Because of this, two different sounds can have the exact same decibel level but sound vastly different in loudness to a listener. This "perceived loudness" is what Phons and Sones aim to quantify. 1. What are Phons? (The Equal-Loudness Level)
is twice as loud as 2 Sones (and four times as loud as 1 Sone). 0.5 Sones is half as loud as 1 Sone. The Phon-to-Sone Conversion
You may have seen graphs called "Fletcher-Munson curves." These lines show how much you have to "crank up" certain frequencies to make them sound as loud as the 1,000 Hz reference.
While the Phon scale aligns frequency response, it is still a . It indicates equivalence but not magnitude proportionality . A sound of 80 phons is not perceived as "twice as loud" as a sound of 40 phons. This limitation necessitated the creation of the Sone.
From the formula, we can derive the critical rule of psychoacoustics: An increase of 10 phons corresponds to a doubling of loudness (in sones).
$$ L_N = 40 + 10 \log_2(N) $$