Stevens anchored the sone unit to a 1000-Hz tone at 40 dB SPL.

Evidence

In this type of experiment, listeners hear two tones: a reference tone (a 40 dB SPL tone) followed by a test tone. In different experimental conditions, test tones are presented at a range of levels. Listeners are told that the reference tone (a 1000-Hz tone presented at 40 dB SPL) should be assigned a rating of 1. Stevens defined this reference sound as one sone, the unit of loudness. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 710

Categorization

• DEFINE: Defines the sone, the fundamental unit of loudness, via its reference stimulus.
• SHARE: Worth sending to anyone curious why loudness is measured in sones — the anchor is a 40 dB SPL 1 kHz tone defined as 1 sone.