A low-frequency multiple-band sound insulator without blocking ventilation along a pipe

Abstract

It is challenging to insulate sound transmission in low frequency-bands without blocking the air flow in a pipe. In this work, a small and light membrane-based cubic sound insulator is created to block acoustic waves in multiple low frequency-bands from 200 to 800 Hz in pipes. Due to distinct vibration modes of the membrane-type faces of the insulator and co-action of acoustic waves transmitting along different paths, large sound attenuation is achieved in multiple frequency-bands, and the maximum transmission loss reaches 25 dB. Furthermore, because the sound insulator with a deep subwavelength size is smaller than the cross-sectional area of the pipe, it does not block ventilation along the pipe.

Figure 1

Experimental apparatus. (a) The model (left) and photo (right) of a MFCB. (b) Experimental apparatus for measuring the transmission loss induced by the MFCB. The distance between the adjacent microphones at the input end (or output end) is $15\text{cm}$ $s_1=s_2=15\text{cm}$ and the distance between the microphone 2 (or 3) to the MFCB is 45 cm $l_1=l_2=45\text{cm}$ (the figure is created using SOLIDWORKS 2016).

Figure 2

Transmission losses induced by the MFCB and the mechanism. (a) Measured transmission losses induced by the MFCBs created with two types of membranes, plastic and latex. (b) Comparison between the simulated and measured transmission losses induced by a MFCB based on latex membranes. (c) Vibration modes of the faces of the MFCB and acoustic intensities (red arrows) around the MFCB obtained at four sound attenuation peaks. (d) Comparison of acoustic intensities between the performance of a MFCB (Blue and red arrows indicate acoustic intensities in and around the MFCB, respectively.) and that of a HQ pipe (the figure is created using MATLAB 2016 and COMSOL 5.5).

Figure 3

Performance of multiple MFCBs. (a) A string of MFCBs established in a pipe. (b) The dispersion of a periodic structure consisting of a string of MFCBs. The lattice constant is $d=15\text{cm}$ and the equivalent Young's modii of the MFCBs are identical, which are $E=4.9\times {10}^{10}\text{Pa}$. (c) Simulated transmission losses induced by a string of five MFCBs with identical parameters. (d) Simulated transmission losses induced by three MFCBs with different Young's moduli. (e) Comparing between simulated and measured transmission losses induced by three MFCBs with different Young's modii of $E_1=6.1\times {10}^{10}\text{Pa}$, $E_2=7.1\times {10}^{10}\text{Pa}$ and $E_3=8.9\times {10}^{10}\text{Pa}$ (the figure is created using MATLAB 2016 and COMSOL 5.5).

Figure 4

Influences of an air flow. (a) Experimental apparatus for evaluating the influence of the MFCB on the ventilation along a pipe. (b) Comparison of the measured and simulated results of the pressure drop induced by a MFCB with different input air flow velocities. (c) Experimental apparatus for measuring the transmission loss induced by a MFCB under an air flow. (d) Measured transmission losses with and without an air flow (the figure is created using SOLIDWORKS 2016, MATLAB 2016 and COMSOL 5.5).