How a bongos makes sound

The sound from the bongos is created by your palm hitting the bongos causing the air inside the bongos to vibrate creating a sound. To make the higher sound you tighten the edges of the drum tightening the skin making a higher pitch. To make a lower sound you loosen the edges to make a lower note. They are joint  instruments one small and one big, the reason that there are two is so you don't have to change the pitch while your playing.