Self-adaptive ultra-sonic touch sensor

A method of configuring a touch sensor includes transmitting an ultra-sonic test signal induced by a first excitation signal towards a touch structure that has a first interface with an enclosed interior volume of the touch sensor and a second interface with an external environment; receiving a plurality of ultra-sonic reflected signals produced from the ultra-sonic test signal and the touch structure, including a first ultra-sonic reflected signal internally reflected by the first interface and a last ultra-sonic reflected signal internally reflected by the second interface; determining a last time of flight corresponding to the last ultra-sonic reflected signal; and selectively configuring a second excitation signal based on the last time of flight. The second excitation signal is used for inducing further ultra-sonic signals.