Acoustic apparatus and operation

A directional acoustic warning device, suitable for creating a greater sound pressure at a selected target than around the target, wherein the device has a sound pressure generating source coupled to a sound guiding structure, wherein the sound pressure generating source comprises an electronic signal generating system connected to a transducer, the transducer being capable of converting electronic signals from the signal generating system to sound pressure waves, and wherein

• the signal generating system is arranged to produce a first signal having a waveform with a frequency which is varied around a first resonant frequency of the human ear canal, and wherein
• the sound guiding structure includes
• an open-ended cone-shaped horn having a narrow end and a wide end, wherein the narrow end of the horn is located toward the sound pressure generating source and, a horn ring of acoustically absorbent material provided at the wide end of the horn, and, a phase plug provided within the horn, and
• a phase plug ring of acoustically absorbent material provided around a part of the phase plug, and wherein
• the dimensions of the horn and the phase plug and the position and dimensions of the horn ring and phase plug ring are selected to maximise the directionality of the sound and minimise sound level exposure to a user of the device and bystanders at frequencies around the first resonant frequency of the human ear canal;
• wherein the phase plug is a bi-conic shape, formed from first and second straight-walled cones each cone having a tip end and a base end which is wider than the tip end, wherein the cones are joined at their base ends to form an apex of the bi-conic shape, and mounted within the horn such that a first cone of the phase plug has a tip end towards the throat of the horn cone.