In a first of its kind, researchers have designed sounds that can be recorded by microphones, even when they are inaudible to humans. Microphones are specifically built to hear the human voice – humans can’t hear at levels higher than 20 kHz. However, the new sound is completely inaudible to humans (40 kHz or above) and yet is audible to any microphone. The sound combines multiple tones that, when interacting with the microphone’s mechanics, create what researchers call a ‘shadow’; it is a sound that the microphones can detect. What this would mean is that military and government officials could secure private and confidential meetings from electronic eavesdropping or cinemas and concerts could prevent unauthorised recording of movies and live performances.