The Just Noticeable Difference (JND), also known as the difference limen, is a concept from psychophysics that defines the threshold of perceptibility. In acoustics, JNDs establish the boundaries between measurable differences and audible differences — a critical distinction for practical sound system work.

For sound level, the JND is approximately 1 dB under ideal laboratory conditions (sine tones, trained listeners, direct comparison). In real-world environments with broadband program material, the practical JND rises to 2-3 dB. This means that EQ adjustments smaller than 1 dB, while measurably different, may be imperceptible to listeners. Changes of 3 dB are reliably noticeable, and 10 dB represents a subjective doubling or halving of loudness.

For frequency, the JND is approximately 0.3% for trained listeners in the 1-4 kHz range where hearing is most acute — about 5 cents (1/20 of a semitone). At frequency extremes and for broadband signals, the JND widens considerably. For reverberation time, the JND is roughly 5% (a 0.05s change in a 1.0s RT60 is barely perceptible).

JNDs are essential for interpreting SonaVyx measurements in practical terms. When the AI diagnostic recommends a 0.5 dB EQ cut at 2 kHz, understanding JND suggests this correction may not be audibly significant on its own, though cumulative small corrections across many bands can be noticeable. The before/after comparison tool contextualizes measured improvements against JND thresholds. The SPL meter displays to 0.1 dB resolution, but the JND reminds us that differences below 1 dB are at the edge of perception.