The phon scale defines loudness relative to equal-loudness matches at 1 kHz

Evidence

In a common implementation of the loudness balancing procedure, we measure the level of a sound of interest (the test stimulus here) that matches the level of a 1000-Hz stimulus (the reference stimulus). For these studies, a 1000-Hz tone is always the reference sound, and it is presented at some fixed stimulus level (e.g., 40 dB SPL). The two sounds (the 1000-Hz reference and the test stimulus) are always presented in succession (never simultaneously) and sometimes to opposite ears if the listener has normal hearing. In a typical experiment, the method of adjustment is used where the listener adjusts the level of the test sound so that its loudness is equal to that of the reference sound. The level of the 1000-Hz tone in dB SPL is the loudness level of the test sound, measured in phons. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 745

The loudness level has been defined such that the sound pressure level of a 1-kHz pure tone in dB has the same loudness level in phon units (SPL, Equation (2.14)). — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 984

Loudness comparisons can lead to more precise results than magnitude estimations. For this reason the loudness level measure was created to characterize the loudness sensation of any sound. It was introduced in the twenties by Barkhausen, the researcher whose name was shortened to create a unit for critical-band rate, the Bark. 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”. Loudness level can be measured for any sound, but best known are the loudness levels for different frequencies of pure tones. Lines which connect points of equal loudness in the hearing area are often called equal-loudness contours. They have been measured in severallaboratories and hold for durations longer than 500 ms. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1169

Categorization

• DEFINE: Defines the phon: loudness level in phons equals the dB SPL of a matching 1000-Hz tone.
• XREF: The phon-as-equal-loudness-reference concept parallels loudness scaling issues in other psychoacoustic sources read; an anchoring-standard idea. — 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 (+4 more)
• corroborated: 5 independent passages support this claim — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 745; Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 984; Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1169

Broader Connections

• Loudness depends on more than physical sound intensity alone #expands (0.75) — Dijk, Pim van, Physiology, Psychoacoustics and…, 2610