Acoustical properties of insulating liquids and gases

The acoustical properties such as transmission-frequency characteristics, impedance and ultrasound velocity, are outlined for several insulating liquids and gases. These properties are important where ultrasonic techniques are used for sensing of acoustic emissions from partial discharges (P.D.) associated with electrical power apparatus. As an example, it was found that sulphur hexafluoride (SF6) is a sound absorbing gas at certain ultrasonic frequencies above 20 kHz, but at audible frequencies near 5 kHz transmits better than air, which is the reason why acoustic sensing of P.D. in SF6 insulated apparatus is best achieved at low frequency. Apart from the ultrasonic sensing of partial discharges, the acoustical properties can be used for identifying the composition of gas mixtures (insulation quality) and the molecular structure of liquids, and these features are discussed in this paper.