Tell me about trace heating?

High frequency attenuation of distant industrial sounds or stereo subwoofers from homes, cars, and music venues. As sound moves through the atmosphere or ground, the high frequencies decrease in amplitude more rapidly than the low frequency ones, which subsequently travel greater distances. The low-frequency sounds can be amplified by walls and structural geometry, and sound like ambiguous rumblings or hums. Industrial machinery such as compressors, pumps and fans can also produce similar types of sounds. Although this is one of the explanations that first come to mind, ordinary microphones have failed to detect the Hum and investigations have failed to convincingly trace the Hum to such sources. Studies in the UK have addressed this issue, and the report about the Kokomo Hum offers examples of sources of acoustic noise.