## 1 Introduction  
Classical absorption and insulation materials present a limited performance in the low frequency range when imposing constraints on their total weight or thickness. However, ventilation or air conditioning units, as well as industrial machines such as compressors and turbines, present a preponderant low-frequency noise content. The use of micro-perforated panels (MPPs) [1] are well suited to such demanding environments as an alternative solution to porous or fibrous materials. Because they work on the principle of Helmholtz resonators, they can achieve remarkable absorption values but are limited to a narrow frequency band. To overcome this limitation, several solutions have been proposed to extend the performance of MPP resonance absorbers over a broader bandwidth toward the low frequency range. The use of multi-layer and parallel partitions has been widely studied by the scientific community [2; 3; 4; 5]. A pilot study was presented by [PERSON] et al. [3] proposing the combination of two different MPPs arranged in parallel and periodically alternated. They provided a parametric study by varying the perforation ratios and depths of the backing cavities and calculated the excess attenuation provided. An array of three parallel-arranged MPPs with varying cavity depths was considered [4] to study how acoustic energy is absorbed by the partition. A parallel absorption mechanism was effective assuming that the distance between the individual MPPs was smaller than a quarter-wavelength of the incident wave. The idea of merging different resonances for optimal sound absorption was further developed to explore the trade-off between sample dimensions, with a maximal constraint on the total thickness and the bandwidth of the absorption spectrum [5]. Using the causality principle, an absorber was designed composed of 16 quarter-wavelength coiled resonators, situated over a square section. With a depth of 59.3 mm, it was able to absorb noise above 345 Hz, provided that a 3 mm acoustic sponge covered the cavities. Another solution based on conventional resonators was proposed by [PERSON] [6], who filled the backing cavity of a perforated plate with a set of tubes of different lengths. This generated an important shift in the absorption coefficient toward the low frequency range, allowing a reduction in the cavity depth. To overcome the above limitations, a different approach is to consider unbacked MPPs, with panels working in free-standing conditions [7; 8; 9; 10]. This configuration does not achieve resonant absorption, but it can provide notable values over a broad frequency range, provided that the physical parameters of the MPP are properly optimized. A selection of the hole diameters lower than 100 \(\upmu\)m and an increase in the perforation ratio was found to be a requisite to obtaining broadband low-frequency absorbers [8]. Previously, ultra-microperforated panels (UMPPs), with perforation diameters of less than 80 \(\upmu\)m, a perforation ratio below 19% and a backing cavity depth of 20 mm, provided wideband absorption that was suitable for a constrained space [9]. The drawback of this solution is the high cost of the manufacturing technique. Recently, MEMS technology was used in bio-microfluidics [10] and applied to silicon MPPs with micrometric holes [9]. However, the efficiency bandwidth of these solutions does not exceed two to three octaves. The objective of the current work is to examine the acoustic performance of micro-capillary plates (MCPs) as ultra-broadband absorbers that are efficient in the low frequency range. The results are assessed against those provided by more standard materials, such as MPPs or UMPPs. Their potential applications include the design of compact low-frequency anechoic terminations. For instance, a 200 mm block of melamine foam with a conical shape was proposed as an anechoic termination to minimize reflections from the duct outlet [11]. It achieved a reflection coefficient ranging from 0.7 down to 0.05 when increasing the frequency.