Sound from ultrasound

From Wikipedia, the free encyclopedia

1 Uses
- 1.1 Commercial advertizing
- 1.2 Military applications
2 History
3 Makes
- 3.1 HyperSonic Sound
- 3.2 Audio Spotlight
4 See also
5 Further resources
6 External links

This page is about modulated ultrasound that can make its carried signal audible without needing a receiver set. This happens when the modulated ultrasound passes through or reflects from anything which behaves nonlinearly and thus acts unintendedly as a demodulator.

Scuba divers and naval submarines have been using modulated ultrasound underwater communicators for many years, but to hear the sound signal needed a receiver set to demodulate the received ultrasound signal.

Researchers since the early 1960's have been experimenting with creating directive low-frequency sound from nonlinear interaction of an aimed beam of ultrasound waves produced by a parametric array. Ultrasound has wavelengths much less than audible sound and thus can be aimed in a much tighter narrow beam than any traditional audible loudspeaker system.

The first modern device was published in 1998 (105th AES Conv, Preprint 4853, 1998), and is now known by the trademark name "Audio Spotlight".

A transducer can be made to project a narrow beam of modulated ultrasound that is powerful enough, (100 to 110 dBSPL) to change the speed of sound in the air that it passes through. The air within the beam behaves nonlinearly and extracts the modulation signal from the ultrasound, resulting in sound that is can be heard only along the path of the beam, or that appears to radiate from any surface that the beam strikes. The practical effect of this technology is that a beam of sound can be projected over a long distance to be heard only in a small well-defined area. A listener outside the beam hears nothing. This effect cannot be achieved with conventional loudspeakers, because sound at audible frequencies cannot be focused into such a narrow beam.

There are some criticisms of this approach. Anyone or anything that disrupts the path of the beam will disturb the dispersion of the signal, and there are limitations, both to the frequency response and to the dispersion pattern of such devices.

[edit] Uses

[edit] Commercial advertizing

To aim a sound signal at a particular passer-by without everybody in the area hearing it.

[edit] Military applications

Military applications have been reported such as a "sonic bullet" weapon that aims a highly-directed high-intensity sound wave, causing debilitating pain. The system is being promoted for use in crowd control or subduing individual suspects without lethal danger. There are reports in online blogospheres that this technology has been picked up by vigilante groups.

[edit] History

This technology was originally developed by the US Navy and Soviet Navy for underwater sonar in the mid-1960s, and was briefly investigated by Japanese researchers in the early 1980s, but these efforts were abandoned due to extremely poor sound quality (high distortion) and substantial system cost. These problems went unsolved until a paper published by Dr. F. Joseph Pompei of the Massachusetts Institute of Technology in 1998 (105th AES Conv, Preprint 4853, 1998) fully described a working device that reduced audible distortion essentially to that of a traditional loudspeaker.

[edit] Makes

At the time of writing there were currently two devices available on the market that use ultrasound to create an audible "beam" of sound: HyperSonic Sound and Audio Spotlight.

It has been reported that here the German company Sennheiser Electronic may be moving into this market.

[edit] HyperSonic Sound

HyperSonic Sound is a trademark of American Technology Corporation.

Elwood Norris, owner of American Technology Corporation, announced he had successfully created a device which achieved ultrasound transmission of sound in 2002. In December 2002 Popular Science named his HyperSonic Sound the best invention of 2002, beating out the Segway scooter. Commercial adaptation of the technology did not occur immediately, despite significant publicity in 2003. This led to millions in continuing losses by ATC.

The Hypersound amplification apparatus is named ‘HSS’ (HyperSonic Sound Technology) by American Technology Corporation.

[edit] Audio Spotlight

At about the same time Norris was developing his technology, F. Joseph Pompei PhD (MIT) developed a similar technology, called Audio Spotlight, and made it commercially available in 2000 by his company Holosonics, which claims to have sold more than 1000 of their "Audio Spotlight" systems.

Major education / entertainment applications are now being developed for this device. Disney are reported to be amongst the first major corporations to buy into this for use at the Epcot Center. (Source - ABC news 21 August 2006)