Realizing a finite array of dipole sources with high acoustic transmission directivity at low frequency.

A finite array of dipole sources is used to enhance the directivity and radiation gains of sound. This letter demonstrates that this idea can be realized as several sub-wavelength slits in a plate with periodic Helmholtz resonators. Moreover, the array gain of this structure can be further improved by adjusting the effective boundary impedance. The directivity of the dipole array is compared with that of a point array, and the advantages of the dipole array are revealed. Prospective applications of this mechanism include achieving directional radiation in high-gain loudspeakers and ultrasonic medical instrumentation.