Abstract: A resonant type transducer transmitting or receiving narrowband continuous waves into or from a propagation medium, comprising a piezoelectric or electro strictive vibrator having a specific acoustic impedance; a single matching layer or multiple matching layers contacting the piezoelectric or electro strictive vibrator as well as the propagation medium to maximise power transfer to and from the transducer and the propagation medium. The equivalent specific acoustic impedance of the matching layers and propagation medium is complex conjugate to the equivalent specific acoustic impedance of the piezoelectric or electro strictive vibrator and backing layer at the frequency of operation resulting in reflection of the travelling wave in the layers resulting in in-phase addition and maximum power transfer.