## 1 Introduction  
Fused Deposition Modeling (FDM) creates 3D-printed objects by extruding melted filaments in layers using a nozzle. Typical materials include polyamide, polycarbonate (PC), polylactic acid (PLA), and polyether ether ketone (PEEK) [1, 2]. Such materials are well-suited for structural, automotive, and aerospace applications [3], due to their customized mechanical properties and high strength-to-weight ratios [4]. Nevertheless, defects like porosity, delamination, and fiber misalignment can be introduced during the layer-by-layer deposition process, which can affect the structural integrity of the material [5]. Other factors like material properties, printing parameters, and cooling dynamics [6] can interact in complicated ways, which can lead to other flaws such as warping, stringing, and under-extrusion [7]. Moreover, [PERSON] et al. [7] found that software, build strategy, and hardware limitations can all lead to process-associated issues. Consequently, the use of non-destructive testing (NDT) methods is vital in the identification and characterization of defects in 3D-printed parts. A number of NDT methods have been used to evaluate additively built parts for imperfections such as thermography [8], X-ray computed tomography [9], eddy current testing [10], and ultrasonic testing [11]. The impulse excitation technique (IET), also known as tap testing, is one of the most simple and efficient ways to find imperfections in composite structures [12]. This technique has undergone updates for various uses, such as identifying insulation flaws, evaluating composite damage [13], and determining dynamic moduli at high temperatures [14]. IET's simplicity, low cost, and great precision make it useful for non-destructive testing [15]. In addition, scientists have created automated technologies to make impact testing more accurate and repeatable [16; 17]. The technique also depends on identifying the vibration and acoustic response that a tapped structure generates [13]. In order to detect defects like holes, fissures, and material irregularities, these methods examine the vibrational properties of objects [18]. In this context, vibro-acoustic techniques have demonstrated a high sensitivity for detecting gaps in safety components, agro-products, and composite structures [12]. They entail subjecting a structure to an impact force and then examining the vibrations or acoustic emissions that result [15; 19]. Therefore, the efficiency of the approach is dependent on the examination of dynamic factors such as the response frequency and damping ratio [14]. Here, the fast Fourier transform (FFT) establishes a relationship between amplitude and frequency by mapping the amplitude of each frequency component to its corresponding frequency. This allows the FFT to provide a frequency domain representation of a signal. Moreover, the technique is flexible enough to accommodate multi-modal analysis, which includes torsional and flexural excitations [20]. Several applications focus on building dispersion curves and material characteristics [21]. In terms of this, FFT analysis of flexural free vibration is also utilized to calculate the viscoelastic characteristics of fiber-reinforced composites [22]. In this context, dynamic mechanical thermal analysis and longitudinal free vibration testing have shown consistency in assessing the viscoelastic characteristics of polymer composites [23]. FFT can also characterize damage in carbon fiber/epoxy composites through acoustic emission analysis [17]. However, FFT has limitations in analyzing non-stationary signals. In addition, [PERSON] et al. [24] emphasize in their work that combining accelerometer and microphone data can enhance our understanding of the underlying physical principles [25]. Furthermore, [PERSON] et al. [26] utilized Macro Fiber Composite (MFC) actuators for active vibration control in 3D-printed structures made from polylactic acid (PLA) and its composites, employing a laser vibrometer to analyze natural frequencies and bending modes.