search

Found

info Overview

Enter frequency (20-12500 Hz) and phon (20-90) to compute SPL on the ISO 226:2003 equal-loudness contour, with hearing threshold, alpha_f and Lu.

📘 How to Use

  1. Enter a frequency between 20 and 12,500 Hz
  2. Drag the loudness slider between 20 and 90 phon
  3. Read the resulting SPL, hearing threshold, alpha_f and Lu

Equal-Loudness Contour SPL Calculator

20 to 12,500 Hz
60

20 to 90 phon

※ ISO 226:2003 Table 1 (29 frequencies) for alpha_f / Tf / Lu interpolated linearly on log frequency.

※ At 1 kHz the phon value equals SPL by definition; low and high frequencies diverge due to ear sensitivity.

※ Outside 20-12,500 Hz the model is undefined; values are clamped at the boundary.

Sound pressure level (SPL)
— dB SPL
—
Hearing threshold Tf
— dB
Exponent alpha_f
—
Transfer Lu
— dB
Copied

grid_view Related

Article

Equal-Loudness Contour SPL Calculator | Convert phon to dB SPL via ISO 226:2003

Enter any frequency and phon value to back-calculate the sound pressure level (SPL, dB) required on the ISO 226:2003 equal-loudness contour. The tool also reports the hearing threshold Tf, contour exponent alpha_f, transfer term Lu, and the SPL difference against the 1 kHz reference.

💡 About this tool

Your ears are far more sensitive at some frequencies than others. A tone that feels like 60 dB SPL at 1 kHz needs much more physical pressure at 50 Hz to feel equally loud. Equal-loudness contours quantify that "perceived sameness," and ISO 226:2003 is the international standard that defines them.

If you calibrate a monitoring room, measure room acoustics, or fit hearing aids, you regularly need to answer one question: how many dB SPL does it take to reach a given phon level at this frequency? This calculator embeds the 29 reference frequencies of ISO 226:2003 Table 1 (with their alpha_f, Lu, and Tf coefficients) and interpolates linearly on log frequency for any value in between. Because it also prints the gap between phon and SPL, you can see at a glance how much extra level the low and high ends demand.

🧐 Frequently Asked Questions

What is the difference between phon and dB SPL? The phon is a unit of perceived loudness level, defined against the SPL of a 1 kHz pure tone. dB SPL is the physical sound pressure. They match at 1 kHz but diverge elsewhere by exactly the amount your ear's sensitivity changes.

Why do phon and SPL always coincide at 1 kHz? Because the phon is defined as "loudness equal to the SPL of a 1 kHz pure tone." At 1 kHz the calculator returns essentially the same value for SPL and phon, and the difference reads as "equal to the reference."

What input range is supported? Frequency runs from 20 to 12,500 Hz, and loudness from 20 to 90 phon. That mirrors the validity range defined by ISO 226:2003. Values past the edges fall outside the model and are clamped at the boundary.

What do alpha_f, Lu, and Tf mean? They are the per-frequency coefficients tabulated in ISO 226:2003 Table 1. alpha_f is the contour exponent, Lu is the magnitude-transfer correction, and Tf is the hearing threshold (lowest audible pressure) at that frequency.

How are off-table frequencies such as 440 Hz handled? The two surrounding table points are interpolated linearly on a log-frequency axis. Equal-loudness contours are smooth in log frequency, so this gives accuracy that is more than adequate in practice.

📚 Fun facts about equal-loudness contours

The first equal-loudness contours were the "Fletcher-Munson curves," published in 1933. Measurement methods were later refined and consolidated into the ISO 226 standard. The 2003 revision pooled data from several research institutions to update the older curves, and the low-frequency shape changed substantially as a result.

The "loudness" button on hi-fi gear is a direct exploit of these curves. At low playback levels your ear loses sensitivity at the bass and treble extremes, so turning the volume down makes the mix sound thin. The loudness compensation boosts those bands back up. Compare a low phon value with a high one in this tool and you will see that gap is largest at the low end.