An adjustable acoustic metamaterial cell using a magnetic membrane for tunable resonance

Abstract

Acoustic metamaterials are growing in popularity for sound applications including noise control. Despite this, there remain significant challenges associated with the fabrication of these materials for the sub-100 Hz regime, because acoustic metamaterials for such frequencies typically require sub-mm scale features to control sound waves. Advances in additive manufacturing technologies have provided practical methods for rapid fabrication of acoustic metamaterials. However, there is a relatively high sensitivity of their resonant characteristics to sub-mm deviations in geometry, pushing the limits of additive manufacturing. One way of overcoming this is via active control of device resonance. Here, an acoustic metamaterial cell with adjustable resonance is demonstrated for the sub-100 Hz regime. A functionally superparamagnetic membrane-devised to facilitate the fabrication process by eliminating magnetic poling requirements-is engineered using stereolithography, and its mechanical and acoustic properties are experimentally measured using laser Doppler vibrometry and electret microphone testing, with a mathematical model developed to predict the cell response. It is demonstrated that an adjustable magnetic acoustic metamaterial can be fabricated at ultra-subwavelength dimensions ( $\le \lambda$ /77.5), exhibiting adjustable resonance from 88.73 to 86.63 Hz. It is anticipated that this research will drive new innovations in adjustable metamaterials, including wider frequency ranges.

Keywords: Acoustic metamaterials; Magnetic membranes; Resonance tunability; Stereolithography printing; Superparamagnetism.

Figure 1

Measured vibration (using laser Doppler vibrometry) of the clamped magnetic membrane. The solid blue line displays the raw data, and the solid black line shows the median trend line (window size, 40). The resonant peak is marked via the red annotation.

Figure 2

Magnetic hysteresis loop (obtained via Superconducting Quantum Interference Device magnetometer) of the custom-made, SLA 3D-printable resin. The solid black line shows the non-poled sample, and the blue and orange dashed lines show, respectively, the samples poled at 150 °C and 300 °C.

Figure 3

Acoustic resonance frequency as a function of applied magnetic field, for both experimental (red stars) and mathematical modelling approaches (dashed blue line). Error bars show recorded magnetic field bounds for each regime—see Supplementary Table S3.

Figure 4

Acoustic power spectra of the acoustic metamaterial cell obtained using an electret microphone for the cases of no applied external magnetic field (0 mT, blue line), and peak magnetic field regime (392.5 mT average, orange line). The signals were recorded over a bandwidth of 70–500 Hz.

Figure 5

(A) Computer-aided design (CAD) file render of the acoustic metamaterial cell components, including the holding support (which includes a bracket to introduce the microphone, allowing for acoustic recordings) and the snap-fit membrane clamp, both 3D-printed using a yellow commercially-available resin, and the magnetic membrane. (B) Assembly of the 3D-printed acoustic metamaterial cell, including the electret microphone with an amplifier.

Figure 6

Diagram of the experimental set up used to determine the operation frequency of the 3D-printed acoustic metamaterial cell.