A phon is a unit of perceived loudness level. By definition, a tone has a loudness of N phons if it sounds as loud as a 1 kHz pure tone played at N dB SPL. At 1 kHz the phon value equals the dB SPL exactly; at other frequencies it diverges because the ear is not equally sensitive.

Why does a 60 Hz tone sound quieter than a 1 kHz tone at the same SPL?

The human ear is most sensitive in the 2–5 kHz region and far less sensitive at low frequencies. At a fixed SPL a 60 Hz tone produces fewer phons than a 1 kHz tone, so it is perceived as quieter. This is the core insight of the Fletcher–Munson and ISO 226 equal-loudness curves.

What model does this calculator use?

It implements the analytical formula and reference coefficient table from ISO 226:2003, which defines equal-loudness-level contours for pure tones presented frontally in a free field. The formula maps a frequency and dB SPL to a loudness level in phons across the standard's 20 Hz to 12.5 kHz range.

How accurate is the result?

ISO 226:2003 is the international standard derived from large listening studies and is accurate for steady pure tones from a frontal free field. Real-world music is broadband and rooms colour the sound, so treat the phon figures as a precise model of average healthy hearing rather than a measurement of your specific listening situation.

What is the practical use of this?

Mixing and mastering engineers use it to understand why bass seems to vanish at low monitoring levels, why loudness compensation (the 'loudness' button) boosts bass and treble, and how to set reference monitoring levels. It quantifies the perceptual tilt you should expect at different playback volumes.

Equal Loudness Level Calculator

A calculator that quantifies one of the most important facts in audio: two pure tones played at the same sound pressure level do not sound equally loud. Using the ISO 226:2003 equal-loudness model, it converts an SPL and frequency into a perceived loudness in phons and compares two tones directly.

How it works

ISO 226:2003 defines, for a set of standard frequencies, three coefficients — αf, Lu, and Tf — that capture how the ear’s sensitivity changes with frequency. For a tone at frequency f and level Lp (dB SPL), the standard computes an intermediate quantity:

Af = 4.47e-3 * (10^(0.025 * Ln) - 1.15)
   + (0.4 * 10^(((Tf + Lu)/10) - 9)) ^ αf

then solves for the loudness level Ln (phons) from the measured SPL by inverting the relationship:

Bf = (0.4 * 10^((Lp + Lu)/10 - 9)) ^ αf
   - (0.4 * 10^((Tf + Lu)/10 - 9)) ^ αf
   + 0.005076
Ln = 40 * log10(Bf) + 94

The calculator linearly interpolates the αf, Lu, and Tf coefficients between the standard’s tabulated frequencies, so any frequency from 20 Hz to 12.5 kHz is supported. At 1 kHz the result equals the input SPL by definition.

Worked example

Play a 1000 Hz tone and a 60 Hz tone, both at 70 dB SPL. The 1 kHz tone is, by definition, 70 phons. The 60 Hz tone evaluates to roughly 57 phons — so even though both are physically the same SPL, the bass tone sounds about 13 phons quieter. Drop the SPL to 40 dB and the gap widens further, which is exactly why bass seems to disappear when you turn a mix down.

Why it matters for mixing

Reference monitoring levels. Mixing too quietly makes you over-boost bass and treble to compensate; the contours show how steep that perceptual tilt is.
Loudness compensation. The “loudness” curve on hi-fi gear is an approximate inverse of these contours at low volume.
Translation. A mix balanced at 85 dB SPL will feel bass-light on a phone at 60 dB and bass-heavy in a loud club — the contours predict the direction of that shift.

Every calculation runs locally in your browser using the published ISO 226:2003 coefficients; nothing is uploaded.