A detailed analysis of the use of an optical cavity to enhance picosecond ultrasonic signals is presented. The optical cavity is formed between a distributed Bragg reflector and the metal thin film samples to be studied. Experimental results for Al and Cu films show enhancement of acoustic signals by up to two orders of magnitude and are in good agreement with calculated results based on a previously established model. This technique provides an efficient method for detecting sound in materials with small piezo-optic coefficients and makes it possible to determine the actual pulse shape of the returning acoustic echoes.