Sound absorption property of open-pore aluminum foams

WANG Lu-cai(王录才)^{1, 2}, WANG Fang(王  芳)², WU Jian-guo(武建国)², YOU Xiao-hong(游晓红)²

1. Department of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China;

2. Department of Materials Science and Engineering, Taiyuan University of Science and Techonology,

Taiyuan 030024, China

Received 28 July 2006; accepted 15 September 2006

Abstract: The sound absorption property of aluminum foam was studied by testing its sound absorption coefficients using standing wave tube method. The open-pore aluminum foams were prepared by infiltration process, with pore size of 0.5 mm to 3.2 mm and porosity of 54.2% to 77%. The frequency of indicted sound wave was ranging from 125 Hz to 10 kHz. The results show that the average values of sound absorption coefficients are all over 0.4 and the aluminum foam has better sound absorption property, its coefficients is influenced by frequency and pore structure, and reaches the maximum at about 1 kHz, with increasing porosity and decreasing cell diameter the sound absorption coefficient values increase.

Key words：aluminum foam; sound absorption property; sound absorption coefficient; pore structure

1 Introduction

The daily noise exposure in our life leads to a demand for new materials offering the possibility of an effective noise reduction. For this purpose metallic foams possess a very interesting application potential because of their unique combination of properties, such as their light mass, high stiffness-to-mass ratio, high energy dissipation, high sound absorption, fire protection, no moisturizing and easy recycling [1-3]. Aluminium foams are one of the most commercially available metal foams at present [4-12]. Open-pore aluminium foams appear to be particularly suitable for the construction of sound absorption and noise control components due to their unique cellular structure and permeability. Many publications have demonstrated the manufacturing methods and the resulting acoustic properties of aluminium foams [1-2, 5-7, 13-14]. However, the knowledge we have gotten up to now including the manufacturing process of the aluminium foams, the resulting acoustic properties, the relationship among the property, the structure and the preparation is far more insufficient for application. Especially, the reports on quantitative description of the relation between sound properties and pore structure and discussion on the sound absorption mechanisms of aluminium foam with super-fine and broad pore size have not be seen yet.

There are many different ways to manufacture aluminium foam. Open pore aluminium foams can be made by casting an aluminium melt into a salt mold that is leached out after the metal and salt composite is cold [2]. The foams have a unique cellular structure and permeability. The pore size and porosity of the foam can be varied through selecting appropriate salt particles and specifying the density of the salt precursor. The acoustic absorption properties for porous aluminium foams depend mainly on properties like porosity, pore- morphology, pore size and air-flow resistance. The correlation of these foam properties with measured acoustic properties and their quantitative prediction theoretically on the results was investigated. The acoustic property measured in sound-absorption coefficient was tested using a plane-wave impedance tube in this study [2, 15].

2 Experimental

2.1 Sample production

The open-pore cylindrical specimens were produced by forming a porous compact out a salt (NaCl) granulate. The porous granulate compact builds the negative form for the metallic foam. The salt compact is pre-heated at temperature of about 500 ℃ for 30 to 60 min. The filler material is infiltrated by aluminum melt (casting alloy AlSi12) using a self-made high-pressure die-casting apparatus, with pouring temperature of aluminum at 760 ℃ and a metal infiltration pressure of about 5 MPa. After solidification and cooling, the composite of metal and salt was machined to the desired shape and size [14]. Then the filler is removed by leaching. The pore size and porosity of the foam can be determined by size of salt particles and specifying the density of the salt precursor. The samples for the acoustic measurements were cylindrical with diameter of 46 mm and thickness of 50 mm.

This process results in a reproducible and homogeneous open-pore structure (Fig.1(a)). Pore sizes are between 0.5 mm and 3.2 mm. After removing the salt the aluminum foam shows connected pore channels throughout the sample (Fig.1(b)). The porosity is ranging from 54.2% to 77%.

Fig.1 Microstructures of open-cell aluminum foam: (a) Open-pore structure; (b) After removing salt

2.2 Testing principle [15]

The sound absorption property of aluminum foam was measured by testing its sound absorption coefficient using a plane-wave impedance tube or standing wave tube. The sound absorption coefficient was defined in terms of the ratio of absorption intensities to incident sound wave intensity. The principle is shown in Fig.2.

Fig.2 Schematic diagram of standing wave tube testing:      1 Sample; 2 Plane-wave impedance tube; 3 Loudspeaker box;  4 Measuring scale; 5 Output indicator plate; 6 Microphone;   7 Acoustic frequency generator; 8 Probe

Firstly, standing wave ratio s or its reciprocal n was measured in a certain frequency. Then, the sound absorption coefficient can be calculated according to

α =                                 (1)

α=                                  (2)

where  α is the sound absorption coefficient, s is the standing wave ratio which is the ratio of maximum of sound wave press to minimum of it, n value is reciprocal of standing wave ratio s.

If the difference between the maximum of sound press and minimum of that can be numerated directly in test, the sound absorption coefficients can be calculated according to

α=                                (3)

where  L is the difference between the maximum of sound wave press and minimum of it.

3 Results and discussion

Table 1 lists the cell structure parameters of aluminum foam samples for the acoustic properties measurements. Table 2 lists the sound absorption of different samples at different indicated sound wave frequency, which was measured in an impedance tube. As listed in Tables 1 and 2, the sound absorption coefficient of open-pore aluminum is affected by three factors, incident sound wave frequency, porosity and pore size of the sample.

3.1 Influence of frequency

Fig.3 obtained from the data listed in Table 2, shows that the sound absorption coefficient is sensitive to wave frequency. As shown in Fig.3, the change of the sound absorption coefficient of foamed aluminum samples of varying cell structure with frequency is identical, i.e. the sound absorption coefficient increases with increasing sound frequency below 1 kHz and the maximum value is observed at around 1 kHz, then it decreases with increasing frequency and the minimum value is observed around 4 kHz. The influence of sound frequency on sound absorption can be explained as follows. Sound absorption means part of the energy of the incident sound wave is absorbed in the material, which is also taken as loss of the incident energy. There are many ways for the sound absorption. One method is the mechanical collision between the sound wave and cell wall that is divided into elastic collision and inelastic collision. The fraction of absorption energy is smaller and the sound absorption coefficient is lower in elastic collision, whereas the fraction of absorption energy is larger and sound absorption coefficient is higher in inelastic collision. In the lower frequency wave band，the wave length is longer so that the incident wave energy is lower and the majority of the collision between sound wave and cell wall is inelastic. With increasing frequency, the loss of energy and sound absorption coefficients become larger. When the frequency is enlarged to some degree, the collision become elastic and the incident loss energy is small owing to the shorter wavelength and higher energy so that the sound absorption coefficients decrease. If the frequency rises much higher, the elastic collision can not lead to lower sound absorption coefficients because of cell-edge vibration due to large collision and reflected wave with higher energy. The reflected and refracted wave will colloid with cell wall repeatedly so that the loss of energy and sound absorption coefficients become larger.

Table 1 Pore size and porosity of open-cell aluminum foam sample

Table 2 Sound absorption coefficients at indicated frequency

Fig.3 Sound absorption coefficient verses frequency of incident sound wave

3.2 Influence of cell structure

The average sound absorption coefficients of aluminum foam sample with different pore structures are listed in Table 3. It is found that the sound absorption coefficient increases with increasing porosity when the pore size is constant by comparing samples 1 and 5, 2 and 6, 4 and 7 and that the sound absorption increase with decreasing pore size when the porosity is constant by comparing the coefficients values of samples 2 and 7, 4 and 5. This influence of pore size and porosity on sound absorption can be understood by the difference in the converted energy from sound energy to thermal energy through friction with inner wall of air and pores. The smaller the cell size, the more the times of collision between the sound wave and cell wall, the longer the path of reflection and refraction and the more the absorption energy, so the sound absorption coefficient is larger. On the other hand, the intensity of reflection and refraction of sound wave will get larger due to the increasing meander degree of open cell when the porosity becomes larger. This also causes the energy loss and corresponds to the sound absorption coefficient increasing.

Table 3 Average absorption coefficient of different samples

3.3 Synthesized factor

As shown in Table 3, the average absorption coefficients of open-cell aluminum foam are larger than 0.4, which indicates it offers some potential for sounder absorber. The absorption coefficient of each sample has a varied optimal value. The reason for this is that the maximum of sound absorption coefficient of aluminum foams occurs at sympathetic vibration of sound wave and foamed aluminum[7]. The resonance frequency of sample is determined by structure, which is not identical with different incidence frequency. Therefore, the cell structural parameter of foamed aluminum for sound absorption should be determined by specific sound source.

4 Conclusions

1) Open-cell aluminium foam offers a favourable potential for sound absorption material because of its combination of properties of better sound absorption (the average absorption coefficients are larger than 0.4), stiffness at light mass, fire protection, no moisturizing and easy recycling, etc.

2) Sound absorption coefficient of each sample is varying with incident wave frequency. And there exists a maximum value when the sound wave frequency is around 1 kHz.

3) Sound absorption coefficient of open-cell aluminium depends on porosity and pore size of the sample. With increasing porosity and decreasing cell diameter the coefficient values increase.

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(Edited by LONG Huai-zhong)

Foundation item: Project(042060) supported by the Science and Technology Program of Shanxi Province, China

Corresponding author: WANG Lu-cai; Tel: +86-351-6999221; E-mail: zsby2@yahoo.com.cn