A filter’s transfer function describes gain in dB independent of sound level.

Evidence

Because we describe filters in terms of their frequency characteristics, they are most easily represented in terms of their transfer function, a representation of the gain provided by the filter as a function of frequency. … A 0-dB gain means no alteration in stimulus level, whereas negative gain implies attenuation. The power of the input (pre-filtered) sound has dB SPL units, and an output (post-filtered) sound has dB SPL units. However, the filter itself is not a representation of sound, and its gain is instead described using dB. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 423

Categorization

• DEFINE: Clarifies what a transfer function is and why filter gain (dB) is distinguished from the sound’s own dB SPL level.