Phon to Sone Converter (Perceived Loudness)

One phon input, one result in sones. The script picks between two rules: at 40 phon and above, sone = 2^((phon − 40)/10); below 40 phon, sone = (phon/40)^2.642. They meet at exactly 1 sone at 40 phon, the anchor of the scale, defined as a 1 kHz tone at 40 dB SPL. From there each extra 10 phon doubles the figure: 50 phon is 2 sones, 60 is 4, 80 is 16, 100 is 64. The field starts at 60 phon, so the page opens showing 4, and it recalculates as you type.

The two units answer different questions. Phon is loudness level, tied to the equal-loudness contours of ISO 226: a sound rated at 70 phon is judged as loud as a 1 kHz tone at 70 dB SPL, whatever its spectrum. But phon inherits the decibel's logarithmic spacing, so 80 phon is nowhere near twice as loud as 40 phon. Sone fixes that. It is a ratio scale coming out of Stevens' magnitude estimation work, where 2 sones really does mean twice as loud as 1 sone. The 2.642 exponent on the low branch is the ISO 532 B (Zwicker) extension, and it is there because loudness grows faster than the doubling rule near threshold: going from 20 to 30 phon moves the result from 0.1602 to 0.4676 sones, a factor close to three rather than two.

The page assumes you already have a phon value, which is where it can mislead. It does not turn dB SPL into phon — that needs the frequency plus an ISO 226 contour — and it does not compute loudness from a spectrum, which is what ISO 532-1 (Zwicker) and ISO 532-2 (Moore and Glasberg) do over third-octave bands. Nothing limits the range either: enter 200 phon and it prints 65536 sones, a physically meaningless figure given that the pain threshold sits around 120 to 130 phon. An empty box is read as zero and prints 0; a negative entry raises a negative base to a fractional power, yields NaN and shows a dash. Trust the output roughly between 3 and 120 phon.