acoustical materials and acoustical treatments for aircraft

THE JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA VOLUME 19, NUMBER 3 MAY, 1947

Acoustical Materials and Acoustical Treatments for Aircraft*

R. H. NICHOLS, JR.,** H. P. SLEEPER, JR.,*** R. L. WALLACE, JR.,** AND H. L. ERICSON**** Cruft Laboratory, Harvard University, Cambridge, Massachusetts

(Received March 4, 1947)

Soundproofing treatments for aircraft must be designed to provide high degrees of both attenuation of sound transmitted through the fuselage and absorption of sound within the cabin. At low frequencies, treatments of weights which are practical for aircraft provide little or no attenuation of transmitted sound, hence the sound absorption at low frequencies should be made as high as possible by spacing the treatment one to three inches from the dural. At intermediate and high frequencies, the sound absorption is approximately the same for most treatments which have a blanket of acoustical material at least one-half inch thick

on the side toward the interior of the cabin, and a highly porous trim cloth. If the trim cloth is not porous, the absorptive properties of the structure will be poor at low frequencies, which must be compensated for by sufficient absorption in the seat upholstering, carpets, etc. The flow resistance of the acoustical material used in the treatment

should not be greater than about 500 g cm -2 sec. -• per inch of thickness.

Attenuation of sound by impervious panels, such as windows, dural, etc., at the intermediate and high frequencies is almost entirely dependent upon the surface density of the panels. The attenuation by an acoustically- treated fuselage structure is at least as great as would be predicted from its surface density. The presence of air spaces, absorptive materials, and impervious septa in the treatment will tend to give greater attenuation than predicted by "weight law." The relative "efficiency" of a treatment may therefore be characterized by a Merit

Factor, which is the ratio of "db better than weight-law" attenuation (at 5000 c.p.s.) to the surface density of the treatment.

For both attenuation and absorption, the most effective of the simple treatments designed at this laboratory consists of two blankets of acoustical material with an

impervious septum between, the treatment being mounted with an air space of one to three inches between it and the dural, and covered with a highly porous trim cloth.

The choice of an acoustical material depends upon many mechanical and economic factors in addition to the acous-

tical requirements. Materials should be chosen on the basis of complete data on their acoustical transmission (attenuation) and absorption characteristics measured by the standard laboratory techniques referred to in this paper. A rapid estimate of the relative effectiveness of various materials in a given treatment may be obtained by measurement of the flow-resistance of samples of equal surface density, at several thicknesses (obtained by com- pressing the samples). From a plot of flow-resistance R rs. thickness T, the value of RT • for each material may be found. The material with the highest value of RT ß will generally provide the highest attenuation in a given treatment.

The validity of the laboratory measurements of acoustic attenuation and absorption discussed in this paper has be•n established by many meast•rements in airplanes of various types, in flight, with a wide variety of acoustical treatments.

I. INTRODUCTION

ROM a purely acoustical viewpoint, a material used on the interior walls of an

airplane cabin for reduction of noise must serve two primary purposes. It must attenuate sound transmitted through it from external noise sources such as propellers and exhausts, and it must also absorb sound which has already been transmitted through the walls or which is

* This paper is based on work done for the Office of Scientific Research and Development under Contract OEMsr-658 with Harvard University. The material was presented in part at the semi-annual national meeting of the American Society of Mechanical Engineers, Aviation Division, New York City, December 2, 1946.

** Now with Bell Telephone Laboratories, Murray Hill, New Jersey and New York City, respectively.

*** Now with Manhattan Project, Los Alamos, New Mexico.

**** Now with Douglas Aircraft Company, Santa Monica, California.

generated by sources within the cabin. High degrees of both attenuation and absorption must be provided by the material and the structure of the acoustical treatment in which it is used if

satisfactory noise-reduction is to be achieved. It is with these two characteristics of acoustical

materials that this laboratory has been primarily concerned in its work on noise reduction in air-

planes. 1-5 The basic problem was to determine

• L. L. Beranek, R. H. Nichols, Jr., H. W. Rudmose, H. P. Sleeper, Jr., R. L. Wallace, Jr., and H. L. Ericson, "Principles of sound control in airplanes," O.S.R.D. Report No. 1543 (1944), 388 pages. (Out of print.)

• L. L. Beranek and H. W. Rudmose, "Sound control in airplanes," (Summary) J. Acous. Soc. Am. 19, 357 (1947).

8 L. L. Beranek and H. W. Rudmose, "Airplane quieting I: Measurement of sound levels in flight," Trans. A.S.M.E. OO, 89 (1947).

4 L. L. Beranek, "Airplane Quieting II' Specification of acceptable noise levels," Trans. A.S.M.E. 69, 97 (1947).

5 H. W. Rudmose and L. L. Beranek, "Noise reduction in aircraft," J. Aero. Sci. 14, 79 (1947).

428

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ACOUSTICAL MATERIALS AND TREATMENTS FOR AIRCRAFT 429

what types of materials are maximally effective, and to design lightweight acoustical treatments which are simple to fabricate and reasonably easy to install and maintain, in which to use the materials. There are, of course, many additional general requirements of practical in•portance of mechanical, thermal, and chemical nature which a material must fulfill.

Materials commonly used for acoustical treatment in airplanes are made of cotton, glass, kapok, milkweed, plastics, mineral products, wool, rubber, and paper. Glass is used in fiber form called "glass wool." Kapok is usually quilted between layers of muslin or combined with wool or other materials to form a felt.

Mineral products are formed into fibrous materials such as highly refined rockwool. Rubber is used in foamed form and paper in an expanded absorbent form. Plastic materials are usually made in fiber form.

Most of the natural fibers such as cotton, kapok, paper, and certain of the plastics must be flame-proofed and are subject in some instances to rot or attack by vermin. Mineral fibers such as rockwool and glasswool usually are satis- fact/)ry from all these viewpoints. It is necessary, however, to permit the use of natural fibers because many of them are basically light and are available in abundant quantity.

The type of construction of an acoustical treatment for an airplane is more or less dictated by the common type of structure of the fuselage. Usually, materials may most easily either be cemented directly against the inside of the dural skin, or attached in panels to the edges of the transverse ribs, in which case an air space is left between the material and the dural skin. The

depth of this air space is determined by the height of the ribs and the thickness of the acoustical treatment. Sound-originating outside

Sound from

outside

sound reduced

material

IIIIIIIII_

(/• •. "-U materie, Oural Air space

TRANSMIS$10N ABSORPTION

Fro. 1. Attenuation and absorptiorr of sound by acoustically-treated fuselage.

/ / /

I / / / / / / /'"- / •

/ILL'// -• • 'Y

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ill///• / /•

il/// / /

vii ! / / / IIII//• / 11/7 ' Vii' • •

i .• .• .4 .o e ABSORPTION

Fro. 2. Theoretical curves showing amount by which sound level in cabin is reduced below that exterior to cabin by uniform walls of absorption coefficient a and transmission coefficient r.

the fuselage strikes the dural skin, and is partially reflected and partially transmitted to the interior of the cabin; the transmitted part is further attenuated in level by passing through the acoustical treatment, as indicated schematically in the first drawing of Fig. 1. •

Sound already present in the cabin will strike the acoustical material first, and be partially absorbed on passing through it to the dural. At the dural, a portion of the sound will be reflected back into the cabin again, and will be further absorbed by the acoustical treatment, as indicated in the second sketch of Fig. 1.

The degree to which a structure reduces the level of sound passing through it is characterized by its "transmission coefficient" r, which is defined as the ratio of the sound energy transmitted through the structure to that which is incident upon it. The attenuation of the sound, in decibels, as measured experimentally in the laboratory, is related to the transmission coefficient r, by the following equation:

n(decibels) = 10 1og10(1/r).


430 NICHOLS, SLEEPER, WALLACE, AND ERICSON

Similarly, the degree of absorption of sound by a structure is characterizi•d by an "absorption coefficient" a, which is defined as the ratio of the sound energy absorbed by the structure to that which is incident upon it.

The importance of having both high attenuation and high absorption in the acoustically treated fuselage structure may be seen from the graph of Fig. 2. The curves are contours of'equal sound-level reduction for various values of'at-

tenuation and absorption by the walls of a com- pletely-enclosed cabin with all walls of uniform acoustical characteristics. These are theoretical

curves for an idealized case, but they serve to illustrate the principle involved. For example, if the walls provide an attenuation of 10 db as measured in the laboratory, but the absorption coefficient of the structure is only 0.1, the reduction in noise level from outside to inside of

the cabin will be only 3 db. If the absorption coefficient were increased to 0.9, a full 10 db of noise-level reduction would be realized.

It is of interest to note that if there is no

absorption of sound within the cabin, the noise- level reduction is zero, regardless of the transmission coefficient of the walls, i.e., the level inside the cabin is the same as that outside. Note

also that if the wall structure causes no reduction

FIG. 3. Schematic cross section of equipment used for measuring the attenuation of sound by sample sections of fuselage walls.

in transmitted sound (r = 1), even large amounts of absorption can produce but little effect in quieting the cabin. Large reductions in sound level are achieved only when the transmission coefficient is very small and the absorption coefficient very large.

It is apparent that the attenuation and absorption characteristics of a treatment will be strongly dependent upon the weight of dural, the depth of air space, and the type of interior

øL 1

._

l, It ' 60

30 I00 I000 I0000

FREQ. in C.P.S.

FIG. 4. Attenuation of sound by single impervious panel. Straight line indicated by arrow shows attenuation predicted from surface 'density alone of panel (weight law attenuation). Single Plexiglas window. Weight approximately 1.55 lb./ft?.

trim cloth, as well as on the physical characteristics of the acoustical material itself. For that

reason, in measuring the acoustical properties of materials it is essential that they be tested under the mounting conditions encountered in airplane installations. The complete structure, including the dural, must be evaluated, since a given acoustical material mounted in one way may be many times more effective than when it is mounted in another way.

The discussions which follow will therefore

pertain to the performance of complete wall structures (including dural) in which the various acoustical material6 are used. Except for ex- ploratory experiments or special tests of certain specific structures, 0.02-inch dural and one particular porous trim cloth were used in all the measurements of attenuation. In general, it has been found that if a given acoustical material gives better performance than another in a structure with 0.02-inch dural skin, the same relative rank-order will hold for the two materials

if applied to a structure with lighter or heavier ß

dural.



The attenuation and a.bsorptio• characteristics of materials and structures will be discussed

separately, in that order. The structures to be considered are classified in the two general types'

E

1. Impervious structures. a uJ 30

a. Single' Dural alone, windows, etc. •, b. Multiple' Dural and doped fabric, etc. •

2. Absorptive structures. • 40 a. Single' Dural with acoustical material cemented z •

to it. • s0 b. Multiple' Dural with one or more acoustical

blankets separated f•om it by an air space.

II. APPARATUS FOR MEASUREMENT OF

ATTENUATION

The equipment which is used for determining the attenuation characteristics of various types of fuselage structure with or without acoustical treatment is shown schematically in Fig. 3. 0 An 18-inch square dural panel is mounted in a frame in an opening in a heavy brick wall. Acoustical treatments may be applied directly to the panel, or may be suspended at a desired distance from the panel, to simulate the type of mounting in which the treatment is attached to the edges of the ribs of the fuselage.

Sound is applied to the bare side of the dural panel (the "primary" side) by means of a bank of nine loudspeakers mounted very close to the panel. The sound passes through the panel and acoustical treatment and then down a sound-

absorbing tube which is 18 inches square in cross section. The purpose of the long perfectly- absorbing termination tube on the secondary side of the structure under test is to prevent reflections and standing waves.

A calibrated microphone is mounted between the loudspeakers and the panel in order that the level of the sound applied to the structure on the primary side may be measured. Similarly, the level of the sound on the secondary side is measured with the aid of another calibrated

microphone mounted in the termination tube adjacent to the acoustical treatment. The attenuation by a structure at any given frequency of applied sound is then the ratio of the primary sound pressure to that on the secondary side, in

6 R. L. Wallace, Jr., H. F. Dienel, and L. L. Beranek, "Measurement of the Transmission of Sound Through Light Weight Structures," J. Acous. Soc. Am. 18, 246A (1946).

I00 300 I000 3000

FREQUENCY IN CYCLES PER SECOND

FIG. 5.

O2

04u-

o.• '• 0.8 Z 10

•.s b 2.0 •5 $0

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decibels. By applying pure tones of various frequencies through the loudspeakers, data may be obtained for plotting a graph of attenuation rs. frequency for a structure. (In determining the attenuation characteristics of airplane window structures, the dural panel was, of course, omitted.)

III. INTERPRETATION OF ATTENUATION DATA

When the attenuation of a single panel of impervious material such as dural, Plexiglas, plywood, etc., is measured in the above equipment and plotted as a function of frequency,.a graph similar to that of Fig. 4 results. The fundamental resonance of the panel is evident, at about 100 c.p.s. The higher order resonances of the panel also appear at higher frequencies, interspersed with certain acoustical resonances in the measuring equipment; above about 700 c.p.s. the majority of the peaks are not to be considered as significant.

It is found that'the average value about which the attenuation rs. frequency curve for a single panel oscillates at the higher frequencies (1000- 10,000 c.p.s.) is that which would be expected from theory if the panel'were a pure mass only, with no stiffness. The heavy straight line indicated by an arrow represents the so-called "weight law" attenuation which would be predicted for the panel. A family of such curves for various values of surface density, •, of a panel is shown in Fig. 5.* These represent quite

* These weight law curves are displaced downward six decibels from those ordinarily published in books on archi-



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DOUBLE STRUCTURE

•i!i!:'.'• Kwilko No. 5, three thicknesses,

/"I.:..:..•:_••iIPlenet, suede-0.035 Ib/ft 2 Total weight of treetment- 0.264 Ib/ft 2

.•0 I00 I000 I0000

FREQUENCY IN C.P.S

Fro. 6. Showing increased attenuation of high frequency sound obtained by adding acoustical treatment to dural panel. Straight line shows attenuation which would be obtained from dural panel of same total weight.

adequately.the average attenuation of a single impervious panel 12 inches square or larger. With smaller panels, the stiffness of the panel may become significant.

There are numerous methods by which the average value of attenuation in a given frequency region may be determined from the curve. The procedure that has been most extensively used at this laboratory involves the use of a planimeter to determine the area under a s•ction of the

curve between two frequencies for which the frequency in question is the geometric mean. For example, to obtain the average attenuation at 5000 c.p.s., the area under the curve between 2500 c.p.s. and 10,000 c.p.s. was measured. From this area, the average attenuation is determined by dividing by the length of the base-line. Aver- age attenuation values are usually determined at 1000 c.p.s., 3000 c.p.s. and 5000 c.p.s.

When acoustical materials, in weights which are reasonable to be considered for use in aircraft, are added to the dural, it is found that the attenuation of the structure is improved primarily at frequencies above 1000 c.p.s. An example is shown in Fig. 6. The heavy straight line (indicated by an arrow) represents in this

tectural acoustics because of the met'hod of measurement used. Pressure doubling over the primary surface of the panel is accounted for in one test method and not in the other.

case, the attenuation which would be obtained from a single impervious panel of the same weight as the total structure (0.834 lb./ft.2). At low frequencies, the average attenuation due to the acoustically-treated structure is essentially the same as would be achieved by simply using a heavier dural skin alone, of the same weight as the structure with treatment. At frequencies above 1000 c.p.s., however, the acoustically- treated structure gives appreciably greater average attenuation than a single dural panel of equal weight.

Since weight is an extremely important factor in airplane design, it is desirable to evaluate acoustical treatments in terms of their relative

effectiveness per un'it weight. The attenuation performance of a structure with a given acoustical treatment may be characterized quite simply by comparing it with the attenuation performance of a single panel of the same surface density. To express it in practical terms, the question that is to be answered is, "For a given total weight of wall structure, how much more attenuation will be achieved with the standard

weight of dural skin and an acoustical treatment than would be obtained by simply making the dural heavier by an equivalent amount?"

For the evaluation of attenuation effectiveness

by such a comparison a quantity called the



FIG. 7.

60 I 30 I00 300 I000

FREQ. IN C.P.S. WITHOUT RUBBER

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18"xlS'!xO.25" PYRALIN WINDOW

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Both ourvee follow wei(ht - law above I000 C. P. $.

Detail ol' rubber mountin( at right.

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WITH RUBBER

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,& RUBBER CO. ,

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DETAIL OF RUBBER MOUNTING

"Merit Factor" of an acoustical treatment has been defined:

Merit Factor = ns/W,

where n5 is the experimentally-measured number of decibels .by which the attenuation at 5000 c.p.s. of a structure exceeds that which would be

obtained with a single panel of the same total surface density, and W is the surface density in lb./ft. •' of the total acoustical treatment (ex- clusive of the dural). The quantity n, is usually referred to as the number of "db better than

weight-law." For example, in Fig. 6, the average attenuation of the structure at 5000 c.p.s. is about 65 db. "Weight-law" attenuation (for a single panel of the same weight) at 5000 c.p.s. is about 48 db, from the straight weight-law line. The difference, n,, is 17 db, the amount "better than weight-law." The Merit Factor is, therefore:

M.F. = n•/W= 17/0.264 = 64.4.

This concept of a Merit Factor is useful in comparing the 'relative attenuation "efficiency"' of various acoustical treatments when applied to a given weight of dural. It may also be used to assign a rank-order of "efficiency" to various acoustical materials, if they are tested in a given type of structure, with the structure

remaining constant except for the acoustical material. ß

The use of the attenuation at 5000 c.p.s. as an indication of the merit of an acoustical treatment

has a good practical foundation, especially When the attenuation is measured by determining its mean value in a fairly wide frequency band, such as that lying between 2500 c.p.s. and 10,000 c.p.s. Research at the Psycho-Acoustic Labora- tory (Harvard) and elsewhere has shown that the middle and high frequency components of airplane noise are the ones which interfere the most with communications and are the moss

annoying to personnel. Fortunately, lightweight acoustical treatments are much more effective at

high frequencies than at low frequencies, and. marked improvement in both ease of communication and comfort can be achieved with them.

IV. ATTENUATION CHARACTERISTICS OF

IMPERVIOUS STRUCTURES

Single Impervious Structures

Single windows, and dural, plywood, or plastic panels are classed as single impervious septa, as has been mentioned previously. If a constant sound pressure is applied over one side of a window or panel the sound transmitted through



0.04" 17ST Dural-0,57 lb/ft. z

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FREQUENCY in C.P.S.

Fro. 8. Addition of light weight impervious lining to dural skin does not increase average attenuation above that of single dural skin of same total weight.

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DOUBLE STRUCTURE

0.04" 17ST Oural- 0.57 Ib/ft 2

Phenol- 0.256 Ib/ft 2 Total weight of treatment- 0.256 Ib/ft z

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FREQUENCY •n C.P.S.

Fro. 9. Addition of heavy inner lining gives greater attenuation than single dural skin of same total weight. Cf. Fig. 8.

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FREQ in C.P.S. FREQ in C.P.S.

Fro. •0. Effect of sp•cin• between p•nes of double windows on •coustic •ttenu•tion. Doubl• Plexiglas wi.dows, •ch • { i.ch thick, total w•i•ht •ppro•im•tdy 1.• lb./itA

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13:43:52


the panel at any particular frequency will depend primarily on the weight per unit area of the panel and to a lesser degree on the thickness, lateral dimensions, Young's modulus, etc., of the material. At low frequencies it is possible with some difficulty to calculate the attenuation rs. frequency characteristics from a knowledge of all of the above factors. The attenuation will be

very small in the vicinity of the resonant frequencies of the septum, and between resonances it will be little greater than would be predicted from assuming that the stiffness was zero and that the panel was a pure mass only. At the higher frequencies (above 700 c.p.s.) for panels greater than twelve inches square the attenuation can be predicted very closely by considering the surface density, •, of the panel alone.

The chart shown in Fig. 5 is useful in pre- dicting the attenuation of sound by single impervious septa such as windows, dural panels, floor structures, etc. The curves were computed on a theoretical basis and are called "weight-law" curves. The term "weight law" was chosen because the physical constants of the septum are assumed to be a function of surface density alone. Given the surface density of a window as 0.2 lb./ft. 2, it can be seen that the attenuation is 23.5 decibels at 1000 c.p.s. and 37.5 decibels at 5000 c.p.s. Experimental data taken for panels larger than twelve inches square coincide reasonably well with these curves at frequencies above 700-c.p.s., though below that frequency they are subject to considerable error because the panel stiffness cannot be neglected, whereas the weight-law curves assume a panel of negligible stiffness.

In the construction of most all-metal air-

TRIM CLOTH

ACOUSTICAL •'MATERIAL

FIG. 11. Structure of type I. Acoustical treatment cemented directly to dural skin of airplane.

SINGLE STRUCTURE

0.04" 17ST Dur&l - 0.57 lb/ft. •

KSB I/2"- O. 15,5 lb/ft. • Se&p&k R •lued to Dur&l. Kwilko No. 7 sewn to •ea•a•.

0

20

• 40

•o

50 I00 1000 I0000

FREQUENOY in O.P. 8

FIG. 12. Attenuation of structure I is not much greater than for impervious panel of same weight. This type of structure is of value primarily because it provides some absorption.

planes, stiffening members (longerons) are riveted or welded to the dural skin. These stiffening members appear to have little effect on the transmission of soilnd through the panel. Trans- mission measurements were made on a panel with longerons and compared to weight-law predic- tions. It was found that the weight-law predic- tions fit the average measured curve quite well provided that the weight of the longerons is neglected.

In an attempt to reduce the transmission of sound through windows and dural panels, considerable attention has been given by industry to the problem of applying damping materials to the sides of the fuselage and of mounting windows in elastic supports. In the early phases of this research these factors were studied ex-

tensively in this laboratory and several devices for damping panel resonances were investigated. These devices all damped to a certain degree those resonances in the frequency range below 500 c.p.s. but had almost no effect at higher frequencies.

A number of types of damping material now in use in the industry were applied to 0.04"



sheets of dural to determine their effect on

sound transmission. Flexible mica plate held by a plastic cement to the dural has been used in the aircraft industry. Tests show it to be quite effectige at low frequencies, considerin• its relatively low weight. Considerable reduction in the transmitted sound was achieved for fre-

quencies below 500 c.p.s. Above 1000 c.p.s. negligible differences were observed. Similar results were observed when a commercial sound

deadener was applied. The weight of this deadener as used was 0.59 lb./ft. • which probably is considerably in excess of that permitted under existing soundproofing weight restricti?ns.

The effects of mounting a window in soft rubber mountings are shown in Fig. 7. All resonant frequencies are shifted downward because of the rubber mounting, but the attenuation is little different from that predicted by weight law in either case. Above 1000 c.p.s. there is no noticeable effect of the rubber on the attenuation of sound.

Double Impervious Structures

The most common example of thia type of structure is a double window. Windows fre-

quently cover such a large area of the pilot's compartment that extra precautions must be

DURAL

SEPTUM

0 03 LB/FT z

TRIM CLOTH

DURAL

ACOUSTICAL MATERIAL

•11'•SE PTUM

ø2 6/Fr' '[RIM CLOTH

FIG. 13. Four simple and effectiye acoustically-treated cabin wall structures.

taken to reduce the noise transmitted through them as much as possible. The following discussion deals with the possibility of reducing sound transmission through such structures by using panes separated by an air space.

Analysis shows that the higher the surface density of either of the panes and the greater the depth of the air space between the windows the greater will be the reduction of the transmitted sound pressure. If the surface density of

the first (outer) window is small compared to the stiffness of the air space between the windows there is no advantage to the double window. To avoid this loss of efficiency either the air space between the windows must be made larger or the surface density Of the outer window be made greater. If the surface density of the second (inner) window is small compared to (Ro/•o), where R0 is the acoustic impedance of air, and • equals 2•r times the frequency, then there is no advantage to its use.

The futility of attempting to reduce sound levels by using a relatively light weight inner pane or lining is illustrated in Fig. 8. The stretched, doped fabric alone does not con- tribute to the attenuation of sound. The weight- law curve in that graph was computed from the theoretical weight-law curves assuming a surface density of 0.6 lb./ft. 2. A heavier lining was tried and the data are plotted in Fig. 9. The attenuation at 3000 c.p.s. was increased by approximately 10 decibels over that predicted by weight law. The weight of the phenol lining was approximately one-half that of the dural.

Transmission measurements on 18" X 18"

double windows, each pane of which is made of •" Plexigla• for two different separations between the panes are shown in Fig. 10. The average added attenuation due to a one-inch air space over that obtained for a window in which a single pane of i" Plexiglas is used is about 14 decibels at 5000 c.p.s. If the spacing is increased from one inch to three inches the attenuation will be increased

four decibels more at 5000 c.p.s. In general, almost as good results could be accomplished at most frequencies by doubling the weight of panes and using this total weight («-inch Plexiglas) in a single window. Furthermore, a complicated frame would not be required, which in itself might equal approximately the weight of two •" windows.

From the experimental data, the following conclusions on sound transmission through imper- viou's structures can be drawn'

(1) At low frequencies strong resonances of an impervious septum make it almost trans- parent to sound. These resonances can be damped by cementing or spraying damping materials on the septum. The value of this type of treatment



FIo. 14. Comparison of this figure with Fig. 6 shows increase in attenuation obtained by add- z ing light weight impervious • septum to acoustical treatment. • &o

o

20

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FREQUENCY IN C. Po $

DOUBLE STRUCTURE

::":.:...•!!:• 0.04" l? ST 0.ral - 0.$'/ !:•Asbest0s septurn- 0.06 Ib/ft 2 ::•!ii':•Kwilko No. 5, th•

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is doubtful, however, in view of the fact that transmission at the higher frequencies is relatively unaffected by the addition of it. Experi- mental data on intelligibility of speech in noise indicate that middle and high frequency noise are of much greater harm to communication efficiency than low frequency noise.

(2) At frequencies above 700 c.p.s. for panels larger than twelve inches square the only important factor contributing to the attenuation of sound by a single impervious septum is its surface density. Mounting conditions, size, shape and kind of material have negligible influence on the attenuation.

(3) There is no acoustical advantage in sup- porting windows in flexible elastic mountings.

(4;) At high frequencies double windows attenuate sound better than single windows of the same total weight. At low frequencies (below 500 c.p.s.) there is little difference. The advantage of double windows is questionable, however, because of the added weight which must go into the frame.

V. ATTENUATION CHARACTERISTICS OF ABSORPT• STRUCTURES

The term "absorptive structure" is applied to structures which comprise the dural and one or more layers of acoustically-absorptive material. The materials commonly used are usually made up in blanket form, and may be used in con- junction with various kinds of trim cloth and impervious septa. All data given in the discussion

are for structures with 0.02-inch dural and a

very porous trim cloth weighing 0.057 lb./sq. ft., unless otherwise stated.

Single Absorptive Structures

A structure which consists of the dural skin with a blanket of acoustically-absorptive material cemented to its inner surface, as shown in Fig. 11, is called a "single absorptive structure." It is one of the five simple structures which have been studied in detail at this laboratory, and is referred to as structure I.

Apart from the increase in attenuation obtained by adding the weight of material to that of the dural, there will be some increase in attenuation at high frequencies, the amount of which depends on the thickness of the material and its flow resistance.*

Experimental attenuation results were obtained on a number of structures of this type, with various thicknesses and weights of several different materials. Data for one structure are

shown in Fig. 12. The values of db better than .weight law at 5000 c.p.s. for a number of such structures were found to range from zero to about 18.

Where the structural and space limitations in the airplane will not allow the use of a spaced

* Flow resistance R is defined as the ratio of the air pressure difference between faces of a material to the linear velocity of air flow through the material: R=p/v where p=pressure differential in dynes/cm •', and v=velocity in cm/sec.


438 NICHOLS, SLEEPER, WALLACE,,AND ERICSON

treatment, the type I structure may and should be used, because it will very definitely assist, in the reduction of undesirable noise. Even though its contribution to attenuation may not be great, as for the structure in Fig. 12, for example, the added acoustical material will give appreciable absorption at high frequencies if it is suf- ficiently thick and has a flow resistance of not less than 400 g cm -•' sec. -• per inch of thickness. The necessity for absorption of sound in the cabin has already been pointed out.

Wherever possible, however, it is better to use a structure in which there is an air space between the dural and the material, such as those discussed immediately following, under "Multiple Absorptive Structures." The attenuation in the frequency range 600-1000 c.p.s. is generally less for the type I than for any equivalent structure with air space. Furthermore, the introduction of an air space increases the absorption coefficient at low frequencies (100-500 c.p.s.) by several times.

Multiple Absorptive Structures

The term "multiple absorptive structure" is used to designate structures which comprise dural and acoustical treatment, with one or more air spaces between components of the structure. The four simple multiple structures which have been studied in detail at the Cruft Laboratory are shown in Fig. 13. In general, adequate soundproofing can be obtained with one of these four structures. Their advantages are (1) that fhey are relatively simple to manufacture and install, and (2) that their approximate performance may be estimated from a knowledge of certain physical constants of their components, as will be discussed later.

The simplest multiple structure consists of the dural skin with an absorptive blanket spaced in from it (structure II). We shall refer to this as a "double absorptive structure."

TABLE I.

If the flow resistance of the acoustical blanket

material is too high, the structure will be no more effective than a double impervious structure, as may be seen from comparison of Fig. 6 with Fig. 9. The surface densities of these two structures are very nearly identical. The increase of attenuation over that predicted by weight law at high frequencies is practically the same for both structures, and is due almost entirely to the air space between the dural and the second element of the structure.

It should be borne in mind, too, that the other function of the second element of the structure

is to absorb sound that is already in the cabin, and an impervious material or one with extremely high flow resistance is of little value for that purpose. Theory shows that, for an absorptive material of a given weight spaced out a given distance from a wall, there is an optimum value of flow resistance which gives good absorption over a wide frequency range. A compromise must therefore be made in the value of flow resistance of the material in order to achieve

the optimum values of both attenuation and absorption to give the most sound level reduction possible in the cabin. An upper limit to the useful range of flow resistance values may be set at about 500 g cm -•' sec. -• per inch of thickness of the material.

A very marked improvement over the structure just described is obtained by attaching an impervious septum to the blanket of acoustical material on the side facing the dUral skin. (Structure III, Fig. 13.) This gives, in effect, a double impervious structure plus a layer of acoustical material on the inside of the cabin.

Such a structure will transmit much less high frequency noise than either a double impervious structure or the simple double absorptive structure discussed above, for the same total surface d•nsity. For example, Fig. 6 and Fig. 14 show the actual attenuation rs. frequency curves

TABLE II.

Merit Factor Merit Factor Material ß Structure II Structure III

Glass Wool 85 i05 Kapok Blanket 40 67 Kapok Felt (60%) 44 70 Mineral Wool 29 5 !

Merit Factor Merit Factor Material Structure II I Structure V

Glass Wool 105 127 Kapok Blanket 67 81 Kapok Felt (60%) 70 78 Mineral Wool 51 54


ACOUSTICAL MATERIALS AND

taken on a double absorptive structure, Fig. 6, with no septum, and, Fig. 14, with an impervious septum of 0.06 lb./sq. ft. surface density. The straight lines indicated by arrows are the weight law curves for the structures. Naturally, the structure with the septurn gives somewhat g. reater total attenuation than the other because of its greater total surface density, but one notes particularly that the structure with the septurn gi'•es more attenuation at high frequencies relative to its own weight-law curve than the structure without Septurn gives relative to its weight law. If "average" curves are drawn through the experimental curves, then for the structure without septurn the attenuation at 5000 c.p.s. is about 17 db greater than weight-law attenuation, and for the structure with septurn, there is an attenuation of about 26 db better than

weight law, a gain of 9 db. This indicates that for a given surface density of treatment the structure with septurn will give the greater attenuation of high frequency sound passing through it.

A comparison of the relative effectiveness of structures II and III is given in Table I, which shows the values of Merit Factor for four typical materials in the two structures. The same dural

and trim cloth were used-in all eight tests, and the septurn used in the type I II structures was asbestos paper of 0.06 lb./sq. ft. surface density. The depth of structure was 3 inches from dural to trim cloth in each case.

By means of a simple re-arrangement of the elements of structure I I I, a still more effective treatment may be obtained. The change consists of putting half of the acoustical blanket on each side of the impervious septurn, as shown in Fig. 13, structure-V. Structure V and structure III are identical in every respect except for the position of the septurn. The improvement in attenuation characteristics which is achieved is

indicated by the data of Table I I, which shows a comparison of values of Merit Factor for structures I I I and V with the identical samples of acoustical materials' listed in Table I. Values

of Merit-Factor for the type IV structure, using the same materials, were 116, 67, 64,' and 49, respectively. (The type IV structure is similar to type V, except that it has a lighter septurn, of only about 0.03 lb./sq. ft. surface density.)

TREATMENTS FOR AIRCRAFT 439

U-z 58

o

z 56

•- •4

5•

5• •

0 • 40 60 80 I00 *.o 40 • eo ZOO

RT x

FIG. 15. Showing correlation between characteristic flow- resistance of acoustical blanket used in type III structure and acoustic attenuation by the structure. These data were obtained with several different types of materials. .

It should be noted that while the Merit

Factor is a useful characterization of the relative

attenuation effectiveness of treatments per unit weight, it does not directly indicate what the total attenuation of a given structure is. The total attenuation of a given structure at 5000 c.p.s. is made up of the weight-law attenuation in db plus the "decibels better than weight law," n•, which is used to compute Merit Factor. A very heavy treatment with a low Merit Factor may easily produce more total attenuation than a' lightweight treatment of high Merit Factor, simply because of the difference in total weight.

Trim Cloth

In order that the trim cloth shall not interfere

with the acoustical absorption prol•erties of the structure, it should have low flow-resistance. In that case, its contribution as an acoustical element is very slight. it may tend to add some effective mass to the structure, but, in general, its weight is a small percentage of the weight of the total structure (including dural), so that its effect on the weight-law curve of the structure is not very •ignificant. The trim cloth may have a very pronounced effect on the Merit Factor of a structure, however, since its surface density is usually a very appreciable portion of the surface density. W, of the total acoustical treatment. Inasmuch as a low flow-resistance trim is

acoustically ineffective, a change in its weight will not materially affect the value of n•, but it will change the value of W, hence the Merit



Factor=ns/W will be altered. Obviously, the degree of. change in Merit Factor will depend upon the relative weights of the trim and of the acoustical material plus septum (if used).

Dural

A change in the weight of dural skin, will, in general, be expected to change the attenuation of a given structure approximately in accordance with the corresponding change in weight law for the structure. Since, in that case, the value of n5 and of W are not affected, the Merit Factor would remain consta. nt, but the total attenuation of the s•tructure would be changed.

Septum

It has been found experimentally that a surface density of about 0.06 lb./sq. ft. for the impervious septum in structures III and V represents a good general compromise value for most purposes. If a more complicated type of structure is acceptable, improvement in attenuation may be found by separating the acoustical material into several layers, instead of one or two, and using lighter septa between the layers. It should always be kept in mind, however, that a reasonable thickness of acoustical material on

the side toward the cabin interior is desirable for

purposes of sound absorption.

Air Space

The depth of air space which exists between the dural and the acoustical treatment is usually dictated by the depth of the rib structure of the airplane. An air space makes a general improvement in the attenuation effectiveness of a treat-

ment, and causes a very marked increase in the absorption coefficient of a structure at the lower frequencies, as will be discussed later. For these reasons, an air space of one inch or more between dural and treatment is highly desirable.

Estimation of Attenuation Effectiveness of

Materials from Physical Characteristics

It is frequently desirable to be able to estimate approximately the relative attenuation effectiveness of various materials, without recourse to the accurate, but rather laborious process of

attenuation measurements described in a fore-

going section. It has been found that the relative attenuation effectiveness of uniform homoge- ueous materials is closely related to the resistance which they offer to the flow of air through them. This is not surprising, since when sound strikes a porous material it forces air in and out through the interstices, and energy is dissipated becaus• of the friction. The "flow.-resistance" of a material is defined as

R=p/v,

where p is the pressure differential between the two parallel faces of the samples in dynes per sq. cm, and v is the linear velocity of the flow of air through the sample in cm per second. The c.g.s. unit of flow-resistance is the acoustic ohm (gcm -2 sec.-l).

For uniform homogeneous fibrous materials, it is found that the flow-resistance R of a sample of given surface density varies with thickness T of the sample when it is compressed, according to the relation'

RT x = constant,

where x is a number whose value depends upon the nature of the material. •. 7 If the attenuation

(at 5000 c.p.s., say) of a given structure of the simple types I-V is measured with a number of different fibrous blankets in it, it is found that there is a definite correlation between the value of attenuation and the values of RT ß of the

various materials. An example, for structure III, is shown in Fig. !5. Similar correlations exist for other simple types of structure. The procedure and equipment for measurement of flow-resistance are very simple in comparison with those required for attenuation measurements.

In practical applications, particularly with very light weight materials, the value of this procedure may be limited because of non- uniformity of materials and the consequent difficulty of obtaining truly representative samples. The data plotted in Fig. 15 were obtained by measuring the average flow-resistance of many samples cut from the identical 18 • X 18 '• blankets used in the attenuation measurements.

7 R. H. Nichols, Jr., "Flow Resistance Characteristics of Fibrous Acoustical Materials," in preparation for publica- tion.


ACOUSTICAL MATERIALS AND

VI. SOUND ABSORPTION BY VARIOUS STRUCTURES

The pressure in a sound wave, striking an acoustical material will tend to do two things. In the first place, the sound pressure will tend to force particles of air through the pores of the material. Because of the frictional resistance

between the air particles and the particles of the material, some of the energy of the sound wave will be dissipated or absorbed in the pores. Secondly, the sound pressure will tend to dis- place the material bodily, a reaction which usually contributes little to the dissipation of energy and consequent absorption of sound.

It is immediately obvious that if there is to be a forcing of air particles through the material, either the material must be heavy enough to remain relatively stationary or it must be me- chanically constrained. The lower the frequency at which the material is to remain relatively immobile, the greater the surface density (or stiffness) must be. Also. the resistance to air flow (flow-resistance) of the material must not be so great that it prevents the flow of air into the pores of the material. On the other hand, if the flow-resistance is too small, there is insuf- ficient friction to cause appreciable loss of sound energy. Hence, for any given weight per unit area (surface density) of material and for any given frequency of incident sound there is an optimum value of flow-resistance for producing a maximum of sound absorption. This value, it appears from experiment, is not too critical and most porous materials can be adjusted to meet the requirements.

In order to absorb sound effectively over a wide frequency range, theory shows that the material should be one or more inches thick. Materials less than one inch thick will absorb

sound, but the absorption coefficient will drop off rapidly as the thickness is decreased.

Experimental data taken on many structures lead to the following conclusions'

1. To absorb sound effectively at low frequencies, the material should be spaced out as far as practicable from the fuselage skin.

2. The material should have a flow-resistance of less

than 500 acoustic ohms (c.g:s. units in g cm -2 sec. -1) per inch of thickness.

3. The material should be of uniform construction, and should not be covered with an impervious covering or

TREATMENTS FOR AIRCRAFT 441

with a covering having a flow-resistance greater than 42 acoustic ohms.

4. The presence of an impervious septurn between two halves of a material does not seriously decrease the absorption coefficient.

5. In mounting the materials in panels in airplanes, every effort should be made to prevent leaks from front to back around the edges. If there are cracks between adjacent panels, the air will be "pumped" back and forth through the cracks rather than through the material. For this reason it is recommended that the materials be

mounted in as large panels as practicable and that the joints be sealed as effectively as convenient.

Measurement of Sound Absorption

The absorption coefficients of an acoustical structure, including the material, the fuselage wall, and the spacing, are very difficult to measure. The values obtained for the absorption coefficients are a function of the method of

ß

measurement, the size of the sample used, the way the sample is mounted, the history of the material, and if small samples are used, the particular portion of the total sample chosen.

Two methods of measuring absorption coefficients are commonly used today. The first is the method approved by the Acoustical Ma- terials Association and is especially applicable to room acoustics. Relatively large samples of material, about 55 square feet, are mounted in the desired structure against the walls of a reverberation chamber. This is a chamber lined

with smooth hard walls to absorb as little sound

as possible before the samples are introduced. The technique of measurement in reverbera-

tion chambers is described thoroughly in the literature. Briefly, a loudspeaker sound source is introduced into the chamber, either in a fixed position or on a swinging pendulum to facilitate the distribution of the sound energy. Corre- spondingly, a microphone, either revolving on a moving boom, or stationary, picks up the incident sound energy. Then the differences in the time required for the sound to decay before and after adding a known area of acoustical material are measured. If the acoustical sample is large and highly absorptive, the absorption coefficient is given approximately by the formula'

a=O.O5V/tS,

where a is the absorption coefficient, V is the volume of the room in cu. ft., $ is the area of the



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FIG. 16, Coefficients oœ absorption oœ normally-incident sound for structures of types I and II with various acoustical materials.

material in sq. ft., and t is the time in seconds required for the sound to decay to one one- millionth of its initial energy, i.e., 60 decibels.

The use of large samples in this method, and the difficulties in duplicating the mounting conditions to be used in an airplane make this method of measurement a clumsy one. Also difficulties arise in the control of experimental conditions such as humidity which make it hard to duplicate measurements closely. However, since this method measures an a for random incidence of the sound energy, it is felt to be a better index of the absorption characteristics in an airplane than methods which measure a,, the absorption coefficient for sound incident normally to the sample under test.

The second method for measuring absorption coefficients is called an impedance tube method. a This method measures the normal incidence ab-

s Hale J. Sabine, "Notes on Impedance Measurement," J. Acous. Soc. Am. 14, 143 (1942).

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FIG. 17. Coefficients of absorption of normally-incident sound for structures of types III and IV with various acoustical materials. Reverberation-chamber data are also plotted for the structures with one material.

sorption coefficient. The impedance tube method has the advantage that only small samples are used, 1 sq. ft. or smaller, and the technique of measurement is simpler than for the reverberation chamber method. From a comparison of the results of the two methods, the second method can be used to estimate the coefficients to be expected in an airplane.

A limitation of the tube method of measurement is that the range of frequencies over which accurate measurements can be made is restricted by the cross-sectional dimensions of the tube. For an 8-inch square cross section, the absorption coefficient can be measured accurately only up to 800 cycles per second. Measurements can be made with fair accuracy to frequencies as high as 1500 c.p.s., but interference is due to cross resonances which prevent measurements at higher frequencies. The range of freriuency measurement could be extended by decreasing the size



of the tube cross section, but then errors due to too small samples and disturbances due to the size of the microphone become large.

Typical results of measurements of absorption by structures of types I, II, III, and V with various materials are shown in Figs. 16 and 17. (No trim cloth was used in these structures.) Results of both types of absorption measurement are plotted for one material in Fig. 17. It is seen that spacing of the material away from the panel makes a very worth while improvement in the absorption at low frequencies (Fig. 16), which is important because of the small attenuation obtained at low frequencies. It is of interest to note that for random incidence of sound (reverberation chamber) the absorption in the middle frequency range is substantially uniform with frequency (Fig. 17). Orie notes also that the different materials all give nearly the same absorptive performance when the structure has an air space.

Absorptive Properties of Trim Cloths

The effects of various types of trim cloth on the absorption of airplane-type acoustical treatment was measured by the reverberation chamber method. The absorption of the test structures was measured first without trim, and then with a

,

number of different types of trim cloth. It was found that any trim cloth which has a flow- resistance of less than 42 acoustic ohms will have

no significant effect on the absorptive characteristics of a structure.

A series of tests on doped fabric trim cloth was also run. Doped fabric trim cloths have been extensively used in the interior of commercial transport airplanes. Because of the ease of cleaning and the pleasing appearance, there is much to be said in their favor. If the floor is

carpeted, and if a large number of people and upholstered chairs are present in the cabin, the total absorption may be high enough to reduce materially the sound levels despite the poor absorbing properties of the impervious trim at the higher frequencies.

In order to retain the features of appearance and ease of cleaning, tests were undertaken to determine methods for improving the absorbing properties without materially sacrificing the other features. In these experiments the ab-

TABLe. III. Absorption coefficients for material covered with doped fabric.

No Perf. Perf. Large Trim Frequency Holes Once Twice Holes Removed

250 0.72 0.84 0.73 0.80 0.59 500 0.73 0.71 0.70 0.79 0.86

1000 0.57 0.57 0.57 0.63 0.75 2500 0.19 0.25 0.30 0.41 0.72 4000 0.13 0.13 0.17 0.31 0.72

sorbing material was mounted in frames and placed in the chamber as before. The front of each frame was covered with doped fabric prior to this test. Absorption coefficients were then measured at the five frequencies, 250, 500, 1000, 2500, and 4000 c.p.s.

The surface of the doped fabric was then perforated by using a special device made by embedding phonograph needles in the surface of a brass cylinder. With it, it was possible to perforate the fabric with small holes &-inch in diameter, one-half inch apart. Measurements were made with these holes and were repeated after going over the fabric a second time so that twice as.many holes, symmetrically placed, were in the fabric. Finally, one-half of the holes were enlarged to x--•" using a different device.

The three sets of data are tabulated in Table

I II. It is seen that for frequencies above 1000 c.p.s., the absorbing efficiency is greatly increased by increasing the number and size of perforation. The definite advantage of using a trim cloth with very low flow-resistance (less than 42 acoustic ohms) can be seen by observing the values in the fifth column.

ACKNOWLEDGMENTS

The authors wish to acknowledge the generous help of personnel of the Air Materiel Command at Wright Field, of the Navy Bureau of Aero- nautics, and of many manufacturers of acoustical materials. Grateful acknowledgment is also made of the many helpful suggestions of Pro- fessor Leo L. Beranek, who, in particular, out- lined the procedure for measuring the attenuation of light-weight panels and was responsible for the concept of "Merit Factor." The assistance of Mr. H. F. Dienel, who took the experimental attenuation data for Fig. 15, is deeply appreci- ated.


acoustical materials and acoustical treatments for aircraft

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