The phon defines loudness level equal to dB SPL at 1 kHz

Evidence

The equal loudness curves presented in Figure 9.2 on page 155 show that the perception of loudness depends on the level and frequency of sinusoidal signals in a manner which cannot be easily expressed in mathematical terms. However, the curves can themselves be used to define the loudness level. The curves represent sound pressure levels of tones that are perceived to have equal loudness with reference values located at 1 kHz with 10 dB spacing in the sound pressure level. The unit of loudness level is the phon. It is defined such that at 1 kHz the sound pressure level in dB and the loudness level in phons have the same magnitudes. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1131

Categorization

• DEFINE: Defines the phon and how equal loudness curves anchor it at 1 kHz.