The purpose of this design is to detect when coconut beetles are eating coconut trees. Each coconut tree will have at least one piezoelectric microphone attached to cover the tree.
There are two possible designs, one where the microphone is enclosed to protect from weather, another is exposed which makes it more sensitive to nearby noise - potentially coconut beetles.
A third design will place the mic on top of an embedded nail, the idea is it will amplify noise from further distances.