Human ear can hear sound frequency up to 18 KHz. The  sound frequency above 20 KHz is categorized as  ultrasonic sound which is not audible. Ultrasonic sound is very directional in nature. We can  utilize this nature to pinpoint the exact origin of the  sound source where the leak is located. Physics tells us that the gas always flows from the higher  pressure region to the lower one. When the pressure  difference is only a small opening, the turbulence created  by the gas through the small hole then generates  ultrasonic sound. In the non pressurized state, the ultrasonic transmitter is  applicable, ultrasonic wave can leak from the small hole. Ultrasonic gas leakage detector is designed based on the  above simple physics.