The detection of acoustic signals is of relevance for a range of practical applications, for example in medical diagnostics. However, whereas rectification of electric current and other energy forms such as thermal flux has been demonstrated(1-6), acoustic rectification has not yet been achieved. Here, on the basis of the earlier theoretical proposal of an 'acoustic diode'(7), we present the first experimental demonstration of a rectified energy flux of acoustic waves. A one-dimensional acoustic rectifier is fabricated by coupling a superlattice with a layer of ultrasound contrast agent microbubble suspension. A significant rectifying effect is observed within two frequency bands at locations that agree well with theoretical predictions. Following optimization of the concentration of the microbubble suspension, rectifying ratios can be as high as similar to 10(4). This realization of an acoustic rectifier should have substantial practical significance, for example in the focusing of ultrasound in medical applications.