Most naturally occurring materials have a disordered atomic structure that interferes with the propagation of both sound and electromagnetic waves. When the waves come into contact with these materials, they bounce around and disperse — and their energy dissipates according to a highly complex interference pattern, diminishing in intensity. That means it's virtually impossible to transmit data or energy intact across wave-scattering media and fully leverage the potential of wave technology.
For example, with a smartphone, the geolocation function does not work as well inside buildings where radio-frequency waves scatter in all directions. Other potential applications include biomedical imaging and geological surveying, where it's important to be able to send waves across highly disordered media.
A system was developed that allows sound waves to travel across such media with no distortion. It uses tiny speakers as acoustic relays to offset the wave scattering. It has been successfully tested on a real acoustic system. The system involves placing acoustic relays at strategic locations so that sound waves can propagate at a constant amplitude, regardless of what may lie in their path. This method could eventually be used to make it possible to hide objects like submarines.
The tiny speakers can be controlled to amplify, attenuate, or shift the phase of the sound waves. That lets them offset the diffusion that results when the waves hit obstacles, thereby reproducing the original sound exactly on the other side of the disordered medium.
The system was tested by building a 3.5-meter-long air-filled tube and placing various kinds of obstacles such as walls, porous materials, and chicanes into it in order to create a highly disordered medium through which no sound waves could pass. The tiny speakers were placed between the obstacles and electronic controls were set up to adjust the speakers’ acoustic properties.
A new control mechanism had to be developed to amplify the sound wave, similar to how optical waves can be amplified with lasers. The new method — the only one of its kind in acoustics — uses programmable circuits to control several speakers simultaneously and in real time.
The method for active acoustic control is similar to that used in noise cancelling headphones and could potentially be used for sounds containing common ambient frequencies. It could also be used to eliminate the waves that bounce off objects like submarines, making them undetectable by sonar. Moreover, the theory underlying the work could have parallel applications in optics or radio frequencies to make objects invisible or to take images through opaque materials.