Inaudible subsonic energy from rumble, drafts, samples and DC wastes mix headroom

Sources below ~20 Hz are common: close-mic drafts on acoustic guitar, traffic/foot-tap vibration through mic stands, snare samples chopped from a mix that still carry a bass-note tail, and DC offset from ageing circuitry (0 Hz, the ultimate subsonic signal). These are inaudible yet raise peak levels, eat headroom, cause limiter pumping, and flap woofers. High-pass filtering removes them before they accumulate.

Examples

A subsonic kick element can eat an extra 3 dB of headroom and flap a woofer without changing the audible sound; a woofer visibly lagging the beat flags a poorly-chosen kick.

Assessment

List four sources of subsonic energy in a session and explain why each is inaudible but harmful.