The focal shift phenomenon in optics describes how the position of the focus point in a focusing system is not simply defined by geometrical ray-based models but is affected by diffraction and is consequently a function of the size of the lens and the frequency of the light. The same effect is also observed in acoustics when looking at the focused field using physical focusing reflectors. This paper describes the focal shift phenomenon applied to the reproduction of focused sources with sound field synthesis systems, and presents a formula for the prediction of the actual rendered focal point position and also a frequency dependent positional correction for the improved rendering of a focused source with a given loudspeaker setup.
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