Abstract: Addressing the application of acoustic waves for detecting voids in tunnel linings, this study conducts numerical simulations and experimental investigations into the influence of void dimensions on acoustic wave characteristics. Using the air-coupled impact-echo method, a two-dimensional finite element model—comprising the secondary lining, waterproofing membrane, primary support, and surrounding rock—was established. The time-domain and frequency-domain response characteristics of acoustic waves were systematically analyzed under varying void widths. Results show that voids induce reflection, diffraction, and superposition of acoustic waves at defect locations, thereby amplifying the time-domain signal amplitude. In the frequency domain, high-frequency peaks in the 4–6 kHz range correspond to overall structural vibration, whereas low-frequency peaks near 1 kHz are associated with localized void vibration. A dimensionless parameter—the frequency-domain signal density ratio β—is proposed to quantify void severity. Experimental validation reveals a strong correlation between β and void width, enabling β to be used for both locating and quantitatively assessing voids in tunnel linings. These findings provide a basis for non-destructive quality assessment of tunnel linings.