This paper presents the results of a study to reduce the background noise level within a large Quiet Room located adjacent to other laboratory testing environments and below a mechanical mezzanine which houses an extensive array of mechanical and electrical equipment including banks of low-temperature chiller compressors, air handling units, and electrical switchgear that serves the entire building complex. This equipment was installed atop the concrete mezzanine floor deck without provisions for isolating vibration. As a result, structure-borne noise from that equipment travels through the floor, radiates from the underside of the floor deck, and intrudes into the Quiet Room below. This causes the background noise level within the Quiet Room to be too high for conducting low sound level measurements and studies on vehicles brought into the Quiet Room.Conclusions of the study are that provisions originally intended to isolate the Quiet Room from the mezzanine floor deck were both insufficient and poorly installed, resulting in excessive noise transmitted into the Quiet Room. Recommended corrective measures include enhancement of the Quiet Room ceiling to improve the sound transmission loss performance of that assembly, and changes made to the mezzanine floor deck to isolate structure-borne noise. This work is being installed in phases and completion of the initial phase has shown very promising results.