Enhancing low frequency sound radiation of electrodynamic loudspeakers with acoustic metamaterials

Abstract

Since the radiation resistance of a vibrating membrane in the air is proportional to frequency in the subwavelength range, low frequency (<100Hz) sound radiating devices generally require a large vibrating diaphragm and a bulky enclosure to be efficient and loud enough for practical use. The tapped horn topology, where the front radiation of an electrodynamic driver adds up with the back radiation after propagating through the internal volume of a speaker box, has been widely used in custom audio to provide low-frequency enhancement to the bass unit. Recently, people have demonstrated the application of acoustic metamaterials based on multiple Helmholtz resonators (HRs) in sound absorption, sound insulation, directivity control, impedance matching, etc. In this presentation, we show that by branching the sound path of a tapped horn speaker box with multiple Helmholtz resonators and fine-tuning the geometry of each individual element, we can produce high and relatively constant sound pressure levels at sub-wavelength frequencies, making our methodology a good candidate for compact subwoofer design. Genetic algorithm (GA) is used to bring forth optimal performance within the geometric framework. Both transfer matrix method and numerical simulation are employed to derive the sound output of a particular subwoofer.