Application Cases

Experiment Name: Research on Acoustic Metamaterials and Topological Acoustics

Experiment Purpose: Finding an appropriate balance between sound absorption performance and ventilation performance is necessary. Efficiently absorbing low-frequency sounds (<1000Hz) while maintaining free fluid flow remains a significant challenge in acoustical engineering. Although the continuous development of acoustic metamaterials has unleashed unprecedented possibilities, and various metamaterial absorbers have been proposed, most of them can only function optimally in the absence of backscattering. Unfortunately, since sound waves are longitudinal waves, this situation requires the complete blockage of fluid channels, allowing them to penetrate any small openings. Otherwise, their absorption performance may be greatly reduced, typically not exceeding 50%. This fundamental trade-off between absorption and ventilation performance undoubtedly limits their application in everyday situations where free airflow is required. Although researchers have proposed some ventilated acoustic barriers with significant transmission loss, they merely reflect sound waves to isolate sound, but these sound waves still exist. Here, to overcome these difficulties, we propose and verify through simulation and experiment a super open-ventilated metamaterial absorber. Experiments have proven that the absorber for low-frequency sounds can ensure high-performance absorption and ventilation at the same time. We explain the mechanism of this resonant cavity through an effective model of coupling loss. In addition, the absorber can be simply stacked according to the principle of the causal relationship between the absorption coefficient and the ultimate thickness, so that it can work in a customized broadband while maintaining good ventilation performance.

Testing Equipment: ATA-304B power amplifier, data acquisition and analyzer, loudspeaker, microphone, etc.

Experiment Process:

A 3×3 lattice array of UVMA units was assembled together as a framework to form a metamaterial absorber similar to an acoustic wall, as shown in Figure 2.1(a). For this lattice-arranged metamaterial, the particularly large cavity above it allows the background fluid (such as air or water) to flow freely through the structure. In the research mentioned in this paper, we assume that the structure is immersed in air. In order to simultaneously achieve efficient absorption of low-frequency sound waves and free flow of air, the sound waves incident on the UVMA unit should be perfectly absorbed. For a single UVMA supercell unit combined into a rectangular lattice, the lattice constants in the x and y directions are L/4 and L, respectively, consisting of four UVMA units, as shown in Figure 2.1(b). For the details of a single UVMA unit, we display the detailed information of the UMVA unit by taking off the cover, as shown in Figure 2.1(c). Each UVMA unit consists of two symmetrically placed shunt tube resonators, which are weakly coupled by a narrow gap connecting them. The cross-sectional view of a single UVMA unit in the xz plane shows structurally identical but oppositely oriented slit tube resonators, as shown in Figure 2.1(d). We have determined the appropriate geometric parameters of the UVMA metamaterial to optimize the structure for the best absorption and ventilation performance for low-frequency sound waves.

The absorption performance of the UVMA units was experimentally verified using the setup shown in Figure 2.1(e). All acoustic measurements were carried out in a rectangular impedance tube using the standard four-microphone double-load method. The impedance tube consists of two aluminum square tubes (with an internal cross-section of 147×147 mm² and a tube thickness of 5 mm), a full-range loudspeaker (China, M5N, HiVi), four microphones (China, BSWA, MP418), a power amplifier (China, Aigtek, ATA304), and a data acquisition and analysis instrument (China, BSWA, MC3242). The plane wave cut-off frequency of the aluminum tube is around 1100 Hz. The lengths of the two aluminum tubes are 600 mm and 400 mm, respectively. A clamped aluminum plate with a thickness of 4 mm was used as a rigid back plate to simulate an acoustically hard boundary termination. When the back aluminum plate was removed, the sound in the tube would radiate to the outside, thus simulating an open-boundary acoustic termination. They served as two different terminal loads in the measurements.

Figure 2.1. (a) Schematic diagram of UVMA supercell assemblies arranged in a rectangular lattice. (b) Schematic diagram of a UVMA supercell unit, consisting of four UVMA units, with lattice constants of L (in the x direction) and L/4 (in the y direction). (c) Perspective view of a single UVMA unit, as indicated by the dashed rectangle in (b). To demonstrate its internal details, the structure was rotated and the end caps were removed. (d) Cross-sectional schematic diagram of a UVMA unit in the xz plane. (e) Experimental setup for acoustic measurements. The impedance tube has a square cross-section (147×147 mm²) and uses the standard four-microphone method for measurement. The inset shows a photograph of the processed sample placed in the impedance tube.

To verify the simulation results of the relationship between the absorption rate of the sample and the parameters, we then carried out experimental measurements of the acoustic properties of the UVMA units in the acoustic wave tube. We only made and studied two types of UVMA samples, which are marked as Sample I and Sample II, respectively. The open rates (open area) of Sample I and Sample II are 72.8% and 69.4%, respectively. As shown in Figure 2.3(a), the transmission and reflection rates of these two samples measured by the four-microphone method (dashed lines) are in very good agreement with the simulation results (solid lines). Both the reflection and transmission spectra show a drop near the resonance frequency, indicating that the UVMA metamaterials efficiently absorb sound at the resonance frequency. As shown in Figure 2.3(b), the simulated and measured absorption results show consistency and fit well together. In the experiment, for Sample I and Sample II, as indicated by the red and purple arrows, respectively, absorption rates of 93.6% and 97.3% were measured at 637 Hz and 472 Hz. For reference, the acoustic properties of two types of melamine sound-absorbing foams (Basotect G+, Germany, BASF) were also measured. They are marked as Foam I and Foam II, respectively, with the same dimensions as Sample I and Sample II, and their absorption spectra are drawn in Figure 2.3(b) with gray solid lines. Compared with high-quality sound-absorbing foams on the market, UVMA units have excellent acoustic absorption performance near low-frequency resonance.

Experimental Results:

As shown in Figure 2.3(d), the sound pressure of the two resonators shows a 90° phase difference at resonance. This phase difference indicates that the symmetric plane (z=0) can be regarded as the superposition of an acoustic soft boundary with a 180° phase difference introduced and an acoustic hard boundary with a 0° phase difference introduced. We used COMSOL to simulate only half of the UVMA unit, with either an acoustic soft boundary or a hard boundary as the termination condition. It was found that the absorption of a single UVMA unit is the average of half of the symmetric and antisymmetric UVMA units, thus confirming that this interpretation is accurate. Since both hard and soft acoustic boundaries act as backscattering conditions, they cause multiple scattering of the incident sound, mixing the resonant modes of the two separate shunt tube resonators. This coupling is the key to obtaining effective absorption. In addition, because the waves reflected between the hard and soft boundaries have a 180° phase difference, they tend to cancel each other out, thereby ensuring near-perfect sound absorption in the case of ventilation at both ports.

Figure: Specification Parameters of the ATA-304C Power Amplifier

This material is organized and released by Aigtek. For more case studies and product details, please continue to follow us. Xi'an Aigtek has become a widely recognized supplier of instruments and equipment in the industry with a broad range of products and considerable scale. Free trials of demo units are supported.