When two black holes merge, the final black hole vibrates and emits gravitational waves in a ringdown phase. In classical general relativity, these vibrations are described by quasi-normal modes whose frequencies and damping times depend only on the black hole's mass and spin, realizing the no-hair picture that no additional structure is visible from outside. Ideas from quantum mechanics and black hole thermodynamics suggest that this classical description may be incomplete and that subtle quantum corrections near the horizon could slightly alter the ringdown signal.This project uses publicly available data from the LIGO and Virgo gravitational wave detectors to test how large such deviations can be while remaining consistent with current observations. The ringdown phase is isolated for several high signal-to-noise merger events. Damped-sinusoid models are fitted to measure the dominant mode's frequency and damping time. These measurements are compared to Kerr predictions inferred from the remnant mass and spin, and any differences are expressed as fractional deviations. By combining results across multiple events, the project obtains upper bounds on these deviations. The derived bounds constrain quantum gravity-motivated models by excluding corrections that would produce shifts larger than the allowed range. Overall, the project shows how gravitational-wave ringdown observations can test efforts to unify general relativity, quantum mechanics, and black hole thermodynamics.