Knock On Wood is an entertaining way to ward off bad spirits. By touching different parts of the wood, you can get different sounds. If you enjoy the sounds, you can use it as an amazing ringtone, notification sound, or alarm. There are many types of wood that you can choose from: 

Bakau

Beech

Birch

Bog Oak

Cherry

Douglas

Knotted

Mansonia

Maple

Mariposa

Padauk

Pine

Poplar

Rosewood

Scots Pine

Sinchewitz

Walnut

If you are not sure if you want to use the specific type of wood, just try it and you will know what sound it is. It is best to use the full version of the app because you will not be able to get all the different types of wood.