A music system that apparently conjures sound out of thin air could replace conventional sound technology, say its developers.  The prototype system has no conventional speakers.  Instead it relies on ultrasonic waves to generate a "sonic hologram", or interference pattern, in midair.  As well as improved sound systems, the technology might also be modified for crows control applications.  The system could target particular individuals with powerful, low-frequency sound waves that temporarily disable them.

American Technology Corporation (ATC) of Poway, California, developed the prototype, which is the brainchild of the company's chief technology officer, Elwood Norris.  The company will unveil the prototype in the US this month, and hopes to have the first versions on sale within a year.

The system emits two ultrasonic waves at different frequencies.  Each set of waves is at a frequency too high to hear, but where they overlap, or interfere, they generate audible sound.  The effect is called acoustical heterodyning and is based on a phenomenon called the Tartini, or difference, tone.  In the 18th century the Italian composer Guiseppe Tartini noted that two interfering sound waves of different frequencies will produce a third sound whose frequency is the difference between the two.

Acoustical heterodyning also occurs with ultrasonic waves.  Two ultrasonic waves at frequencies too high to be audible, such as 200 kilohertz and 201 kilohertz, would produce an audible sound of 1 kilohertz.

The ultrasonic waves in the prototype are generated by piezoelectric crystals, or transducers.  an oscillating voltage applied to the crystal makes it vibrate in a similar way to a loudspeaker.  One crystal emits a 100-kilohertz fixed signal, while the second varies between 100 kilohertz and 120 kilohertz.  This generates difference tones of between 0 hertz and 20 kilohertz, which covers the full range of human hearing.

"The sensation is nothing short of amazing," says Norris.  "If you point the sound at a wall, a room full of people will point to the same spot as the source of the sound.  You can then move the sound out into the centre of the room and up over the top of the audiences' heads."

The system produces sound indirectly, which should eliminate the normal distortion caused by speakers, says Norris.  He believes it could improve telephones and hearing aids, and add something extra to cinema sound.  The mobile sound source could be used to represent, for example, a jet aircraft crossing over the heads of the audience.

"There is no reason why it shouldn't work," says Peter Fryer, head of research at B&W Loudspeakers in Steyning, Sussex.  "But the high frequencies may be so high that the waves may not propagate very far as they are absorbed in air.  The proof of the pudding will be in the eating."

The system may also have applications in crowd control.  Powerful, low-frequency sound can cause disorientation and nausea.  In the 1960s, the US tried unsuccessfully to use low-frequency sound from helicopters to disable enemy soldiers in the Vietnamese jungle.  But the sound sources needed were so intense that they almost shook the aircraft apart, and most of the sound was absorbed by those nearest to the loudspeakers.  According to Norris, acoustical heterodyning could pinpoint an individual up to 200 or 300 metres away by positioning the interference zone correctly.