When gas moves from a high-pressure area to a low-pressure area through a hole , it expands very rapidly and produces a turbulent flow, resulting in an audible "hissing" sound. The "hissing" sound is broadband acoustic sound, which ranges from the audible frequency range (20 Hz to 20 kHz) into the ultrasonic frequency range (16 kHz to 10 MHz).
The level of ultrasound produced by a gas leak is determined by three main factors (mainly 1 and 2):
- The pressure drop across the leak
- The physical size of the leak
- Gas specific properties such as: molecule weight and the specific heat ratio of the gas
For the turbulent flow to take place and produce ultrasound the pressure on one side of the leak has to be at least twice the pressure of the other side of the leak. Besides, if the hole is too big, the pressure drop across the hole will be too little, and no ultrasound will be produced.
The three factors above - mainly the pressure and the leak size - will decide whether enough ultrasound is generated to trigger an ultrasonic (acoustic) gas leak detector. The standard for these parameters is referred to as the leak rate - and this standard can be calculated by means of a mass flow formula.