A bypass duct similar to a Herschel-Quincke tube can be used to increase the transmission loss of mufflers and insertion loss of enclosures at selected frequencies. In many cases, the duct can be short and thought of as a leak. It is shown that the optimal length and cross-sectional area can be determined by using a simple optimization technique known as the Vincent Circle. To prove the concept, a muffler was designed and optimized using transfer matrix theory. Then, the optimized muffler was constructed and the transmission loss was measured using the two-load method. The measured results compared well with predictions from transfer matrix theory for the muffler. A similar study was performed for a partial enclosure. A small bypass duct was introduced on an interior panel. It was demonstrated using boundary element analysis that a bypass duct can shift the trough of the insertion loss curve to another frequency without greatly changing the shape and size of the enclosure.