音響帰還を起しにくい拡声用スピーカの一設計法
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概要
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In modern halls and theaters, so called proscenium loudspeaker systems are used for sound reinforcement. Most of the proscenium loudspeaker systems consist of many medium size component loudspeakers distributed over the front part of the ceiling to obtain a desired directivity. This paper shows a design method for anti-acoustic feedback loudspeaker system. The sound pressure level frequency response of the arrayed loudsperker cluster in the direction of the stage have peaks in the medium frequency range as shown in Fig. 2. It follows that an acoustic feedback can easily happen at some peak frequency. This is caused by the fact that sound radiating from the component loudspeakers in the direction of the stage are in nearly the same phase in this frequency. Therefore, to eliminate the acoustic feedback, it is necessary to suppress the peaks in the frequency response. As a result of the study of several methods to suppress the peaks, it becomes evident that an unequally spaced array of the component loudspeakers is the most useful. In this study, sound pressure level frequency responses of unequally spaced linear array point sound sources, with geometrical or arithmetical progression spaces, were investigated. Fig. 4 and Fig. 6 are the calculated sound pressure level frequency responses of the unequally spaced linear array point sound sources. with geometrical and arithmetical progression space, (n=6, minimum space=20cm) at a point far distant from the center of the array and in a direction of 60-degrees off axis from the main lobe. If the larger common ratio or common difference is used, the lower peaks in the medium frequency range can be obtained. If a more larger common ratio or common difference were used, however, small peaks would appear in the high frequency range, and the total size of the loudspeaker system would be large. Therefore, there are most suitable common ratios and common differences for the numbers of component loudspeakers. Table I shows the suitable common ratios and common differences. Fig. 9 shows an example of a proscenium loudspeaker cluster with equally spaced linear array. Fig. 10 shows the measured loop gain frequency response just below acoustic feedback of the sound reinforcement system with the proscenium loudspeaker cluster shown in Fig. 9. The peaks at 2, 500 Hz and 4, 000 Hz show that acoustic feedbacks can easily happen at these frequencies. Fig. 13 shows an example of the proscenium loudspeaker cluster with arithmetical progression spaced curved array. The calculated sound pressure level frequency response in the direction of the stage in this case does not have large peaks as shown in Fig. 14. Consequently, the loop gain frequency response is almost flat, as shown in Fig. 15. If the sound pressure level frequency reponses in the direction of the stage shown in Fig. 12 and Fig. 14 are compared, it is evident that the acoustic feedback stability can be improved 5 to 6 dB by the use of the unequally spaced array.
- 社団法人日本音響学会の論文
- 1971-01-10