In this work, the application of ultra-thin low frequency (UTLF) resonators to reduce low-frequency tonal noise components in HVAC systems is presented. A single UTLF absorber consists of a mass, a flexible membrane, a cavity, and a resistive layer. The resonator produces a near-perfect absorption in a narrow region around its resonance frequency. The proposed treatment consists of the addition, on one side of the HVAC duct, of an array of two kinds of UTLF resonators, tuned to two target frequencies. The acoustic performances of the system are mimicked in a three-dimensional numerical model, starting from a single resonator. The resonator impedance is tuned to the Cremer impedance of the duct by varying the design parameters to obtain optimal damping of the plane wave mode. The resonance frequency of the system is adjusted changing the mass and the depth of the cavity. Combining many resonators along the duct, one may achieve a high sound transmission loss in the target frequency due to the locally reactive behavior.