Stevens, S.S. (1935). The relation of pitch to intensity. J. Acoust. Soc. Am. 6, 150-154

Measurements were made of the amount by which the pitch of tones ranging from 150 to 12,000 cycles is changed by an increase in intensity. Observers were presented alternately with two tones of different frequency and required to make them sound equal in pitch by varying the intensity of one of the tones. The results, when plotted as equal pitch contours, show (1) the pitch of high tones increases with intensity, (2) the pitch of low tones decreases with intensity, (3) the point at which the effect reverses varies with intensity level. The fact that the points of reversal correspond quite closely with the points of greates sensitivity of the ear, as shown by contours of equal loudness, suggests that the change in pitch with intensity is due to the resonant characteristics of the ear.