An acoustic directional coupler is a device which separates out the incident and the reflected waves in a waveguide. A slot-coupled type consists of two adjacent waveguides coupled with a slot-window between them. By tapering the coupling distribution of the slot-window smoothly to zero at each end, the directivity is improved at the insignificant expense of a wider main lobe. As a new attempt, commonly used windows for FIR digital filters are utilized as coupling functions for the slot-coupled type. Under the assumption of the loose-coupling, the directivity characteristics of various slot-windows such as rectangular, Bartlett, Hamming, Hanning, Blackman and Kaiser are presented. It is also clarified both theoretically and experimentally that the frequency band width of the slot-coupled type is much broader than that of the two-hole, the binomial array and the Tchebycheff array types. Therefore, the slot-coupled acoustic directional coupler is potentially useful for a swept-frequency reflectometer.
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