直線配列音源の近傍音場特性

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It is well-known that directive array sources are useful for indoor and outdoor sound reinforcement systems, and such the arrays are used in many cases. But the characteristics of the directive array has not yet been fully known. In this paper, the near field sound pressure characteristics of the linear array source are made clear to give instructions for designing a sound reinforcement system. The curved array is superior to the linear array not to cause large change with frequencies in the directive pattern. But the effectiveness of the directive array is easily observed in many cases even though the liner array is used. Moreover, the fundamental properties of the directive array can be found in the linear array. For this reason the near field sound pressure characteristics of the linear array are investigated. The curved array which is superior to the linear array will be taken up in the next chance. In order to synthesize a good directive pattern, the length of the array must be much longer than the wavelength of the lowest frequency. Therefore, a very long array having the length of several meters is sometimes used. In such a case, the objective region of the sound reinforcement must be considered to be the region of near field. The sound pressure distribution and the frequency characteristic of the linear array within the objective region are remarkably different from those of single loudspeaker. In order to generally indicate the sound field characteristic of the linear array, the continuous line source of length l shown in Fig. 1 is taken up as a model of the sound source. The results of the numerical computation of the near field sound pressure distribution and the frequency characteristic of the line source are shown Fig. 2 to 7. Some important properties are found in the figures. That is, the smoothed sound pressure distribution in the near field is nearly inversely proportional to the square roots of the distance in the range kl>50, x/l<10(Fig. 2). The attenuation of the sound pressure by the distance from the source can be reduced within a limited range by use of a slightly slant line source (Figs. 4 and 5). The longer the length of line source is, the flatter the near field sound pressure distribution becomes. But the sound pressure frequency characteristics must be compensated by an electric network, because the efficiency is fairly increased at low frequencies. Next, the near field transient phenomena of the line source are investigated. The results of numerical computation of the transient phenomena of the line source are shown in Fig. 20 to 23 and the experimental waveforms are shown in Figs. 24 and 25. It is found that the transient phenomenon of the line source is out of question from the practical point of view, if the length of the line source is shorter than about 10m, though the transient characteristics of the linear array are much different from those of single loudspeaker. Some interesting results are observed in the transient phenomena which are explanated physically with a vector diagram (Fig. 19).
社団法人日本音響学会の論文
1970-03-10

直線配列音源の近傍音場特性

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