直接音消去法による吸音率の現場測定 : 垂直入射吸音率

抄録

As the method of measuring the sound absorption coeffient of acoustic materials, we have mainly used the reverberation chamber method for the random incidence and the standing wave tube method for the normal incidence. As to the method of measuring the acoustical characteristics of a material for oblique incidence, numerous investigations have been developed by many authors from different viewpoints. However, a particular room is always required in those methods except the tube method. so that the measurements will be much influenced by the room characteristics and an on-the-spot measurement cannot be made. Furthermore, the estimation of sound absorption coefficient is more laborious in the above methods than in a new method, which will be discussed in this paper. For these reasons, the new method of obtaining the sound absorption coefficient is proposed here, in which it is possible to obtain the coefficient through an on-the-spot measurement (field measurement) that has been considered very difficult. This paper describes that for tone bursts of duration of 10 milliseconds the reflection from a sample can be obtained by combining the outputs from two undirectional microphones through a phase inverter (see Fig. 2) and by comparing each amplitude of some intermediate cycles with that of the corresponding cycle of the direct sound, which is measured separately, we can estimate the sound pressure reflection coefficient, from which the absorption coefficient is obtained. The results for several kinds of samples which were obtained by this method agree with those from the tube method within a few percent at several test frequencies from 250 Hz to 4 kHz (see Figs. 10, 11, 12), and the result for a cloth with air space behind it is also consistent with the absorption coefficient already known (see Fig. 13). Further the reflection coefficient of a rigid square panel obtained by this method agrees well with the coefficient calculated from an approximate formula based on the Fraunhofer diffraction after Maekawa and Sakurai. It is also found that a diffraction phenomenon doesn't appear before the reflection from the panel edge arrives at the receiving point. This method of measurement may be conveniently applied not only to the ordinary measurement of the absorption characteristics of materials, but also to the study of room acoustics through an on-the-spot measurement.