Acoustic methods for leak detection and tightness testing

Air tightness testing has importance in industrial environments where in compressed air systems leaks account for a high percentage of energy loss. These losses can effectively be reduced by locating and repairing of leaks. Furthermore, tightness is a criterion for the quality of different kinds of seals and joints. For leak detection and tightness testing acoustic methods (passive and active) can be applied. Variations of the technology enable access to different problems. The methods for leak detection can be graded up by the estimation of leak sizes and energetic losses. Similarly, slot size and length of tightness leaks are responsible for energetic losses in buildings (window and door seals) or for dysfunctions in technical systems such as cabins, heat exchangers and others. Leaks can be found easily by means of traditional ultrasound leak detectors which operate in a narrow frequency band (e.g. 40 kHz). However, this simple technology is inadequate for quantitative estimations of leak and tightness losses. Therefore, new theoretically based approaches have been developed and experimentally exemplified. Sound will be created by passing of air through leaks (for high and low pressure as well). Compressed air exits an orifice generating a turbulent jet, which emits a broadband sound. Acoustic leak finder devices can localize these leaks. However, quantification of their size remains a crucial task (with respect to energy loss and priority of repair). From the point of view of aeroacoustical theory a quantification of fluid parameters of leak jets can only be achieved by evaluation of a broad frequency band (20 kHz to 100 kHz). A new testing procedure and evaluation algorithm for finding and quantification of leaks will be presented. Tightness of many technical systems can be tested by means of actively transmitted (ultra-)sound. Again, quantification remains a demanding task. The sound field strongly depends on depth, width and geometry of an orifice and is affected by diffraction, reflection, transmission and interference. The method is discussed with respect to the quantification of leak size. It is shown which demands are to be met on the sound source and the generated acoustic fields.