Planar acoustic notch filter for low frequency sound wave suppression

In this study, a subwavelength acoustic notch filter with a tapered neck is proposed. This type of structure is capable of suppressing tonal noise at relatively low frequencies, but with a thickness of 1/16 of the targeting sound wavelength. Unlike the traditional acoustic Helmholtz resonator, which is bulky for low frequency noise control, the proposed structure can be formed as a planar acoustic notch filter, occupying a small volume and making the system compact. It is found that when the proposed units are mounted on a waveguide, the achieved noise reduction performance has a close relationship to the separation distance of the resonators. Near total reflection can occur when the propagation phase lag comes up to 90 degrees if acoustic damping is neglected. The damping influence has also been discussed, indicating damping should be reduced for this application. In the experimental study, since the tapered form can reduce damping, it is demonstrated that such sample can have better sound amplification performance compared to the uniform sample. The tapered samples are then used to suppress tonal noise emitted by a loudspeaker. At the optimal configuration, measured results show sound pressure can be suppressed by 84 percent when the units start to work. The proposed planar acoustic notch filter can be used when ventilation and noise suppression are simultaneously required in a narrow space.