An experiment is described to illustrate the concept of acoustic impedance. Pulses of sound, frequency 670 kHz, are propagated through water and through paraffin (kerosene) and the speeds of propagation are measured by the time-of-flight method. The densities of the liquids are also measured and values for the acoustic impedances are calculated. The amplitude reflection coefficient for the water-paraffin interface is measured and compared with the theoretical value derived from the impedances. Agreement at the level of five per cent is obtainable in about an hour's work. The cost of apparatus, excluding standard laboratory equipment, is about Pounds 50.