Sound insulation property of Al–Si closed-cell aluminum foam sandwich panels

Applied Acoustics 68 (2007) 1502–1510 www.elsevier.com/locate/apacoust

Sound insulation property of Al–Si closed-cell aluminum foam sandwich panels Haijun Yu a

a,*

, Guangchun Yao a, Xiaolin Wang b, Yihan Liu a, Hongbin Li a

School of Materials and Metallurgy, Northeastern University, P.O. Box 117 Liaoning, Shenyang 110004, PR China b Institute of Acoustics, Chinese Academy of Sciences, Beijing 100080, PR China

Received 15 November 2005; received in revised form 26 July 2006; accepted 27 July 2006 Available online 18 October 2006

Abstract Al–Si closed-cell aluminum foam sandwich panels (1240 mm · 1100 mm) of different thicknesses and different densities were prepared by molten body transitional foaming process in Northeastern University. The experiments were carried out to investigate the sound insulation property of Al–Si closed-cell aluminum foam sandwich panels of different thicknesses and different densities under different frequencies (100–4000 Hz). Results show that sound reduction index (R) is small under low frequencies, large under high frequencies; thickness affects the sound insulation property of material obviously: when the thicknesses of Al–Si closed-cell aluminum foam sandwich panels are 12, 22, and 32 mm, the corresponding weighted sound reduction indices (RW) are 26.3, 32.2, and 34.6 dB, respectively, the rising trend tempered; the increase of density of Al–Si closed-cell aluminum foam can also increase the sound insulation property: when the densities of aluminum foam are 0.31, 0.51, and 0.67 g/cm3, the corresponding weighted sound reduction indices (RW) are 28.9, 34.3, and 34.6 dB, the increasing value mitigating.  2006 Elsevier Ltd. All rights reserved. Keywords: Al–Si; Closed-cell aluminum foam; Sandwich panel; Weighted sound reduction index (RW)

*

Corresponding author. E-mail address: [email protected] (H. Yu).

0003-682X/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apacoust.2006.07.019

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1. Introduction The research and development of metal foams started since 1940’s. The earliest report was the patent technology of Sosnick [1] who prepared metal foams through the gasification of low melting point substance. In 1956, Borksten Research Laboratory Inc., first prepared aluminum foam through molten body direct foaming process [2]. Then Foamalum Corp. and Italy Corp. put this technology into practice [3]. In 1987 Japan made a breakthrough in the research of metal foam materials [4–8]. In addition, IFAM [9] in Germany and Alcan International Limited [10] in Canada both made great progress in the production technique of metal foam materials, most of which were about aluminum foam materials [11]. Compared with traditional materials, aluminum foam materials have the advantages of low density, high stiffness, against impact, low thermal conductivity, low magnetic conductivity, and fine damping, so they have become one of the fields in high technology material research all over the world. Owing to the difficulty of preparing large and uniform specimen, the researches in the past focused only on the mechanics and energy absorption of aluminum foam material [12,13], and rarely studied the acoustics especially sound insulation property. Because of its special structure, aluminum foam has great potential application in the fields such as sound insulation and noise reduction. Closed-cell aluminum foams have good sound insulation property, but the perforation in aluminum foam panel may decrease the sound reduction index (R); sandwich panel of aluminum foam can well solve this problem. So in this paper, the author researches the sound insulation property of Al–Si closed-cell aluminum foam sandwich panels of different thicknesses and different densities prepared by molten body transitional foaming process in Northeastern University [14,15], and provides some information for its application in the fields of sound insulation and noise reduction. 2. Experimental 2.1. Preparation of aluminum foam sandwich panel Al–Si closed-cell aluminum foams were produced by molten body transitional foaming process in Northeastern University, China [13]. Al–Si closed-cell aluminum foam of a range of relative thicknesses and densities will be called below the NEU foam. There are five-step techniques to prepare NEU foam: (i) melting alloy of aluminum–silicon and calcium in furnace; (ii) adding titanium hydride (TiH2) to the molten body; (iii) transferring the molten to the bubbly case; (iv) pushing bubbly case to the maintaining furnace and foaming in it; and (v) aluminum foam post processing [14,15]. For convenience, the NEU foam sandwich panels of thickness 12, 22, and 32 mm (density 0.67 g/cm3) and density 0.51, 0.31 g/cm3 (thickness 30 mm) with two pieces of aluminum panel (thickness 1 mm) on both sides by using high strength epoxy are labeled specimen 1, 2, 3, 4, and 5, respectively (see Table 1). The essential parameters of NEU foams are systematically recorded before experiments, including the NEU foam’s density, cell diameter, wall thickness of cell, etc. (see Table 1). Fig. 1a–c are macro pictures of NEU foams with density 0.67, 0.51, and 0.31 g/cm3, respectively. It can be easily seen that the morphology of the three different NEU foams are distinct from each other; the cell morphology is relatively in good condition; the main cell diameter gets larger and larger, and the wall of cell thinner and thinner.

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Table 1 Parameters of NEU foam panels Specimen

Density of NEU foam (g/cm3)

Thickness of NEU foam (mm)

Main cell diameter (mm)

Wall thickness of cell (mm)

Length (mm) · Width (mm)

1 2 3 4 5

0.67 0.67 0.67 0.51 0.31

10 20 30 30 30

3 3 3 5 11

0.53 0.53 0.53 0.44 0.42

1240 · 1100 1240 · 1100 1240 · 1100 1240 · 1100 1240 · 1100

Fig. 1. Macrograph of NEU foam of different densities: (a) q = 0.67 g/cm3; (b) q = 0.51 g/cm3; and (c) q = 0.31 g/cm3.

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Fig. 2. Sketch map of sound insulation testing.

2.2. Testing of sound insulation property Sound insulation properties of the NEU foam sandwich panels are tested at Institute of Acoustics, Chinese Academy of Sciences [16]. Fig. 2 is the sketch map of testing. Fig. 3 is the specimen picture during testing. The sound pressure signals of the receiving room and emitting room are collected through Real Time Analyzer 840 sound pressure analytical apparatus. The weighted sound reduction index (RW) measurements are performed according to ISO 140/3 procedure [17]. 3. Results and discussion 3.1. Effect of frequency The thickness of each aluminum panel is 1 mm and the thickness of NEU foam panel is much larger. Moreover, the aluminum panels are attached to the NEU foam panel by using high strength epoxy, so the air chambers between NEU foam and aluminum panels have a negligible effect, as a result the NEU foam sandwich panels can be treated as single

Fig. 3. Specimen picture during testing.

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separation element. If an infinite simple separation element is held to behave like a group of juxtaposed masses, having independent displacement, and null damping forces, the sound reduction index (R) for plane wave incidence follows a law, known as Mass Law [18,19]. Thus the Mass Law can be introduced here simply as a reference to research the sound insulation property of NEU foam sandwich panels in this study. A simple one-degree-of-freedom model readily illustrates that when f  fn (critical frequency) the stiffness dominates, when f  fn the damping dominates, and when f  fn the mass dominates [20]. The random-incidence sound reduction index (R) can be evaluated from the following approximate equation, where R0 is the vertical incidence sound reduction index; m is the surface density of board material; f is the frequency [20]. R ¼ R0  5 dB ¼ 20lgðm  f Þ  48 dB

ð1Þ

Fig. 4a–e are the sound insulation property results of the five specimens, from which it can be concluded that sound reduction index (R) increases when the frequency rises from 100 Hz to 4000 Hz; sound reduction index (R) can reach about 20 dB at lower frequencies, 50 dB at higher frequencies, which indicates that the sound reduction index (R) of NEU foam sandwich panels are subjected to frequency. Results also show that there are no significant fluctuations in the curves of sound insulation property of NEU foam sandwich panels. Compared with entity material, aluminum foams have good damping effect; the loss coefficient (g) is one order of magnitude higher than that of entity material [21]. The consumption of vibration by aluminum foam is mainly through the friction between crack faces (see Fig. 5) in the structure transmitting vibration energy to thermal energy and then dispersing into the surrounding environment. In addition to the vibration of crack faces, the vibration of cell wall also contributes to the damping effect of aluminum foam. 3.2. Effect of thickness Besides frequency, thickness of NEU foam sandwich panels also plays an important part in sound insulation. Eq. (1) indicates that the sound reduction index (R) is influenced mainly by surface density and frequency. When frequency is fixed, the relation between R and m are logarithmic, from which it can be concluded that sound reduction index (R) of NEU foam sandwich panels increases with the addition of surface density, but the increasing trend becomes mild. Owing to the direct proportional relation between surface density and thickness of boards, the weighted sound reduction index (RW) of sandwich panels of different thickness grows larger when thickness is added, but does not increase equivalently with the evenly added thickness. When the thicknesses of NEU foam sandwich panels are 12, 22, and 32 mm, the corresponding weighted sound reduction indices (RW) are 26.3, 32.2, and 34.6 dB, respectively, the rising trend tempered (see RW in Fig. 6). Thus the increase of sound insulation property cannot be achieved by adding the thickness of NEU foam sandwich panel blindly. At present, in many cities of the world the traffic noise has reached 85 dB, whereas the maximum limitation of traffic noise in most countries is about 60 dB [22]. Based on the present tests, NEU foam sandwich panel (thickness, 22 mm) can already sufficiently decrease the traffic noise to the value below the ordained one, so further increasing the thickness of NEU foam sandwich panel to enhance its sound insulation property is a waste of material.

H. Yu et al. / Applied Acoustics 68 (2007) 1502–1510 50

a

40

40

Rw=32.2

30

Rw=26.3

R(dB)

20

25 20 15

10

Sound insulation index Reference curve (ISO 717-1)

10

Sound insulation index reference curve (ISO 717-1)

5

0

Frequency f(Hz)

c

45

RW =34.6

35

4000

3000

2000

600 700 800 900 1000

RW =34.3

30 25 20

Sound insulation index Reference curve (ISO 717-1)

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15

Sound insulation index Reference curve (ISO 717-1)

50 60 70 80 90 100

4000

3000

2000

500 600 700 800 900 1000

400

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0 50 60 70 80 90 100

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4000

20

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R(dB)

30

R(dB)

d

40

40 35

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Frequency f(Hz) 50

50 45

400

300

200

60 70 80 90 100

50

4000

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e

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RW =28.9

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Sound insulation index Reference curve (ISO 717-1)

5

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500 600 700 800 900 1000

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Fig. 4. ISO14O-3 and 717-1 sound reduction index (R) vs. frequency of the various specimens: (a) specimen 1; (b) specimen 2; (c) specimen 3; (d) specimen 4; and (e) specimen 5.

3.3. Effect of density Fig. 7 is the sound insulation property of NEU foam sandwich panels (NEU foams, thickness = 30 mm, q = 0.31, 0.51, 0.67 g/cm3). The weighted sound reduction index

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Fig. 5. Cracks on cell wall of NEU foam.

The Weighted Sound Reduction Index (dB)

36

34

NEU foams: 3 ρ=0.67g/cm Thickness: 10mm 20mm 30mm Aluminum faces: 3 ρ=2.7g/cm Thickness: 1mm+1mm

32

30

28

26

10

15

20

25

30

35

Thickness of Specimen (mm)

Fig. 6. Weighted sound reduction index (RW) vs. different thickness of NEU foam sandwich panels.

(RW) increases obviously from 28.9 to 34.3 dB when the density of NEU foam is added from 0.31 to 0.51 g/cm3; no obvious change (from 34.3 to 34.6 dB) occurs when the density turns from 0.51 to 0.67 g/cm3. The increasing trend of the weighted sound reduction index (RW) gradually becomes mitigated. The reason why the addition of density results in the increase of the weighted sound reduction index (RW) can also be explained by Eq. (1): just like the addition of thickness, the addition of density can also increase the surface density of NEU foam sandwich panels which leads to the increase of weighted sound reduction index (RW). NEU foam sandwich panel (NEU foam, q = 0.51 g/cm3) already has fine sound insulation effect (with RW 34.3 dB) in practical situation, and further increasing of density cannot raise the weighted sound reduction index (RW) obviously, as a result there is no necessity to enhance the sound insulation property of NEU foam sandwich panel by adding the density of NEU foam again.

The Weighted Sound Reduction Index (dB)

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34

NEU foams: 3 =0.31g/cm 3 =0.51g/cm 3 =0.67g/cm Thickness: 30mm Aluminum faces: 3 =2.7g/cm Thickness: 1mm+1mm

32

30

28 0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

Density of NEU foam (g/cm3)

Fig. 7. Weighted sound reduction index (RW) vs. different density of NEU foam sandwich panels.

4. Conclusions

(1) The results indicate that frequency has a great effect on sound insulation property of NEU foam sandwich panels: sound reduction index (R) of NEU foam sandwich panels increases when the frequency ranges from 100 Hz to 4000 Hz: about 20 dB under low frequency and around 50 dB under high frequency. (2) Experiments with panels of different thicknesses and densities display that the weighted sound reduction index (RW) increases with the addition of thickness and density, but the increasing trend becomes mitigated when the thickness and density are added equivalently, so adding thickness and density has their limitations on enhancing sound insulation property of NEU foam sandwich panels.

Acknowledgement The present authors wishes to thank the financial support provided by National 863 high technology research and development program of China (2002AA334060). References [1] [2] [3] [4] [5] [6] [7] [8] [9]

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