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The Use of Airborne Ultrasonics for Generating Audible Sound Beams

JAES 01/1999; 47:726-731.
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    ABSTRACT: A highly focused sound beam can be created from the nonlinear interaction of ultrasound waves by using a parametric loudspeaker. An audio signal is produced by the self-demodulation of amplitude modulated sound waves in air. Several preprocessing techniques have been discussed in previous literature to reduce distortion. A comparative analysis of these preprocessing techniques is presented in this paper. Some preceding preprocessing techniques rely on equalization and square-rooting. Newer techniques, instead of using only simple AM, have an additional AM signal with a carrier that is orthogonal to the original one. It will be clearly seen that the distortion of the modified AM is greatly reduced from the normal AM. Simulation results show the effectiveness of the preprocessing techniques.
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    ABSTRACT: Research has indicated that airborne ultrasonic sound impinging on the ear drums of humans has the potential to cause undesirable effects. The United States of America's Occupational Health and Safety Administration have changed their guidelines to permit an additional 30dB increase in the acceptable ultrasonic amplitudes under certain conditions. This paper contains a review of current recommended acceptable exposure limits from standards organisations around the world.
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    ABSTRACT: The control of directivity represents a new stake in the reproduction of sound sources by an electroacoustic device. This paper first reminds the principle of a general method for reproducing the acoustical field radiated by a source. The present study, is devoted to the bandwidth improvement and to the optimization of the directivity control. It rests on the preliminary constitution of a set of basic directivities from which it is possible to tune the cardioicity and the three-dimensional orientation of the synthesized directivity pattern. For the simulation of musical instruments, it is important to maintain independent control on the spatial and spectral characteristics. For that, an analysis/synthesis method of the power spectrum is used to provide the automatic design of a correction filter to applied on near field recording before feeding the source. Results are discussed considering room acoustics criteria and examples of musical instruments.
    01/2001;