Why you should always compare at equal loudness

Try it: play the same clip twice, the second slightly louder. Almost everyone prefers the second. It seems brighter, richer, more "present". Yet nothing changed but the level.

A very real perception bias

Our sensitivity to frequencies depends on listening level: the louder the sound, the more present lows and highs seem compared with the mids. A louder sound therefore also seems fuller and more detailed. Equal-loudness contours describe this, and they underpin modern loudness measurement.

The direct consequence: whenever a process raises the level, it seems to improve the sound. An EQ adding a little gain, a compressor with generous make-up gain, a saturator… they all win the comparison for the wrong reason.

How to guard against it

• Before comparing with and without processing, adjust the output gain so both versions have the same perceived level.
• In mastering, compare with a reference at equal loudness, measured in LUFS, rather than on the peak meter.
• Compare briefly and often: the ear quickly adapts to whatever it hears for long.
• Distrust your first impression when a setting suddenly "sounds better": check the level first.

What Hearwise does

In comparison games, clips are level-matched (by RMS, or by LUFS for reference comparison): only differences in timbre, dynamics or space remain. Games where level is the question itself, such as the LUFS challenge, keep their differences on purpose.