How do slit drums produce several different pitches?

In the context of African music, slit drums are idiophones crafted from hollowed-out logs. They produce different pitches primarily through the careful thinning of the instrument's flanks at specific locations. By varying the thickness of the edges along the slit, the drum can be tuned to yield multiple distinct pitches—sometimes as many as four.12

These instruments are valued for their resonance and carrying power, which allow them to be used for signaling and transmitting messages by reproducing the inflections of human speech.12