Loudness falls to zero at threshold in quiet, forcing a vertical asymptote

Evidence

At levels below 10 dB, the loudness of the I-kHz tone decreases drastically and reaches zero at an SPL of 3 dB. This “zero” corresponds on a logarithmic scale to a value at minus infinity. In other words, the loudness function must reach a vertical asymptote for the threshold in quiet that corresponds to a level of 3 dB. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1187

Categorization

• DEFINE: Explains why the loudness function renders threshold-of-quiet as a vertical asymptote, a modeling constraint that would otherwise look like an artifact.