Each note of a kalimba, mbira, thumb piano, etc. is a separate idiophone, and in orchestral terms, the instrument as a whole belongs in the bar percussion family. Furthermore, the thumbs are not exclusively used, as some instruments are played with the thumbs and other fingers also.
This thumb piano consist of a wooden board to which metal tines of varying lengths are affixed. It has a mechanism for readily tuning the tines to different scales. The longest tines are typically in the center, with shorter (and thus higher-pitched) tines arranged alternately in ascending order towards both sides of the instrument. The thumb piano is most commonly held in both hands, with both thumbs being used to pluck tines either simultaneously or in turn
This thumb piano is made of different woods - the back is real polished Coconut and yes its round. The sound board is hand carved with a with a resonate hole. The hollow resonating chamber increases the volume.
You can hear one of these on the TOCA site here -
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