This thesis presents the optimization of a quarter-wavelength tube (QWT) system for the absorption of low frequency sound in the range of 88 to 355 Hz. Low frequency noise, often generated by sources such as traffic, heavy machinery, and HVAC systems, present significant challenges in building acoustics due to the large material thickness required for absorption. This study proposes a compact, efficient, and customizable solution using quarter-wavelength tubes. These tubes can be connected with each other not only in series, but also in parallel. The configuration considered in this thesis is a combination of the two different combination types.

The goal of this research is twofold:
• To maximize the absorption of QWT systems in the aforementioned low frequency bandwidth.
• To minimize the volume of these QWT systems to improve the possibility of incorporating them into sound absorbing panels and to reduce the material use of the systems.

To achieve this, a methodology consisting of three different types of analysis is used.
• Analytical analysis: A Python code is designed for the optimization of the QWT system. This code consists of two parts: The part where the absorption coefficients of a QWT system is calculated using the recursive formulation, and the part where the dimensions and number of tubes of the system are optimized using the evolutionary algorithm DEAP. The most optimal system is used for the next analyses.
• Numerical analysis: Finite element analysis (FEA) was performed in COMSOL Multiphysics (in short: COMSOL), using both Pressure Acoustics physics with Narrow Region Acoustics and Thermoviscous Acoustics interfaces. These simulations provide more accurate absorption coefficient results than the analytical Python code, as three-dimensional thermoviscous effects are considered in more detail.
• Experimental analysis: Scaled prototypes were 3D printed using PLA and tested in an impedance tube to measure their real-world performance.

The final optimized QWT system using DEAP achieved broadband low frequency absorption while being significantly more compact than traditional absorbers. This system consists of one main tube connected in series to five parallel tubes. Optimization caused a decrease of 2731 cm3 of air volume in the QWT system, while the absorption coefficient only slightly decreased. Less air volume in the system means that the different QWTs can be smaller, so less material is needed.

The analytical and numerical results aligned very well, but the experimental results were slightly off. This may be caused by the lack of complete airtightness of the model or the presence of undissolved support material (PVA).

From an engineering perspective, this research showcases how QWT systems can be optimized to effectively absorb low frequencies. Different boundaries can be entered into the optimization code, which makes it possible to design an optimized QWT system for every situation. This is especially useful where space or thickness is limited for traditional sound absorption.

In conclusion, this thesis bridges theoretical acoustics, computational modelling, and practical engineering, resulting in a tunable, space-saving soundproofing solution that can be adapted for real-life application. Further research may include the application of different materials or testing such systems on a large scale.