The degree of speckle reduction achievable by spatial and frequency compounding is a function of the rates of speckle pattern change induced by varying the imaging system’s illumination angle and acoustic frequency, respectively. We have measured these rates under a variety of conditions and derived a method of maximizing speckle reduction using the average of partially correlated speckle patterns. Our experimental results agree well with theoretical predictions of these phenomena and indicate that, under limited conditions, improved target detection is possible using spatial compounding. Frequency compounding appears to be counterproductive in improving target detectability. © 1987 SPIE.