Measured noise levels beneath type B flat metal roof assemblies during rainfall

Flat metal roof assemblies are common in steel building design. Since they can produce high levels of rainfall generated noise, quiet roof assemblies are desirable over noise sensitive spaces. To determine the relative effects of assembly variations, a pair of fixtures were constructed to measure the noise levels beneath 2 x 4 ft sections of selected roof assemblies subjected to rainfall. Each test item was assembled from precut 2 x 4 ft sections of Type B metal roof (with or without a concrete layer), cover boards, and insulation, membrane and ballast as required. The fixtures were equipped with tipping buckets to measure rainfall rate. A microphone located inside each box and connected to a digital recorder, picked up ambient noise, rainfall generated roof assembly noise and the tipping bucket signal. To calibrate the recordings, the sound pressure level of the tipping bucket signal at the recording microphone was measured before testing. Recordings were analyzed to calculate indicated octave band sound power levels versus rainfall rate for the various tested roof assemblies.