Sound waves are mechanical waves , mostly sinosoidal which means that they behave like a sine or cosine wave function ...when two sound waves meet they produce interference i.e if one sound wave can be represented by sinA and other by sinB (A=2pi X f1, B=2pi X f2; f1 & f2 being the respective frequencies) then the resultant wave at the point of interference will be sinA + sinB .. You can expand this result using trignometric identities of sine addition and at a certain frequency this resultant would reach a maximum ... This frequency is further called the resonant frequency ...
