What Happens When fD approaches fCR

In summary then, When f surpasses the so called limit of frequency discrimination fD, we distinguish two separate tones. However at that limit the sensation of roughness still persists, especially in the low pitch range. Do not confuse fD with the limen of frequency discrimination, the JND for pitch.

Only after surpassing a still larger frequency difference fCR called the critical band, the roughness sensation disappears and both tones sound smooth. This transition from roughness to smoothness is gradual.