Abstract: A sounder for use in mobile apparatus comprising an electronic signal generator and a electroacoustic transducer to generate an audible sound, wherein the sound comprises continuous repetitions of a pre-determined section of substantially broadband sound.

Abstract: A sounder for use in mobile apparatus comprising an electronic signal generator and a electroacoustic transducer to generate an audible sound, wherein the sound comprises continuous repetitions of a pre-determined section of substantially broadband sound.

Abstract: Apparatus for attenuating a sound wave propagating in a given direction along a fluid-containing duct comprises two sound sources spaced along the duct in a given direction; a sound detector between the two sources at a null point at which sound emanating only from the two sources substantially cancels; and means for utilizing the output of the sound detector including a phase-shifter and a frequency-sensitive amplifier to control the operation of the sound sources so that they emit sound in relative antiphase and at equal amplitudes and at such phases relative to the phase of the detected sound that the resultant of the sound radiations emitted in the given direction substantially attenuates the sound wave propagating along the duct.