I believe you have to get a high frequency speaker as well, i dont know what you call it. Maybe the ones in those bug deterrant things you plug into the wall will work, i dont know??
Piezos are often used but an small speker wod work.You dont need high power i made one whith a speker that was waery silent in audio range but wen i thurned the potencimeter to high freq. there was about 7 dogs going nuts!
You can use a ultra sound speaker. They used them in old TVs for remote control(must have made the dogs crazy) and the use them for parking helpers today(radar parking)... I just don`t know if they are omnidirectional
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their not omnidirectional, it's beamwidth is around 90 degrees, but it bounces off solid objects very well and that property makes the ultrasonic waves travel futher.It would be very nice if you could use ultrasonic because it's designed for frequencies just above the human hearing range(above 20KHz) but more expensive compared to ceramic transducers and normal speakers
BTW: Do you know about a dog's hearing frequency range?