Imagine that a singer sings the same note twice, one an octave above the other. You can hear a difference between these two sounds. That s because their pitch is different. Pitch depends on the frequency of a sound wave.
Frequency is the number of wavelengths that fit into one unit of time.
Remember that a wavelength is equal to one compression and one rarefaction. Even though the singer sang the same note, because the sounds had different frequencies, we heard them as different. Frequencies are measured in Hertz. One Hertz is equal to one cycle of compression and rarefaction, per second. High sounds have high frequencies and low sounds have low frequencies.