Fully clockwise (wide open), the filter pass frequency is set to 6 kHz, which will essentially let everything useful come through. As you start turning counter-clockwise, you gradually lose high frequencies, with the signal being reduced by 12 dB per octave above the pass frequency. The center position is roughly 1.3-1.4 kHz, and at fully counter-clockwise, everything above 350 Hz is being reduced (approximately the fundamental frequency of an F played on the first fret of the high E string with standard guitar tuning).