日本音響学会誌
Online ISSN : 2432-2040
Print ISSN : 0369-4232
水中送受波器の感度校正値におよぼす無響水槽壁面からの反射波の影響について
菅田 成雄
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ジャーナル フリー

1966 年 22 巻 5 号 p. 265-275

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When underwater sound transducers are calibrated in an anechoic tank, the radio of the open-circuit output voltage of a hydrophone to voltage across the terminals of a projector must be measured. Measured values will be inevitably accompanied with errors caused by sound waves reflected from the walls of the tank. The purpose of this paper is to draw general properties of errors of this nature from an idealized simple model of an anechoic tank. The simple model of an anechoic tank is rectangular in shape (height L_1, length L_2 and depth L_3), the inside walls of which are covered with homogeneous absorbing material (Sounds pressure refletivity is independent of an incident angle. ). It is assumed that reflection from the walls of the tank is geometrical and sound waves are reflected only once. A projector and a hydrophone are placed on a line that is pallarel to the longest center axis of the tank. (1) In the case the projector and the hydrophone are both nondirectional(See Fig. 1) From Eq's 4, 9 and 11 when the center of the projector and the hydrophone coincide with the center of the tank, an error will be reduced to a minimum. Then, if Eq. 14 holds true, the output voltage of the hydrophone caused by reflected sound waves or the error will be given by Eq. 15. Also, if Eq. 16 holds true, the error expressed in dB will be given by Eq. 17, in which E_0 denotes the output voltage of the hydrophone caused by direct sounds waves from the projector, and d denotes the distance between the projector and the hydrophone. It will be senn from Eq. 15 or 17 that when the distance between the projector and the hydrophone is much smaller than the dimentions of the tank, the error will be in propotion to the distance, while the larger the dimensions of the tank are and the smaller sound pressure reflectivity is, the smaller the error is. (2) In the case the projrctor is monodirectional and the hydrophone is nondirectional(See Fig. 2) If Eq. 19 holds true, the error will be given by Eq. 20, in which G_<S1> and G_<S3> denote a sound pressure directivity factor in the θ_1 direction and θ_3 direction of the projector respectively. Also, if Eq. 21 holds true, the error expressed in dB will be given by Eq. 22. (3) In the case the projector and the hydrophone are both monodirectional (See Fig. 2) If Eq. 24 holds true, the error will be given by Eq. 25, in which G_<R1> and G_<R3> denote a sound pressure directivity factor in the θ_1 direction and θ_3 direction of the hydrophone respectively. Also, if Eq. 26 holds true, the error expressed in dB will be given by Eq. 27. It will be seen from Eq's 15, 20 and 25 that the more monodirectional transducers are used, the smaller the error is. For confirming the above mentioned results, measurements were performed in an actual anechoic tank. The anechoic tank is rectangular in shape (1. 5m × 3m × 1. 5m) and the inside walls of the tank are lined with absorbing pine wedges (See Fig. 3). Nondirectional transducers used for mesurements remain nondirectional up to about 60 kc. Also, a directivity pattern of one of monodirectional transducers is shown in Fig. 4. The block diagram of the measuring system is shown in Fig. 5. An error caused by sound waves reflected from the walls of the tank is determined as follows. When a projector and a hydrophone the distance between which is kept constant is moved along the longest center line of the tank and indicated values of the VTVM are read to calculate the average value, the error shall be given in the maximum absolute value of the difference between the average value and the indicated values. The maximum, minimum, and average values measured with a nondirectional projector and a nondirectional hydrophone in the above mentioned anechoic tank are shown in Fig. 7. The average values are on 20logd^<-1> with deviation of about 0. 2dB for frequencies of more than 10 kc (d denotes

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© 1966 一般社団法人 日本音響学会
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