Ultrasonic

Frequencies beyond 20 kHz are inaudible to the human ear — and therein lies their trickiness and usefulness. On set and in editing, we often work unconsciously with ultrasound because it is present everywhere in the real world: bats navigate with it, rodents communicate, and modern digital cameras record it unless we actively counteract it.

Ultrasound becomes practically relevant primarily in two scenarios. Firstly, when simulating rodent repellents or technical alarm systems — if a scene shows that invisible frequencies are meant to drive away rats or mice, we cut high-frequency sine waves (from 25 kHz upwards) into the soundtrack. These remain inaudible in the cinema mix but document that the device is "working." The dramatic effect then arises from the visual reaction of the animals, not from our hearing. Secondly, in spatial sound design — Dolby Atmos and similar immersive formats use high-frequency content to create ambient atmosphere and subtle environmental attractors that operate below the conscious hearing threshold but create an unconscious presence. This is sound design on a psychoacoustic level.

An important practical detail: Not all playback systems reproduce ultrasound. Cinema chains with older Dolby systems often filter out frequencies >20 kHz; modern streamers and home theater setups can theoretically pass them but don't always do so. This means you can embed an ultrasound track in an Atmos master without it causing problems everywhere — it's practically a fallback layer for special systems. In any case, these extreme edges are negligible for loudness management (according to ITU BS.1770).

Common mistake: Confusion with high-pass filters. Ultrasound is not filtering; it is genuine, uncompressed frequency material. If you need a rodent repellent sound, you generate it digitally or buy it — not by EQing existing sounds. The quality of the source determines whether the frequencies remain stable enough to sound consistent in Atmos rendering.