日本音響学会誌
Online ISSN : 2432-2040
Print ISSN : 0369-4232
36 巻, 6 号
選択された号の論文の7件中1~7を表示しています
  • 新田 恒雄, 田中 正俊
    原稿種別: 本文
    1980 年 36 巻 6 号 p. 297-303
    発行日: 1980/06/01
    公開日: 2017/06/02
    ジャーナル フリー
    Free-field measurements for a loudspeaker system in a normal room (frequency responses, directional characteristics, phase characteristics and group delay characteristics) are studies. Two methods for reducing reflected waves are investigated. 1) Comb Lifter method : filtering in quefrency domain using three CLs with gentle slope. 2) Moving Average method : smoothing in frequency domain using MA with variable averaging points. The LC method has an advantage of relative design ease, that is, CL constants are related to dimensions of a room. The MA method is able to vary easily its averaging points, or frequency window lengths F_w_'_s. That is, a shorter F_w is used for the lower frequency region and a longer F_w is used for the higher frequency region to reduce reflected waves effectively. The MA method is applicable to rapid processing. Approximate free-field characteristics in full audio frequency band are obtained with reduced reflection from the floor, walls and ceiling. These measuring methods can be carried out in any normal room without the use of an anechoic chamber. They are useful for the development of loudspeaker systems.
  • 福島 正忠
    原稿種別: 本文
    1980 年 36 巻 6 号 p. 304-310
    発行日: 1980/06/01
    公開日: 2017/06/02
    ジャーナル フリー
    Surface duct sound propagation in water is an important problem in underwater acoustics. There are many articles on this problem, but few ones consider the effect of lossy surface which represents an actual ocean surface with wind. In this report the general theory to obtain the approximate depth functions and eigenvalues taking account of the lossy surface is described. The depth function is expressed as a sum of perturbed terms with lossy parameters. In the case of a parabolic sound velocity profile and of a loss free surface, of which exact solutions can be easily obtained, these approximate solutions are compared with the exact ones. Except for the vicinity of the turning point, the present approximate solutions agree well with the exact ones. The effect of lossy surface to the depth functions is sensitive in the neibourhood of sea surface. The present analysis shows that the lossy surface is very important and can not be neglected to design the SONAR system and to predict the performance of SONAR system. It is assumed that the water layer is horizontally stratified medium, and the sound velocity profile is monotonously increasing function with the depth coordinate only. The latter is very important to analyze the effect of lossy surface to obtain the eigenvalues and the depth functions of surface-duct-trapping-mode, because the effect of sea bottom can be neglected by virtue of such profile.
  • 福島 正忠
    原稿種別: 本文
    1980 年 36 巻 6 号 p. 311-316
    発行日: 1980/06/01
    公開日: 2017/06/02
    ジャーナル フリー
    Recently theories for shallow water normal mode propagation of sound have been studied for low frequency and long rage problems and some experiments on mode enhancement have been reported. The normal mode theory contains mathematical difficulties in order to design model SONAR systems, and the ray theory, on the other hand, is not applicable for these problems. In this paper, based on the normal mode theory, some formulae are derived to design model SONAR systems and to analyze acoustic field in shallow water. Pekeris model is used for shallow water. Comparing approximate and exact solutions of vertical wave number eigenvalues for sand and mud bottoms, it is shown that approximate formulae are quite useful. Using theses approximate eigenvalues mode attenuation coefficients due to lossly bottom are derived and compared with the experimental values. They agree well with each other. It is also shown that higher modes decrease rapidly with the distance due to bottom absorption and therefore the dominant contribution of sound field at long range is the lower modes in real shallow water.
  • 大石 弥幸, 久野 和宏, 池谷 和夫
    原稿種別: 本文
    1980 年 36 巻 6 号 p. 317-321
    発行日: 1980/06/01
    公開日: 2017/06/02
    ジャーナル フリー
    The field around a rigid oblate spheroid placed in diffuse sound field is investigated. Theoretically it is assumed for simplicity that the diffuse sound field is equivalent to the random imcident plane wave on the object. Especially the sound pressure distribution near a disc, which is a special type of oblate spheroids, is calculated in detail. The results show that the increase of sound pressure level on the surface of a rigid disc approaches to 3 dB for sufficiently high frequencies. And the field more than one diameter apart from the disc seems to be diffuse similarly to the original incident field. The sound pressure levels around a rigid disc are measured in the reverberant chamber. The results are in good agreements with the predicted ones.
  • 立道 有年, 岡崎 明彦
    原稿種別: 本文
    1980 年 36 巻 6 号 p. 322-330
    発行日: 1980/06/01
    公開日: 2017/06/02
    ジャーナル フリー
    An axisymmetric free vibration of circular plates with partial coverage of constrained viscoelastic layers is analyzed by employing transfer matrix method. Loss factors of partially covered circular plates are determined by evaluating strain energies of each layer from the calculated modes of vibration, and damping properties of plates are discussed. Detailed considerations are aimed at the influence of area and location of the laminated portion, thickness of each layer, and edge conditions on the loos factors of plates. An eminent result on this analysis is that, if the suitable portion of a plate is covered with constrained viscoelastic layers, the partial coverage of comparatively small area ratio can be expected to give higher damping effect that the uniform coverage. It is also pointed out that, when the area ratio of the laminated portion isn't very small, the loss factor of the plate with central coverage dominates that of plates with other coverage of the same area ratio without regard to the edge conditions. The experimental data is also presented, and close agreement with the theoretical predictions is found.
  • 似鳥 高司, 相沢 憲四郎
    原稿種別: 本文
    1980 年 36 巻 6 号 p. 331-335
    発行日: 1980/06/01
    公開日: 2017/06/02
    ジャーナル フリー
  • 柴田 南雄
    原稿種別: 本文
    1980 年 36 巻 6 号 p. 336-339
    発行日: 1980/06/01
    公開日: 2017/06/02
    ジャーナル フリー
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