Effects of Geometrical and Numerical Parameters of Macro Holes on SPM Sound

42

are irregular, as shown in Figure 4-5b, the tortuosity and the number of interconnected pores can be greater.

Consequently, the effect of Al on SPM sound absorption increases as Al dosage increases.

4.2.3 Effects of Geometrical and Numerical Parameters of Macro Holes on SPM Sound

43 (a)

(b)

Figure 4-10. Effects of surface porosity on SPM sound absorption: (a) 15 mm hole diameter (D15) and (b) 8 mm hole diameter (D8).

4.2.3.2 Effects of Macro-Sized Hole Depth

The results of the sound absorption tests for SPM with different hole depths are shown in Figure 4-11. In the absence of Al, the peak frequency location of the negative area depends on the hole depth. In the case

44

of P30-D15, the peak frequency locations move from 1,200 Hz to 800 Hz as the hole depth increases. That is, the resonance frequency is dependent on hole geometry. The same tendency is also observed with P15- D8.

When Al is added, negative coefficient values are eliminated and data present positive curves. Notably, the peak locations of the positive curve are close to the resonance frequency, especially in the case of P30-D15.

This implies that the main frequencies of the sound absorbed by the macro-sized hole are affected by hole design. It can also be explained, as stated in the literature, those different frequencies of the sound of which sound absorption was improved by macro-sized holes. Atalla et al. and LeninBabu and Padmanabhan showed sound absorption enhancement in frequency ranges of 250–500 Hz (low) [127] and 500–1,600 Hz (mid or high) [28], respectively. Differing results regarding improved frequency can be attributed to the matrix material as well as to the geometry of macro-sized holes. Consequently, it can be concluded that hole depth is a critical design variable because it can control the main sound-absorbing frequency.

(a)

45 (b)

Figure 4-11. Effects of hole depth on SPM sound absorption: (a) 30% porosity -15 mm of hole diameter (P30-D15) and (b) 15% porosity -8 mm of hole diameter (P15-D8).

4.2.3.3 Effects of Macro-Sized Hole Size

The results of sound absorption tests for SPM with different hole sizes are shown in Figure 4-12. The effects of hole size on sound absorption are minimal when design surface porosity is 7.5%. The SAAR values are constant for SPM 0.2% Al and 0% Al cases. However, in the 15% design surface porosity case, the sound absorption coefficient curve shifts to the left as the hole size decreases from 15 mm to 8 mm. This is assumed to be because the geometry of the macro-sized hole is changed. Therefore, it can be concluded that hole size is another critical SPM design variable for controlling sound absorption frequency.

46 (a)

(b)

Figure 4-12. Effects of hole size on SPM sound absorption: (a) 15% porosity (P15) and (b) 7.5% porosity (P7.5).

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4.2.3.4 Effects of the Holes’ Inner Surface Area and Gap

Design variable studies have found that surface porosity is not always correlated with sound absorption of SPM with 0.2% Al. For example, from the summary of the result of sound absorption tests in Table 4-2, P30-D15-H80 and P15-D8-H80 are shown to have similar SAAR values of around 0.7 even though their surface porosities are distinctly different. This is a critical point of SPM design in terms of compressive strength. Hence, it needs to be further discussed.

The distance between the holes affects SPM sound absorption. Figure 4-13a shows the relationship between the SAAR value and the distance between macro-sized holes. SPM has a higher SAAR value with a shorter distance. This is because the mortar surface can reflect sound even though its porosity is increased by Al, and the chance of reflection can be higher when the distance is longer. This implies that the distribution of macro-sized holes is an essential factor in SPM sound absorption. The requirement of a minimal distance between holes restricts the hole size. For example, in the 15% design surface porosity case, the distance increases from 17 mm to 32 mm if the hole size increases from 8 mm to 15 mm. Consequently, the holes’

array should be determined carefully to allow for even hole distribution.

The inner surface area of macro-sized holes also affects SPM sound absorption. The P7.5-D8 specimen data are exceptional in Figure 4-13a because the SAAR value is relatively low even though its distance is short. This is because the total inner surface area of the holes is small. When the data in Figure 4-13a are sorted by inner surface area, the SPM with a lower design surface porosity has a lower SAAR value. This implies that the inner surface area of the macro-sized holes needs to exceed a certain level to have effective SPM sound absorption.

In summary, surface porosity and pore size should be set so that there is a small distance between holes.

Furthermore, it should be pointed out that a higher surface porosity does not guarantee a higher SPM sound absorption. When hole size increases, the distance between holes also increases, which degrades SPM sound absorption. Hence, higher SPM sound absorption can be achieved not only with smaller hole sizes but also with higher surface porosity. However, this type of design could be difficult to manufacture and, therefore, the ability to construct SPM on a large scale and in large quantity needs to be studied.

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Table 4-2. Summary of the Results of Sound Absorption Tests for SPM Specimens with 0.2% Al and Their Design Properties

Specimens

Actual Porosity

(%)

Number of Holes

Distance Between Holes

(mm)

Hole Depth

(mm)

The Inner Surface Area of an Entire

Hole (mm²)

SAAR

P30-D15 30.6 156 22 80 220,539.8 0.70

P15-D15 15.3 78 33 80 110,269.9 0.58

P7.5-D15 8.2 39 44 80 55,135.0 0.55

P15-D8 14.3 236 17 80 94,901.2 0.69

P7.5-D8 8.0 144 25 80 57,905.8 0.49

(a) (b)

Figure 4-13. Relationships between the SAAR value of SPM with 0.2% Al and its design variables derived from data in Table 2: (a) distance between macro-sized holes and (b) SPM with 0.2% Al.