Spatial sound decomposer

The team conceived the emitter as bits of acoustically reflecting material within a certain area, with voids between them filled with air. A monopolar sound source located at the centre of the structure emitted frequencies between 7,600 Hz and 12,800 Hz (which is within treble audio frequency range) with equal power throughout. Because of reciprocity, the emission and reception properties of such a device are reversible — hence the design as an emitter, which is more feasible to realize experimentally than a receiver.

Christiansen and colleagues solved the design task for such an emitter numerically as an iterative process in which they compared the desired target emission pattern to the acoustic pattern produced by the proposed structure. By minimizing a well-defined figure of merit, the team obtained the optimal geometry. Continuous pressure fields were assumed, as well as negligible mechanical vibrations in the device. The result of this formidable computational exercise was a square mask with 28 subwavelength features, which they then fabricated as a 10 cm by 10 cm structure on a substrate (pictured). The researchers demonstrated that this device had the desired spatio-spectral response and a high efficiency over the complete frequency range of the source.