Critical Bandwidth   

The difference in frequency between two pure tones at which the sensation of "roughness" disappears and the tones sound smooth is known as the critical band. 
  above example
a "110 hz" drone with five  
increases in pitch beginning after
one second to establish the drone.
The range of critical bandwidth varies between an interval of a whole tone and a minor third, although at the low end of the frequency range critical bandwidth can exceed an interval of a major third. This is why close intervals in the bass register in music are difficult to distinguish and can muddy the resulting texture.
Another feature of critical bandwidth is that when two pure tones of similar intensity and with a frequency difference falling within the critical bandwidth are sounded the resulting perception of the loudness will be less than if the difference between them exceeded the critical bandwidth. 
The phenomena of critical bandwidth occurs when two frequencies are close enough to stimulate the same section of the basilar membrane or, conversely, far enough to stimulate different portions.