| active sonar whose principal output is typically a very directional low-frequency beam formed by the nonlinear interaction in space of two high-frequency signals transmitted simultaneously in narrow beams in the same direction|
Note 1 to entry: When the transmitted signal consists of two harmonic primary waves, the principal secondary waves resulting from the nonlinear interaction are formed at frequencies equal to the sum and difference-frequencies of the primary waves.
Note 2 to entry: Two characteristic properties of the difference-frequency wave are extreme directionality and large bandwidth at a low frequency.
Note 3 to entry: In order to facilitate the nonlinear interaction, high-amplitude sound waves shall be generated by the transducer, with the waves exhibiting finite-amplitude propagation in the water medium. The efficiency of the parametric conversion depends on the nonlinearity parameter of the medium, which for water is rather low.
Note 4 to entry: Notwithstanding the typically low efficiency of parametric conversion, exploitation of bandwidth can enable a substantial processing gain.
Note 5 to entry: The nonlinear interaction generates virtual sources that are phased so that they add coherently in the end-fire direction, leading to the parametric sonar array sometimes being referred to as a "virtual end-fire array".
Note 6 to entry: Note that the use in underwater acoustics of the words "transmit", "transmitter" and "transmission" to describe the process of sound radiation by a source is commonplace, but this deviates from the usage prescribed in IEV 702-02-05 where the words "emit", "emitter" and "emission" are preferred.