I've created a transmission protocol in order to transmit information bits between a PC speaker and a smartphone, or between 2 smartphones using ultrasounds. I've implemented a QFSK system which uses 4 frequencies and a 3 kHz bandwidth. Now I would like to build my own Transmitter/Receiver using Rasberry PI or any other SBC. I'm having problems selecting the right speaker for my transmisor because I want the system to operate in the ultrasound region so it won't disturb the users.
I've found some ultrasound 40 kHz transducers but they are mainly used for motion and distance sensing and have a 5.0kHz transmssion bandwidth. Although this devices bandwidth seem to work for me I don't know if you can use them to send a custom made audio signal like the one i want to send. I would also like to have a speaker with greater operation bandwidth, so I could implement a more bandwidth consuming modulation such as 16fsk in order to increase the bit rate.
Are there speakers with a flat frequency response, lets say, between 20 and 48 kHz and could be loud enough for reaching a microphone that is 2 or 3 m away?
It would be appreciated if they were also small and directional. Does such device exist? Maybe all of this features cannot be attained by single device, then one with most of these would do.