Performance Improvement of Sound-Absorbing Materials Using Natural Bamboo Fibers and Their Application

Performance improvement of sound-absorbing materials using natural bamboo fibers and their application

T. Koizumi, N. Tsujiuchi & K. Fujita Department of Mechanical Engineering, Doshisha University, Japan

Abstract

An acoustic material has been newly developed from the viewpoint of environmental protection. For this purpose, we used a natural resource, bamboo fiber, to manufacture sound-absorbing material. Until now, the sound-absorbing material has been developed using the crushed bamboo fibers. This material, however, is too heavy. To solve this problem, we have tried to develop a material using the explosion method. The normal incidence sound absorption coefficient of the bamboo fiber material was measured to be confirmed the basic properties. In addition, the characteristic impedance and the propagation constant of the material were measured. The contour lines of the sound absorption coefficient were calculated to find the relation of thickness and air space of the material with respect to density. In addition, we considered the influence of random incidence, and evaluated the material by adapting it to actual loudspeakers. The results show that the weight problem was solved, that the bamboo wool material works effectively in an actual product. Consequently, fundamental design criteria have been confirmed using the newly developed bamboo fiber material for sound absorption. Keywords: sound absorption coefficient, bamboo wool, fiber diameter, loudspeaker, acoustic property.

1 Introduction

Acoustic materials play a number of roles that are important in acoustic engineering such as the control of room acoustics and traffic noise. Acoustic materials are divided into three types: porous material, boards, and resonance- type boards. Porous materials are the most effective for noises that occur over a

High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5 462 High Performance Structures and Materials II broad frequency range. Acoustic materials have been developed for their use in sound absorption. On the other hand, acoustic material is claimed to have additional benefits, such as energy conservation, the advanced use and the re-use of resources from the viewpoint of earth protection [1]. Health concerns have been raised over glass fibers that become airborne and adhere to the body during building construction. Since glass wool is not the best material in terms of human health and the environment, we have adopted the safer, natural bamboo. Because it grows so quickly, we can harvest bamboo annually. Bamboo is effective as a resource because it is natural and easy to dispose of; there is no anxiety over ground pollution even if it is buried in the earth, and it does not emit poisonous fumes when burned. In this paper, an alternative sound-absorbing material is proposed that alleviates the load on the environment.

2 Sound absorption coefficient

When the homogeneous layer has the thickness L , the characteristic impedance

Zc , the propagation constant γ , and this layer has the acoustic impedance Z c in the back, the acoustic impedance of surface material Z c is expressed as: coshγ + sinhγLZLZ 2 c 1 = ZZ c (1) 2 sinhγ + c coshγLZLZ while the sound absorption coefficient is expressed as [2]: 2 ()ZZ −1/ α 1−= 1 air (2) ()1 ZZ air +1/

Dual channel Personal Amplifier FFT analyzer computer

Dx Lx LL0

Z1 Z2

Mic.1 Mic.2 Speaker

Impedance tube

Air space Porous material Movable piston

Figure 1: Block diagram of the impedance tube and the sample.

3 Measurement method

We used the measurement system shown in Figure 1 [3]. A random signal was generated from the loudspeaker, and the transfer function H between the two microphones was extracted using a two-channel FFT analyzer. The reflection coefficient and the sound absorption coefficient are expressed as follows, respectively, where k is the wave number for air.

− jkDk − eH 2 ()+DLkj R = e kk (3) jkDk − He 2 α 1−= R (4)

4 Experimental results and discussions

4.1 Bamboo wool sound-absorbing material

Until now, the sound-absorbing material has developed using the crushed bamboo fibers. In order to obtain a sound absorption effect similar to that of glass wool, however, the bamboo fiber material needs to be about four times as dense as glass wool [4, 5]. To solve this weight problem, we have attempted to develop a novel material using the explosion method. The resulting material is called “bamboo wool.”

4.1.1 Bamboo wool material The fiber was separated from the woody phase using the explosion method, and the very fine fibers, that is the bamboo wool material, were extracted using the home mixer. The thin composite fiber, polyolefin, was used as the binder material. The sheath part of this composite fiber consists of low-melting-point polyethylene, and its core is made of high-melting-point polypropylene. This difference between the two melting points allows a shaped fiber form to hold together. After thoroughly mixing the bamboo wool material and the binder material (10 % wt.), we molded the sound-absorbing material using a metal mold in a hot press machine.

1.0 1.0 0.9 0.9 0.8 0.8 0.7 0.7 Thickness Density 0.6 0.6 50 mm 32 kg/m3 Air space 0.5 Thickness 0.5 0 mm 0.4 50 mm 0.4 Density 0.3 Air space 0.3 20 kg/m3 0 mm 3 0.2 0.2 30 kg/m 25 mm 3 0.1 0.1 40 kg/m

Sound absorption coefficient absorption Sound 3 Sound absorption coefficient 50 mm 50 kg/m 0.0 0.0 125 250 500 1k 2k 4k 125 250 500 1k 2k 4k Frequency [Hz] Frequency [Hz]

(a) Air space depth. (b) Apparent density. Figure 2: Sound absorption coefficient.

The sound absorption coefficient was measured by changing the air space depth and the apparent density of the material. Results show that the sound absorption coefficient increases in all frequency ranges and the first peak value of sound absorption coefficient moves from the high- to the low-frequency range as the air space depth of the sample increases or the density of the sample increases. These basic properties of porous sound-absorbing material are confirmed. These results are shown in Figure 2 (a) and (b), respectively.

4.1.2 Comparison with glass wool and bamboo fiber Table 1 gives the results of measurements of the fiber diameters of the bamboo wool, glass wool and the bamboo fiber using a digital microscope. Although the bamboo wool and glass wool have similar fiber diameters, we can confirm that the bamboo fiber has a fiber diameter over ten times larger. Figure 3 shows a comparison of the sound absorption characteristics of the bamboo wool, glass wool and the bamboo fiber. Also, although the densities of the bamboo wool and glass wool are the same, it is confirmed that the density of the bamboo fiber needs to be about four times as dense as them to have the similar effect. These results are explained by the sound absorption principle of porous material. For rigid-framed porous material, this absorption is mainly attributed to thermo- elastic damping and viscosity loss generated while the sound propagates through a large number of small air cavities in the material [6]. Of course, the number of the bamboo fibers per unit area can increase when the fiber diameter decreases. In addition, the energy loss increases when the surface friction increases. Consequently, the sound absorption coefficient increases.

Table 1: Comparison of fiber diameters.

Material Range of fiber size [μm] Bamboo wool 10.84 – 17.66 Glass wool 11.07 – 12.73 Bamboo fiber 125 – 210

1.0 0.9 0.8 0.7 0.6 Density 0.5 Bamboo wool 0.4 32 kg/m3 0.3 Glass wool 3 0.2 32 kg/m Bamboo fiber 0.1 3

Sound absorption coefficient 120 kg/m 0.0 125 250 500 1k 2k 4k Frequency [Hz]

Figure 3: Comparison of bamboo wool, diameters

Taking the above results, into account, we can solve the weight problem posed by using the bamboo fiber material because we confirmed that the bamboo wool material has a similar sound absorption effect to glass wool.

4.1.3 Characteristic impedance and propagation constant The characteristic impedance and the propagation constant determine the state of the sound wave [7]. To clarify the acoustic effect in the bamboo wool material, the characteristic impedance and the propagation constant were measured for a material of densities ranging from 20 to 50 kg/m3. Results indicate that the characteristic impedance and the sound attenuation increase, and the sound velocity decreases, as the density rises.

Figure 4: Optimization thickness and air space depth.

4.1.4 Optimization The sound absorption coefficient was calculated for the thickness and the air space depth of between 0 and 100 mm using the characteristic impedance and the propagation constant. The contour line was expressed as the sound absorption coefficient of averaged frequency between 500 and 4,000 Hz [8]. Although the sound absorption coefficient increases as the thickness increases, the air space is

High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5 466 High Performance Structures and Materials II barely influenced at all when the density is between 20 and 40 kg/m3. Also, the largest domain of sound absorption coefficient of more than 0.9 is obtained, when the density is 40 kg/m3. However, when the density is 50 kg/m3, the domain of sound absorption coefficient of more than 0.9 suddenly decreases. This result shows that the optimal density of the bamboo wool material exists between 40 and 50 kg/m3. These results are illustrated in Figure 4 (a) - (d).

4.2 Adaptation to product

The above evaluations were all based on the normal incidence sound absorption coefficient. However, the sound-absorbing materials are used for cars, buildings and loudspeakers, etc., and the random incidence of sound waves is carried out to the actual material. Because it is easy to handle, we noticed the loudspeaker. The bamboo wool material was evaluated by adapting it to the loudspeaker. The role of the sound-absorbing material in the loudspeaker is to hold the standing wave inside the box. Three identical loudspeakers (RSM-90: Roland) were purchased and designated A, B, and C. Only the inside of the loudspeaker box C was changed: with the existing polyester material, without anything and with the bamboo wool material. Three conditions of loudspeaker C were tested together with the ready- made loudspeakers A and B, and evaluated each one in relation to its frequency characteristics, phase characteristics, impedance characteristics, distortion characteristics and the cumulative decay spectrum. The distortion characteristic is defined as “the ratio between the execution value of the distortion component of an output signal when adding sine waves to the loudspeaker, and the execution value of all output signals.” No clear differences were found with respect to phase characteristics, impedance characteristics, or distortion characteristics.

4.2.1 Frequency characteristic Since the errors by the test-day and the individual differences appeared greatly, it was not possible to compare only frequency characteristics results of loudspeaker C. Then, the loudspeaker A and B in which the conditions did not change were noticed. By comparing these, it was confirmed that the individual differences do not change regardless of the test-day. Moreover, to simplify the comparison, the average frequency characteristics of loudspeakers A and B and the frequency characteristics of loudspeaker C that removed the individual differences were compared. Results of the second time and the third time are shown in Figure 5 (a) and (b), respectively. Figure 5 (a) shows that the big dips are confirmed to lie within the frequency range of 400 to 700 Hz. These dips are influenced by the sound-absorbing material. Since flat frequency characteristics are better, it is thought that the sound-absorbing material was used to hold these dips. From the Figure 5 (b), it is confirmed that the dips of second time are held. Therefore, the bamboo wool material is confirmed to be effective.

90 90

85 85

80 80 Av. A-B Av. A-B C’ C’ 75 75 Magnitude [dB] Magnitude [dB] 70 70

65 65 100 1k 10k 100 1k 10k

Frequency [Hz] Frequency [Hz] (a) 2nd time. (d) 3rd time.

Figure 5: Frequency characteristic.

4.2.2 Cumulative decay spectrum The cumulative decay spectrum can be obtained from the impulse response. FFT is performed using all data and the values that calculated the frequency response become basis. The data which transposed the waveform data from t = 0 to arbitrary time t = tn to 0 is made. The frequency response in arbitrary time can be obtained by performing FFT using this data. And it is the cumulative decay spectrum that displayed these values in three dimensions side by side in order [9]. The aspect in the loudspeaker box can be observed using the cumulative decay spectrum. In consideration of the frequency characteristics results, the results are illustrated using the data to 4,000 Hz, those for loudspeaker C are shown in Figure 6 (a) - (c). Since the characteristics of the green domain do not change regardless of individual differences and the errors by the test-day, the green domain was noticed. Under the condition without the sound-absorbing material, a large green domain can be observed, therefore this result shows the sound inside the box remained there for a long time. Compared with this result, for the bamboo wool material, a small green domain can be observed. Therefore the bamboo wool material was confirmed to be effective. Taking the above results, we confirmed that the bamboo wool material works effectively in an actual product.

4 4 4

3 3 3 30- 20-30 2 2 2 10-20 1 1 1 -0 Magnitude [dB] Magnitude Frequency [kHz] Frequency Frequency [kHz] Frequency [kHz] Frequency 0 0 0 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 Time [ms] Time [ms] Time [ms] (a) Polyester. (b) Nothing. (c) Bamboo wool.

Figure 6: Frequency characteristic.

4.2.3 Tone quality evaluation Since no clear differences in the physical characteristics between the polyester material and the bamboo wool material could be confirmed from the loudspeaker tests, tone quality evaluation was performed by the one-pair comparing method [10]. For four stimulus sounds: base, drums, Japanese flute (Shinobue) and male’s English narration, the subject evaluated seven adjectives: “clear-cut”, “delicate”, “deep”, “soft”, “metallic”, “powerful” and “desirability”. Here, the evaluation gave in nine categories to +4 to -4 about all adjectives for every stimulus. It conducted one experiment at a time in the listening room. Subjects are nine 21- 22-year-old men who are experienced in music. The result of the analysis of variance is shown in Table 2. From Table 2, in all except “desirability”, 5% significance is confirmed in the main effect. Therefore, it is shown that the differences are in the tone quality between the loudspeakers. Then, since the tone quality difference between loudspeakers was specified, the confidence interval about adjectives other than “desirability” was calculated. Results are shown in Table 3 (a)-(f). From Table 3 (a)-(f), since zero is not included in the confidence interval in the adjective of “soft”, “metallic” “powerful”, we confirmed that there is no significant at the loudspeakers A-B, and there is the significant at the loudspeakers A-C and B-C. Therefore, by adapting the loudspeaker to the bamboo wool material, it shows that “metallic” and “powerful” tone quality can be obtained although there is no “soft” tone quality. In addition, by seeing the parameter estimator of Table 3 (a)-(f), the loudspeaker C has “clear-cut” tone quality and it is confirmed that there is no “deep” tone quality. Moreover, from the analysis of variance results, since there is no significant about "desirability", it cannot necessarily be said that the bamboo wool material improves tone quality. However, it is possible that the bamboo wool sound-absorbing material sufficiently adapted for the loudspeaker, and in respect of the loudspeaker used by this sturdy, the result of clearing the tone quality was obtained. Table 2: Analysis of variance.

Table 3: Parameter estimator and confidence interval.

(a) Clear-cut.

Parameter estimator Confidence interval A -0.02 A-B 0.74 - 0.03 B -0.41 A-C -0.09 - -0.80 C 0.43 B-C -0.48 - -1.19

(b) Delicate. Parameter estimator Confidence interval A 0.34 A-B 1.00 - 0.22 B -0.27 A-C 0.81 - 0.03 C -0.07 B-C 0.19 - -0.58

(c) Deep. Parameter estimator Confidence interval A 0.25 A-B 0.56 - -0.12 B 0.04 A-C 0.89 - 0.21 C -0.29 B-C 0.67 - -0.01

(d) Soft. Parameter estimator Confidence interval A 0.16 A-B 0.17 - -0.41 B 0.28 A-C 0.88 - 0.31 C -0.44 B-C 1.00 - 0.43

(e) Metallic. Parameter estimator Confidence interval A -0.21 A-B 0.23 - -0.33 B -0.16 A-C -0.31 - -0.87 C 0.38 B-C -0.25 - -0.82

(f) Powerful. Parameter estimator Confidence interval A -0.06 A-B 0.81 - -0.04 B -0.45 A-C -0.15 - -1.01 C 0.51 B-C -0.54 - -1.39

5 Conclusion

In this paper, alternative sound-absorbing materials using the bamboo wool were developed. The performances of the bamboo wool material were evaluated by measuring the normal incidence sound absorption coefficient and by adapting it to the loudspeaker. The following conclusions were drawn from this study.

1. The basic sound absorption characteristics of the bamboo wool material were confirmed. 2. The optimal thickness and air space were calculated using the characteristic impedance and the propagation constant. 3. The effectiveness of the bamboo wool material was confirmed, even when adapted for an actual product. 4. The fundamental design criteria were confirmed using the newly developed bamboo fiber material for the purpose of practical use.

Acknowledgements

This research is partially supported by a grant to the research project at Doshisha University named "Development of functional bamboo fibers and their eco- composites" from the Ministry of Education and Science, Japan. Also, Chisso Corporation is appreciated for their material support.

References

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