US4395600A Auditory subliminal message system and method

What it covers (plain English)

A store installs microphones to monitor how loud the environment is. The system then adjusts the volume of a subliminal anti-shoplifting message (kept below conscious hearing) to track the ambient noise:

When the store gets louder, the subliminal message slowly rises in level.

When it gets quieter, the subliminal message drops quickly so it doesn’t become audible.

A masking sound (e.g., music/white noise) is used and is also level-tracked to keep the message imperceptible.
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