NASA/TM-2009-215954

Development of Quiet Honeycomb Panels

Daniel L. Palumbo and Jacob Klos

Langley Resear°ch Center, Hampton, Virginia

December 2009
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NASA/TM-2009-215954

Development of Quiet Honeycomb Panels

Daniel L. Palumbo and Jacob Klos

Langley Resear°ch Center, Hampton, Virginia

National Aeronautics and

Space Administration

Langley Research Center

Hampton, Virginia 23681-2199

December 2009
Available from:

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076-1320

443-757-5802
I. Abstract

Sandwich honeycomb composite panels are lightweight and strong, and, therefore, provide a reasonable alternative to the aluminum

ring frame/stringer architecture currently used for most aircraft airframes. The drawback to honeycomb panels is that they radiate noise

into the aircraft cabin very efficiently provoking the need for additional sound treatment which adds weight and reduces the material's

cost advantage. A series of honeycomb panels were made which incorporated different design strategies aimed at reducing the

honeycomb panels' radiation efficiency while at the same time maintaining strength. The majority of the designs were centered around

the concept of creating areas of reduced stiffness in the panel by adding voids and recesses to the core. The effort culminated with a

reinforced/recessed panel which had 6 dB higher transmission loss than the baseline solid core panel while maintaining comparable

strength. Attempts were made to damp the panels' vibration energy by the addition of li ghtweight particles to the honeycomb cells.

These designs were very difficult to build given the particles' tendency to pollute the bond interface between the honeycomb and the

face sheet. Well constructed panels exhibited very little benefit from the treatment that could not be attributed to the added mass alone.
H. Introduction

The application of composite materials to aircraft structures will decrease aircraft mass and change how aircraft are designed and

constructed. The expectation is that the new materials will reduce the life cycle cost of the aircraft through lower manufacturing and

operational costs. A representative model of current aircraft sidewall construction is shown on the left in Fig. 1. The sidewall is

composed of 3 major components; the aircraft skin, longitudinal stringers and circumferential ring frames. All the components are

aluminum and are riveted to gether. The mass per Lunt area of the stiffened aluminum panel is 1.2 lb/ft2 (5.9 kg/m2).

Fig. 1 Stiffened aluminum sidewall construction (left) is typical of most aircraft today. On the right is a

composite honeycomb sandwich panel similar in construction to Hawker Premier fuselage.

Contrast the aluminum sidewall to the honeycomb sandwich composite panel shown on the right in Fig. 1. Visually the differences

are striking. The interior is smooth and the panel is much thinner than its aluminum counterpart, <1" (2.54cm) for the composite panel

vs. ?25' (6.35cm) for aluminum panel. This gives the designers the freedom to set a smaller diameter fuselage for a comparable

interior space, saving even more weight beyond the 35% reduction the materials already provide, (the mass per unit area of the

honeycomb panel is 0.79 lb/ft2 (3.9 kg/m2)).

However, this weight savings comes at a cost in increased levels of interior noise. This trend is observed in transmission loss

measurements where the acoustic power incident on the `source' side of a panel is compared to the power on the `receiving' side. A

high transmission loss implies reduced interior noise. Fig. 2(a) shows the transmission loss of the stiffened aluminum panel compared

to a flat honeycomb composite panel of construction similar to the curved panel in Fig. 2. As can be seen, the honeycomb panel loses

as much as 10 dB in transmission loss to the aluminum panel. This deficiency will have to be compensated by added acoustic

treatment, reducing the weight benefit the manufacturer and it's customers expected to achieve. Note that the acoustic treatment weight

penalty is particularly onerous in this case because the honeycomb sidewall already provides sufficient thermal insulation. The

insulation blankets in an aluminum aircraft are needed to provide thermal isolation from the -50° C external temperatures. The

acoustic damping provided by the blankets is an added benefit with little added cost in either dollars or weight. The cost penalty for

acoustic damping in a honeycomb composite aircraft is now solely born by the noise requirement.

For these reasons, it is important to understand why the honeycomb panel has such poor noise performance and to investigate ways

in which the noise penalty can be reduced while maintaining the weight advantage these new materials bring to aerospace vehicle

design. The following section will discuss some of the theory behind the honeycomb composite panel's behavior. Subsequent sections

will present the results of testing various instances of honeycomb panels that were built to understand how the goal of increased

transmission loss without appreciable weight gain might be achieved.
Exterior Panel i i i i i

I I I I I

I I I I I

I I I I I

I I

I

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I I I I I

4 _ _ _ _ _ 4 _ _ _ _ _ 1 _ _ _ _ _

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----^-- -- 4 --- -4------------

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- —I -- - - --- J - - - - - -L - - - - - - - - - - -

I I I I

I I

_____-_____ — Honeycomb, 12.66 lb

- Stiffened AI Panel, 19.31 lb

500 1000 1500 2000 2500 3000

Frequency, Hz

30

28

26

24 m

v

y 22

O

J

.NO 20

rn

N 18

^o

F= ^ e

10'_

(a) 0

Average Difference in TL Over Frequency Band

100-500 500-1000 1000-2000 2000-5000

Fig. 2 Transmission loss of stiffened aluminum panel and solid core honeycomb panel (a), band average difference in

TL (b), and components of honeycomb sandwich panel (c).

A. Structural Acoustics Background

A sandwich core composite panel is composed of three components, the core and the inner and outer face sheets, Fig. 2(c). The

components can be made of many different kinds of materials depending on the application. For example, the core is often foam in

lightweight partitions, but is stiffer honeycomb in load bearing panels. Likewise, the face sheets can be sheet metal, fiberglass or

carbon fiber. The benefit of the sandwich core design is that a lightweight and semi-rigid core material acts to increase the stiffness of

the face sheets by virtue of their constrained displacement away from the composite panel's neutral axis. In this way the strength of the

composite panel is greater than the sum of its parts.

The light weight and increased stiffness results in higher wave

kl = Flexural wavenumber in sandwich plate speeds, and thus lower wavenumbers, in the material. This is

ks = Wavenumber in absence of bending k b = Wavenumber in absence of shear significant because once the wavenumber in the panel falls below the

k - Acousticwavenumber ks wavenumber in air, the panel radiates sound more efficiently. The

wavenumber,,'frequency spectrum can be divided into 3 domains

k, depending on the type of wave propagating', Fig. 3. The lower

Face plate bending frequencies are dominated by bending waves whose nature is

determined by the composite panel properties. The mid-frequencies

Core shear kb contain shear waves that are governed mostly by core properties.

The higher frequencies are dominated by flexural waves in the face

Section bending 

face plates y pyre

sheets. For the honeycomb panels tested here, the shear wave is

considered to be the major source of the panel's increased radiation

efficiency.

The effect of the decreased mass and increased stiffness of the

honeycomb panel can be seen in the respective panels' wavenumber

Frequency w

spectra; Fig. 4. In the wavenumber spectrum plots displayed here

Fig. 3 Dispersion curves for sandwich panels from Fahy'. and elsewhere, the total power in the panels is calculated as the sum

of the squared velocity over the surface of the panel. The velocity

was normalized by the input force during acquisition. The color axis

is in dB taking the total power in the solid core honeycomb panel as reference. The majority of the vibration energy in the stiffened
1

(b) 

30

(a)

0.25 0.5 0.75

Distance. in

-20 -10 0 10 20 30

Wavenumber, rad/m

aluminum panel is well above the sonic wavenumber (the black line), characteristic of a subsonic panel. The energy is also scattered

due to the discrete nature of the panel's construction. In contrast, the power in the honeycomb panel, though much less than the

aluminum panel; is concentrated around the sonic wavenumber, indicatin g its lighter weight, increased stiffness and more uniform

construction. The panel is seen to be substantially supersonic by 400 Hz.

100 200 300 400 500 600 700 800 900 1000

Frequency, Hz

Stiffend Aluminum Panel; Total Power= 73 dB

{ I

i IY

F ld 

120

110

0 100

90

0

80

0 L 70

60

0 50

40

0

30

0 20

10

0 (b)

Solid Core; Total Power = 46 dB

100 200 300 400 500 600 700 800 900 1000

Frequency, Hz

120 zI

110 IJ

100 '1{

90

80

70 L

60

50

40

30

20

10

(a)

0

-10

-20

-30

-40

-5A

-60

-70

Fig. 4 Wavenumber spectra of stiffened aluminum panel (a) and solid core panel (b) compared to sonic wavenumber,

black line.

The uniform construction of the honeycomb panel enables the excitation of almost ideal modes. The velocity response and related

wavenumber spectrum for the honeycomb panel at 340 Hz is shown In Fig. 5. Note the very well defined 2,2 mode. The wavenumber

spectrum of this response shows the energy concentrated around the sonic circle. This curve illustrates why a panel's radiation is not

always reduced by increasing panel stiffness. For a given excitation; a panel's velocity response will be inversely proportional to its

stiffness. Intuitively, one might expect the sound radiation to decrease as panel stiffness increases. However, as stiffness increases, the

panel's energy, as represented by the modal nodes in Fig. 5(b), moves towards the sonic circle, increasing the proportion of the total

panel energy that is radiating efficiently. This increase in radiation efficiency overwhelms the decrease in vibration up to the point

where most of the energy is within the sonic circle. After that, an increase in stiffness will reduce noise radiation as expected.

25

1

0.75

a^U

0.5

Q

0.25

20

15

E 10

-a

L AL:

a^ 0

E 5 !^

C

> 10

^s

-20

Fig. 5 Velocity response (a) and wavenumber spectrum (b) of solid core honeycomb panel at 340

Hz, 2,2 mode. Sonic circle shown as green line in (b).

This behavior is demonstrated in Fig. 6 where transmission loss is plotted for panels with decreased (a) and increased (b) stiffness

with respect to the baseline honeycomb panel. In all cases of the decreased stiffness, the transmission loss is increased. In most cases

4
of increased stiffness, the transmission loss is increased as well. The baseline panel is seen to have been designed with close to the

worst case stiffness when considering noise radiation.

25

20

15

0

.a

0 10

_ 5

N

on

U

-5

100.130.16 0.20.250.32 0.4 0.50.63 0.8 1 1.3 1.6 2 2.5 3.1

On-Third Octave Ra i Canter Prennanry H-T,

(a)

16

— 8

— 4

— 2

— 1.75

1.5

— 1.25

1

).130.16 0.20.250.32 0.4 0.50.63 0.8 1 1.3 1.6 2 2.5 3.1

Ono- Third Octave Baud Center Frequency, kHz

(b)

Fig. 6 Effect on transmission loss of decreasing, (a), and increasing, (b), stiffness of solid core honeycomb

panel.

B. Approach to the problem

The design challenge presented was how to reduce the panel's radiation without increasing its mass and without decreasing its load

bearing capability. The approach taken was to reduce the surface area of the panel which supported supersonic waves. This was

accomplished by milling the core in different locations so that one or both face sheets were not bonded to the core in these locations.

The initial 2 desi gns which resulted from this approach were the voided and recessed core panels, Fig- 7 (a) and (b) respectively- The

voided core had areas where the core was completely removed. This resulted in a core stricture which resembled the aluminum panel's

ring frame/stringer architecture. The voids also created areas of double wall features which would have a positive acoustic benefit.

To make the recessed core, core material was milled to a prescribed depth, from one side of the panel. This was done initially to

accommodate automated lay-up tools which would need a continuous surface on at least one side of the panel to lay down the carbon

fiber tape- As will be seen in later sections, the remaining core not only provided acoustic benefit by damping transverse modes in the

cavity, but also served as a base for increasing the panel's stiffness throu gh the addition of a third, internal face sheetAnother consideration in taking this approach was that it would break up the baseline panel's uniforni architecture and scatter the

panel's vibration energy into many wavenumbers, thus reducing the energy concentrated around the sonic circle- While this effect was

observed, its benefit was hard to quantify. Finally, it was expected that the voided and recessed panels would lose stiffness with respect

to the baseline panel. To address this, finite element models of the panels were created and an optimal design was achieved which gave

up some of the acoustic gains for additional stiffness2,3.

It should be noted that all the panels were made using the same core material with the same core dimensions. The composite panel's

stiffness could have been increased, for example, by increasin g the core thickness. This dimension of the design space was not

explored.
Wave Speed

(a)

wave ^,peea

(b)

Exterior Panel

0.017

Dimensions in meters

Material thickness: 0.040"(0.001 m)

(b)

Fig. 8 Components of stiffened aluminum panel, ring frame

(a) and stringer (b).

Fig. 7 Components of voided (a) and recessed (b) core composite panels.

C. Panel specifications and test procedures

A flat model of a stiffened aluminum sidewall (pictured in Fig. 1(a)) was used as a basis for comparison to the honeycomb panel.

As mentioned earlier, the panel is composed of 3 distinct components, the skin, ring frames and stringers. All the components are

aluminum. The skin is 0.050" (1.27mm) thick. The ring frames (the vertical stiffeners in Fig. 1(a)) have a cross section shown in Fig.

8(a) and are also made from 0.050" (1.27mm) material. The stringers have a cross section shown in Fig. 8(b), but are made from 0.040"

(1.02nun) material. The components are riveted together as is typical of aircraft construction. The complete panel is 4'x4'

(1.22ntix1.22m) with a 1" (2.54 cm) flange provided for clamping into the test window (the `working' section of the panel is then 46"

(1.17m)) square. The sub-panel dimensions are 17.25 in. (43.8 cm) x 5.5 in (14 cm) and the completed panel weighs 19.31 lbs (8.8 kg).

0.021
Dimensions in meters

(0.00367,0.003175) (0.00734,0.003175)

(0.001835,0) (0.009175,0)

The honeycomb panel is also composed of 3 major pieces, the 2 face

sheets and the core. In practice, the face sheets are typically carbon

fiber lay-ups. For test purposes, the face sheets were made from

0.020" (0.51mm) aluminum. This gives the panel the same

approximate stiffness as one with carbon face sheets, with little

added mass. The core is 0.75" (1.91cm) thick Nomex with

hexagonal cells. See Fig. 9 for cell dimensions. The density of the

core was 3 lbs/ft. 3 (38 kg/m3 . The core and one face sheet is 46"

(1.17m) square. The second face sheet is 48" (1.22m) square

providing a 1" (2.54cm) flange that can be clamped into the test

window. The baseline honeycomb panel weighed 12.66 lb (5.7 kg).

The panels were tested for transmission loss and velocity response in

NASA Langley's Structural Acoustic Loads and Transmission

(SALT) facility. The SALT facility is composed of an anechoic

chamber (receiving room), Fig. 10, and a reverb chamber (source

room), Fig. 11, with a test window which is visible in both figures

separating the two

Fig. 9 Dimensions of Nomex core hexagonal cell.

Fig. 10 The anechoic chamber in the SALT facility.

Transmission loss is calculated as the ratio of acoustic power radiated by the receiving side of the panel to the acoustic power

incident on the source side. The incident acoustic intensity on the source side was derived from the average power in the reverb

chamber as estimated by an array of 12 randomly spaced microphones. The transmitted normal acoustic intensity was measured in the

anechoic chamber using Bruel & Kjaer 2683 intensity probes on a 2 in. x 3 in. (5.08 cm x 7.62 cm) grid 5 in. (12.7 cm) from the panel's
surface. A 121nm spacer was used with the intensity probe which resulted in a 15 dB pressure-residual intensity index over the analysis

band. The measured pressure-intensity index was 5 dB.

6-Q: I

Fig. 11 The reverb chamber in the SALT facility.

The wavenumber spectra are calculated as the spatial Fourier transform of the velocity response over the panel. The panels were

excited with pseudo-random noise having a bandwidth of 100 to 2000 Hz using a 10 N shaker through a PCB 288D01 impedance head

located at a point on the panel over solid core. In most cases this was 8 in (203 cm) form one side and 16 in (40.6 cm) from the other.

The normal velocity structural response was measured with a Polytec PSV-300 scanning laser vibrometer. Velocity measurements

were taken on a 1 in. x 1 in. (2.54 cm x 2.54 cm) grid.

III. Voided Core Panels

A. Voided Core, R3 10" (25.4cm) squares, panel 2,11.15 lbs (5.2 kg)

The honeycomb core with milled 10" (25.4cm) voids is shown in Fig. 12(a). The transmission loss (TL) of the assembled panel is

compared to the solid core panel in Fig. 12(b). The acoustic behavior of the voided panel is complicated by the many resonances, both

structural and acoustic, introduced by the design. The voids will exhibit a mass-air-mass resonance due to the double wall construction

and undamped cavity. This resonance can be seen as a pronounce dip in the TL at 400 Hz, followed by a distinct increase in TL. The

increased TL achieved by the action of the double-wall is reduced by a second dip in TL which is seen to occur at about 650 Hz. This

resonance is due to transverse modes in the cavity. The voided panel achieves a 6.9 dB increase in TL between 1 kHz and 2 kHz.

Above 2 kHz, the increase in TL drops to 4.6 dB. The increase in TL is achieved in spite of a 12% decrease in mass compared to the

solid core panel.
120 r ^I

1'10

100

90

80

E 

70

m 

60

50

40

Voided Core; Trial Power= 58 d8

7^1 , 

h

10

301

20

10 1

100 200 300 400 500 600 700 800 900 1000

Frequency. Hz

Fig. 13 Wavenumber spectrum of 30 10" voided panel.

30

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I I I 1 1 1 1 1 1 I I I

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I I I 1 1 1 1 1 1 I I I 1 1 1 1

I I I 1 1 1 1 1 1 I

I I I I I I I I I I

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I I I 1 1 1 1 1 1 I I I

10 I I I 1 1 1 1 1 1 I I I '

I I I 1 1 1 1 1 1 I I I

I I I 1 1 1 1 1 1

Solid Core I I I 1 1 1 1 1 1

3x3 10 in Voids

5 IJ

2 3

10 10

Frequency, Hz

25

Ma

vi

6 20 J

CON

Vl

E 15

Average Difference in TL Over Frequency Band

100-500 500-1000 1000-2000 2000->5000

0.5 6.0 6.9 4.6

(a)

(c)

Fig. 12 Nomex core with 10" voids (a), t ransmission loss (b), and band average difference in TL, (c).

The wavenumber spectrum of the velocity response of the voided

core panel in Fig. 13 has more in common with the aluminum

panel, Fig. 4(a); than the solid core panel, Fig. 4(b). The total

vibrational power is increased by 12 dB over the solid core panel.

This is due to the unconstrained vibration of the panel over the

voids. This part of the panel is substantially subsonic and produces

most of the energy observed in Fig. 13 above the sonic line.

An open question at this point is what the relative strength of the

voided panel would be with respect to the aluminum and solid core

panels. Assuming that the recessed core panel (described in

"Recessed Core, 3x3 10" (25.4cm) squares, panel 4, 12.15 lbs (5.5

kg)" on page 12) has a strength similar to the voided core, it can be

inferred from deflection tests done on the recessed core panel that

the voided core panel lacked substantial strength (see "Deflection

tests" on page 16). Even if the core thickness is increased to

recover stiffness, the panel sections over the voids would have to

be reinforced to sustain pressurization loads. An optimum

combination of thicker core and reinforced voids might result in a

viable design.

B. Voided core, 3x3 6" (15.2cm) squares, panel 7, 12.06 lbs (5.5kg)

This variant of the 10" voided core panel was produced to test the effect of varying the dimension of the voided area on panel TL

performance. The reduced surface area of the voids reduces the effects of the resonances associated with the voids such that the massair-mass resonance which was so pronounced in the 10" panel is not readily visible in the 6" panel. The reduced dimension of the void

increases the transverse modal resonance to above 1 kHz thereby increasing the effect of the mass-air-mass resonance at lower

frequencies. As might be expected, this panel exhibits reduced TL performance compared to the 10" voide core panel.
(a)

(c)

30 I I I I I I I I I I I I

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F T -1 x______1____1_ 1 I

I I I I 1 1 1 1 1 I I

I I I I 1 1 1 1 1 I

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I I I I I 1 1 1 1 I I I L _ _ _ L _ _ I_ _ I J _ L I I I I I I I I I I I

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I I I I I I I I I I I I -------------------------------- I I I I 1 1 1 1 1 I I I

I I I I 1 1 1 1 1

1 1 1 Solid Core I I I I 1 1 1 1 1

1 1 1 3x3 6i Voided

103

Frequency, Hz

Average Difference in TL Over Frequency Band

100-500 500-1000 1000-2000 2000-5000

1.2 4.9 3.9 2.6

25

M

vi

o 20 J

CON

N

E 15 NC

H

10

(b) 5 d

10

Fig. 14 Voided core with 6" square voids in 30 configuration (a), transmission loss (b) and band difference in TL, (c).

C. Voided core, 5x5 6" (15.2cm) square, panel 14, 10.8 lbs (4.9 kg)

The 5x5 void core panel was produced as a reference for the reinforced panel described in section "Reinforced core, 5x5 6"

(15.2cm) squares, panel 15, 13.2 lbs (6.0 kg)" on page 14. The panel achieves close to a 7 dB increase in TL in the range 500 Hz to

2000 Hz after which the increase in TL drops to 5 dB.

(a)

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---- - -r 1 t t ----1--- 7--Y- I I I I I I I I I I

I I I I I I I I I

I I I I I I I I I I I I

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I I I I I I I I I I I I ----------------------------------- I I I I I I I I I I I I

I I I I I I I I I Solid Core I I I I I I I I I 5x5 bin Voids

5L

(b) 102

30

25

M0

N

O 20 J

CO

NN

E 15 Nc

I10

103

Frequency, Hz

Average Difference in TL Over Frequency Band

(c) ^

100 500 500-1000 1000-2000 2000-5000

^ 1 6.5 /.0 5.2

Fig. 15 Voided core with 6" squares in 5x5 configuration (a), transmission loss (b) and band average difference in TL, (c).

10
D. Voided core, 30 10" (2.5.4cm) square, fiberglass filled, panel 6, 11.37 lbs (5Z kg)

Filling the voids with fiberglass removes the effects of acoustic resonances in the voids resulting in a 10 dB gain in TL above 500

Hz. A small dip in TL at the mass-air-mass resonance of 400 Hz is still visible. The combination of thicker core, thicker exterior face

sheet and fiberglass filling may produce a strong yet substantially quieter panel.

30 I I I I I I I I I I I

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25

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m I I I I I I I I I a I I I I I I I I I I I I vi I I I I I I I I I I I I

o 20 1___L__I- I_ J_ L 111______1__

J I I I I I I I I I I I I

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O I I I I I I I I I I

N I I I I I I I I N I 15 - - - -r I r t ----I---7-- r- N I I I I I I I I I I c I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

10 I I I I I I I I I I I

Solid Core

3x3 10in Fiberglass Filled

5

(b) 102

103

Frequency, Hz

Fig. 16 Voided core with fiberglass fill (a),

transmission loss (b), and band difference in TL, (c).

(C)

Average Difference in TL Over Frequency Band

100-500 500-1000 1000-2000 2000-5000

2.6 10.3 9.5 5.8

E. Comparison of voided core panels' performance

The difference in TL between the baseline ; solid core; panel and the modified panels is plotted in Fig. 17. Below 500 Hz the panels'

behaviors are similar. Above 500 Hz the fiberglass filled panel has the best performance with the 3x3 6" panel returning the poorest

performance.

11
T ---, --- ,- _^ I I I I ^ I I I I I I I I

I I I I ^ I I I I I I I I

I I I I I I I I I I I I

I I I I II ^ I I I I I I

I I I I I I^III^ I I I

4-----4---I-- -4

I I I I' I I I

I I I

III

I - I --- -^- I I I I ^^ I' I I ^ I I I II '. i

t

1I I I ^ I I ^` I^ i I^

II I I I I

l

I I h I I I I I I I i I

'^I I

I^ !1

^.I'^1' '.

I I I I I I I I

^ I I I I I ^1 I , -,_ ^ I I I I I I I I ,w I I I I I I I I

I r III I I I

I I I I I I I I A!

II I I I I I I

I I I I I I I I I

-^ 3x3 10 in Voided

3x3 6 in Voided

I I I 5x5 6 in Voided

3x3 10 in Fiberglass Filled

5 2 3

10 10

Frequency, Hz

Average Difference in TL Over Frequency Band

Panel 100-500 500-1000 1000-2000 2000-5000

30, 10" 0.5 6.0 6.9 4.6

30, 6" 1.2 4.9 3.9 2.6

5x5, 6" 1.5 6.5 7.0 5.2

Fiber Filled 2.6 10.3 9.5 5.8

Fig. 17 Voided core panels' transmission loss with respect to baseline

solid core panel (a) and band average differences in TL, (b).

IV. Recessed Core Panels

A. Recessed Core, 3x3 10" (25.4cm) squares, panel 4, 12.15 lbs (5S kg)

The recessed core design was originally conceived to facilitate constriction of assemblies using automated tape machines. In this

process, the tape machine lays down the interior face sheet, the honeycomb is applied, then the tape machine lays down the exterior face

sheet. This process would not work well with the voided core as the tape would sag over the voided areas. Using recessed core,

however, the honeycomb can be laid down with the recesses facing the interior. This would present a continuous surface on the

exterior, so the tape machine would have no trouble applying the exterior layers of tape. The recesses in this panel are cut to a depth of

1/4" (0.64cm). During testing it was observed that the orientation of the recesses, i.e., whether they faced the source room or the

receiving room, did not alter the transmission loss. Orientation would, of course ; matter during vibration measurements which were

done over the recessed side of the panel.

The recessed core had unanticipated acoustics benefits. The transmission loss, Fig. 18(b), is similar to the fiberglass filled panel;

Fig. 16(b), with the exception of a dip in the TL around 600 Hz. This resonance is due to the same transverse modes that occur in the

voided core panel that are now largely eliminated by the core except in the recess. The `exterior' face sheet is bonded to the core so the

effective mass and stiffness of the exterior face sheet increased, altering the mass-air-mass resonance and the effectiveness of the

double wall.

15

P

O

U

°— 10 0

cn

m

0

v 5 0

J

c

O

N

E 0 Wc

12
25

m

a

vi

o 20 J

CO

Cn

n

E 15

w

c

I10

(b) 510z

Recessed 30; Total Power= 52 dB

120

110

100

90

80

70

60

50

40

30

20

10

L

C

100 200 300 400 500 600 700 800 900 1000

Frequency, Hz

30

(a)

(c)

I I I I I I I I I

I I I I I I I I I

I I I I I I I I I

I I I I I I I I I I

I I I I I I I I

I I I I I I I I I

I I I I I I I I I I

I I I I I I I I I I I _I_ _I_ L 111 _ _ _ _ _ _I_ I I I I I I I I I I

I I I I I I I I I I

I I I I I I I I I I

I I I I I I I I I

I I I I I I I I I I

I I I I I I I I I

I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I

Solid Core I I I I I I I I I

Recessed Core

103

Frequency, Hz

Average Difference in TL Over Frequency Band

100-500 500-1000 1000-2000 2000-5000

-).i i.

Fig. 18 Recessed core with 3x3 10" squares recesses, 0.25" deep (a), transmission loss (b) and band average difference

Compared to the voided core, the wavenumber spectrum of the

recessed core, Fig. 19 ; has 6 dB less vibrational power overall and

much less vibrational power around the sonic line throughout the

10 frequency band, as would be expected given that the TL of the

recessed core is 3-5 dB greater than the voided core.

70

Fig. 19 Wavenumber spectrum of 30 10" recessed panel.

B. Reinforced core, 30 10" (25.4cm) squares, panel 11, 14.1 lbs (6.4 kg)

Once it was determined that a ample amount of TL headroom could be obtained using the recessed core design, the effort turned to

restoring the lost stiffness. Stiffness can be restored to the panel by reinforcing the recessed core with then aluminum panels. To test

the effectiveness of this approach, the 3x3 10" recessed panel was modified by the addition of 0.016" (0.41mn) aluminum reinforcing

panels to the recessed areas of the core, Fig. 20(a). The TL of the reinforced panel, Fig. 20(b), is reduced 3-6 dB compared to the

recessed panel ; Fig. 18(b), despite increasin g mass 15%. The mass of the reinforced panel is about 10% greater than the baseline, solid

core panel. The decrease in TL (and increase in mass) is due primarily to the introduction of the reinforcing interior panel. The TL

13
30

(b) 5 2

102

25

M

ui

o 20 J

CO

N

E 15 NCN

10

penalty can largely be compensated by the inclusion of fiberglass in the recesses as was done in a subsequent panel, see "Reinforced,

fiberglass filled, 3x3 10" (25.4cm) squares, panel 20, 13.8 lbs (6.3 kg)" on page 15.

I I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I

I I I I I I I I I I I I 1___ __I J_ 1

I I I I I I I I I I

I I I I I I I I I I

I I I I I I I I I

I I I I I I I I I

I I I I I I I I I

I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I -------------------------------- I I I I I I I I I I I I

I I I I I I Solid Core I I I I I I 3x3 10 in Reinforced

103

Frequency, Hz

(a) Average Difference in TL Over Frequency Band

100-500 500-1000 1000-2000 2000-5000

3.0 6.4 5.1 4.2

Fig. 20 Reinforced recessed core with 10" square recesses, 0.25" deep (a), transmission loss (b) and band averaged

difference in TL, (c).

Reinforced Recessed 3x3; Toial Power = 53 d8

(c)

100 200 300 400 500 600 700 800 900 1000

Frequency, Hz

The total vibrational power in the reinforced panel ; Fig. 21 ; is just 1

dB greater than the recessed panel but more of that power is

10 concentrated around the sonic line, increasing the radiated sound

power. Some of the low frequency modes were moved closer to the

20 sonic line and higher frequency modes (possibly coming from the

interior panel) pop up around the sonic line. All these factors

30 contribute to the observed decrease in TL.

70

120

110

100

90

80

E

r 70

m 

60

50

40

30

20

1 F11

Fig. 21 Wavenumber spectrum of 30 10" reinforced panel.

C. Reinforced core, 5x5 6" (15.2cm) squares, panel 15, 13.2 lbs (6.0 kg)

In an attempt to assess the potential of the reinforced-recessed core design, an optimization effort was conducted using finite

element models 23. This study was limited to varying the layout of the recessed areas and did not consider other design parameters,

such as thicker core material. The result of the study was a design which featured 6" (15.2cm) square recesses reinforced by 0.016"

(0.4mm) aluminum. The recesses were arranged in a 5x5 array as shown in Fig. 22(a). The panel was only 5% heavier than the

baseline, solid core, panel. This was achieved by increasing the recess depth to 1/2" (1.27cm). The 5x5 panel had a 1-2 dB advantage

in TL over the 30 panel.

14
30

5L

(b) 102

25

ma

vi

o 20 J

C

O

en

en

E 15

c

I10

Reinforced Recessed wlFiberglass; Total Power= 48 dB

100 200 300 4100 500 600 700 800 900 1000

Frequency, HE

120

110

100

90

80

L 70

60

m

50

40

30

20

10

(b)

I I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I

I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I

I I I I I I I I I

I I I I I I I I I I — — —r I t 7 ----I---7--t- I I I I I I I I I

I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I

Solid Core I I I I I I I 5x5 6 in Reinforced

103

Frequency, Hz

Fig. 22 Reinforced recessed core with 25 6" recesses, Average Difference in TL Over Frequency Band

0.5" deep (a), transmission loss (b) and band average 100-500 500-1000 1000-2000 2000-5000

difference in TL, (c). (c) 3.7 1 7.1

D. Reinforced, fiberglass filled, 30 10" (25.4cm) squares, panel 20, 13.8 lbs (6.3 kg)

As the benefit of filling panel voids with fiberglass had been demonstrated on a voided core panel; the last panel in this series was a

reinforced core panel with the 1/4" recesses filled with fiberglass, Fig. 23(a). Unfortunately, at the time this panel was produced, the

SALT was not configured for TL tests, and only vibration tests were conducted. The resulting wavenumber spectrum, Fig. 23(b), has 5

dB less total vibration power than the 3x3 reinforced panel; and much less power around the sonic line at the higher frequencies. The

fiberglass does not affect the panel's global anodes at the lower frequencies.

Fig. 23 Reinforced recessed core with 10 inch squares, fiberglass filled (a), wavenumber spectrum (b).

E. Comparison of recessed core panels' performance

15
5

(b) 102

2

`OU

1n -10 ai

-5

a

C

7

O

1n

a 0

+4-+7

a

15

- 102 103

(a) Frequency, Hz

15

P

0U

72 10 O

ai

U 5

1nO

CON

E 0

C

H

As TL testing was not performed on the fiberglass treated reinforced panel, the comparisons must be done in 2 phases. The first

compares the available TL data of the 3x3 recessed and reinforced panels to the 5x5 panel, Fig. 24(a). The 5x5 panel achieves increases

of TL of 3 dB over the 3x3 reinforced panel for 2 modes just below and above 400 Hz and 2 dB increase above 800 Hz. This increase

in TL reduces as frequency increases. The improved performance of the 5x5 panel is due in part to the decrease in stiffness as discussed

in the next section. The second phase compares the radiated sound power of the 3x3 panels to the solid core panel, Fig. 24(b) (note that

the ordiante axis is inverted. This was done to make the results in (b) similar to transmission loss in (a)). The fiberglass treated panel

tracks the untreated panel up to 600 Hz where gains of 3 dB are achieved through damping of the voids' modes.

I I I I I I I I I

I I I I I I I I I

I I I I I I I I

I I I I I I I

I I I I I I I

I I I I I I I I I

I I I I ^ I I

I I I I^ ^ L I I 1 11111 1 111111 Al l

I I I

I I I I I -- 4--- -4- I-- -

I I I I I 1 I I I I I I .^ y I I I I I I

I I I I I I I I I

I I I I I I I I I I I I I I -4__1_-4_L-4_1---------

U3 10 in Recessd

- 3x3 10 in Reinforced

3x3 10 in Fiberglass + Reinforced

103

Frequency, Hz

Average Difference in TL Over Frequency Band Average Difference in RSP Over Frequency Band

(C) (d)

Panel 100-500

500-

1000

1000-

2000

2000-

5000

x ec .. 1 .1 .

3x3 Rein 3.0 6.4 5.1 4.2

5x5 Rein 3.7 7.1 7.5 4.9

Panel 100-500 500-1000 1000-2000

x ec .. ..

30 Rein 2.0 4.0 3.7

30

Rein+Fiber

2.5 6.9 5.2

Fig. 24 Comparison of recessed core panels' performance, transmission loss (a), radiated sound power (b), band

difference in TL, (c) and band difference in RSP, (d).

F. Deflection tests

An important requirement for the honeycomb panels is their ability to bear loads. As it became apparent that ample acoustic

performance could be gained by milling sections of the core, attention turned to restoring the panel's load bearing ability. Finite

element tests indicated that reinforcing the core recesses with thin aluminum sheets could, depending on the configuration, restore the

panel's strength. To test how well the reinforcing worked, deflection tests were performed on the solid core, recessed core and

reinforced core panels. The deflection tests were done at 3 points as shown in Fig. 25 on the `external' side of the panel (unmilled side)

with the panel mounted in the SALT test window. Pressure was applied through a 5 lb (22.2 N) load cell and deflection measured with

a dial indicator. The results of the tests are shown in Table 1 in μ in ,'lb. (0.056 micron/kg). The recessed panel deflection rate is an

order of magnitude larger than the solid core panel. The recessed panel may have gained transmission loss, but it lost significant

stiffness. The reinforced panel deflection rates are much closer to baseline with the maximum difference occurring at point 2 which

deflected 40% more than the baseline. The optimized 5x5 panel's maximum difference also occurred at point 2, but was 100% greater

than baseline. Recall that the 5x5 panel recesses were twice as deep as the 3x3 panel (1/2" vs. U'4") reducing the thickness, and;

therefore, the stiffness, of the reinforced section.

16
Fig. 25 Test points for deflection tests, 30 panels, (a) and 5x5 panels (b).

Table 1: Deflection Data

Pt 1

I1 in lb.

Pt 2

^t llL%1b.

Pt 3

^t llL%1b.

Solid Core 300 390 500

Recessed, 3x3 10" 5000 3700 1200

Reinforced, 3x3 10" 360 540 660

Optimized, 5x5 6" 560 800 950

Domed, 5x5 6" 460 450 670

17
30

25

m

ui

a 20 J

G

ON

E 15

c

F10

5

V. Domed Core Panels

Previous panel features were squares which produced sharp corners and edges. This type of construction is avoided in practice

because it leads to concentrations of stresses and eventually delamination of the composite. To address the issue, a series of panels

were assembled with domed recessed areas.

A. 5x5 6" (15.2cm) domes, polycarbonate disks, panel 16,13.4 lbs (6.1 kg)

This panel featured 6" (15.2cm) diameter domes cut to a maximum depth of 1/2" 1.27cm). The recesses were reinforced with

polycarbonate disks made using a stereo lithography machine. This panel achieved TL behavior similar to the 30 10" reinforced panel

with a gain in TL of about 3-5 dB over baseline above 500 Hz. The domed panel was subjected to deflection tests similar to those

performed on the recessed panels, Fig. 25(b) and Table 1. The domed panel had a maximum deflection 50% greater than the baseline

solid core panel at point 1 ; a figure comparable to the 30 reinforced panel and an improvement of 20%-40% over the 5x5 optimized

panel.

(b) 102

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I

I I I I I I I I I I

I I I I I I I I I I

I I I I I I I I I I I 1___L_ __I_ _L ______I__ I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I

I I I I I I I I I

I ---- - -r I r t ----I---7-- r- I I I I I I I I I I

I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I Solid Core

5x5 Polyearb Domes

103

Frequency, Hz

(a) Average Difference in TL Over Frequency Band

100-500 500-1000 1000-2000 2000-5000

(c) o.

Fig. 26 Reinforced recessed core with 6" domes, 0.5" deep, polycarbonate inserts (a), transmission loss (b) and band

average of difference in TL, (c).

18
Domed 5x6 Poly; Total Power= 52 dB

100 200 300 400 500 600 700 600 9010 1000

Frequency, Hz

120

110

100

90

so L 70

60

s0

40

30

20

10

10

20

70

The wavenumber spectrum of the domed panel is similar to the 3x3

10" reinforced panel as well, Fig. 27. Of interest, the low frequency

modes seem to be attenuated.

Fig. 27 Wavenumber spectrum of 5x5 polycarbonate domed

B. 5x5 6" (15.2cm) domes, carbon inserts, panel 17,13.2 lbs (6.0 kg)

In an attempt to increase the stiffness of a domed panel, the polycarbonate domes were replaced with carbon fiber domes. The

modification did not appreciably alter the TL. The polycarbonate panel has more TL in the range of 1-2 kHz, but less in the range 500-

1000 Hz. Unfortunately deflection tests were not conducted on this panel.

30

(a)

(c)

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I -------------------------------- I I I I I I I I I I I -I

I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I 1 _ _ _ L _ _ J _ L 1 I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I

I I I I I I I I

I ----- — —r I r r ----I---7- - r- I I I I I I I I I

I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I -- ------------------------------- I I I I I I I I I I I I

I I I I I I I Solid Core I I I I I I I 5x5 Carbon Domes

5

(b) 10 103

Frequency, Hz

Average Difference in TL Over Frequencv Band

100-500 500-1000 1000-2000 2000-5000

2.6 6.6 3.9 3.0

25

m

ui

6 20 J

CON

In

E 15 w

c

10

Fig. 28 Reinforced recessed core with 6" domes, 0.5" deep, carbon fiber inserts (a), transmission loss (b) and band

average of difference in TL, (c).

19
Looking at the wavenumber spectrum, Fig. 29, it appears that the

carbon fiber inserts did increase the panel stiffness. The total

vibration power in the panel is 3 dB less than the polycarbonate

10 based panel and the low frequency modes hold more energy, but are

still subsonic.

Domed 5x5 Carbon. Total Power = 49 dB

120

110

100

90

80

70

60

50

40

30

20

10

a

v

70 100 201] 300 400 500 600 700 800 91DCI 1000

Frequency, Hz

C. Staggered 6" (15.2cm) domes, carbon inserts, panel 18, 12.23

lbs (5.5 kg)

The row,"Column organization used for panel recesses thus far has the

disadvantage that it creates fold lines along which panel anodes can

form. A staggered pattern was created in an attempt to disrupt the

regular pattern, Fig. 30 (a). Although the panel produced marginal

gains in TL, Fig. 30(b), over the other domed panels, the TL is

noticeably lacking modal peaks. Performance might be increased

further as this pattern is still regular along the diagonal and; thus,

introduces fold lines along the diagonal axes. At this point in panel

development, it became obvious that arbitrary distributions of mass

and stiffness could be achieved using the reinforced/recessed

approach and an optimization effort with greater degrees of freedom

than had been employed thus far was necessary.

Fig. 29 Wavenumber spectrum of 515 carbon domed panel.

30

25

m-a

N

0 20 J

C

N

N

F 15 NC

10

5

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I I -------------------------------- - I I I I I I I I I I I I

I I I I I I I I I I

I I I I I I I I I I

I I I I I I I I I

I I I I I I I L _ _ _ L _ _ _ J _ L 1 1 1 _ _ _ _ _ _ _ _ L _. I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I

I I I I I I I I

I ---- — —r I I t t ----I---7-- r- I I I I I I I I I I

I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I Solid Core

I I I I I I I Staggered Domes

(b) 102 103

Frequency, Hz

(a) Average Difference in TL Over Frequency Band

100-500 500-1000 1000-2000 2000-5000

(c) .1

Fig. 30 Reinforced recessed core, 6" carbon domes, staggered configuration (a), transmission loss (b) and band

average of difference in TL, (c).

20
Domed 5x5 Carbon Stagger; Total Power = 50 dB

120

110

100

90

80

70

60

50

40

30

20

10

100 200 300 400 500 600 700 800 900 1000

Frequency, Hz

(c)

The staggered panel had 1 dB more vibration energy than the carbon

insert panel and had more energy scattered in the higher

wavenumbers at mid-frequencies where energy was moved array

10 from the sonic line. At hi gh frequencies it appears more energy is

concentrated around the sonic line.

E

D. 5x5 6" (15.2cm) domes, carbon face sheet, panel 19, 13.8 lbs

(6.3 kg)

This panel was built to see if the reinforcements could be built into

the face sheets. The idea was that flat aluminum discs could be

placed over the recesses at a later time, completing the double wall.

The face sheet was molded in carbon fiber. Unfortunately, due to a

deficiency in specification, the panel was built with the carbon

inserts as well. This increased the panel mass and reduced the value

of the data. With the panel configured as it appears in Fig. 32(a),

several dB of TL were gained over the solid core panel. This may be

due to the panel being slightly less stiff and more massive resulting

in a slightly lower wave speed. The panel was never tested with the

flat discs installed.

Fig. 31 Wavenumber spectrum of staggered domed panel.

(b)

30 I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I I I I I I I I I I I — I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I

i --- L__ L J L 1 1 1 _ _ _ _ _ _I I I I I I I I I I I I

I I I I I I I

I I I I I I

I I I I I I I I

I

I I I I I I I I I

I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I I I I I I

I I I

Solid Core

I I I Integrated Carbon Facesheet 5 2 

10

Average Difference in TL Over Frequency Band

100-500 500-1000 1000-2000 2000-5000

25

M0

U

U) 20

J

C

ON

fn

E 15

fn

C

(6H

10

103

Frequency, Hz

Fig. 32 Reinforced recessed core, 6" carbon domes, carbon face sheet (a), transmission loss (b) and band average of

difference in TL, (c).

21
Domed 56 Carbon Facesheet; Total Power = 49 M

120

110

100

90

80

s 70

i^

60

m

50

40

30

20

10

100 200 300 400 500 600 700 800 900 1000

Frequency. Hz

Although the panel has 3 dB more vibrational energy than the solid

i core panel, the TL is slightly better. Reduced stiffness and increased

mass has reduced the wave speed in the panel just enough to produce

10 lower radiation efficiencies, Fig. 33.

Fig. 33 Wavenumber spectrum of domed carbon face sheet

panel.

12

I I I I I I I I I I I I

N

^

------ 1 _ _ _ 1 1 _1_ J L J _1_1_ _ _ _ _ _ _ L --- 1 _ _ 1 _

^ I I I I I I I I I I ^ I I I I ^I I I I I I

8 ------1 - _____ L --- 1 _ _ 17r

^ ^ _ G'I I ^ I I I I II

I ) I 1 1 0 I I I I I

I

I

`y 1

^

7tI

(Ii

2

^1

I^- O

J

1

p L- ___I_^_ 1 _1

-I111I 1 1 1 1 --.T ^, 1 1

N _`2 _ ^1

N

I I I I

5x5 Polycarb Dome

m

4 - 5x5 Carbon Dome

-g __ _i___1__1__i_ Staggered Carbon Dome

5x5 Carbon Facesheet

(a)

8

10z 103

Frequency, Hz

Average Difference in TL Over Frequency Band

E. Comparison of domed core panels'

performance

The TL gain of the 4 domed panels relative to the

solid core panel is plotted in Fig. 34. The

staggered panel appears to have a slight edge

even though it has 8% less treated area. The TL

performance of these panels is comparable to the

5x5 optimized panel indicating that domes can be

used effectively to reduce noise radiation.

(b'

Panel 100-500 500-1000 1000-2000 2000-5000

Polycarb 2.9 5.4 4.9 2.8

Carbon 2.6 6.6 3.9 3.0

Staggered 3.4 7.1 4.6 3.0

Face sheet 2.0 2.9 0.8 1.5

Fig. 34 Comparison of transmission loss of doomed panels with respect to

solid core panel (a) and band averaged difference in TL, (b).

22
VI. Particulate Damping

There is some evidence in the literature that vibration reduction can be achieved using particle damping. The most cormnon

approach is to use lightweight particles to impart damping through the motion and collisions of the partieles 4 . A second approach is

through radiation damping whereby the particle treatment increases the radiation efficiency of the structure b . A series of tests were

conducted to determine if the composite honeycomb panel could be damped by including li ghtweight particles in the core. Two types

of particles were tested. One was a microsphere made of polyimide' that was shown to have excellent hi gh temperature characteristics

and seemed to offer, through its viscoelastic properties, the promise of adding damping to the panel'. The second particle system was

perlite. This material is lightweight and flame resistant. The major drawback to using these materials in sandwich core panels is

confining the material during assembly. To insure a good bond between the face sheet and the core, the contacting edges of the

honeycomb cells must be free of contaminates. The light weight of the particles and their tendency to take an electrostatic charge, made

it difficult to assemble the last face sheet without the particles getting into the epoxy. This caused problems with several panels as will

be explained in the next sections.

A. Double wall panel with microsphere filled honeycomb

As a first test of the microspheres' structural acoustic damping ability, a section of Nomex honeycomb was filled with

microspheres. The microspheres were contained using a thin fabric. The resulting treatment was tested in a double wall system which

was created by an `exterior' panel of 0.020" (0.51mm) aluminum and an `interior' panel of 0.032 (0.81rnm) aluminum. The panels were

4'x4' (122mx1.22m) and were separated by 1" (2.54cm). The double wall system was tested for TL with an air gap and treatments of

fiberglass, empty honeycomb and the microsphere filled honeycomb, Fig. 3.5. Of the 3 treatments, the microsphere filled honeycomb

produced the lowest TL, even though it was the heaviest treatment. The difference in performance of the 3 systems can be seen to be

largely due to the point at which the mass-air-mass resonance occurs as the slope of the 3 TL curves after resonance is very similar.

Note that the empty honeycomb treatment actually has a double dip, one at the `air' resonance and another presumed to be at the

honeycomb resonance. The microspheres effectively eliminate the air resonance, leaving a largely structural path between the 2 panels

at a higher resonant frequency.

70

60

m 50

to

W 40 J

ON

30

E

m

20

F10

I I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I - ,I

I I I I I I I I I I+ I

I I I I I I I I I -'I'' I

I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I ^' I I

I I I I I I I I I

I I I I I I //' I I

I I I I I ^ I I I I

I__ J__ I_ J I I I I

I I I I

J_I__ I I I I I I

I I I I I

I I I I I I I

I_ _ _ _I_ _ J I

,J/ I I ^ I I I I I

Air Gap

^ I I I I I I Fiberglass

Empty Honeycomb _I_ _ _ _I_ _ J _ _ I_ J _ L JI Microsphere Filled

(a) 102 103

Frequency, Hz

(b)

Average Difference in TL over Band wrt Air Gap

Panel 100-500 500-1000 1000-2000 2000-5000

Fiberglass a.

Honeycomb -1.2 11 .9 19.4 14.9

Microshperes -0.4 7.4 15.3 12.9

Fig. 35 Transmission loss of double wall panels, (a) and band average

difference in TL with respect to air gap, (b).

23
(a)

1. NASA panel, panel 12, 17.8 lbs (8.1 leg)

Great pains were taken in assembling this panel to ensure that the

honeycomb edge was clean and a good bond to the second face sheet

could be obtained. However, this meant that the perlite treatment

was exposed to air in an uncontrolled environment for a long time.

As a result, the finished panel was 40% heavier than baseline. The

added mass reduced the value of anv data taken from the panel to the

point where no further tests were done.

B. Solid core honeycomb with microsphere fill, panel 9, 13.77 lbs (6.2 kg)

Microspheres were added to a solid core panel and tested for TL. The panel exhibited little sound transmission benefit from the

treatment besides a 3 dB gain in TL above 2 kHz that cannot be attributed entirely to the panel's 9% increase in mass over baseline.

The carbon face sheet, domed, panel is about the same mass and gains a slight 1 dB at higher frequencies, Fig. 32(b). Significantly,

the TL curve of the carbon face sheet panel is parallel to the baseline cur v e, implying the gain is solely due to the increased mass. The

high frequency TL of the microsphere treated panel has an increased slope compared to baseline, indicating a different mechanism at

work.

30

25

(b)

M

vi

o 20 J

CON

N

^ 15

c

M

F

10

I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I I

1___ L__I__I_ J_ L 111______ I I I I I I I I I I

I I I I I I I I I I

I I I I I I I I I

I I I I I I I I

I

- - - r I ----r--7--r -

I I I I I I I I

I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I

I I I I I I I I I I I I

I I I I I I I

Solid Core I I I I I I I Microsphere Filled

5

103

Frequency, Hz

102

Fig. 36 Honeycomb filled with microspheres (a),

transmission loss (b) and band average difference (c)

in TL, (c).

Average Difference in TL Over Frequency Band

100-500 500-1000 1000-2000 2000-5000

0.1 0.5 0.3 3.3

C. Solid core honeycomb with Perlite fill.

The perlite was very difficult to work with. It not only polluted the epoxy but absorbed moisture from the air, drastically increasing

its mass.

Fig. 37 NASA panel with Perlite treatment about 40%

complete.

24
2. Rocketdyne panels, panels 21-23

Pratt and Whitney Rocketdyne had experience assembling

honeycomb panels treated with perlite and were contracted to build 3

panels to test the effectiveness of a uniformly applied treatment and

an optimally applied treatment. Rocketdyne had access to

hydrophobic perlite which was said to reduce the material's

tendency to absorb moisture. Unfortunately, they were not careful to

avoid epoxy contamination as all the panels had severe face sheet

bonding defects (e.g., Fig. 38) that made the data taken from the

panels difficult to interpret.

Fig. 38 Rocketdyne panel with 30% Perlite fill. Area of

unbonded face sheet detailed.

VII. Patents

As of this writing there are 2 outstanding patent applications on this technology. Patent application, "Composite panel having

subsonic transverse wave speed characteristics", was submitted in May 2005 and deals primarily with the voided and recessed core

designs. Patent application, "Composite panel with reinforced recesses", was submitted November 2007 and covers all the variants of

the reinforced panels.

VIII. Conclusions

The reinforced/recessed panel desi gn has shown promise. It has been demonstrated that substantial noise reduction can be obtained

while panel strength and mass are maintained. The ultimate utility of this design approach depends on how well the panel features can

be optimized for a particular application's requirements. The optimization performed thus far did not encompass the entire design

space. For example, panel thickness was not varied from one design to the next and feature shape and orientation were uniform for

each design. It is believed that the `embedded double-wall' design will always return increased TL.

IX. References

Tally, Frank, `Sound and Stuctural Vibration, Radiation, Transmission and Response", Academic Press, New York, 1985.

2Grosveld, Ferdinand W., Palumbo, Daniel L.; Klos, Jacob and Castle, William D., `Finite Element Development of Honeycomb Panel

Configurations with Improved Transmission Loss", INTER-NOISE 2006. Honolulu, HI, 3-6 Dec. 2006.

3Grosveld, Ferdinand W., Finite Element and Development and Specifications of a Patched, Recessed Nomex Core Honeycomb Panel for

Increased Sound Transmission Loss", NASA/CR-2007-214898, Sept. 2007.

4Panossian, H., Optimized Non-Obstructive Particle Damping (NOPD) Treatment for Composite Honeycomb Structures, Presented in the SDM 06

Conference, Newport, RI, May 2006.

'Palumbo, D, Park, J, Structural and Acoustic Damping Characteristics of Polyimide Microspheres, 46th AIAA/ASME/ASCE/AHS/ASC

Structures, Structural Dynamics & Materials Conference, April 2005, AI.AA-2005-2328.

6Fricke, J.R.. Lodengraf Damping - An Advanced Vibration Damping Technology, Sound and Vibration, July 2000. pp 22-27.

'Weiser, E., Grimsley, B.W., Pipes, R.B., and Williams, M.K., Polyimide Foams from Friable Balloons, 47th International SAMPE

Symposium and Exhibition, Long Beach, CA, May, 2002, pp. 1151-1162.

25
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1. REPORT DATE (DD-MM-YYYY) 2. REPORT TYPE 3. DATES COVERED (From - To)

01-12-2009 Technical Memorandum

4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER

Development of Quiet Honeycomb Panels

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S) 5d. PROJECT NUMBER

Palumbo, Daniel L.; KIDS, Jacob

5e. TASK NUMBER

5f. WORK UNIT NUMBER

561581.02.08.07.18.02

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION

NASA Langley Research Center REPORT NUMBER

Hampton; VA 23681-2199

L-19785

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National Aeronautics and Space Administration NASA

Washington, DC 20.546-0001

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NUMBER(S)

NASA/TM-2009-215954

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category 71

Availability: NASA CASI (443) 757-5802

13. SUPPLEMENTARY NOTES

14.ABSTRACT

Sandwich honeycomb composite panels are lightweight and strong, and, therefore, provide a reasonable alternative to the

aluminum ring framelstringer architecture currently used for most aircraft airframes. The drawback to honeycomb panels is

that they radiate noise into the aircraft cabin very efficiently provoking the need for additional sound treatment which adds

weight and reduces the material's cost advantage. A series of honeycomb panels were made which incorporated different

design strategies aimed at reducing the honeycomb panels' radiation efficiency while at the same time maintaining its strength.

The majority of the designs were centered around the concept of creatin g areas of reduced stiffness in the panel by adding

voids and recesses to the core. The effort culminated with a reinforced./recessed panel which had 6 dB higher transmission loss

than the baseline solid core panel while maintaining comparable strength. y

15. SUBJECT TERMS

Transmission loss; Wavenumber spectrum: Voided core; Recessed core

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF RESPONSIBLE PERSON

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