An optimal design and application of sound-absorbing material made of exploded bamboo fibers

An acoustic material made of exploded bamboo fibers has been newly developed from an ecological viewpoint. The basic acoustical properties of the material as the sound absorption coefficient, the characteristic impedance, and the propagation constant were measured for the frequency range between 50 and 6400 Hz by the transfer function method using two-microphone impedance tubes. In order to design an optimal arrangement of the material, the normal incidence sound absorption coefficient was calculated from the results of impedance measurements for the parameters of bulk density, thickness, and backing air space. We also tried to find the best bulk density of the material, perforated facing, and air space. We applied the designed arrangement of material to actual loudspeakers. The basic properties of loudspeakers such as the frequency characteristics and the cumulative decay spectrum were measured. By controlling the sound absorption coefficient at target frequencies, the optimal arrangement of the material more effectively eliminate standing waves which exist in loudspeakers than the conventional materials in actual products. Sensory evaluations for loudspeakers were carried out based on Scheffe’s method of paired comparison. Consequently, the validity of fundamental design criteria have been confirmed for the sound absorption of the newly developed bamboo fiber material under the restricted conditions. 1.Introduction Acoustic materials play a number of important roles 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 broad frequency range. Acoustic materials have been developed for their use in sound absorption. On the other hand, acoustic material supposedly has such additional benefits as energy conservation, advanced uses and the reuse of resources from an ecological viewpoint [1]. Health concerns have also been raised over airborne glass fibers that adhere to the body during building construction. Since glass wool is not the best material for human health and the environment, we have adopted the safe, natural bamboo, which grows quickly, so it can be harvested annually. Bamboo is effective as a resource because it is natural and easy to dispose of; it doesn’t cause 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 eases the load on the environment. 2. Sound absorption coefficient When the homogeneous layer has the thickness L, the characteristic impedance , the propagation constant c Z , and this layer has the acoustic impedance in the back, the acoustic impedance of surface material is expressed as: 2 Z