A method of using acoustic signals in the form of waves or pulses to non-destructively measure the thickness of a bonding layer sandwiched between and bonding together overlying and underlying materials different from the bonding layer especially when the thickness of the bonding layer is so small that the features (maxima, minima, time position) of the echo from the interface of the bonding layer and the overlying material is indistinguishable, i.e., not independently observable from the features (maxima, minima, time position) of the echo from the interface of the bonding layer and the underlying material.