Directional sound: large sources or parametric arrays (loudspeaker arrays), ultrasound heterodyning, audio spotlight, LRAD



Directional sound is a technological principle which consists of the concentration/focusing of sound waves or, in other words, the decrease of their dispersion (spread). The purpose is to generate a beam with increased directivity. For all wave sources, including sound sources, the larger the source compared to the wavelength it generates, the more directional the beam is. Also, sound waves of higher frequency and therefore lower wavelength, like ultrasound, are spreading less. For this reason, ultrasound has been used as a carrier for sound information via modulation or conventional mixing termed heterodyning ("sound from ultrasound") in order to achieve directional audible sound delivery. 



Figure 1: From Wikipedia: "Sound from an array spreads less than sound from a point source, by the Huygens–Fresnel principle applied to diffraction" (Image ref.).



Additionally, using an array of sound sources like loudspeakers, it is possible to create a beam of increased directionality. Researchers have experimented since the 1960s on directional sound using ultrasound heterodyning with parametric arrays. The term "Audio Spotlight" coined for this technology by a theoretical study in the 1980s, in now a trademark. Holosonics, founded by an MIT graduate, is an example of a company specialized in Audio Spotlight speaker systems for corporations and venues like museums. Additionally, a billboard advertisment for A&E's TV show "Paranormal State" used directional sound to generate a message that could be heard only by a person walking by the advertisment. 


In the military and law enforcement, the Long Range Acoustic Device (LRAD) (ref.) is an acoustic hailing device which uses directional sound to send instructions or warnings to disperse crowds (Figure 1). If verbal instructions do not produce a result, the LRAD can generate a loud warning tone that approaches or surpasses the threshold of pain. When used in this deterrence manner, the LRAD is a non-lethal weapon.


Holosonics and A&E's advertisment are cited in this article by journalist and author Sharon Weinberger. Also, this video discusses the article, as well as the technique of voice morphing.




Figure 2: Long Range Acoustic Device (LRAD) used by the US Navy (Wikipedia).