Device (Shopkick) that make sound that humans cant hear

I want a device similar to what ShopKick (iphone app) uses to detect when people enter the building. The sound box gives off a high pitch tune that humans or pets cannot hear, but the sound can be picked up by a smartphone microphone. The device should be able to plug into usa sockets. The box should be small as possible and deliver a sound that can be detected over a 5000 sq ft area. The device should deliver a constant consistent tune without stopping or interruption. The device should be able to plug into a computer where you can change or add the tune to the device. Device only need small memory to hold one tune at a time. Here is an article of the shopkick device: [login to view URL] Not sure of my budget for a product design...opent to all bids. Will be looking for someone to build also after product design (if you can do both). Would like the cost of makeing the device to be less than $50 a box. The cheaper the better. Would like test done to make sure sound is not heard by babies, teens, pets, and adults. Also would liek to know if sound can be heard when other music is playing.