Loudness is measured subjectively against 1 kHz reference tones

Evidence

Loudness level of a sound is the sound pressure level of a I-kHz tone in a plane wave and frontal incident that is as loud as the sound; its unit is “phon”. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1169

Based on this definition, it is possible to measure the equal loudness curves using subjects who compare a 1-kHz reference tone at a given level and a test tone of another frequency by adjusting the latter to have the same perceived loudness. Curves in Figure 9.2 are averaged for a large number of subjects under conditions standardized in ISO-226 (2003). — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 985

Categorization

• DEFINE: Defines the phon unit and the operational definition of loudness level via a 1-kHz reference.
• XREF: Relates to sound pressure level and dB scaling from earlier chapters; connects to the overall decibel framework of psychoacoustics. — connects to: The phon unit anchors loudness to perceived equality across frequencies rather than raw SPL — Eric J. Heller, Why You Hear What You Hear- An…, 2002; Loudness is a subjective percept standardized against 1-kHz tone levels in phons; The phon unit measures perceived loudness, not physical sound level (+10 more)
• SHARE: The matching-to-reference experimental design and ISO-226 standardization is a neat psychophysics method detail for someone into measurement.
• corroborated: 3 independent passages support this claim — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1169; Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 985

Broader Connections

• The phon scale defines loudness relative to equal-loudness matches at 1 kHz #expands (0.95) — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 745
• Loudness is measured by listeners assigning numbers relative to a reference sound. #corroborates (0.90)
• Loudness perception requires a tenfold intensity increase to double #qualifies (0.72) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 652