The phon unit anchors loudness to perceived equality across frequencies rather than raw SPL

Evidence

Human sensitivity to sound intensity can be summarized by equal loudness curves, which necessarily apply only to pure tones. (There are too many variables and ambiguities if we try to define equal loudness for complex tones.) The idea is to choose a standard frequency, 1000 Hz, and define a new loudness unit, the phon, to be equal to the sound pressure level (SPL) in dB at that frequency. That sets the phon, because when the frequency is changed, and we adjust the SPL to reach the same loudness we experienced before, we will find that, say, 40 phons at 200 Hz is not 40 dB, but rather we find we need the SPL above 50 dB to reach the same loudness. We are less sensitive at 200 Hz than we are at 1000. — Eric J. Heller, Why You Hear What You Hear- An…, 2002

Categorization

• DEFINE: Defines the phon as a loudness unit equated to SPL in dB at 1000 Hz, and applies only to pure tones.
• corroborated: 2 independent passages support this claim — Eric J. Heller, Why You Hear What You Hear- An…, 2002
• XREF: connects to: A-weighted sound power exaggerates perceived loudness differences in marketing — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1778; Loudness is a subjective percept standardized against 1-kHz tone levels in phons; The phon scale defines loudness relative to equal-loudness matches at 1 kHz — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 745 (+4 more)

Broader Connections

• Loudness is a subjective percept standardized against 1-kHz tone levels in phons #corroborates (0.95)
• The phon scale defines loudness relative to equal-loudness matches at 1 kHz #corroborates (0.95) — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 745
• The phon unit measures perceived loudness, not physical sound level #corroborates (0.90)
• Loudness is measured subjectively against 1 kHz reference tones #corroborates (0.90) — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1169
• Equal-loudness contours are a fixed property of human hearing #complicates (0.80) — Toole, Floyd E, Sound reproduction- the acoustics and…, 909
• Perceived loudness does not map one-to-one onto acoustic amplitude #expands (0.80) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 616