Visible light can be transmitted in a beam (torch).
The same can be done with audible sound.
The sound can also be made to disappear in the propagation direction.
(This can not be done with light).
The concept is to cancel out the directive sound beam produced by a parametric sound source, by transmitting a second parametric signal which produces the audible sound in anti-phase. Due to differences in frequencies in the first and second sources the audible sound will be generated at different distances, giving a region in the propagation direction where the sound can be heard.
The input wave is sin(w1t)+sin(w2t)+Asin(w3t+p)+Asin(w4t). In the plot below, the frequencies wi are 38, 40, 58 and 60 kHz. The amplitude parameter A is changed. The frequency pair 60 and 58 creates the audible sound 60-58 = 2 kHz, then the frequency pair 40 and 38 cancels the audible sound.Plotted below is the difference frequency 2 kHz as function of distance for A=1.525.
The solutions are based on an exact analytical arbitrary multi-frequency solution found in Multi-frequency plane, nonlinear and dissipative waves at arbitrary distances, Hedberg, C.M., J. Acoust. Soc. Am. 106, 3150-3155 (1999).
The disappearing sound received a grant from Teknikbrostiftelsen Lunds 2000 for university associated product development.
Resonators and other nonlinear system
Nonlinear standing waves in a layer excited by periodic motion of its boundary, O.V. Rudenko, C.M. Hedberg and B.O. Enflo, Acoustical Physics, v. 47, No. 4, 525-533 (2001).
Pulse response of a nonlinear layer, C.M. Hedberg and O.V. Rudenko, J. Acoust. Soc. Am. 110(5), 2340-2350 (2001).
Enhancements of energy and Q-factor of a nonlinear resonator with increase in its losses, O.V. Rudenko, A.L. Sobisevich, L.E. Sobisevich and C.M. Hedberg, Doclady, Physics (Reports of the Russian Academy of Sciences) Vol. 383, No. 3, pp. 330-333 (2002).
Resonant properties of a nonlinear dissipative layer excited by a vibrating boundary: Q-factor and frequency response, B.O. Enflo, C.M. Hedberg and O.V. Rudenko, submitted to JASA in May 2002.
Interaction between low and high-frequency modes in a nonlinear system: gas-filled cylinder covered by movable piston, O.V. Rudenko and C.M. Hedberg, accepted for Nonlinear Dynamics May 2003.
Nonlinear Dynamics of Grains in a Liquid-Saturated Soil, Hedberg, C.M., in Nonlinear Acoustics at the Beginning of the 21st Century, Eds. O.V.Rudenko and O.A.Sapozhnikov. pp. 735-738, Moscow State University, 2002.