Masked subliminals rely on volume differencing to make the subliminal portion of the audio subliminal. Ultrasonics rely on pitch differencing.

When you listen to a masked subliminal, you're hearing the messages at the same pitch as normally spoken, but much less volume than you are used to, and your conscious mind ignores them, while they are detected, decoded and responded to by your subconscious mind.

Ultrasonic subliminals, by contrast, are just as loud as anything else you play on your player, but they are shifted to what is known as the "umbra ultrasonic" range of pitch. They are now playing at such a high pitch that your eardrum no longer effectively resonates in response to them because it is too big to resonate with such a high frequency. Instead, the tiny hairs inside your ear that help you maintain balance are being vibrated by these pitches, but they do not directly connect to the auditory areas of the brain in the same way, so you do not consciously detect or understand what is being said.