The HRTF or Head-related-transfer-function is a technology that make sounds seem more natural and spatial, as if we are hearing them in a real acoustic space. When you hear a sound, various factors such as size and shape of head, ears, and ear canals transform the sound and affect how it is perceived, boosting some frequencies and attenuating others. The HRTF is meant to recreate this phenomenon of how our head changes the sound that reaches our ears.
Generally speaking, the HRTF boosts frequencies from 2–5 kHz with a primary resonance of +17 dB at 2,700 Hz. But the response curve is more complex than a single bump, affects a broad frequency spectrum, and varies significantly from person to person.
Users will notice that they are able to change between two HRTF profiles (A and B) only when on the new "Club Indey" and "Yellow Matter Studios" rooms. That is because the HRTF B profile is a new profile included only on these two rooms. HRTF A is the universal HRTF we have been using for all VSX rooms up to this point, and cannot be changed on any previous rooms (yet).
HRTF B will have more midrange (around 2.5K) than HRTF A. This is very easy to notice when switching between the two profiles while playing pink noise: Pink noise generator
This mid-range boost is also noticeable when listening to a rock song that features distorted guitars that are mid-heavy by nature. Rock song reference
We recommend that users take various commercial releases they are familiar with, and listening to them on both profiles and taking note of how the mid-range and vocal levels feel. Then choosing an HRTF profile accordingly.