The classic wooden cleft / wood block sound, heard in many Igbo musical genres, although made particularly famous by Fela Kuti's Afrobeat genre from the 1970s/1980s.
These instruments are the true pulse of Afrobeat!
Check this link to hear Okpokolo on an Igbo Ogene ensemble:
Medium (approx 8.5 inches)
Large (approx 11.5 inches)