Acoustic illusions

Researchers have devised an ingenious method of using acoustics to conceal and simulate objects. When listening to music, we don't just hear the notes produced by the instruments, we are also immersed in its echoes from our surroundings. Sound waves bounce back off the walls and objects around us, forming a characteristic sound effect - a specific acoustic field. This explains why the same piece of music sounds very different when played in an old church or a modern concrete building. Architects have long been capitalising on this fact when building, say, concert halls. However, the principle can also be transferred to other applications: objects hidden underground can be visualised by measuring how sound waves from a known source are reflected. Active and passive manipulation .