著者

菊池 年晃 高橋 純夫 中西 俊之

出版者

The Marine Acoustics Society of Japan

雑誌

海洋音響研究会報 (ISSN:18847927)

巻号頁・発行日

vol.12, no.3, pp.118-124, 1985

---

The acoustic transmission line method is widely used for measurements of absorption coefficients and specific acoustic impedance of sound absorbing materials. As the diameter of the tube must not be greater than about half the wave length of sound, this method is not available at high frequency. This paper describes a new method for measuring acoustic impedance of the samples larger than the wave length. Firstly, the pressure amplitude and phase in the nearfield of a rectangular piston source are examined theoretically and experimentally. It was shown that the pressure amplitude along the axis of square piston source for k<SUB>1</SUB> b=20 (k<SUB>1</SUB> : wave number, 2b : width of piston) are nearly smooth at around k<SUB>1</SUB> x=12 (x : distance from the piston). Secondly, the square rubber samples 20cm in width with free backing plate are prepared. The standing wave ratio along the axis of rubber sample and phase shift at the surface of it are measured at frequency range 30-40kHz. The absorption coefficients of the samples are obtained by comparing measured impedance circles with those by calculation.

著者

小浜 俊一 江川 護

出版者

The Marine Acoustics Society of Japan

雑誌

海洋音響学会誌 (ISSN:09165835)

巻号頁・発行日

vol.20, no.4, pp.228-233, 1993

---

In the case of the passive sonar, the LOFAR-Grams of received signals using frequency analysis and variable density visualization technique is widely used for target signal detection. The auto-spectrum analysis method is a signal processing means for the LOFAR-Grams.<BR>As the means to improve the ratio of signal to noise levels (S/N), there is a process of averaging (cumulative processing) analyzed signals used in the LOFAR-Grams.<BR>The discrimination signals smoothed cumulatively in the auto-spectrum method are apt to be difficult due to the nature of noise components, and this auto-spectrum method is not used in this case.<BR>We proposed a new method here, and analyzed experimentally several types of signals of the passive sonar utilizing this method for the LOFAR-Grams.<BR>The results showed that the process gain by cumulative method was obtained in accordance with calculation, and the delay type cross-spectrum method gave processing gain 3 dB higher than the auto-spectrum method.<BR>It was considered that this technique was to be applied extensively to the passive sonar in the future.