpe11er noise it is recommended that the freight
compartments of the fus??1age be planned to be the closest to the propel-
lers, ar,d that the walls at those points be more rigid, avoiding, if
possible, hatches and windows therein.
It h-_-, been e t~'.liahed ',h..t individual exhaust stacks produce an
incompnr:.bly t,?.Fh?_?r noise Live'i than manifolds or, even nore so, manifolds
with mufflers. %'ar::tions :n :t-ck .resign do affect the noise leve, but
only as i clear from the curves in Figure 22.