What's the difference between Resonant box and flat board ?
The difference between the two is the appearance of the sound hole and the lack of a sound hole. The resonant box has a sound hole which creates "wah-wah" effects that can be more manually encouraged by covering or uncovering the sound hole.
One the other hand, the flat board does not have a sound hole. Historically the lack of the sound hole on the flat board was the most common one found in Africa. To amplify the sound of the flat board they would stick the kalimba onto a hollowed out gourd to create the amplified sound.