Usage The Audio Spotlight can be used in two major ways: As directed audio, sound is directed at a specific listener or area, to provide a private or area specific listening space. As projected audio, sound is projected against a distant object, creating an audio image. This audio image is literally a projected loudspeaker - sound appears to come directly from the projection, just like light. The Audio Spotlight consists of a thin, circular transducer array and a specially designed signal processor and amplifier. The transducer is about half an inch thick, nonmagnetic, and lightweight. The signal processor and amplifier are integrated into a unit about the same size as a traditional audio amplifier, and has similar power requirements. Technology Because it is impossible to generate extremely narrow beams of audible sound without extremely large loudspeaker arrays, we instead generate the sound indirectly, using the nonlinearity of the air to convert a narrow beam of ultrasound into a highly directive, audible beam of sound. The device transmits a narrow beam of ultrasound (blue), which, due to the inherent nonlinearity of the air itself, distorts (changes shape) very slightly as it travels. This distortion creates, along with new ultrasonic frequencies, audible artifacts (green) which can be mathematically predicted, and therefore controlled. By constructing the proper ultrasonic beam, this nonlinearity can be used to create, within the beam itself, an audible sound beam containing any sound desired. This is presently done in real-time using low cost circuitry, a specially designed amplifier, and transducers developed at MIT specifically for this project. Hyperdirectivity The directivity, or narrowness, of an acoustic wave generated by a circular transducer is proportional to the ratio of the diameter of the transducer to the wavelength of the sound. So a transducer much larger than the wavelength of the sound creates a very narrow beam. -94-