## 1 Introduction  
The World Health Organization's report on night-noise guidelines for Europe and the burden of disease from environmental noise affirms that about 20% of the European population is exposed to road traffic noise at levels exceeding 65 dBA (equivalent sound pressure level [SPL] with A-frequency weighting) [1]. This noise level is higher than the maximum acceptable outdoor value in living environments in many European countries [2]. The perception of noise as annoying depends primarily on noise levels and the source producing such noise [3]. Noise can affect human health [4], cause sleep disorders [5], annoyance [6], learning impairment [7, 8], hypertension ischemic heart disease [9, 10], diabetes risk [11], and loss of hearing [12]. This is why there is an urgent need to control noise pollution, especially in sensitive areas such as near hospitals and schools. Traditionally, the most common method for traffic noise abatement has been noise barriers, which can significantly reduce the noise exposure of a single receiver. There is growing interest in low-noise pavements to reduce traffic noise because the benefits of silent pavements are enjoyed by many receivers. Road pavement is a principal factor in traffic noise. Parameters such as texture [13] and granulometry properties [14] are very important when assessing superficial properties, and these cannot be disregarded in the evaluation of pavement quality [15; 16; 17]. Sonic crystal barriers are a more recent development. These are a modern and sustainable solution for traffic noise mitigation, but the technology still needs further development [18; 19; 20]. Noise barriers can be formed using earth mounds (\"perms\") positioned along roads, high walls, or a combination of both. These barriers do not eliminate traffic noise, but they reduce it substantially, improving the quality of life of people who live adjacent to busy roads. However, different structures (caps) attached to the tops of the barriers can enhance their noise isolation performance with a barely perceptible increase in the height of the original barrier (depending on the shape of the structure). Caps thus improve noise mitigation and maintain existing aesthetics, and can be incorporated during the initial design phase or by retrofitting existing barriers. It is important to note that, given that walls are subject to maximum height restrictions, simply increasing a wall's height to avoid the need for a cap is not always an appropriate solution for noise attenuation. The existing literature has examined the performance of various caps. For example, [PERSON] [21; 22; 23; 24], [PERSON] [25], [PERSON] [26], [PERSON] [27], and [PERSON] [28] considered different diffracting structures (e.g., multiple-edge, T-profile, and cylindrical caps). The Japanese experience of reducing road traffic noise from the 1990s to 2000s is also relevant, as this period saw several commercial developments of edge-modified barriers [29]. Nevertheless, since then, research into this area has progressively decreased because the tested models (e.g., modified cylinders, T-shaped, Y-shaped, and mushroom-type) have failed to achieve the expected noise reduction efficiencies [30]. According to the Quietness and Economics Stimulate Infrastructure Management (QUESTIM) noise barrier survey [31], which was later confirmed by [32], the aforementioned cap elements are largely unused across Europe because of the lack of noise-modeling software, the lack of data confirming their effectiveness, and cost reasons. The above authors [21; 22; 23; 24; 25; 26; 27; 28] typically analyzed single or multi edge-modified devices, but without systematically checking possible combinations of such structures and/or without extending the frequency range of evaluation further than 4-5 kHz.