"Pitch" is the highness or lowness of sound. Pitch is how humans hear different frequencies. Frequency is determined by the number of vibrations per second. The highest key on a piano, for instance, vibrates 4,000 times per second. It has a frequency of 4000 hertz (Hz), or 4 kilohertz (kHz). Lower keys have lower frequencies. A note an octave higher than another note has twice the frequency of that note. 