The paper deals with the design of a sound recognition system focused on an ultra low power hardware implementation in a button like miniature form factor. We present the results of the first design phase focused on selection and experimental evaluation of sound classes and algorithms suitable for low power realization. We also present the VHDL model of the hardware showing that our method can be implemented with minimal resources. Our approach is based on spectrum analysis to distinguish between a subset of sound sources with a clear audio signature. It also uses intensity analysis from microphones placed at different locations to correlate the sounds with user activity.