A passive compression wave imaging system comprises an array of sensor elements arranged in a sparse array and a processor arranged to: store a plurality of samples of the output from each of the sensor elements over a sample period; derive from the stored samples a value for each of a set of image pixels; wherein for each of the image pixels the processing means is arranged to: define a plurality of different sets of weights for the elements of the sparse array; calculate a component of a pixel value from each of the sets of weights and the stored samples; and sum the components of the pixel value to produce a final pixel value.