A standing-wave thermoacoustic engine is essentially composed of a stack, two heat exchangers, and tubes; the stack has many narrow flow channels and is located inside one of the tubes. One end and the other end of the stack are thermally connected to hot and cold heat sources via the heat exchangers, respectively. When the ratio of temperatures of the heat sources exceeds a critical value, the gas inside the tubes spontaneously oscillates and the stack generates acoustical energy using heat from the hot heat source. In this study, the critical temperature ratio needed for exciting the spontaneous gas oscillation was numerically calculated by changing the stack's length, flow channel radius, and position. Further, the thermal efficiency with the critical temperature ratio was calculated. These calculations allowed us to design and construct the engine in accordance with the heat source temperatures. The constructed engine worked with the heat sources having the temperature ratio 1.7 and achieved 8% of the Carnot efficiency. These obtained values quantitatively agreed with the design ones.