1. A product recognition system comprising: a product image data collector arranged to capture image data which is representative of an image of a product item; an acoustic energy source arranged to emit acoustic energy towards the product item; and a product acoustic data collector arranged to (i) capture acoustic energy deflected from the product item, (ii) process the captured acoustic data which has been deflected from the product item to provide product acoustic data which is representative of one or more characteristics of the product item, and (iii) compare the product acoustic data with a store of reference acoustic data to provide one or more subsets of items against which the captured image data can be compared to identify the product item.