Research & Development
February 2007031
T
he daily noise exposure in our life leads to a demand fornew 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 light weight, high stiffness-to-weight ratio, high energy dissipation, high sound absorption, fire protection, non-moisturizing and easy recycling. Aluminum foams are one of the most commercially available metal foams at present. 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 acoustic properties of aluminium foams [1-2, 7]. However, the knowledge acquired up to now is far more from sufficient for application, including the manufacturing process of the aluminum foams, acoustic properties, and the relationship of the properties with the structure and preparation.
There are many different ways to manufacture aluminum foam. Open pore aluminum foams can be made by casting an aluminum melt into a salt mold which 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 of porous aluminum foams depend mainly on foam properties (parameters) such as porosity, pore-morphology, pore size and air-flow resistance. The correlation of these foam
Sound absorption property of
open-pore aluminum foams
*WANG Fang, WANG Lu-cai, WU Jian-guo, YOU Xiao-hong
(Department of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, Shanxi, P. R. China)
Female, born in 1972, lecturer. Research interest: porous and functional materials.
E-mail: lwjiyi@126.com
Received date: 2006-07-03; Accepted date: 2006-10-29
*WANG Fang
Abstract:
This paper presents a study on sound absorption property of aluminum foam by evaluating its sound absorption coefficients using standing wave tube method. Experimental results showed that the average values of sound absorption coefficients (over the test frequency range) are all above 0.4, which indicate very good sound absorption property of the aluminum foams. The sound absorption coefficient is affected by frequency and pore structure, and reaches its maximum value at around 1 000 Hz. With the increase of porosity and decrease of cell diameter, the sound absorption coefficient values increase.Key words:
aluminum foam; sound absorption property; sound absorption coefficients; plane-wave impedance tubeCLC number: TG146.2+1 Document Code: A Article ID: 1672-6421(2007)01-031-03
properties with measured acoustic properties has been investigated, and theoretical analysis on the results has also been conducted. The acoustic property has been measured in sound-absorption coefficient determined with a plane-wave impedance tube in the present study [3].
1 Experimental procedure
1.1 Sample preparation
Open-pore cylindrical specimens were produced firstly by forming a porous compact of salt (NaCl) granulate. The porous granulate compact builds the negative form for the metallic foam. The salt compact was pre-heated at temperature of about 500ņ for 30 to 60 min. The filler material was infiltrated with aluminum melt (casting alloy AlSi12) using a self-made high pressure die casting apparatus. The pouring temperature of aluminum was 760ņ, and the metal infiltration pressure was about 5 MPa. After solidification and cooling, the metal-salt composite was machined to the desired shape and size [4]. Then the filler was removed by leaching. The pore size and porosity of the foam can be determined by the size of salt particles and the specified density of the salt precursor. The samples for the acoustic measurement were cylindrical with diameter of 46 mm and thickness of 50 mm.
This process generated a reproducible and homogeneous open-pore structure (see Fig.1a) with open-pore sizes between 0.5 mm to 3.2 mm. After the salt removal, the aluminum foam shows connected pore channels throughout the sample (Fig. 1b). The volume of porosity ranges from 54.2% to 77%.
1.2 Testing principle
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Vol.4 No.1
032
coefficient is defined in terms of the ratio of absorption intensity to incident sound wave intensity. The testing principle is shown in Fig. 2.
Fig.1 Microstructure of open-pore aluminum foam before removal of the salt (a) and after removal of the salt (b)
α= 4S
(S+1)2 (1)
α= 4n
(1+n)2 (2)
α=4+21
M031
1+21M031 (3)
a
b
Firstly, standing wave ratio S or its reciprocal n is measured in a certain frequency. Then, the sound absorption coefficient can be calculated according to formula (1) or (2)
Where α is the sound absorption coefficient, and S the standing wave ratio, or the ratio between the maximum and minimum sound wave presses.
If the value difference between the maximum and minimum sound wave presses can be measured directly in test, the sound absorption coefficient can be calculated according to the formula (3) below:
Here L is the value difference between the maximum and minimum sound wave presses.
1.3 Testing results
Table 1 shows the cell structure parameters of aluminum foam samples for the acoustic measurements. Table 2 lists the sound absorption coefficients of different samples at different indicated sound wave frequencies, measured in an impedance tube.
2 Discussion
As shown in Tables 1 and 2, the sound absorption coefficient of open-pore aluminum is affected by three factors, the incident sound wave frequency, porosity and pore size of samples.
2.1 The influence of frequency
Figure 3 shows the correlation between the sound absorption coefficient and the wave frequency. It can be seen that the sound absorption coefficient is sensitive to wave frequency. As shown in Fig. 3, the change patterns of the coefficient with frequency are identical for foamed aluminum samples with different cell structures. The sound absorption coefficient increases with increase of sound frequency below 1 000 Hz and reaches the maximum value around 1 000 Hz; then it decreases with the increase of frequency and finds its minimum value around 4 000 Hz. The influence of sound frequency on sound absorption can be explained as follows: Sound absorption means that part of the energy of the incident sound wave is absorbed in the material, a result as loss of the incident energy. There are many mechanisms for this to happen, one of which is the mechanical collision between sound wave and cell wall. Such mechanical collision can be divided into elastic collision and inelastic collision. In elastic collision the fraction of absorption energy is smaller and sound absorption coefficient is lower than those in inelastic collision (in another words, the fraction of absorption energy is larger and sound absorption coefficient is higher in inelastic collision). In the lower frequency wave band, the wavelength is longer so that the incident wave energy is lower and the majority of the collision between sound wave and cell wall is inelastic. As the frequency increases, the loss of energy and sound absorption coefficients become larger. When the frequency is increased to some degree, the collision becomes elastic, leading to smaller loss of incident energy and decreased
Frequency Hz
Samples
250
500
1 000
4 000
10 000 1
0.44
0.47
0.58
0.33
0.43 2
0.28
0.30
0.77
0.33
0.68 3
0.33
0.36
0.68
0.25
0.60 4
0.36
0.40
0.62
0.30
0.51 5
0.40
0.43
0.60
0.33
0.49 6
0.36
0.40
0.64
0.27
0.56 7
0.30
0.33
0.73
0.33
0.68 8
0.47
0.51
0.52
0.36
0.43 Table 2 Sound absorption coefficients at indicated
frequencies
Structure
Samples
Porosity, %
Diameter, mm 1
60.6
1.0 2
77
0.05 3
72.6
0.4 4
65.6
1.6 5
64.6
1.0 6
68.2
0.05 7
75.8
1.6 8
54.2
3.2 Table 1 Pore size and porosity of open-pore aluminum
foam samples
Fig. 2 The sketch of standing wave tube testing principle
1. Sample; 2. A plane-wave impedance tube; 3. Loudspeaker box; 4. Measuring scale; 5. Output indicator plate;
Research & Development
February 2007033
Table 3 Average absorption coefficient of different sample
Structure Samples
Porosity, %
Diameter, mm
Average absorption coefficient
1
60.6
1.0
0.448
2
77
0.05
0.538
3
72.6
0.4
0.444
4
65.6
1.6
0.438
5
64.6
1.0
0.45
6
68.2
0.05
0.446
7
75.8
1.6
0.474
8
54.2
3.2
0.458
Berg A Maysenholder W, Haesche H H W. Noise Reduction by Open-pore Aluminum Foams. Cellular Metals: Manufacture, Properties and Applications, Bremen: MIT-Verlag, 2003: 487-492.
Ashby M F, Evans A G, Fleck N A, Gibson L J, Hutchinson J W, Wadley HNG. Metal Foams:A Design Guide. Boston: Butter-worth-Heinemann, 2000.
MA Da-you, SHEN Feng. Acoustics Handbook. Beijing: Science Press, 2004. (in Chinese)
WANG Lu-cai, YOU Xiao-hong. Research on the Producing Methods of Aluminum Foams - a New Engineering Material. Proceedings of the International Conference on Advanced Manufacturing Technology. Science Press, New York Ltd.1999: 1149 - 1153.
Tongji University. Measuring norm of sound absorption coefficient and impedence ratio using standing wave tube method. Measure Standard, 1986 (4). (in Chinese)
ZHAO Ting-liang, Xu Lian-tang, et al. Sound absorption property of aluminum foams. Combustion Engine Engineering, 1995, 16 (2): 55-59. (in Chinese)
[1]
[2]
[3] [4]
[5]
[6]
References
2.2 The influence of cell structure
The average sound absorption coefficients of aluminium foam samples with different pore structures are shown in Table 3. It is clear that when the pore size is constant, the sound absorption coefficient increases with the increasing of porosity by comparing samples 1 and 5, 2 and 6, and 4 and 7. On the other hand, when the porosity is constant, the sound absorption increases with the reducing of pore size by comparing the coefficient values of samples 2 and 7, and 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 collision occurrence between sound wave and cell wall, the longer the path of reflection and refraction and the more the absorption energy, and thus the greater the sound absorption coefficient. On the other hand, the reflection and refraction of sound wave will intensify due to the increasing bending degree of open cell with the porosity. This also causes the energy loss and the corresponding increase in the sound absorption coefficient.
2.3 Synthesized factor
As shown in Table 3, the average absorption coefficients of open-pore aluminium foams are greater than 0.4, indicating the aluminium foam material offers some potential for sound 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 aluminium foams occurs at synthetic vibration of sound wave and foamed aluminium [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 aluminium for sound absorption should be determined by specific sound source.
3 Conclusions
Under the condition of this study, the following results have been obtained by testing the sound absorption coefficients of aluminium foams using a plane-wave impedance tube.
(1) Open-pore aluminium foam offers a favourable potential as sound absorption material because of its excellent combination of properties, such as relatively high value (greater than 0.4) of average absorption coefficients, good stiffness at light weight, fire protection, non-moisturizing and easy recycling.
(2) Sound absorption coefficient of each sample varies with incident wave frequency, and the coefficient reaches its
maximum when sound wave frequency is around 1 000 Hz. (3) Sound absorption coefficient of open-pore aluminium depends also on porosity and pore size of samples. With the increase of porosity and/or the decrease of pore diameter, the coefficient values increase.
Sound absorption coefficient verses frequency of incident sound wave Fig. 3
