I am not an experienced ear tuner. I got 4.5 Hz (15.5 cents) the first time a few weeks ago, 2.25 Hz (7.8 cent) this afternoon, and then 2.4 Hz (8.3 cents) this evening. So perhaps with more practice, I will become perfect!
A discussion on cents and 'just noticeable difference s pitch (jnd)' can be found at http://hyperphysics.phy-astr.gsu.edu/hb ... cents.html
. It says 'Rossing describes measurements of pitch discrimination with pure tones at about 80 dB for frequencies between 1 and 4 kHz. The jnd is found to be about 0.5% of the pure tone frequency, which corresponds to about 8¢ . He [Rossing] also states that the jnd has been found to depend upon the frequency, the sound level, the duration of the tone, the suddenness of the frequency change, the musical training of the listener, and the method of measurement
.' Elsewhere, the article also suggests 'You can hear about a nickel's worth of difference
' which means jnd (pitch) = 5 cents (obviously US money).
: Rossing, Thomas D., The Science of Sound 2nd ed, Addison-Wesley 1990