Subsonic Resonance

Some sound arrives below hearing. Infrasound, the band beneath roughly twenty hertz, is produced by machinery, ventilation systems, traffic, wind over structures, and large bodies of moving air, and though the ear registers nothing, the body is not deaf to it. At sufficient levels it is felt as pressure, unease, or a vibration in the chest, and one well-known line of research found that infrasonic frequencies could induce feelings of anxiety and strange presence in listeners who had no idea any sound was present.

The famous anecdote involves a laboratory ventilation fan resonating at about nineteen hertz, which produced chills, oppression, and even peripheral visual anomalies until the fan was identified and switched off. Whatever the full explanation, the direction of the finding is sober enough: environments can carry an inaudible mechanical signature, and the signature can register as mood.

Buildings hum more than their occupants know. Plant rooms, compressors, pumps, transformers, and long duct runs all generate low-frequency energy, and lightweight construction transmits it readily. Residents of such buildings rarely complain about sound; they complain about the room, describing it as tense or tiring, because the source sits below the threshold where causes get names. Low-frequency noise complaints, where they do surface, are notoriously hard to diagnose for exactly this reason.

The design response is unglamorous and effective: isolate the machines. Resilient mounts, mass in the separating construction, duct design that does not turn the building into an organ pipe, and equipment placed away from the rooms where people rest. A home's lowest octaves should belong to weather and distant life, not to its own mechanical pulse. Silence, it turns out, has a floor several octaves below what the ear reports.