"It is difficult to establish how many cents are perceptible to humans; this accuracy varies greatly from person to person. One author stated that humans can distinguish a difference in pitch of about 5-6 cents.
The threshold of what is perceptible also varies as a function of the timbre
of the pitch: in one study, changes in tone quality negatively impacted student musicians' ability to recognize as out-of-tune pitches that deviated from their appropriate values by +/- 12 cents.
It has also been established that increased tonal context enables listeners to judge pitch more accurately.
When listening to pitches with vibrato
, there is evidence that humans perceive the mean frequency as the center of the pitch.
One study of vibrato in western vocal music found a variation in cents of vibrato typically ranged between +/-34 cents and +/-123 cents, with a mean variation of +/-71 cents; the variation was much higher on Verdi
Normal adults are able to recognize pitch differences of as small as 25 cents very reliably. Adults with amusia
, however, have trouble recognizing differences of less than 100 cents and sometimes have trouble with these or larger interval
So it would appear that if you plus or minus 3-5 cents, you're pretty much 'in tune'. Unless you're playing a unison, where you can better hear the 'beat' frequencies.