Acoustic measurement as an external control technique appears to be advantageous in internal leak detection along a cylinder piston seal under static conditions. It is proven in valve flow monitoring, and this article from the 6-8 Dec. 1983 Los Angeles 39th National Conference on Fluid Power, and the experiments described and evaluated here, at 105 bar and 38 DEGREE C, employ a PAC 4120 valve leak detector. (R.H.)