Hi-fi speakers range from piezoelectric tweeters of various kinds of mid-range speakers and woofers which generally rely on circuits at large enclosures to produce quality sound, whether it is dynamic, electrostatic or some other transducers- based design. Engineers have struggled for nearly a century to produce a speaker design with the ideal 20Hz -20,000Hz capability of human hearing and also produce narrow beam of audible sound.
The Audio spotlight developed by American Technology Corporation uses Ultrasonic energy to create extremely narrow beams of sound that behaves like beam of light. Audio spotlight exploits property of non-linearity of air. A device known as parametric array employs the non-linearity of air to create audible by products from inaudible ultrasound, resulting an extremely directive and beam like sound. This source can projected about an area much like a spotlight and creates an actual specialized sound distant from a transducer. The ultrasound column acts as airborne speaker, and as the beam moves through the air gradual distortion takes place in a predictable way. This gives rise to audible components that can be accurately predicted and precisely controlled.
This audio spotlight technology creates focused beams of sound similar to light beams coming out of a flashlight. Specific listeners can be targeted with sound without other hereby hearing it, i.e. to focus the sound into coherent and highly directional beam. It makes use of non- linearity property of air.
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CHAPTER 02
2.1 THEORY
The regular loudspeakers produce audible sound by directly moving the air molecules. The audible portions of sound tend to spread out in all directions from the point of origin. They do not travel as narrow beams which is why you don?t need to be right in front of a radio to hear music. In fact, the beam angle of audible sound is very wide, just about 360 degrees. This effectively means the sound that you hear will be propagated through air equally in all directions.
In order to focus sound into a narrow beam, you need to maintain a low beam angle that is dictated by wavelength. The smaller the wavelength, the less the beam angle, and hence, the more focused the sound. Unfortunately, most of the human-audible sound is a mixture of signals with varying wavelengths between 2 cms to 17 meters (the human hearing ranges from a frequency of 20 Hz to 20,000 Hz). Hence, except for very low wavelengths, just about the entire audible spectrum tends to spread out at 360 degrees. To create a narrow sound beam, the aperture size of the source also matters a large loudspeaker will focus sound over a smaller area. If the source loudspeaker can be made several times bigger than the wavelength of the sound transmitted, then a finely focused beam can be created. The problem here is that this is not a very practical solution. To ensure that the shortest audible wavelengths are focused into a beam, a loudspeaker about 10 meters across is required, and to guarantee that all the audible wavelengths are focused, even bigger loudspeakers are needed.
Here comes the acoustical device ?AUDIO SPOTLIGHT? invented by Holosonics Labs founder Dr. F. Joseph Pompei (while a graduate student at MIT), who is the master brain behind the development of this technology.
2.2 NON-LINEARITY OF AIR
Audio spotlighting exploits the property of non-linearity of air. When inaudible ultrasound pulses are fired into the air, it spontaneously converts the inaudible ultrasound into audible sound tones, hence proved that as with water, sound propagation in air is just as non-linear, and can be calculated mathematically.
??????????? A device known as a parametric array employs the non-linearity of the air to create audible by-products from inaudible ultrasound, resulting in an extremely directive, beamlike wide-band acoustical source. This source can be projected about an area much like a spotlight, and creates an actual sound distant from the transducer. The ultrasound column acts as an airborne speaker, and as the beam moves through the air, gradual distortion takes place in a predictable way. This gives rise to audible components that can be accurately predicted and precisely controlled. However, the problem with firing off ultrasound pulses, and having them interfere to produce audible tones is that the audible components created are nowhere similar to the complex signals in speech and music.
Human speech, as well as music, contains multiple varying frequency signals, which interfere to produce sound and distortion. To generate such sound out of pure ultrasound tones is not easy.
This is when teams of researchers from Ricoh and other Japanese companies got together to come up with the idea of using pure ultrasound signals as a carrier wave, and superimposing audible speech and music signals on it to create a hybrid wave. If the range of human hearing is expressed as a percentage of shift from the lowest audible frequency to the highest, it spans a range of 100,000%. No single loudspeaker element can operate efficiently or uniformly over this range of frequencies. In order to deal with this speaker manufacturers carve the audio spectrum into smaller sections. This requires multiple transducers and crossovers to create a 'higher fidelity' system with current technology.
(Airborne ultrasounds of 28 kHz are envelope-modulated with audio signals. Inherent non-linearity of the air works as a de-modulator. Thus de-modulated sounds impinge on our eardrums. We can hear those sounds! )
Using a technique of multiplying audible frequencies upwards and superimposing them on a "carrier" of say, 200,000 cycles the required frequency shift for a transducer would be only 10%. Building a transducer that only needs to produce waves uniformly over only a 10% frequency range. ?In this technology we can ?put sound where we want?
Using sound with vision improves retention rates by up to 60%, but how do you get round the issue of noise pollution to the surrounding area? By using Audio spotlight - which concentrates the sound just as a spotlight does so only those in the "beam" can hear your message. Use it outside your shop window, or under your billboard. People can hear, but can't always know where the sound is coming from. Creative opportunities
AUDIO SPOTLIGHT TRANSDUCER
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